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---
title: Qiskit 0.20 release notes
description: Changes made in Qiskit 0.20
in_page_toc_max_heading_level: 4
---
# Qiskit 0.20 release notes
## 0.20.1
<span id="id405" />
### Terra 0.15.2
<span id="release-notes-0-15-2-bug-fixes" />
<span id="id406" />
#### Bug Fixes
* When accessing the `definition` attribute of a parameterized `Gate` instance, the generated `QuantumCircuit` had been generated with an invalid `ParameterTable`, such that reading from `QuantumCircuit.parameters` or calling `QuantumCircuit.bind_parameters` would incorrectly report the unbound parameters. This has been resolved.
* `SXGate().inverse()` had previously returned an ‘sx\_dg’ gate with a correct `definition` but incorrect `to_matrix`. This has been updated such that `SXGate().inverse()` returns an `SXdgGate()` and vice versa.
* `Instruction.inverse()`, when not overridden by a subclass, would in some cases return a `Gate` instance with an incorrect `to_matrix` method. The instances of incorrect `to_matrix` methods have been removed.
* For `C3XGate` with a non-zero `angle`, inverting the gate via `C3XGate.inverse()` had previously generated an incorrect inverse gate. This has been corrected.
* The `MCXGate` modes have been updated to return a gate of the same mode when calling `.inverse()`. This resolves an issue where in some cases, transpiling a circuit containing the inverse of an `MCXVChain` gate would raise an error.
* Previously, when creating a multiply controlled phase gate via `PhaseGate.control`, an `MCU1Gate` gate had been returned. This has been had corrected so that an `MCPhaseGate` is returned.
* Previously, attempting to decompose a circuit containing an `MCPhaseGate` would raise an error due to an inconsistency in the definition of the `MCPhaseGate`. This has been corrected.
* `QuantumCircuit.compose` and `DAGCircuit.compose` had, in some cases, incorrectly translated conditional gates if the input circuit contained more than one `ClassicalRegister`. This has been resolved.
* Fixed an issue when creating a [`qiskit.result.Counts`](/api/qiskit/0.45/qiskit.result.Counts "qiskit.result.Counts") object from an empty data dictionary. Now this will create an empty [`Counts`](/api/qiskit/0.45/qiskit.result.Counts "qiskit.result.Counts") object. The [`most_frequent()`](/api/qiskit/0.45/qiskit.result.Counts#most_frequent "qiskit.result.Counts.most_frequent") method is also updated to raise a more descriptive exception when the object is empty. Fixes [#5017](https://github.com/Qiskit/qiskit/issues/5017)
* Extending circuits with differing registers updated the `qregs` and `cregs` properties accordingly, but not the `qubits` and `clbits` lists. As these are no longer generated from the registers but are cached lists, this lead to a discrepancy of registers and bits. This has been fixed and the `extend` method explicitly updates the cached bit lists.
* Fix bugs of the concrete implementations of meth:\~qiskit.circuit.ControlledGate.inverse method which do not preserve the `ctrl_state` parameter.
* A bug was fixed that caused long pulse schedules to throw a recursion error.
<span id="id407" />
### Aer 0.6.1
No change
<span id="id408" />
### Ignis 0.4.0
No change
<span id="id409" />
### Aqua 0.7.5
No change
<span id="ibm-q-provider-0-8-0" />
### IBM Q Provider 0.8.0
No change
<span id="qiskit-0-20-0" />
## 0.20.0
<span id="terra-0-15-1" />
### Terra 0.15.1
<span id="release-notes-0-15-0-prelude" />
<span id="id410" />
#### Prelude
The 0.15.0 release includes several new features and bug fixes. Some highlights for this release are:
This release includes the introduction of arbitrary basis translation to the transpiler. This includes support for directly targeting a broader range of device basis sets, e.g. backends implementing RZ, RY, RZ, CZ or iSwap gates.
The [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") class now tracks global phase. This means controlling a circuit which has global phase now correctly adds a relative phase, and gate matrix definitions are now exact rather than equal up to a global phase.
<span id="release-notes-0-15-0-new-features" />
<span id="id411" />
#### New Features
* A new DAG class [`qiskit.dagcircuit.DAGDependency`](/api/qiskit/0.45/qiskit.dagcircuit.DAGDependency "qiskit.dagcircuit.DAGDependency") for representing the dependency form of circuit, In this DAG, the nodes are operations (gates, measure, barrier, etc…) and the edges corresponds to non-commutation between two operations.
* Four new functions are added to [`qiskit.converters`](/api/qiskit/0.45/converters#module-qiskit.converters "qiskit.converters") for converting back and forth to [`DAGDependency`](/api/qiskit/0.45/qiskit.dagcircuit.DAGDependency "qiskit.dagcircuit.DAGDependency"). These functions are:
* [`circuit_to_dagdependency()`](/api/qiskit/0.45/converters#qiskit.converters.circuit_to_dagdependency "qiskit.converters.circuit_to_dagdependency") to convert from a [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") object to a [`DAGDependency`](/api/qiskit/0.45/qiskit.dagcircuit.DAGDependency "qiskit.dagcircuit.DAGDependency") object.
* [`dagdependency_to_circuit()`](/api/qiskit/0.45/converters#qiskit.converters.dagdependency_to_circuit "qiskit.converters.dagdependency_to_circuit") to convert from a [`DAGDependency`](/api/qiskit/0.45/qiskit.dagcircuit.DAGDependency "qiskit.dagcircuit.DAGDependency") object to a [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") object.
* [`dag_to_dagdependency()`](/api/qiskit/0.45/converters#qiskit.converters.dag_to_dagdependency "qiskit.converters.dag_to_dagdependency") to convert from a [`DAGCircuit`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit "qiskit.dagcircuit.DAGCircuit") object to a [`DAGDependency`](/api/qiskit/0.45/qiskit.dagcircuit.DAGDependency "qiskit.dagcircuit.DAGDependency") object.
* [`dagdependency_to_dag()`](/api/qiskit/0.45/converters#qiskit.converters.dagdependency_to_dag "qiskit.converters.dagdependency_to_dag") to convert from a [`DAGDependency`](/api/qiskit/0.45/qiskit.dagcircuit.DAGDependency "qiskit.dagcircuit.DAGDependency") object to a [`DAGCircuit`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit "qiskit.dagcircuit.DAGCircuit") object.
For example:
```python
from qiskit.converters.dagdependency_to_circuit import dagdependency_to_circuit
from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
circuit_in = QuantumCircuit(2)
circuit_in.h(qr[0])
circuit_in.h(qr[1])
dag_dependency = circuit_to_dagdependency(circuit_in)
circuit_out = dagdepency_to_circuit(dag_dependency)
```
* Two new transpiler passes have been added to [`qiskit.transpiler.passes`](/api/qiskit/0.45/transpiler_passes#module-qiskit.transpiler.passes "qiskit.transpiler.passes") The first, [`UnrollCustomDefinitions`](/api/qiskit/0.45/qiskit.transpiler.passes.UnrollCustomDefinitions "qiskit.transpiler.passes.UnrollCustomDefinitions"), unrolls all instructions in the circuit according to their [`definition`](/api/qiskit/0.45/qiskit.circuit.Instruction#definition "qiskit.circuit.Instruction.definition") property, stopping when reaching either the specified `basis_gates` or a set of gates in the provided [`EquivalenceLibrary`](/api/qiskit/0.45/qiskit.circuit.EquivalenceLibrary "qiskit.circuit.EquivalenceLibrary"). The second, [`BasisTranslator`](/api/qiskit/0.45/qiskit.transpiler.passes.BasisTranslator "qiskit.transpiler.passes.BasisTranslator"), uses the set of translations in the provided [`EquivalenceLibrary`](/api/qiskit/0.45/qiskit.circuit.EquivalenceLibrary "qiskit.circuit.EquivalenceLibrary") to re-write circuit instructions in a specified basis.
* A new `translation_method` keyword argument has been added to [`transpile()`](/api/qiskit/0.45/compiler#qiskit.compiler.transpile "qiskit.compiler.transpile") to allow selection of the method to be used for translating circuits to the available device gates. For example, `transpile(circ, backend, translation_method='translator')`. Valid choices are:
* `'unroller'`: to use the [`Unroller`](/api/qiskit/0.45/qiskit.transpiler.passes.Unroller "qiskit.transpiler.passes.Unroller") pass
* `'translator'`: to use the [`BasisTranslator`](/api/qiskit/0.45/qiskit.transpiler.passes.BasisTranslator "qiskit.transpiler.passes.BasisTranslator") pass.
* `'synthesis'`: to use the [`UnitarySynthesis`](/api/qiskit/0.45/qiskit.transpiler.passes.UnitarySynthesis "qiskit.transpiler.passes.UnitarySynthesis") pass.
The default value is `'translator'`.
* A new class for handling counts result data, [`qiskit.result.Counts`](/api/qiskit/0.45/qiskit.result.Counts "qiskit.result.Counts"), has been added. This class is a subclass of `dict` and can be interacted with like any other dictionary. But, it includes helper methods and attributes for dealing with counts results from experiments and also handles post processing and formatting of binary strings at object initialization. A [`Counts`](/api/qiskit/0.45/qiskit.result.Counts "qiskit.result.Counts") object can be created by passing a dictionary of counts with the keys being either integers, hexadecimal strings of the form `'0x4a'`, binary strings of the form `'0b1101'`, a bit string formatted across register and memory slots (ie `'00 10'`), or a dit string. For example:
```python
from qiskit.result import Counts
counts = Counts({"0x0': 1, '0x1', 3, '0x2': 1020})
```
* A new method for constructing [`qiskit.dagcircuit.DAGCircuit`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit "qiskit.dagcircuit.DAGCircuit") objects has been added, `from_networkx()`. This method takes in a networkx `MultiDiGraph` object (in the format returned by `to_networkx()`) and will return a new [`DAGCircuit`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit "qiskit.dagcircuit.DAGCircuit") object. The intent behind this function is to enable transpiler pass authors to leverage networkx’s [graph algorithm library](https://networkx.github.io/documentation/stable/reference/algorithms/index.html) if a function is missing from the [retworkx API](https://retworkx.readthedocs.io/en/latest/api.html). Although, hopefully in such casses an issue will be opened with [retworkx issue tracker](https://github.com/Qiskit/retworkx/issues) (or even better a pull request submitted).
* A new kwarg for `init_qubits` has been added to [`assemble()`](/api/qiskit/0.45/compiler#qiskit.compiler.assemble "qiskit.compiler.assemble") and `execute()`. For backends that support this feature `init_qubits` can be used to control whether the backend executing the circuits inserts any initialization sequences at the start of each shot. By default this is set to `True` meaning that all qubits can assumed to be in the ground state at the start of each shot. However, when `init_qubits` is set to `False` qubits will be uninitialized at the start of each experiment and between shots. Note, that the backend running the circuits has to support this feature for this flag to have any effect.
* A new kwarg `rep_delay` has been added to [`qiskit.compiler.assemble()`](/api/qiskit/0.45/compiler#qiskit.compiler.assemble "qiskit.compiler.assemble"), `qiskit.execute.execute()`, and the constructor for `PulseQobjtConfig`.qiskit This new kwarg is used to denotes the time between program executions. It must be chosen from the list of valid values set as the `rep_delays` from a backend’s [`PulseBackendConfiguration`](/api/qiskit/0.45/qiskit.providers.models.PulseBackendConfiguration "qiskit.providers.models.PulseBackendConfiguration") object which can be accessed as `backend.configuration().rep_delays`).
The `rep_delay` kwarg will only work on backends which allow for dynamic repetition time. This will also be indicated in the [`PulseBackendConfiguration`](/api/qiskit/0.45/qiskit.providers.models.PulseBackendConfiguration "qiskit.providers.models.PulseBackendConfiguration") object for a backend as the `dynamic_reprate_enabled` attribute. If `dynamic_reprate_enabled` is `False` then the `rep_time` value specified for [`qiskit.compiler.assemble()`](/api/qiskit/0.45/compiler#qiskit.compiler.assemble "qiskit.compiler.assemble"), `qiskit.execute.execute()`, or the constructor for `PulseQobjtConfig` will be used rather than `rep_delay`. `rep_time` only allows users to specify the duration of a program, rather than the delay between programs.
* The `qobj_schema.json` JSON Schema file in `qiskit.schemas` has been updated to include the `rep_delay` as an optional configuration property for pulse qobjs.
* The `backend_configuration_schema.json` JSON Schema file in mod:qiskit.schemas has been updated to include `rep_delay_range` and `default_rep_delay` as optional properties for a pulse backend configuration.
* A new attribute, [`global_phase`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#global_phase "qiskit.circuit.QuantumCircuit.global_phase"), which is is used for tracking the global phase has been added to the [`qiskit.circuit.QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") class. For example:
```python
import math
from qiskit import QuantumCircuit
circ = QuantumCircuit(1, global_phase=math.pi)
circ.u1(0)
```
The global phase may also be changed or queried with `circ.global_phase` in the above example. In either case the setting is in radians. If the circuit is converted to an instruction or gate the global phase is represented by two single qubit rotations on the first qubit.
This allows for other methods and functions which consume a [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") object to take global phase into account. For example. with the [`global_phase`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#global_phase "qiskit.circuit.QuantumCircuit.global_phase") attribute the [`to_matrix()`](/api/qiskit/0.45/qiskit.circuit.Gate#to_matrix "qiskit.circuit.Gate.to_matrix") method for a gate can now exactly correspond to its decompositions instead of just up to a global phase.
The same attribute has also been added to the [`DAGCircuit`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit "qiskit.dagcircuit.DAGCircuit") class so that global phase can be tracked when converting between [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") and [`DAGCircuit`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit "qiskit.dagcircuit.DAGCircuit").
* Two new classes, [`AncillaRegister`](/api/qiskit/0.45/qiskit.circuit.AncillaRegister "qiskit.circuit.AncillaRegister") and [`AncillaQubit`](/api/qiskit/0.45/qiskit.circuit.AncillaQubit "qiskit.circuit.AncillaQubit") have been added to the [`qiskit.circuit`](/api/qiskit/0.45/circuit#module-qiskit.circuit "qiskit.circuit") module. These are subclasses of [`QuantumRegister`](/api/qiskit/0.45/qiskit.circuit.QuantumRegister "qiskit.circuit.QuantumRegister") and [`Qubit`](/api/qiskit/0.45/qiskit.circuit.Qubit "qiskit.circuit.Qubit") respectively and enable marking qubits being ancillas. This will allow these qubits to be re-used in larger circuits and algorithms.
* A new method, [`control()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#control "qiskit.circuit.QuantumCircuit.control"), has been added to the [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit"). This method will return a controlled version of the [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") object, with both open and closed controls. This functionality had previously only been accessible via the [`Gate`](/api/qiskit/0.45/qiskit.circuit.Gate "qiskit.circuit.Gate") class.
* A new method [`repeat()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#repeat "qiskit.circuit.QuantumCircuit.repeat") has been added to the [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") class. It returns a new circuit object containing a specified number of repetitions of the original circuit. For example:
```python
from qiskit.circuit import QuantumCircuit
qc = QuantumCircuit(2)
qc.h(0)
qc.cx(0, 1)
repeated_qc = qc.repeat(3)
repeated_qc.decompose().draw(output='mpl')
```
The parameters are copied by reference, meaning that if you update the parameters in one instance of the circuit all repetitions will be updated.
* A new method [`reverse_bits()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#reverse_bits "qiskit.circuit.QuantumCircuit.reverse_bits") has been added to the [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") class. This method will reverse the order of bits in a circuit (both quantum and classical bits). This can be used to switch a circuit from little-endian to big-endian and vice-versa.
* A new method, [`combine_into_edge_map()`](/api/qiskit/0.45/qiskit.transpiler.Layout#combine_into_edge_map "qiskit.transpiler.Layout.combine_into_edge_map"), was added to the [`qiskit.transpiler.Layout`](/api/qiskit/0.45/qiskit.transpiler.Layout "qiskit.transpiler.Layout") class. This method enables converting converting two [`Layout`](/api/qiskit/0.45/qiskit.transpiler.Layout "qiskit.transpiler.Layout") objects into a qubit map for composing two circuits.
* A new class, `ConfigurableFakeBackend`, has been added to the `qiskit.test.mock.utils` module. This new class enables the creation of configurable mock backends for use in testing. For example:
```python
from qiskit.test.mock.utils import ConfigurableFakeBackend
backend = ConfigurableFakeBackend("Tashkent",
n_qubits=100,
version="0.0.1",
basis_gates=['u1'],
qubit_t1=99.,
qubit_t2=146.,
qubit_frequency=5.,
qubit_readout_error=0.01,
single_qubit_gates=['u1'])
```
will create a backend object with 100 qubits and all the other parameters specified in the constructor.
* A new method [`draw()`](/api/qiskit/0.45/qiskit.circuit.EquivalenceLibrary#draw "qiskit.circuit.EquivalenceLibrary.draw") has been added to the [`qiskit.circuit.EquivalenceLibrary`](/api/qiskit/0.45/qiskit.circuit.EquivalenceLibrary "qiskit.circuit.EquivalenceLibrary") class. This method can be used for drawing the contents of an equivalence library, which can be useful for debugging. For example:
```python
from numpy import pi
from qiskit.circuit import EquivalenceLibrary
from qiskit.circuit import QuantumCircuit
from qiskit.circuit import QuantumRegister
from qiskit.circuit import Parameter
from qiskit.circuit.library import HGate
from qiskit.circuit.library import U2Gate
from qiskit.circuit.library import U3Gate
my_equiv_library = EquivalenceLibrary()
q = QuantumRegister(1, 'q')
def_h = QuantumCircuit(q)
def_h.append(U2Gate(0, pi), [q[0]], [])
my_equiv_library.add_equivalence(HGate(), def_h)
theta = Parameter('theta')
phi = Parameter('phi')
lam = Parameter('lam')
def_u2 = QuantumCircuit(q)
def_u2.append(U3Gate(pi / 2, phi, lam), [q[0]], [])
my_equiv_library.add_equivalence(U2Gate(phi, lam), def_u2)
my_equiv_library.draw()
```
* A new Phase instruction, `SetPhase`, has been added to [`qiskit.pulse`](/api/qiskit/0.45/pulse#module-qiskit.pulse "qiskit.pulse"). This instruction sets the phase of the subsequent pulses to the specified phase (in radians. For example:
```python
import numpy as np
from qiskit.pulse import DriveChannel
from qiskit.pulse import Schedule
from qiskit.pulse import SetPhase
sched = Schedule()
sched += SetPhase(np.pi, DriveChannel(0))
```
In this example, the phase of the pulses applied to `DriveChannel(0)` after the `SetPhase` instruction will be set to $\pi$ radians.
* A new pulse instruction `ShiftFrequency` has been added to [`qiskit.pulse.instructions`](/api/qiskit/0.45/pulse#module-qiskit.pulse.instructions "qiskit.pulse.instructions"). This instruction enables shifting the frequency of a channel from its set frequency. For example:
```python
from qiskit.pulse import DriveChannel
from qiskit.pulse import Schedule
from qiskit.pulse import ShiftFrequency
sched = Schedule()
sched += ShiftFrequency(-340e6, DriveChannel(0))
```
In this example all the pulses applied to `DriveChannel(0)` after the `ShiftFrequency` command will have the envelope a frequency decremented by 340MHz.
* A new method [`conjugate()`](/api/qiskit/0.45/qiskit.circuit.ParameterExpression#conjugate "qiskit.circuit.ParameterExpression.conjugate") has been added to the [`ParameterExpression`](/api/qiskit/0.45/qiskit.circuit.ParameterExpression "qiskit.circuit.ParameterExpression") class. This enables calling `numpy.conj()` without raising an error. Since a [`ParameterExpression`](/api/qiskit/0.45/qiskit.circuit.ParameterExpression "qiskit.circuit.ParameterExpression") object is real, it will return itself. This behaviour is analogous to Python floats/ints.
* A new class [`PhaseEstimation`](/api/qiskit/0.45/qiskit.circuit.library.PhaseEstimation "qiskit.circuit.library.PhaseEstimation") has been added to [`qiskit.circuit.library`](/api/qiskit/0.45/circuit_library#module-qiskit.circuit.library "qiskit.circuit.library"). This circuit library class is the circuit used in the original formulation of the phase estimation algorithm in [arXiv:quant-ph/9511026](https://arxiv.org/pdf/quant-ph/9511026.pdf). Phase estimation is the task to to estimate the phase $\phi$ of an eigenvalue $e^{2\pi i\phi}$ of a unitary operator $U$, provided with the corresponding eigenstate $|psi\rangle$. That is
$$
U|\psi\rangle = e^{2\pi i\phi} |\psi\rangle
$$
This estimation (and thereby this circuit) is a central routine to several well-known algorithms, such as Shor’s algorithm or Quantum Amplitude Estimation.
* The [`qiskit.visualization`](/api/qiskit/0.45/visualization#module-qiskit.visualization "qiskit.visualization") function [`plot_state_qsphere()`](/api/qiskit/0.45/qiskit.visualization.plot_state_qsphere "qiskit.visualization.plot_state_qsphere") has a new kwarg `show_state_labels` which is used to control whether each blob in the qsphere visualization is labeled. By default this kwarg is set to `True` and shows the basis states next to each blob by default. This feature can be disabled, reverting to the previous behavior, by setting the `show_state_labels` kwarg to `False`.
* The [`qiskit.visualization`](/api/qiskit/0.45/visualization#module-qiskit.visualization "qiskit.visualization") function [`plot_state_qsphere()`](/api/qiskit/0.45/qiskit.visualization.plot_state_qsphere "qiskit.visualization.plot_state_qsphere") has a new kwarg `show_state_phases` which is set to `False` by default. When set to `True` it displays the phase of each basis state.
* The [`qiskit.visualization`](/api/qiskit/0.45/visualization#module-qiskit.visualization "qiskit.visualization") function [`plot_state_qsphere()`](/api/qiskit/0.45/qiskit.visualization.plot_state_qsphere "qiskit.visualization.plot_state_qsphere") has a new kwarg `use_degrees` which is set to `False` by default. When set to `True` it displays the phase of each basis state in degrees, along with the phase circle at the bottom right.
* A new class, [`QuadraticForm`](/api/qiskit/0.45/qiskit.circuit.library.QuadraticForm "qiskit.circuit.library.QuadraticForm") to the [`qiskit.circuit.library`](/api/qiskit/0.45/circuit_library#module-qiskit.circuit.library "qiskit.circuit.library") module for implementing a a quadratic form on binary variables. The circuit library element implements the operation
$$
|x\rangle |0\rangle \mapsto |x\rangle |Q(x) \mod 2^m\rangle
$$
for the quadratic form $Q$ and $m$ output qubits. The result is in the $m$ output qubits is encoded in two’s complement. If $m$ is not specified, the circuit will choose the minimal number of qubits required to represent the result without applying a modulo operation. The quadratic form is specified using a matrix for the quadratic terms, a vector for the linear terms and a constant offset. If all terms are integers, the circuit implements the quadratic form exactly, otherwise it is only an approximation.
For example:
```python
import numpy as np
from qiskit.circuit.library import QuadraticForm
A = np.array([[1, 2], [-1, 0]])
b = np.array([3, -3])
c = -2
m = 4
quad_form_circuit = QuadraticForm(m, A, b, c)
```
* Add [`qiskit.quantum_info.Statevector.expectation_value()`](/api/qiskit/0.45/qiskit.quantum_info.Statevector#expectation_value "qiskit.quantum_info.Statevector.expectation_value") and [`qiskit.quantum_info.DensityMatrix.expectation_value()`](/api/qiskit/0.45/qiskit.quantum_info.DensityMatrix#expectation_value "qiskit.quantum_info.DensityMatrix.expectation_value") methods for computing the expectation value of an [`qiskit.quantum_info.Operator`](/api/qiskit/0.45/qiskit.quantum_info.Operator "qiskit.quantum_info.Operator").
* For the `seed` kwarg in the constructor for [`qiskit.circuit.library.QuantumVolume`](/api/qiskit/0.45/qiskit.circuit.library.QuantumVolume "qiskit.circuit.library.QuantumVolume") [numpy random Generator objects](https://numpy.org/doc/stable/reference/random/generator.html) can now be used. Previously, only integers were a valid input. This is useful when integrating [`QuantumVolume`](/api/qiskit/0.45/qiskit.circuit.library.QuantumVolume "qiskit.circuit.library.QuantumVolume") as part of a larger function with its own random number generation, e.g. generating a sequence of [`QuantumVolume`](/api/qiskit/0.45/qiskit.circuit.library.QuantumVolume "qiskit.circuit.library.QuantumVolume") circuits.
* The [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") method [`compose()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#compose "qiskit.circuit.QuantumCircuit.compose") has a new kwarg `front` which can be used for prepending the other circuit before the origin circuit instead of appending. For example:
```python
from qiskit.circuit import QuantumCircuit
circ1 = QuantumCircuit(2)
circ2 = QuantumCircuit(2)
circ2.h(0)
circ1.cx(0, 1)
circ1.compose(circ2, front=True).draw(output='mpl')
```
* Two new passes, [`SabreLayout`](/api/qiskit/0.45/qiskit.transpiler.passes.SabreLayout "qiskit.transpiler.passes.SabreLayout") and [`SabreSwap`](/api/qiskit/0.45/qiskit.transpiler.passes.SabreSwap "qiskit.transpiler.passes.SabreSwap") for layout and routing have been added to [`qiskit.transpiler.passes`](/api/qiskit/0.45/transpiler_passes#module-qiskit.transpiler.passes "qiskit.transpiler.passes"). These new passes are based on the algorithm presented in Li et al., “Tackling the Qubit Mapping Problem for NISQ-Era Quantum Devices”, ASPLOS 2019. They can also be selected when using the [`transpile()`](/api/qiskit/0.45/compiler#qiskit.compiler.transpile "qiskit.compiler.transpile") function by setting the `layout_method` kwarg to `'sabre'` and/or the `routing_method` to `'sabre'` to use [`SabreLayout`](/api/qiskit/0.45/qiskit.transpiler.passes.SabreLayout "qiskit.transpiler.passes.SabreLayout") and [`SabreSwap`](/api/qiskit/0.45/qiskit.transpiler.passes.SabreSwap "qiskit.transpiler.passes.SabreSwap") respectively.
* Added the method [`replace()`](/api/qiskit/0.45/qiskit.pulse.Schedule#replace "qiskit.pulse.Schedule.replace") to the [`qiskit.pulse.Schedule`](/api/qiskit/0.45/qiskit.pulse.Schedule "qiskit.pulse.Schedule") class which allows a pulse instruction to be replaced with another. For example:
```python
.. code-block:: python
```
> from qiskit import pulse
>
> d0 = pulse.DriveChannel(0)
>
> sched = pulse.Schedule()
>
> old = pulse.Play(pulse.Constant(100, 1.0), d0) new = pulse.Play(pulse.Constant(100, 0.1), d0)
>
> sched += old
>
> sched = sched.replace(old, new)
>
> assert sched == pulse.Schedule(new)
* Added new gate classes to [`qiskit.circuit.library`](/api/qiskit/0.45/circuit_library#module-qiskit.circuit.library "qiskit.circuit.library") for the $\sqrt{X}$, its adjoint $\sqrt{X}^\dagger$, and controlled $\sqrt{X}$ gates as [`SXGate`](/api/qiskit/0.45/qiskit.circuit.library.SXGate "qiskit.circuit.library.SXGate"), [`SXdgGate`](/api/qiskit/0.45/qiskit.circuit.library.SXdgGate "qiskit.circuit.library.SXdgGate"), and [`CSXGate`](/api/qiskit/0.45/qiskit.circuit.library.CSXGate "qiskit.circuit.library.CSXGate"). They can also be added to a [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") object using the [`sx()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#sx "qiskit.circuit.QuantumCircuit.sx"), [`sxdg()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#sxdg "qiskit.circuit.QuantumCircuit.sxdg"), and [`csx()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#csx "qiskit.circuit.QuantumCircuit.csx") respectively.
* Add support for `Reset` instructions to [`qiskit.quantum_info.Statevector.from_instruction()`](/api/qiskit/0.45/qiskit.quantum_info.Statevector#from_instruction "qiskit.quantum_info.Statevector.from_instruction"). Note that this involves RNG sampling in choosing the projection to the zero state in the case where the qubit is in a superposition state. The seed for sampling can be set using the [`seed()`](/api/qiskit/0.45/qiskit.quantum_info.Statevector#seed "qiskit.quantum_info.Statevector.seed") method.
* The methods [`qiskit.circuit.ParameterExpression.subs()`](/api/qiskit/0.45/qiskit.circuit.ParameterExpression#subs "qiskit.circuit.ParameterExpression.subs") and [`qiskit.circuit.QuantumCircuit.assign_parameters()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#assign_parameters "qiskit.circuit.QuantumCircuit.assign_parameters") now accept [`ParameterExpression`](/api/qiskit/0.45/qiskit.circuit.ParameterExpression "qiskit.circuit.ParameterExpression") as the target value to be substituted.
For example,
```python
from qiskit.circuit import QuantumCircuit, Parameter
p = Parameter('p')
source = QuantumCircuit(1)
source.rz(p, 0)
x = Parameter('x')
source.assign_parameters({p: x*x})
```
```python
┌──────────┐
q_0: ┤ Rz(x**2) ├
└──────────┘
```
* The [`QuantumCircuit()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") method [`to_gate()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#to_gate "qiskit.circuit.QuantumCircuit.to_gate") has a new kwarg `label` which can be used to set a label for for the output [`Gate`](/api/qiskit/0.45/qiskit.circuit.Gate "qiskit.circuit.Gate") object. For example:
```python
from qiskit.circuit import QuantumCircuit
circuit_gate = QuantumCircuit(2)
circuit_gate.h(0)
circuit_gate.cx(0, 1)
custom_gate = circuit_gate.to_gate(label='My Special Bell')
new_circ = QuantumCircuit(2)
new_circ.append(custom_gate, [0, 1], [])
new_circ.draw(output='mpl')
```
* Added the [`UGate`](/api/qiskit/0.45/qiskit.circuit.library.UGate "qiskit.circuit.library.UGate"), [`CUGate`](/api/qiskit/0.45/qiskit.circuit.library.CUGate "qiskit.circuit.library.CUGate"), [`PhaseGate`](/api/qiskit/0.45/qiskit.circuit.library.PhaseGate "qiskit.circuit.library.PhaseGate"), and [`CPhaseGate`](/api/qiskit/0.45/qiskit.circuit.library.CPhaseGate "qiskit.circuit.library.CPhaseGate") with the corresponding [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") methods [`u()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#u "qiskit.circuit.QuantumCircuit.u"), [`cu()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#cu "qiskit.circuit.QuantumCircuit.cu"), [`p()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#p "qiskit.circuit.QuantumCircuit.p"), and [`cp()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#cp "qiskit.circuit.QuantumCircuit.cp"). The [`UGate`](/api/qiskit/0.45/qiskit.circuit.library.UGate "qiskit.circuit.library.UGate") gate is the generic single qubit rotation gate with 3 Euler angles and the [`CUGate`](/api/qiskit/0.45/qiskit.circuit.library.CUGate "qiskit.circuit.library.CUGate") gate its controlled version. [`CUGate`](/api/qiskit/0.45/qiskit.circuit.library.CUGate "qiskit.circuit.library.CUGate") has 4 parameters to account for a possible global phase of the U gate. The [`PhaseGate`](/api/qiskit/0.45/qiskit.circuit.library.PhaseGate "qiskit.circuit.library.PhaseGate") and [`CPhaseGate`](/api/qiskit/0.45/qiskit.circuit.library.CPhaseGate "qiskit.circuit.library.CPhaseGate") gates are the general Phase gate at an arbitrary angle and it’s controlled version.
* A new kwarg, `cregbundle` has been added to the [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function and the [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") method [`draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw"). When set to `True` the cregs will be bundled into a single line in circuit visualizations for the `text` and `mpl` drawers. The default value is `True`. Addresses issue [#4290](https://github.com/Qiskit/qiskit/issues/4290).
For example:
```python
from qiskit import QuantumCircuit
circuit = QuantumCircuit(2)
circuit.measure_all()
circuit.draw(output='mpl', cregbundle=True)
```
* A new kwarg, `initial_state` has been added to the [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function and the [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") method [`draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw"). When set to `True` the initial state will now be included in circuit visualizations for all drawers. Addresses issue [#4293](https://github.com/Qiskit/qiskit/issues/4293).
For example:
```python
from qiskit import QuantumCircuit
circuit = QuantumCircuit(2)
circuit.measure_all()
circuit.draw(output='mpl', initial_state=True)
```
* Labels will now be displayed when using the ‘mpl’ drawer. There are 2 types of labels - gate labels and control labels. Gate labels will replace the gate name in the display. Control labels will display above or below the controls for a gate. Fixes issues #3766, #4580 Addresses issues [#3766](https://github.com/Qiskit/qiskit/issues/3766) and [#4580](https://github.com/Qiskit/qiskit/issues/4580).
For example:
```python
from qiskit import QuantumCircuit
from qiskit.circuit.library.standard_gates import YGate
circuit = QuantumCircuit(2)
circuit.append(YGate(label='A Y Gate').control(label='Y Control'), [0, 1])
circuit.draw(output='mpl')
```
<span id="release-notes-0-15-0-upgrade-notes" />
<span id="id416" />
#### Upgrade Notes
* Implementations of the multi-controlled X Gate ( [`MCXGrayCode`](/api/qiskit/0.45/qiskit.circuit.library.MCXGrayCode "qiskit.circuit.library.MCXGrayCode"), [`MCXRecursive`](/api/qiskit/0.45/qiskit.circuit.library.MCXRecursive "qiskit.circuit.library.MCXRecursive"), and [`MCXVChain`](/api/qiskit/0.45/qiskit.circuit.library.MCXVChain "qiskit.circuit.library.MCXVChain")) have had their `name` properties changed to more accurately describe their implementation: `mcx_gray`, `mcx_recursive`, and `mcx_vchain` respectively. Previously, these gates shared the name `mcx` with [`MCXGate`](/api/qiskit/0.45/qiskit.circuit.library.MCXGate "qiskit.circuit.library.MCXGate"), which caused these gates to be incorrectly transpiled and simulated.
* By default the preset passmanagers in [`qiskit.transpiler.preset_passmanagers`](/api/qiskit/0.45/transpiler_preset#module-qiskit.transpiler.preset_passmanagers "qiskit.transpiler.preset_passmanagers") are using [`UnrollCustomDefinitions`](/api/qiskit/0.45/qiskit.transpiler.passes.UnrollCustomDefinitions "qiskit.transpiler.passes.UnrollCustomDefinitions") and [`BasisTranslator`](/api/qiskit/0.45/qiskit.transpiler.passes.BasisTranslator "qiskit.transpiler.passes.BasisTranslator") to handle basis changing instead of the previous default [`Unroller`](/api/qiskit/0.45/qiskit.transpiler.passes.Unroller "qiskit.transpiler.passes.Unroller"). This was done because the new passes are more flexible and allow targeting any basis set, however the output may differ. To use the previous default you can set the `translation_method` kwarg on [`transpile()`](/api/qiskit/0.45/compiler#qiskit.compiler.transpile "qiskit.compiler.transpile") to `'unroller'`.
* The [`qiskit.converters.circuit_to_gate()`](/api/qiskit/0.45/converters#qiskit.converters.circuit_to_gate "qiskit.converters.circuit_to_gate") and :func\`qiskit.converters.circuit\_to\_instruction\` converter functions had previously automatically included the generated gate or instruction in the active `SessionEquivalenceLibrary`. These converters now accept an optional `equivalence_library` keyword argument to specify if and where the converted instances should be registered. The default behavior has changed to not register the converted instance.
* The default value of the `cregbundle` kwarg for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function has been changed to `True`. This means that by default the classical bits in the circuit diagram will now be bundled by default, for example:
```python
from qiskit.circuit import QuantumCircuit
circ = QuantumCircuit(4)
circ.x(0)
circ.h(1)
circ.measure_all()
circ.draw(output='mpl')
```
If you want to have your circuit drawing retain the previous behavior and show each classical bit in the diagram you can set the `cregbundle` kwarg to `False`. For example:
```python
from qiskit.circuit import QuantumCircuit
circ = QuantumCircuit(4)
circ.x(0)
circ.h(1)
circ.measure_all()
circ.draw(output='mpl', cregbundle=False)
```
* [`Schedule`](/api/qiskit/0.45/qiskit.pulse.Schedule "qiskit.pulse.Schedule") plotting with [`qiskit.pulse.Schedule.draw()`](/api/qiskit/0.45/qiskit.pulse.Schedule#draw "qiskit.pulse.Schedule.draw") and [`qiskit.visualization.pulse_drawer()`](/api/qiskit/0.45/qiskit.visualization.pulse_drawer "qiskit.visualization.pulse_drawer") will no longer display the event table by default. This can be reenabled by setting the `table` kwarg to `True`.
* The pass [`RemoveResetInZeroState`](/api/qiskit/0.45/qiskit.transpiler.passes.RemoveResetInZeroState "qiskit.transpiler.passes.RemoveResetInZeroState") was previously included in the preset pass manager [`level_0_pass_manager()`](/api/qiskit/0.45/transpiler_preset#qiskit.transpiler.preset_passmanagers.level_0_pass_manager "qiskit.transpiler.preset_passmanagers.level_0_pass_manager") which was used with the `optimization_level=0` for [`transpile()`](/api/qiskit/0.45/compiler#qiskit.compiler.transpile "qiskit.compiler.transpile") and `execute()` functions. However, [`RemoveResetInZeroState`](/api/qiskit/0.45/qiskit.transpiler.passes.RemoveResetInZeroState "qiskit.transpiler.passes.RemoveResetInZeroState") is an optimization pass and should not have been included in optimization level 0 and was removed. If you need to run [`transpile()`](/api/qiskit/0.45/compiler#qiskit.compiler.transpile "qiskit.compiler.transpile") with [`RemoveResetInZeroState`](/api/qiskit/0.45/qiskit.transpiler.passes.RemoveResetInZeroState "qiskit.transpiler.passes.RemoveResetInZeroState") either use a custom pass manager or `optimization_level` 1, 2, or 3.
* The deprecated kwarg `line_length` for the [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function and [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method has been removed. It had been deprecated since the 0.10.0 release. Instead you can use the `fold` kwarg to adjust the width of the circuit diagram.
* The `'mpl'` output mode for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") now requires the [pylatexenc](https://pylatexenc.readthedocs.io/en/latest/latexencode/) library to be installed. This was already an optional dependency for visualization, but was only required for the `'latex'` output mode before. It is now also required for the matplotlib drawer because it is needed to handle correctly sizing gates with matplotlib’s [mathtext](https://matplotlib.org/3.2.2/tutorials/text/mathtext.html) labels for gates.
* The deprecated `get_tokens` methods for the [`qiskit.qasm.Qasm`](/api/qiskit/0.45/qasm#qiskit.qasm.Qasm "qiskit.qasm.Qasm") and `qiskit.qasm.QasmParser` has been removed. These methods have been deprecated since the 0.9.0 release. The `qiskit.qasm.Qasm.generate_tokens()` and `qiskit.qasm.QasmParser.generate_tokens()` methods should be used instead.
* The deprecated kwarg `channels_to_plot` for [`qiskit.pulse.Schedule.draw()`](/api/qiskit/0.45/qiskit.pulse.Schedule#draw "qiskit.pulse.Schedule.draw"), `qiskit.pulse.Instruction.draw()`, `qiskit.visualization.pulse.matplotlib.ScheduleDrawer.draw` and [`pulse_drawer()`](/api/qiskit/0.45/qiskit.visualization.pulse_drawer "qiskit.visualization.pulse_drawer") has been removed. The kwarg has been deprecated since the 0.11.0 release and was replaced by the `channels` kwarg, which functions identically and should be used instead.
* The deprecated `circuit_instruction_map` attribute of the [`qiskit.providers.models.PulseDefaults`](/api/qiskit/0.45/qiskit.providers.models.PulseDefaults "qiskit.providers.models.PulseDefaults") class has been removed. This attribute has been deprecated since the 0.12.0 release and was replaced by the `instruction_schedule_map` attribute which can be used instead.
* The `union` method of [`Schedule`](/api/qiskit/0.45/qiskit.pulse.Schedule "qiskit.pulse.Schedule") and `Instruction` have been deprecated since the 0.12.0 release and have now been removed. Use [`qiskit.pulse.Schedule.insert()`](/api/qiskit/0.45/qiskit.pulse.Schedule#insert "qiskit.pulse.Schedule.insert") and `qiskit.pulse.Instruction.meth()` methods instead with the kwarg\`\`time=0\`\`.
* The deprecated `scaling` argument to the `draw` method of [`Schedule`](/api/qiskit/0.45/qiskit.pulse.Schedule "qiskit.pulse.Schedule") and `Instruction` has been replaced with `scale` since the 0.12.0 release and now has been removed. Use the `scale` kwarg instead.
* The deprecated `period` argument to [`qiskit.pulse.library`](/api/qiskit/0.45/pulse#module-qiskit.pulse.library "qiskit.pulse.library") functions have been replaced by `freq` since the 0.13.0 release and now removed. Use the `freq` kwarg instead of `period`.
* The `qiskit.pulse.commands` module containing `Commands` classes was deprecated in the 0.13.0 release and has now been removed. You will have to upgrade your Pulse code if you were still using commands. For example:
| Old | New |
| ------------------------------------ | ---------------------------- |
| `Command(args)(channel)` | `Instruction(args, channel)` |
| \`\`\`python | |
| Acquire(duration)(AcquireChannel(0)) | |
| \`\`\` | \`\`\`python |
| Acquire(duration, AcquireChannel(0)) | |
````|
| ```python
Delay(duration)(channel)
``` | ```python
Delay(duration, channel)
``` |
| ```python
FrameChange(angle)(DriveChannel(0))
``` | ```python
# FrameChange was also renamed ShiftPhase(angle, DriveChannel(0))
``` |
| ```python
Gaussian(...)(DriveChannel(0))
``` | ```python
# Pulses need to be `Play`d Play(Gaussian(...), DriveChannel(0))
``` |
````
* All classes and function in the `qiskit.tool.qi` module were deprecated in the 0.12.0 release and have now been removed. Instead use the [`qiskit.quantum_info`](/api/qiskit/0.45/quantum_info#module-qiskit.quantum_info "qiskit.quantum_info") module and the new methods and classes that it has for working with quantum states and operators.
* The `qiskit.quantum_info.basis_state` and `qiskit.quantum_info.projector` functions are deprecated as of Qiskit Terra 0.12.0 as are now removed. Use the `qiskit.quantum_info.QuantumState` and its derivatives [`qiskit.quantum_info.Statevector`](/api/qiskit/0.45/qiskit.quantum_info.Statevector "qiskit.quantum_info.Statevector") and [`qiskit.quantum_info.DensityMatrix`](/api/qiskit/0.45/qiskit.quantum_info.DensityMatrix "qiskit.quantum_info.DensityMatrix") to work with states.
* The interactive plotting functions from [`qiskit.visualization`](/api/qiskit/0.45/visualization#module-qiskit.visualization "qiskit.visualization"), `iplot_bloch_multivector`, `iplot_state_city`, `iplot_state_qsphere`, `iplot_state_hinton`, `iplot_histogram`, `iplot_state_paulivec` now are just deprecated aliases for the matplotlib based equivalents and are no longer interactive. The hosted static JS code that these functions relied on has been removed and they no longer could work. A normal deprecation wasn’t possible because the site they depended on no longer exists.
* The validation components using marshmallow from `qiskit.validation` have been removed from terra. Since they are no longer used to build any objects in terra.
* The marshmallow schema classes in [`qiskit.result`](/api/qiskit/0.45/result#module-qiskit.result "qiskit.result") have been removed since they are no longer used by the [`qiskit.result.Result`](/api/qiskit/0.45/qiskit.result.Result "qiskit.result.Result") class.
* The output of the [`to_dict()`](/api/qiskit/0.45/qiskit.result.Result#to_dict "qiskit.result.Result.to_dict") method for the [`qiskit.result.Result`](/api/qiskit/0.45/qiskit.result.Result "qiskit.result.Result") class is no longer in a format for direct JSON serialization. Depending on the content contained in instances of these classes there may be types that the default JSON encoder doesn’t know how to handle, for example complex numbers or numpy arrays. If you’re JSON serializing the output of the `to_dict()` method directly you should ensure that your JSON encoder can handle these types.
* The option to acquire multiple qubits at once was deprecated in the 0.12.0 release and is now removed. Specifically, the init args `mem_slots` and `reg_slots` have been removed from [`qiskit.pulse.instructions.Acquire`](/api/qiskit/0.45/qiskit.pulse.instructions.Acquire "qiskit.pulse.instructions.Acquire"), and `channel`, `mem_slot` and `reg_slot` will raise an error if a list is provided as input.
* Support for the use of the `USE_RETWORKX` environment variable which was introduced in the 0.13.0 release to provide an optional fallback to the legacy [networkx](https://networkx.github.io/) based [`qiskit.dagcircuit.DAGCircuit`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit "qiskit.dagcircuit.DAGCircuit") implementation has been removed. This flag was only intended as provide a relief valve for any users that encountered a problem with the new implementation for one release during the transition to retworkx.
* The module within [`qiskit.pulse`](/api/qiskit/0.45/pulse#module-qiskit.pulse "qiskit.pulse") responsible for schedule->schedule transformations has been renamed from `reschedule.py` to `transforms.py`. The previous import path has been deprecated. To upgrade your code:
```python
from qiskit.pulse.rescheduler import <X>
```
should be replaced by:
```python
from qiskit.pulse.transforms import <X>
```
* In previous releases a [`PassManager`](/api/qiskit/0.45/qiskit.transpiler.PassManager "qiskit.transpiler.PassManager") did not allow `TransformationPass` classes to modify the `PropertySet`. This restriction has been lifted so a `TransformationPass` class now has read and write access to both the `PropertySet` and `DAGCircuit` during [`run()`](/api/qiskit/0.45/qiskit.transpiler.PassManager#run "qiskit.transpiler.PassManager.run"). This change was made to more efficiently facilitate `TransformationPass` classes that have an internal state which may be necessary for later passes in the [`PassManager`](/api/qiskit/0.45/qiskit.transpiler.PassManager "qiskit.transpiler.PassManager"). Without this change a second redundant `AnalysisPass` would have been necessary to recreate the internal state, which could add significant overhead.
<span id="release-notes-0-15-0-deprecation-notes" />
<span id="id417" />
#### Deprecation Notes
* The name of the first positional parameter for the [`qiskit.visualization`](/api/qiskit/0.45/visualization#module-qiskit.visualization "qiskit.visualization") functions [`plot_state_hinton()`](/api/qiskit/0.45/qiskit.visualization.plot_state_hinton "qiskit.visualization.plot_state_hinton"), [`plot_bloch_multivector()`](/api/qiskit/0.45/qiskit.visualization.plot_bloch_multivector "qiskit.visualization.plot_bloch_multivector"), [`plot_state_city()`](/api/qiskit/0.45/qiskit.visualization.plot_state_city "qiskit.visualization.plot_state_city"), [`plot_state_paulivec()`](/api/qiskit/0.45/qiskit.visualization.plot_state_paulivec "qiskit.visualization.plot_state_paulivec"), and [`plot_state_qsphere()`](/api/qiskit/0.45/qiskit.visualization.plot_state_qsphere "qiskit.visualization.plot_state_qsphere") has been renamed from `rho` to `state`. Passing in the value by name to `rho` is deprecated and will be removed in a future release. Instead you should either pass the argument positionally or use the new parameter name `state`.
* The `qiskit.pulse.pulse_lib` module has been deprecated and will be removed in a future release. It has been renamed to [`qiskit.pulse.library`](/api/qiskit/0.45/pulse#module-qiskit.pulse.library "qiskit.pulse.library") which should be used instead.
* The [`qiskit.circuit.QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") method `mirror()` has been deprecated and will be removed in a future release. The method [`qiskit.circuit.QuantumCircuit.reverse_ops()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#reverse_ops "qiskit.circuit.QuantumCircuit.reverse_ops") should be used instead, since mirroring could be confused with swapping the output qubits of the circuit. The [`reverse_ops()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#reverse_ops "qiskit.circuit.QuantumCircuit.reverse_ops") method only reverses the order of gates that are applied instead of mirroring.
* The `qubits()` and `clbits()` methods of [`qiskit.dagcircuit.DAGCircuit`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit "qiskit.dagcircuit.DAGCircuit") have been deprecated and will be removed in a future release. They have been replaced with properties of the same name, `qiskit.dagcircuit.DAGCircuit.qubits` and `qiskit.dagcircuit.DAGCircuit.clbits`, and are cached so accessing them is much faster.
* The `get_sample_pulse` method for `qiskit.pulse.library.ParametricPulse` derived classes (for example [`GaussianSquare`](/api/qiskit/0.45/qiskit.pulse.library.GaussianSquare "qiskit.pulse.library.GaussianSquare")) has been deprecated and will be removed in a future release. It has been replaced by the `get_waveform` method (for example `get_waveform()`) which should behave identically.
* The use of the optional `condition` argument on [`qiskit.dagcircuit.DAGNode`](/api/qiskit/0.45/qiskit.dagcircuit.DAGNode "qiskit.dagcircuit.DAGNode"), [`qiskit.dagcircuit.DAGCircuit.apply_operation_back()`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit#apply_operation_back "qiskit.dagcircuit.DAGCircuit.apply_operation_back"), and [`qiskit.dagcircuit.DAGCircuit.apply_operation_front()`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit#apply_operation_front "qiskit.dagcircuit.DAGCircuit.apply_operation_front") has been deprecated and will be removed in a future release. Instead the `control` set in [`qiskit.circuit.Instruction`](/api/qiskit/0.45/qiskit.circuit.Instruction "qiskit.circuit.Instruction") instances being added to a [`DAGCircuit`](/api/qiskit/0.45/qiskit.dagcircuit.DAGCircuit "qiskit.dagcircuit.DAGCircuit") should be used.
* The `set_atol` and `set_rtol` class methods of the `qiskit.quantum_info.BaseOperator` and `qiskit.quantum_info.QuantumState` classes (and their subclasses such as [`Operator`](/api/qiskit/0.45/qiskit.quantum_info.Operator "qiskit.quantum_info.Operator") and [`qiskit.quantum_info.DensityMatrix`](/api/qiskit/0.45/qiskit.quantum_info.DensityMatrix "qiskit.quantum_info.DensityMatrix")) are deprecated and will be removed in a future release. Instead the value for the attributes `.atol` and `.rtol` should be set on the class instead. For example:
```python
from qiskit.quantum_info import ScalarOp
ScalarOp.atol = 3e-5
op = ScalarOp(2)
```
* The interactive plotting functions from [`qiskit.visualization`](/api/qiskit/0.45/visualization#module-qiskit.visualization "qiskit.visualization"), `iplot_bloch_multivector`, `iplot_state_city`, `iplot_state_qsphere`, `iplot_state_hinton`, `iplot_histogram`, `iplot_state_paulivec` have been deprecated and will be removed in a future release. The matplotlib based equivalent functions from [`qiskit.visualization`](/api/qiskit/0.45/visualization#module-qiskit.visualization "qiskit.visualization"), [`plot_bloch_multivector()`](/api/qiskit/0.45/qiskit.visualization.plot_bloch_multivector "qiskit.visualization.plot_bloch_multivector"), [`plot_state_city()`](/api/qiskit/0.45/qiskit.visualization.plot_state_city "qiskit.visualization.plot_state_city"), [`plot_state_qsphere()`](/api/qiskit/0.45/qiskit.visualization.plot_state_qsphere "qiskit.visualization.plot_state_qsphere"), [`plot_state_hinton()`](/api/qiskit/0.45/qiskit.visualization.plot_state_hinton "qiskit.visualization.plot_state_hinton"), `plot_state_histogram()`, and [`plot_state_paulivec()`](/api/qiskit/0.45/qiskit.visualization.plot_state_paulivec "qiskit.visualization.plot_state_paulivec") should be used instead.
* The properties `acquires`, `mem_slots`, and `reg_slots` of the [`qiskit.pulse.instructions.Acquire`](/api/qiskit/0.45/qiskit.pulse.instructions.Acquire "qiskit.pulse.instructions.Acquire") pulse instruction have been deprecated and will be removed in a future release. They are just duplicates of [`channel`](/api/qiskit/0.45/qiskit.pulse.instructions.Acquire#channel "qiskit.pulse.instructions.Acquire.channel"), [`mem_slot`](/api/qiskit/0.45/qiskit.pulse.instructions.Acquire#mem_slot "qiskit.pulse.instructions.Acquire.mem_slot"), and [`reg_slot`](/api/qiskit/0.45/qiskit.pulse.instructions.Acquire#reg_slot "qiskit.pulse.instructions.Acquire.reg_slot") respectively now that previously deprecated support for using multiple qubits in a single [`Acquire`](/api/qiskit/0.45/qiskit.pulse.instructions.Acquire "qiskit.pulse.instructions.Acquire") instruction has been removed.
* The `SamplePulse` class from [`qiskit.pulse`](/api/qiskit/0.45/pulse#module-qiskit.pulse "qiskit.pulse") has been renamed to [`Waveform`](/api/qiskit/0.45/qiskit.pulse.library.Waveform "qiskit.pulse.library.Waveform"). `SamplePulse` is deprecated and will be removed in a future release.
* The style dictionary key `cregbundle` has been deprecated and will be removed in a future release. This has been replaced by the kwarg `cregbundle` added to the [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function and the [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") method [`draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw").
<span id="release-notes-0-15-0-bug-fixes" />
<span id="id418" />
#### Bug Fixes
* The [`qiskit.circuit.QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") method [`num_nonlocal_gates`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#num_nonlocal_gates "qiskit.circuit.QuantumCircuit.num_nonlocal_gates") previously included multi-qubit [`qiskit.circuit.Instruction`](/api/qiskit/0.45/qiskit.circuit.Instruction "qiskit.circuit.Instruction") objects (for example, [`Barrier`](/api/qiskit/0.45/qiskit.circuit.library.Barrier "qiskit.circuit.library.Barrier")) in its count of non-local gates. This has been corrected so that only non-local [`Gate`](/api/qiskit/0.45/qiskit.circuit.Gate "qiskit.circuit.Gate") objects are counted. Fixes [#4500](https://github.com/Qiskit/qiskit/issues/4500)
* [`ControlledGate`](/api/qiskit/0.45/qiskit.circuit.ControlledGate "qiskit.circuit.ControlledGate") instances with a set `ctrl_state` were in some cases not being evaluated as equal, even if the compared gates were equivalent. This has been resolved so that Fixes [#4573](https://github.com/Qiskit/qiskit/issues/4573)
* When accessing a bit from a [`qiskit.circuit.QuantumRegister`](/api/qiskit/0.45/qiskit.circuit.QuantumRegister "qiskit.circuit.QuantumRegister") or [`qiskit.circuit.ClassicalRegister`](/api/qiskit/0.45/qiskit.circuit.ClassicalRegister "qiskit.circuit.ClassicalRegister") by index when using numpy integer types \<[https://numpy.org/doc/stable/user/basics.types.html](https://numpy.org/doc/stable/user/basics.types.html)>\`\_\_ would previously raise a `CircuitError` exception. This has been resolved so numpy types can be used in addition to Python’s built-in `int` type. Fixes [#3929](https://github.com/Qiskit/qiskit/issues/3929).
* A bug was fixed where only the first `qiskit.pulse.configuration.Kernel` or `qiskit.pulse.configuration.Discriminator` for an `qiskit.pulse.Acquire` was used when there were multiple Acquires at the same time in a [`qiskit.pulse.Schedule`](/api/qiskit/0.45/qiskit.pulse.Schedule "qiskit.pulse.Schedule").
* The SI unit use for constructing `qiskit.pulse.SetFrequency` objects is in Hz, but when a [`PulseQobjInstruction`](/api/qiskit/0.45/qiskit.qobj.PulseQobjInstruction "qiskit.qobj.PulseQobjInstruction") object is created from a `SetFrequency` instance it needs to be converted to GHz. This conversion was missing from previous releases and has been fixed.
* Previously it was possible to set the number of control qubits to zero in which case the the original, potentially non-controlled, operation would be returned. This could cause an `AttributeError` to be raised if the caller attempted to access an attribute which only [`ControlledGate`](/api/qiskit/0.45/qiskit.circuit.ControlledGate "qiskit.circuit.ControlledGate") object have. This has been fixed by adding a getter and setter for [`num_ctrl_qubits`](/api/qiskit/0.45/qiskit.circuit.ControlledGate#num_ctrl_qubits "qiskit.circuit.ControlledGate.num_ctrl_qubits") to validate that a valid value is being used. Fixes [#4576](https://github.com/Qiskit/qiskit/issues/4576)
* Open controls were implemented by modifying a [`Gate`](/api/qiskit/0.45/qiskit.circuit.Gate "qiskit.circuit.Gate") objects [`definition`](/api/qiskit/0.45/qiskit.circuit.Gate#definition "qiskit.circuit.Gate.definition"). However, when the gate already exists in the basis set, this definition was not used, which resulted in incorrect circuits being sent to a backend after transpilation. This has been fixed by modifying the `Unroller` pass to use the definition if it encounters a controlled gate with open controls. Fixes [#4437](https://github.com/Qiskit/qiskit/issues/4437)
* The `insert_barriers` keyword argument in the [`ZZFeatureMap`](/api/qiskit/0.45/qiskit.circuit.library.ZZFeatureMap "qiskit.circuit.library.ZZFeatureMap") class didn’t actually insert barriers in between the Hadamard layers and evolution layers. This has been fixed so that barriers are now properly inserted.
* Fixed issue where some gates with three or more qubits would fail to compile in certain instances. Refer to #4577 \<[https://github.com/Qiskit/qiskit/issues/4577](https://github.com/Qiskit/qiskit/issues/4577) for more detail.
* The matplotlib (`'mpl'`) output backend for the [`qiskit.circuit.QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") method [`draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") and the [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function was not properly scaling when the kwarg `scale` was set. Fonts and line widths did not scale with the rest of the image. This has been fixed and all elements of the circuit diagram now scale properly. For example:
```python
from qiskit import QuantumCircuit
circuit = QuantumCircuit(2)
circuit.h(0)
circuit.cx(0, 1)
circuit.draw(output='mpl', scale=0.5)
```
Fixes [#4179](https://github.com/Qiskit/qiskit/issues/4179).
* Fixes issue where initializing or evolving [`qiskit.quantum_info.Statevector`](/api/qiskit/0.45/qiskit.quantum_info.Statevector "qiskit.quantum_info.Statevector") and [`qiskit.quantum_info.DensityMatrix`](/api/qiskit/0.45/qiskit.quantum_info.DensityMatrix "qiskit.quantum_info.DensityMatrix") classes by circuits by circuit containing `Barrier` instructions would raise an exception. Fixes [#4461](https://github.com/Qiskit/qiskit/issues/4461)
* Previously when a [`QuantumCircuit`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") contained a [`Gate`](/api/qiskit/0.45/qiskit.circuit.Gate "qiskit.circuit.Gate") with a classical condition the transpiler would sometimes fail when using `optimization_level=3` on [`transpile()`](/api/qiskit/0.45/compiler#qiskit.compiler.transpile "qiskit.compiler.transpile") or `execute()` raising an `UnboundLocalError`. This has been fixed by updating the [`ConsolidateBlocks`](/api/qiskit/0.45/qiskit.transpiler.passes.ConsolidateBlocks "qiskit.transpiler.passes.ConsolidateBlocks") pass to account for the classical condition. Fixes [#4672](https://github.com/Qiskit/qiskit/issues/4672).
* In some situations long gate and register names would overflow, or leave excessive empty space around them when using the `'mpl'` output backend for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function. This has been fixed by using correct text widths for a proportional font. Fixes [#4611](https://github.com/Qiskit/qiskit/issues/4611), [#4605](https://github.com/Qiskit/qiskit/issues/4605), [#4545](https://github.com/Qiskit/qiskit/issues/4545), [#4497](https://github.com/Qiskit/qiskit/issues/4497), [#4449](https://github.com/Qiskit/qiskit/issues/4449), and [#3641](https://github.com/Qiskit/qiskit/issues/3641).
* When using the ```style` kwarg on the :meth:`qiskit.circuit.QuantumCircuit.draw` or :func:`qiskit.visualization.circuit_drawer` with the ``'mpl'``` output backend the dictionary key `'showindex'` set to `True`, the index numbers at the top of the column did not line up properly. This has been fixed.
* When using `cregbunde=True` with the `'mpl'` output backend for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function and measuring onto a second fold, the measure arrow would overwrite the creg count. The count was moved to the left to prevent this. Fixes [#4148](https://github.com/Qiskit/qiskit/issues/4148).
* When using the `'mpl'` output backend for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function [`CSwapGate`](/api/qiskit/0.45/qiskit.circuit.library.CSwapGate "qiskit.circuit.library.CSwapGate") gates and a controlled [`RZZGate`](/api/qiskit/0.45/qiskit.circuit.library.RZZGate "qiskit.circuit.library.RZZGate") gates now display with their appropriate symbols instead of in a box.
* When using the `'mpl'` output backend for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function controlled gates created using the [`to_gate()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#to_gate "qiskit.circuit.QuantumCircuit.to_gate") method were not properly spaced and could overlap with other gates in the circuit diagram. This issue has been fixed.
* When using the `'mpl'` output backend for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function gates with arrays as parameters, such as `HamiltonianGate`, no longer display with excessive space around them. Fixes [#4352](https://github.com/Qiskit/qiskit/issues/4352).
* When using the `'mpl'` output backend for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function generic gates created by directly instantiating [`qiskit.circuit.Gate`](/api/qiskit/0.45/qiskit.circuit.Gate "qiskit.circuit.Gate") method now display the proper background color for the gate. Fixes [#4496](https://github.com/Qiskit/qiskit/issues/4496).
* When using the `'mpl'` output backend for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function an `AttributeError` that occurred when using `Isometry` or `Initialize` has been fixed. Fixes [#4439](https://github.com/Qiskit/qiskit/issues/4439).
* When using the `'mpl'` output backend for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function some open-controlled gates did not properly display the open controls. This has been corrected so that open controls are properly displayed as open circles. Fixes [#4248](https://github.com/Qiskit/qiskit/issues/4248).
* When using the `'mpl'` output backend for the [`qiskit.circuit.QuantumCircuit.draw()`](/api/qiskit/0.45/qiskit.circuit.QuantumCircuit#draw "qiskit.circuit.QuantumCircuit.draw") method and [`qiskit.visualization.circuit_drawer()`](/api/qiskit/0.45/qiskit.visualization.circuit_drawer "qiskit.visualization.circuit_drawer") function setting the `fold` kwarg to -1 will now properly display the circuit without folding. Fixes [#4506](https://github.com/Qiskit/qiskit/issues/4506).
* Parametric pulses from `qiskit.pulse.library.discrete` now have zero ends of parametric pulses by default. The endpoints are defined such that for a function $f(x)$ then $f(-1) = f(duration + 1) = 0$. Fixes [#4317](https://github.com/Qiskit/qiskit/issues/4317)
<span id="release-notes-0-15-0-other-notes" />
<span id="id421" />
#### Other Notes
* The [`qiskit.result.Result`](/api/qiskit/0.45/qiskit.result.Result "qiskit.result.Result") class which was previously constructed using the marshmallow library has been refactored to not depend on marshmallow anymore. This new implementation should be a seamless transition but some specific behavior that was previously inherited from marshmallow may not work. Please file issues for any incompatibilities found.
<span id="id422" />
### Aer 0.6.1
<span id="release-notes-0-6-0-prelude" />
<span id="id423" />
#### Prelude
This 0.6.0 release includes numerous performance improvements for all simulators in the Aer provider and significant changes to the build system when building from source. The main changes are support for SIMD vectorization, approximation in the matrix product state method via bond-dimension truncation, more efficient Pauli expectation value computation, and greatly improved efficiency in Python conversion of C++ result objects. The build system was upgraded to use the [Conan](https://conan.io/) to manage common C++ dependencies when building from source.
<span id="release-notes-0-6-0-new-features" />
<span id="id424" />
#### New Features
* Add density matrix snapshot support to “statevector” and “statevector\_gpu” methods of the QasmSimulator.
* Allow density matrix snapshots on specific qubits, not just all qubits. This computes the partial trace of the state over the remaining qubits.
* Adds Pauli expectation value snapshot support to the “density\_matrix” simulation method of the `qiskit.providers.aer.QasmSimulator`. Add snapshots to circuits using the `qiskit.providers.aer.extensions.SnapshotExpectationValue` extension.
* Greatly improves performance of the Pauli expectation value snapshot algorithm for the “statevector”, “statevector\_gpu, “density\_matrix”, and “density\_matrix\_gpu” simulation methods of the `qiskit.providers.aer.QasmSimulator`.
* Enable the gate-fusion circuit optimization from the `qiskit.providers.aer.QasmSimulator` in both the `qiskit.providers.aer.StatevectorSimulator` and `qiskit.providers.aer.UnitarySimulator` backends.
* Improve the performance of average snapshot data in simulator results. This effects probability, Pauli expectation value, and density matrix snapshots using the following extensions:
* `qiskit.providers.aer.extensions.SnapshotExpectationValue`
* `qiskit.providers.aer.extensions.SnapshotProbabilities`
* `qiskit.providers.aer.extensions.SnapshotDensityMatrix`
* Add move constructor and improve memory usage of the C++ matrix class to minimize copies of matrices when moving output of simulators into results.
* Improve performance of unitary simulator.
* Add approximation to the “matrix\_product\_state” simulation method of the `QasmSimulator` to limit the bond-dimension of the MPS.
There are two modes of approximation. Both discard the smallest Schmidt coefficients following the SVD algorithm. There are two parameters that control the degree of approximation: `"matrix_product_state_max_bond_dimension"` (int): Sets a limit on the number of Schmidt coefficients retained at the end of the svd algorithm. Coefficients beyond this limit will be discarded. (Default: None, i.e., no limit on the bond dimension). `"matrix_product_state_truncation_threshold"` (double): Discard the smallest coefficients for which the sum of their squares is smaller than this threshold. (Default: 1e-16).
* Improve the performance of measure sampling when using the “matrix\_product\_state” `QasmSimulator` simulation method.
* Add support for `Delay`, `Phase` and `SetPhase` pulse instructions to the `qiskit.providers.aer.PulseSimulator`.
* Improve the performance of the `qiskit.providers.aer.PulseSimulator` by caching calls to RHS function
* Introduce alternate DE solving methods, specifiable through `backend_options` in the `qiskit.providers.aer.PulseSimulator`.
* Improve performance of simulator result classes by using move semantics and removing unnecessary copies that were happening when combining results from separate experiments into the final result object.
* Greatly improve performance of pybind11 conversion of simulator results by using move semantics where possible, and by moving vector and matrix results to Numpy arrays without copies.
* Change the RNG engine for simulators from 32-bit Mersenne twister to 64-bit Mersenne twister engine.
* Improves the performance of the “statevector” simulation method of the `qiskit.providers.aer.QasmSimulator` and `qiskit.providers.aer.StatevectorSimulator` by using SIMD intrinsics on systems that support the AVX2 instruction set. AVX2 support is automatically detected and enabled at runtime.
<span id="release-notes-0-6-0-upgrade-notes" />
<span id="id425" />
#### Upgrade Notes
* Changes the build system to use the [Conan package manager](https://conan.io/). This tool will handle most of the dependencies needed by the C++ source code. Internet connection may be needed for the first build or when dependencies are added or updated, in order to download the required packages if they are not in your Conan local repository.
When building the standalone version of qiskit-aer you must install conan first with:
```python
pip install conan
```
* Changes how transpilation passes are handled in the C++ Controller classes so that each pass must be explicitly called. This allows for greater customization on when each pass should be called, and with what parameters. In particular this enables setting different parameters for the gate fusion optimization pass depending on the QasmController simulation method.
* Add `gate_length_units` kwarg to `qiskit.providers.aer.noise.NoiseModel.from_device()` for specifying custom `gate_lengths` in the device noise model function to handle unit conversions for internal code.
* Add Controlled-Y (“cy”) gate to the Stabilizer simulator methods supported gateset.
* For Aer’s backend the jsonschema validation of input qobj objects from terra is now opt-in instead of being enabled by default. If you want to enable jsonschema validation of qobj set the `validate` kwarg on the `qiskit.providers.aer.QasmSimualtor.run()` method for the backend object to `True`.
* Adds an OpSet object to the base simulator State class to allow easier validation of instructions, gates, and snapshots supported by simulators.
* Refactor OpSet class. Moved OpSet to separate header file and add `contains` and `difference` methods based on `std::set::contains` and `std::algorithm::set_difference`. These replace the removed invalid and validate instructions from OpSet, but with the order reversed. It returns a list of other ops not in current opset rather than opset instructions not in the other.
* Improves how measurement sampling optimization is checked. The expensive part of this operation is now done once during circuit construction where rather than multiple times during simulation for when checking memory requirements, simulation method, and final execution.
<span id="release-notes-0-6-0-bug-fixes" />
<span id="id426" />
#### Bug Fixes
* Remove “extended\_stabilizer” from the automatically selected simulation methods. This is needed as the extended stabilizer method is not exact and may give incorrect results for certain circuits unless the user knows how to optimize its configuration parameters.
The automatic method now only selects from “stabilizer”, “density\_matrix”, and “statevector” methods. If a non-Clifford circuit that is too large for the statevector method is executed an exception will be raised suggesting you could try explicitly using the “extended\_stabilizer” or “matrix\_product\_state” methods instead.
* Disables gate fusion for the matrix product state simulation method as this was causing issues with incorrect results being returned in some cases.
* Fixes a bug causing incorrect channel evaluation in the `qiskit.providers.aer.PulseSimulator`.
* Fixes several minor bugs for Hamiltonian parsing edge cases in the `qiskit.providers.aer.pulse.system_models.hamiltonian_model.HamiltonianModel` class.
<span id="id427" />
### Ignis 0.4.0
<span id="release-notes-0-4-0-prelude" />
<span id="id428" />
#### Prelude
The main change made in this release is a refactor of the Randomized Benchmarking code to integrate the updated Clifford class [`qiskit.quantum_info.Clifford`](/api/qiskit/0.45/qiskit.quantum_info.Clifford "qiskit.quantum_info.Clifford") from Terra and to improve the CNOT-Dihedral class.
<span id="release-notes-0-4-0-new-features" />
<span id="id429" />
#### New Features
* The `qiskit.ignis.verification.randomized_benchmarking.randomized_benchmarking_seq()` function was refactored to use the updated Clifford class [`Clifford`](/api/qiskit/0.45/qiskit.quantum_info.Clifford "qiskit.quantum_info.Clifford"), to allow efficient Randomized Benchmarking (RB) on Clifford sequences with more than 2 qubits. In addition, the code of the CNOT-Dihedral class `qiskit.ignis.verification.randomized_benchmarking.CNOTDihedral` was refactored to make it more efficient, by using numpy arrays, as well not using pre-generated pickle files storing all the 2-qubit group elements. The `qiskit.ignis.verification.randomized_benchmarking.randomized_benchmarking_seq()` function has a new kwarg `rand_seed` which can be used to specify a seed for the random number generator used to generate the RB circuits. This can be useful for having a reproducible circuit.
* The `qiskit.ignis.verification.qv_circuits()` function has a new kwarg `seed` which can be used to specify a seed for the random number generator used to generate the Quantum Volume circuits. This can be useful for having a reproducible circuit.
<span id="release-notes-0-4-0-upgrade-notes" />
<span id="id430" />
#### Upgrade Notes
* The `qiskit.ignis.verification.randomized_benchmarking.randomized_benchmarking_seq()` function is now using the updated Clifford class [`Clifford`](/api/qiskit/0.45/qiskit.quantum_info.Clifford "qiskit.quantum_info.Clifford") and the updated CNOT-Dihedral class `qiskit.ignis.verification.randomized_benchmarking.CNOTDihedral` to construct its output instead of using pre-generated group tables for the Clifford and CNOT-Dihedral group elements, which were stored in pickle files. This may result in subtle differences from the output from the previous version.
* A new requirement [scikit-learn](https://scikit-learn.org/stable/) has been added to the requirements list. This dependency was added in the 0.3.0 release but wasn’t properly exposed as a dependency in that release. This would lead to an `ImportError` if the `qiskit.ignis.measurement.discriminator.iq_discriminators` module was imported. This is now correctly listed as a dependency so that `scikit-learn` will be installed with qiskit-ignis.
* The `qiskit.ignis.verification.qv_circuits()` function is now using the circuit library class [`QuantumVolume`](/api/qiskit/0.45/qiskit.circuit.library.QuantumVolume "qiskit.circuit.library.QuantumVolume") to construct its output instead of building the circuit from scratch. This may result in subtle differences from the output from the previous version.
* Tomography fitters can now also get list of Result objects instead of a single Result as requested in [issue #320](https://github.com/qiskit-community/qiskit-ignis/issues/320/).
<span id="release-notes-0-4-0-deprecation-notes" />
<span id="id431" />
#### Deprecation Notes
* The kwarg `interleaved_gates` for the `qiskit.ignis.verification.randomized_benchmarking.randomized_benchmarking_seq()` function has been deprecated and will be removed in a future release. It is superseded by `interleaved_elem`. The helper functions `qiskit.ignis.verification.randomized_benchmarking.BasicUtils`, `qiskit.ignis.verification.randomized_benchmarking.CliffordUtils` and `qiskit.ignis.verification.randomized_benchmarking.DihedralUtils` were deprecated. These classes are superseded by `qiskit.ignis.verification.randomized_benchmarking.RBgroup` that handles the group operations needed for RB. The class `qiskit.ignis.verification.randomized_benchmarking.Clifford` is superseded by [`Clifford`](/api/qiskit/0.45/qiskit.quantum_info.Clifford "qiskit.quantum_info.Clifford").
* The kwargs `qr` and `cr` for the `qiskit.ignis.verification.qv_circuits()` function have been deprecated and will be removed in a future release. These kwargs were documented as being used for specifying a [`qiskit.circuit.QuantumRegister`](/api/qiskit/0.45/qiskit.circuit.QuantumRegister "qiskit.circuit.QuantumRegister") and [`qiskit.circuit.ClassicalRegister`](/api/qiskit/0.45/qiskit.circuit.ClassicalRegister "qiskit.circuit.ClassicalRegister") to use in the generated Quantum Volume circuits instead of creating new ones. However, the parameters were never actually respected and a new Register would always be created regardless of whether they were set or not. This behavior is unchanged and these kwargs still do not have any effect, but are being deprecated prior to removal to avoid a breaking change for users who may have been setting either.
* Support for passing in subsets of qubits as a list in the `qubit_lists` parameter for the `qiskit.ignis.verification.qv_circuits()` function has been deprecated and will removed in a future release. In the past this was used to specify a layout to run the circuit on a device. In other words if you had a 5 qubit device and wanted to run a 2 qubit QV circuit on qubits 1, 3, and 4 of that device. You would pass in `[1, 3, 4]` as one of the lists in `qubit_lists`, which would generate a 5 qubit virtual circuit and have qv applied to qubits 1, 3, and 4 in that virtual circuit. However, this functionality is not necessary and overlaps with the concept of `initial_layout` in the transpiler and whether a circuit has been embedded with a layout set. Moving forward instead you should just run [`transpile()`](/api/qiskit/0.45/compiler#qiskit.compiler.transpile "qiskit.compiler.transpile") or `execute()` with initial layout set to do this. For example, running the above example would become:
```python
from qiskit import execute
from qiskit.ignis.verification import qv_circuits
initial_layout = [1, 3, 4]
qv_circs, _ = qv_circuits([list(range3)])
execute(qv_circuits, initial_layout=initial_layout)
```
<span id="release-notes-0-4-0-bug-fixes" />
<span id="id432" />
#### Bug Fixes
* Fix a bug of the position of measurement pulses inserted by py:func:qiskit.ignis.characterization.calibrations.pulse\_schedules.drag\_schedules. Fixes [#465](https://github.com/qiskit-community/qiskit-ignis/issues/465)
<span id="id433" />
### Aqua 0.7.5
<span id="release-notes-0-7-5-new-features" />
<span id="id434" />
#### New Features
* Removed soft dependency on CPLEX in ADMMOptimizer. Now default optimizers used by ADMMOptimizer are MinimumEigenOptimizer for QUBO problems and SlsqpOptimizer as a continuous optimizer. You can still use CplexOptimizer as an optimizer for ADMMOptimizer, but it should be set explicitly.
* New Yahoo! finance provider created.
* Introduced `QuadraticProgramConverter` which is an abstract class for converters. Added `convert`/`interpret` methods for converters instead of `encode`/`decode`. Added `to_ising` and `from_ising` to `QuadraticProgram` class. Moved all parameters from `convert` to constructor except `name`. Created setter/getter for converter parameters. Added `auto_define_penalty` and `interpret` for\`\`LinearEqualityToPenalty\`\`. Now error messages of converters are more informative.
* Added an SLSQP optimizer `qiskit.optimization.algorithms.SlsqpOptimizer` as a wrapper of the corresponding SciPy optimization method. This is a classical optimizer, does not depend on quantum algorithms and may be used as a replacement for `CobylaOptimizer`.
* Cobyla optimizer has been modified to accommodate a multi start feature introduced in the SLSQP optimizer. By default, the optimizer does not run in the multi start mode.
* The `SummedOp` does a mathematically more correct check for equality, where expressions such as `X + X == 2*X` and `X + Z == Z + X` evaluate to `True`.
<span id="release-notes-0-7-5-deprecation-notes" />
<span id="id435" />
#### Deprecation Notes
* GSLS optimizer class deprecated `__init__` parameter `max_iter` in favor of `maxiter`. SPSA optimizer class deprecated `__init__` parameter `max_trials` in favor of `maxiter`. optimize\_svm function deprecated `max_iters` parameter in favor of `maxiter`. ADMMParameters class deprecated `__init__` parameter `max_iter` in favor of `maxiter`.
* The ising convert classes `qiskit.optimization.converters.QuadraticProgramToIsing` and `qiskit.optimization.converters.IsingToQuadraticProgram` have been deprecated and will be removed in a future release. Instead the `qiskit.optimization.QuadraticProgram` methods `to_ising()` and `from_ising()` should be used instead.
* The `pprint_as_string` method for `qiskit.optimization.QuadraticProgram` has been deprecated and will be removed in a future release. Instead you should just run `.pprint_as_string()` on the output from `to_docplex()`
* The `prettyprint` method for `qiskit.optimization.QuadraticProgram` has been deprecated and will be removed in a future release. Instead you should just run `.prettyprint()` on the output from `to_docplex()`
<span id="release-notes-0-7-5-bug-fixes" />
<span id="id436" />
#### Bug Fixes
* Changed in python version 3.8: On macOS, the spawn start method is now the default. The fork start method should be considered unsafe as it can lead to crashes in subprocesses. However P\_BFGS doesn’t support spawn, so we revert to single process. Refer to #1109 \<[https://github.com/qiskit-community/qiskit-aqua/issues/1109](https://github.com/qiskit-community/qiskit-aqua/issues/1109)> for more details.
* Binding parameters in the `CircuitStateFn` did not copy the value of `is_measurement` and always set `is_measurement=False`. This has been fixed.
* Previously, SummedOp.to\_matrix\_op built a list MatrixOp’s (with numpy matrices) and then summed them, returning a single MatrixOp. Some algorithms (for example vqe) require summing thousands of matrices, which exhausts memory when building the list of matrices. With this change, no list is constructed. Rather, each operand in the sum is converted to a matrix, added to an accumulator, and discarded.
* Changing backends in VQE from statevector to qasm\_simulator or real device was causing an error due to CircuitSampler incompatible reuse. VQE was changed to always create a new CircuitSampler and create a new expectation in case not entered by user. Refer to #1153 \<[https://github.com/qiskit-community/qiskit-aqua/issues/1153](https://github.com/qiskit-community/qiskit-aqua/issues/1153)> for more details.
* Exchange and Wikipedia finance providers were fixed to correctly handle Quandl data. Refer to #775 \<[https://github.com/qiskit-community/qiskit-aqua/issues/775](https://github.com/qiskit-community/qiskit-aqua/issues/775)> for more details. Fixes a divide by 0 error on finance providers mean vector and covariance matrix calculations. Refer to #781 \<[https://github.com/qiskit-community/qiskit-aqua/issues/781](https://github.com/qiskit-community/qiskit-aqua/issues/781)> for more details.
* The `ListOp.combo_fn` property has been lost in several transformations, such as converting to another operator type, traversing, reducing or multiplication. Now this attribute is propagated to the resulting operator.
* The evaluation of some operator expressions, such as of `SummedOp``s and evaluations with the ``CircuitSampler` did not treat coefficients correctly or ignored them completely. E.g. evaluating `~StateFn(0 * (I + Z)) @ Plus` did not yield 0 or the normalization of `~StateFn(I) @ ((Plus + Minus) / sqrt(2))` missed a factor of `sqrt(2)`. This has been fixed.
* `OptimizationResult` included some public setters and class variables were `Optional`. This fix makes all class variables read-only so that mypy and pylint can check types more effectively. `MinimumEigenOptimizer.solve` generated bitstrings in a result as `str`. This fix changed the result into `List[float]` as the other algorithms do. Some public classes related to optimization algorithms were missing in the documentation of `qiskit.optimization.algorithms`. This fix added all such classes to the docstring. #1131 \<[https://github.com/qiskit-community/qiskit-aqua/issues/1131](https://github.com/qiskit-community/qiskit-aqua/issues/1131)> for more details.
* `OptimizationResult.__init__` did not check whether the sizes of `x` and `variables` match or not (they should match). This fix added the check to raise an error if they do not match and fixes bugs detected by the check. This fix also adds missing unit tests related to `OptimizationResult.variable_names` and `OptimizationResult.variables_dict` in `test_converters`. #1167 \<[https://github.com/qiskit-community/qiskit-aqua/issues/1167](https://github.com/qiskit-community/qiskit-aqua/issues/1167)> for more details.
* Fix parameter binding in the `OperatorStateFn`, which did not bind parameters of the underlying primitive but just the coefficients.
* `op.eval(other)`, where `op` is of type `OperatorBase`, sometimes silently returns a nonsensical value when the number of qubits in `op` and `other` are not equal. This fix results in correct behavior, which is to throw an error rather than return a value, because the input in this case is invalid.
* The `construct_circuit` method of `VQE` previously returned the expectation value to be evaluated as type `OperatorBase`. This functionality has been moved into `construct_expectation` and `construct_circuit` returns a list of the circuits that are evaluated to compute the expectation value.
<span id="id437" />
### IBM Q Provider 0.8.0
<span id="release-notes-0-8-0-new-features" />
<span id="id438" />
#### New Features
* `IBMQBackend` now has a new `reservations()` method that returns reservation information for the backend, with optional filtering. In addition, you can now use `provider.backends.my_reservations()` to query for your own reservations.
* `qiskit.providers.ibmq.job.IBMQJob.result()` raises an `IBMQJobFailureError` exception if the job has failed. The exception message now contains the reason the job failed, if the entire job failed for a single reason.
* A new attribute `client_version` was added to `IBMQJob` and [`qiskit.result.Result`](/api/qiskit/0.45/qiskit.result.Result "qiskit.result.Result") object retrieved via `qiskit.providers.ibmq.job.IBMQJob.result()`. `client_version` is a dictionary with the key being the name and the value being the version of the client used to submit the job, such as Qiskit.
* The `least_busy()` function now takes a new, optional parameter `reservation_lookahead`. If specified or defaulted to, a backend is considered unavailable if it has reservations in the next `n` minutes, where `n` is the value of `reservation_lookahead`. For example, if the default value of 60 is used, then any backends that have reservations in the next 60 minutes are considered unavailable.
* `ManagedResults` now has a new `combine_results()` method that combines results from all managed jobs and returns a single [`Result`](/api/qiskit/0.45/qiskit.result.Result "qiskit.result.Result") object. This `Result` object can be used, for example, in `qiskit-ignis` fitter methods.
<span id="release-notes-0-8-0-upgrade-notes" />
<span id="id439" />
#### Upgrade Notes
* Timestamps in the following fields are now in local time instead of UTC:
* Backend properties returned by `qiskit.providers.ibmq.IBMQBackend.properties()`.
* Backend properties returned by `qiskit.providers.ibmq.job.IBMQJob.properties()`.
* `estimated_start_time` and `estimated_complete_time` in `QueueInfo`, returned by `qiskit.providers.ibmq.job.IBMQJob.queue_info()`.
* `date` in [`Result`](/api/qiskit/0.45/qiskit.result.Result "qiskit.result.Result"), returned by `qiskit.providers.ibmq.job.IBMQJob.result()`.
In addition, the `datetime` parameter for `qiskit.providers.ibmq.IBMQBackend.properties()` is also expected to be in local time unless it has UTC timezone information.
* `websockets` 8.0 or above is now required if Python 3.7 or above is used. `websockets` 7.0 will continue to be used for Python 3.6 or below.
* On Windows, the event loop policy is set to `WindowsSelectorEventLoopPolicy` instead of using the default `WindowsProactorEventLoopPolicy`. This fixes the issue that the `qiskit.providers.ibmq.job.IBMQJob.result()` method could hang on Windows. Fixes [#691](https://github.com/Qiskit/qiskit-ibmq-provider/issues/691)
<span id="release-notes-0-8-0-deprecation-notes" />
<span id="id441" />
#### Deprecation Notes
* Use of `Qconfig.py` to save IBM Quantum Experience credentials is deprecated and will be removed in the next release. You should use `qiskitrc` (the default) instead.
<span id="release-notes-0-8-0-bug-fixes" />
<span id="id442" />
#### Bug Fixes
* Fixes an issue wherein a call to `qiskit.providers.ibmq.IBMQBackend.jobs()` can hang if the number of jobs being returned is large. Fixes [#674](https://github.com/Qiskit/qiskit-ibmq-provider/issues/674)
* Fixes an issue which would raise a `ValueError` when building error maps in Jupyter for backends that are offline. Fixes [#706](https://github.com/Qiskit/qiskit-ibmq-provider/issues/706)
* `qiskit.providers.ibmq.IBMQBackend.jobs()` will now return the correct list of `IBMQJob` objects when the `status` kwarg is set to `'RUNNING'`.
* The package metadata has been updated to properly reflect the dependency on `qiskit-terra` >= 0.14.0. This dependency was implicitly added as part of the 0.7.0 release but was not reflected in the package requirements so it was previously possible to install `qiskit-ibmq-provider` with a version of `qiskit-terra` which was too old. Fixes [#677](https://github.com/Qiskit/qiskit-ibmq-provider/issues/677)
<span id="qiskit-0-19-6" />
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