91 lines
3.9 KiB
Plaintext
91 lines
3.9 KiB
Plaintext
---
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title: plot_state_paulivec
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description: API reference for qiskit.visualization.plot_state_paulivec
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in_page_toc_min_heading_level: 1
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python_api_type: function
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python_api_name: qiskit.visualization.plot_state_paulivec
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---
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<span id="qiskit-visualization-plot-state-paulivec" />
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# qiskit.visualization.plot\_state\_paulivec
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<Function id="qiskit.visualization.plot_state_paulivec" isDedicatedPage={true} github="https://github.com/Qiskit/qiskit/tree/stable/1.1/qiskit/utils/lazy_tester.py#L611-L722" signature="qiskit.visualization.plot_state_paulivec(state, title='', figsize=None, color=None, ax=None, *, filename=None)">
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Plot the Pauli-vector representation of a quantum state as bar graph.
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The Pauli-vector of a density matrix $\rho$ is defined by the expectation of each possible tensor product of single-qubit Pauli operators (including the identity), that is
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$$
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\rho = \frac{1}{2^n} \sum_{\sigma \in \{I, X, Y, Z\}^{\otimes n}}
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\mathrm{Tr}(\sigma \rho) \sigma.
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$$
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This function plots the coefficients $\mathrm{Tr}(\sigma\rho)$ as bar graph.
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**Parameters**
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* **state** ([*Statevector*](qiskit.quantum_info.Statevector "qiskit.quantum_info.Statevector") *or*[*DensityMatrix*](qiskit.quantum_info.DensityMatrix "qiskit.quantum_info.DensityMatrix") *or ndarray*) – an N-qubit quantum state.
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* **title** ([*str*](https://docs.python.org/3/library/stdtypes.html#str "(in Python v3.12)")) – a string that represents the plot title
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* **figsize** ([*tuple*](https://docs.python.org/3/library/stdtypes.html#tuple "(in Python v3.12)")) – Figure size in inches.
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* **color** ([*list*](https://docs.python.org/3/library/stdtypes.html#list "(in Python v3.12)") *or*[*str*](https://docs.python.org/3/library/stdtypes.html#str "(in Python v3.12)")) – Color of the coefficient value bars.
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* **ax** ([*matplotlib.axes.Axes*](https://matplotlib.org/stable/api/_as_gen/matplotlib.axes.Axes.html#matplotlib.axes.Axes "(in Matplotlib v3.9.1)")) – An optional Axes object to be used for the visualization output. If none is specified a new matplotlib Figure will be created and used. Additionally, if specified there will be no returned Figure since it is redundant.
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**Returns**
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The matplotlib.Figure of the visualization if the `ax` kwarg is not set
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**Return type**
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[`matplotlib.figure.Figure`](https://matplotlib.org/stable/api/_as_gen/matplotlib.figure.Figure.html#matplotlib.figure.Figure "(in Matplotlib v3.9.1)")
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**Raises**
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* [**MissingOptionalLibraryError**](exceptions#qiskit.exceptions.MissingOptionalLibraryError "qiskit.exceptions.MissingOptionalLibraryError") – Requires matplotlib.
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* [**VisualizationError**](visualization#qiskit.visualization.VisualizationError "qiskit.visualization.VisualizationError") – if input is not a valid N-qubit state.
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**Examples**
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```python
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# You can set a color for all the bars.
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from qiskit import QuantumCircuit
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from qiskit.quantum_info import Statevector
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from qiskit.visualization import plot_state_paulivec
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qc = QuantumCircuit(2)
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qc.h(0)
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qc.cx(0, 1)
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state = Statevector(qc)
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plot_state_paulivec(state, color='midnightblue', title="New PauliVec plot")
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```
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```python
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# If you introduce a list with less colors than bars, the color of the bars will
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# alternate following the sequence from the list.
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import numpy as np
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from qiskit.quantum_info import DensityMatrix
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from qiskit import QuantumCircuit
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from qiskit.visualization import plot_state_paulivec
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qc = QuantumCircuit(2)
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qc.h(0)
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qc.cx(0, 1)
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qc = QuantumCircuit(2)
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qc.h([0, 1])
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qc.cz(0, 1)
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qc.ry(np.pi/3, 0)
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qc.rx(np.pi/5, 1)
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matrix = DensityMatrix(qc)
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plot_state_paulivec(matrix, color=['crimson', 'midnightblue', 'seagreen'])
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```
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</Function>
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