36 lines
1.3 KiB
Plaintext
36 lines
1.3 KiB
Plaintext
---
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title: partial_trace (v0.31)
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description: API reference for qiskit.quantum_info.partial_trace in qiskit v0.31
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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.quantum_info.partial_trace
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---
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<span id="qiskit-quantum-info-partial-trace" />
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# qiskit.quantum\_info.partial\_trace
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<Function id="qiskit.quantum_info.partial_trace" isDedicatedPage={true} github="https://github.com/qiskit/qiskit/tree/stable/0.18/qiskit/quantum_info/states/utils.py" signature="partial_trace(state, qargs)">
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Return reduced density matrix by tracing out part of quantum state.
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If all subsystems are traced over this returns the [`trace()`](qiskit.quantum_info.DensityMatrix#trace "qiskit.quantum_info.DensityMatrix.trace") of the input state.
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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")) – the input state.
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* **qargs** (*list*) – The subsystems to trace over.
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**Returns**
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The reduced density matrix.
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**Return type**
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[DensityMatrix](qiskit.quantum_info.DensityMatrix "qiskit.quantum_info.DensityMatrix")
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**Raises**
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**QiskitError** – if input state is invalid.
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</Function>
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