Choi

class Choi(data, input_dims=None, output_dims=None)

Bases: qiskit.quantum_info.operators.channel.quantum_channel.QuantumChannel

Choi-matrix representation of a Quantum Channel.

The Choi-matrix representation of a quantum channel $\mathcal{E}$ is a matrix

\Lambda = \sum_{i,j} |i\rangle\!\langle j|\otimes \mathcal{E}\left(|i\rangle\!\langle j|\right)

Evolution of a DensityMatrix $\rho$ with respect to the Choi-matrix is given by

\mathcal{E}(\rho) = \text{Tr}_{1}\left[\Lambda (\rho^T \otimes \mathbb{I})\right]

where $\text{Tr}_1$ is the partial_trace() over subsystem 1.

See reference [1] for further details.

References

C.J. Wood, J.D. Biamonte, D.G. Cory, Tensor networks and graphical calculus for open quantum systems, Quant. Inf. Comp. 15, 0579-0811 (2015). arXiv:1111.6950 [quant-ph]

Initialize a quantum channel Choi matrix operator.

Parameters

**(**QuantumCircuit or (data) – Instruction or BaseOperator or matrix): data to initialize superoperator.
input_dims (tuple) – the input subsystem dimensions. [Default: None]
output_dims (tuple) – the output subsystem dimensions. [Default: None]

Raises

QiskitError – if input data cannot be initialized as a Choi matrix.

Additional Information:

If the input or output dimensions are None, they will be automatically determined from the input data. If the input data is a Numpy array of shape (4**N, 4**N) qubit systems will be used. If the input operator is not an N-qubit operator, it will assign a single subsystem with dimension specified by the shape of the input.

Methods

adjoint

Choi.adjoint()

Return the adjoint quantum channel.

Note

This is equivalent to the matrix Hermitian conjugate in the SuperOp representation ie. for a channel $\mathcal{E}$ , the SuperOp of the adjoint channel $\mathcal{{E}}^\dagger$ is $S_{\mathcal{E}^\dagger} = S_{\mathcal{E}}^\dagger$ .

compose

Choi.compose(other, qargs=None, front=False)

Return the operator composition with another Choi.

Parameters

other (Choi) – a Choi object.
qargs (list or None) – Optional, a list of subsystem positions to apply other on. If None apply on all subsystems (default: None).
front (bool) – If True compose using right operator multiplication, instead of left multiplication [default: False].

Returns

The composed Choi.

Return type

Choi

Raises

QiskitError – if other cannot be converted to an operator, or has incompatible dimensions for specified subsystems.

Note

Composition (&) by default is defined as left matrix multiplication for matrix operators, while dot() is defined as right matrix multiplication. That is that A & B == A.compose(B) is equivalent to B.dot(A) when A and B are of the same type.

Setting the front=True kwarg changes this to right matrix multiplication and is equivalent to the dot() method A.dot(B) == A.compose(B, front=True).

conjugate

Choi.conjugate()

Return the conjugate quantum channel.

Note

This is equivalent to the matrix complex conjugate in the SuperOp representation ie. for a channel $\mathcal{E}$ , the SuperOp of the conjugate channel $\overline{{\mathcal{{E}}}}$ is $S_{\overline{\mathcal{E}^\dagger}} = \overline{S_{\mathcal{E}}}$ .

copy

Choi.copy()

Make a deep copy of current operator.

dot

Choi.dot(other, qargs=None)

Return the right multiplied operator self * other.

Parameters

other (Operator) – an operator object.
qargs (list or None) – Optional, a list of subsystem positions to apply other on. If None apply on all subsystems (default: None).

Returns

The right matrix multiplied Operator.

Return type

Operator

expand

Choi.expand(other)

Return the reverse-order tensor product with another Choi.

Parameters

other (Choi) – a Choi object.

Returns

the tensor product $b \otimes a$ , where $a$

is the current Choi, and $b$ is the other Choi.

Return type

Choi

input_dims

Choi.input_dims(qargs=None)

Return tuple of input dimension for specified subsystems.

is_cp

Choi.is_cp(atol=None, rtol=None)

Test if Choi-matrix is completely-positive (CP)

is_cptp

Choi.is_cptp(atol=None, rtol=None)

Return True if completely-positive trace-preserving (CPTP).

is_tp

Choi.is_tp(atol=None, rtol=None)

Test if a channel is trace-preserving (TP)

is_unitary

Choi.is_unitary(atol=None, rtol=None)

Return True if QuantumChannel is a unitary channel.

output_dims

Choi.output_dims(qargs=None)

Return tuple of output dimension for specified subsystems.

power

Choi.power(n)

Return the power of the quantum channel.

Parameters

n (float) – the power exponent.

Returns

the channel $\mathcal{{E}} ^n$ .

Return type

SuperOp

Raises

QiskitError – if the input and output dimensions of the SuperOp are not equal.

Note

For non-positive or non-integer exponents the power is defined as the matrix power of the SuperOp representation ie. for a channel $\mathcal{{E}}$ , the SuperOp of the powered channel $\mathcal{{E}}^n$ is $S_{{\mathcal{{E}}^n}} = S_{{\mathcal{{E}}}}^n$ .

reshape

Choi.reshape(input_dims=None, output_dims=None, num_qubits=None)

Return a shallow copy with reshaped input and output subsystem dimensions.

Parameters

input_dims (None or tuple) – new subsystem input dimensions. If None the original input dims will be preserved [Default: None].
output_dims (None or tuple) – new subsystem output dimensions. If None the original output dims will be preserved [Default: None].
num_qubits (None or int) – reshape to an N-qubit operator [Default: None].

Returns

returns self with reshaped input and output dimensions.

Return type

BaseOperator

Raises

QiskitError – if combined size of all subsystem input dimension or subsystem output dimensions is not constant.

tensor

Choi.tensor(other)

Return the tensor product with another Choi.

Parameters

other (Choi) – a Choi object.

Returns

the tensor product $a \otimes b$ , where $a$

is the current Choi, and $b$ is the other Choi.

Return type

Choi

Note

The tensor product can be obtained using the ^ binary operator. Hence a.tensor(b) is equivalent to a ^ b.

to_instruction

Choi.to_instruction()

Convert to a Kraus or UnitaryGate circuit instruction.

If the channel is unitary it will be added as a unitary gate, otherwise it will be added as a kraus simulator instruction.

Returns

A kraus instruction for the channel.

Return type

qiskit.circuit.Instruction

Raises

QiskitError – if input data is not an N-qubit CPTP quantum channel.

to_operator

Choi.to_operator()

Try to convert channel to a unitary representation Operator.

transpose

Choi.transpose()

Return the transpose quantum channel.

Note

This is equivalent to the matrix transpose in the SuperOp representation, ie. for a channel $\mathcal{E}$ , the SuperOp of the transpose channel $\mathcal{{E}}^T$ is $S_{mathcal{E}^T} = S_{\mathcal{E}}^T$ .

Attributes

atol

Default value: 1e-08

data

Return data.

dim

Return tuple (input_shape, output_shape).

num_qubits

Return the number of qubits if a N-qubit operator or None otherwise.

qargs

Return the qargs for the operator.

rtol

Default value: 1e-05

settings

Return settings.

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