Ansatz generation (qiskit_addon_aqc_tensor.ansatz_generation)

Utility for generating a general, parametrized, ansatz circuit which matches the two-qubit connectivity of an input circuit.

AnsatzBlock	Ansatz block.
OneQubitAnsatzBlock	One-qubit ansatz block.
TwoQubitAnsatzBlock	Two-qubit ansatz block.
ZXZ	One-qubit ansatz block based on the ZXZ decomposition.
KAK	Two-qubit ansatz block based on the KAK decomposition.

generate_ansatz_from_circuit(qc, /, *, qubits_initially_zero=False, parameter_name='theta')

Generate an ansatz from the two-qubit connectivity structure of a circuit.

See the explanatatory material for motivation.

Parameters:

• qc (QuantumCircuit) – A circuit, which is assumed to be unitary. Barriers are ignored.

• qubits_initially_zero (bool) – If True, the first Z rotation on each qubit is fixed to zero because such a rotation has no effect on the state \(|0\rangle\).

• parameter_name (str) – Name for the ParameterVector representing the free parameters in the returned ansatz circuit.

Return type:

tuple[QuantumCircuit, list[float]]

Returns:

(ansatz, parameter_values) such that ansatz.assign_parameters(parameter_values) is equivalent to qc up to a global phase.

Example:

Consider the following circuit as an example:

from qiskit import QuantumCircuit

qc = QuantumCircuit(6)
qc.rx(0.4, 0)
qc.ryy(0.2, 2, 3)
qc.h(2)
qc.rz(0.1, 2)
qc.rxx(0.3, 0, 1)
qc.rzz(0.3, 0, 1)
qc.cx(2, 1)
qc.s(1)
qc.h(4)
qc.draw("mpl")

(Source code)

If the above circuit is passed to generate_ansatz_from_circuit(), it will return an ansatz with parametrized two-qubit KAK rotations in the same locations as the input:

from qiskit_addon_aqc_tensor import generate_ansatz_from_circuit

ansatz, initial_params = generate_ansatz_from_circuit(
    qc, qubits_initially_zero=True, parameter_name="x"
)
ansatz.draw("mpl")

(Source code)

Note that in the generated ansatz, all consecutive single-qubit gates are collapsed into the same ZXZ block, and all consecutive two-qubit gates are collapsed into a single KAK block, up to single-qubit rotations.

Further, the generate_ansatz_from_circuit() function provides parameters which, when bound to the ansatz, will result in a circuit equivalent to the original one, up to a global phase:

ansatz.assign_parameters(initial_params).draw("mpl")

(Source code)