ExcitationPreserving

class qiskit.circuit.library.ExcitationPreserving(num_qubits=None, mode='iswap', entanglement='full', reps=3, skip_unentangled_qubits=False, skip_final_rotation_layer=False, parameter_prefix='θ', insert_barriers=False, initial_state=None, name='ExcitationPreserving', flatten=None)

Bases: TwoLocal

The heuristic excitation-preserving wave function ansatz.

The ExcitationPreserving circuit preserves the ratio of $|00\rangle$, $|01\rangle + |10\rangle$ and $|11\rangle$ states. To this end, this circuit uses two-qubit interactions of the form

for the mode 'fsim' or with $e^{-i\phi} = 1$ for the mode 'iswap'.

Note that other wave functions, such as UCC-ansatzes, are also excitation preserving. However these can become complex quickly, while this heuristically motivated circuit follows a simpler pattern.

This trial wave function consists of layers of $Z$ rotations with 2-qubit entanglements. The entangling is creating using $XX+YY$ rotations and optionally a controlled-phase gate for the mode 'fsim'.

See RealAmplitudes for more detail on the possible arguments and options such as skipping unentanglement qubits, which apply here too.

The rotations of the ExcitationPreserving ansatz can be written as

Examples

>>> ansatz = ExcitationPreserving(3, reps=1, insert_barriers=True, entanglement='linear')
>>> print(ansatz.decompose())  # show the circuit
     ┌──────────┐ ░ ┌────────────┐┌────────────┐                             ░ ┌──────────┐
q_0: ┤ RZ(θ[0]) ├─░─┤0           ├┤0           ├─────────────────────────────░─┤ RZ(θ[5]) ├
     ├──────────┤ ░ │  RXX(θ[3]) ││  RYY(θ[3]) │┌────────────┐┌────────────┐ ░ ├──────────┤
q_1: ┤ RZ(θ[1]) ├─░─┤1           ├┤1           ├┤0           ├┤0           ├─░─┤ RZ(θ[6]) ├
     ├──────────┤ ░ └────────────┘└────────────┘│  RXX(θ[4]) ││  RYY(θ[4]) │ ░ ├──────────┤
q_2: ┤ RZ(θ[2]) ├─░─────────────────────────────┤1           ├┤1           ├─░─┤ RZ(θ[7]) ├
     └──────────┘ ░                             └────────────┘└────────────┘ ░ └──────────┘

>>> ansatz = ExcitationPreserving(2, reps=1, flatten=True)
>>> qc = QuantumCircuit(2)  # create a circuit and append the RY variational form
>>> qc.cry(0.2, 0, 1)  # do some previous operation
>>> qc.compose(ansatz, inplace=True)  # add the excitation-preserving
>>> qc.draw()
                ┌──────────┐┌────────────┐┌────────────┐┌──────────┐
q_0: ─────■─────┤ RZ(θ[0]) ├┤0           ├┤0           ├┤ RZ(θ[3]) ├
     ┌────┴────┐├──────────┤│  RXX(θ[2]) ││  RYY(θ[2]) │├──────────┤
q_1: ┤ RY(0.2) ├┤ RZ(θ[1]) ├┤1           ├┤1           ├┤ RZ(θ[4]) ├
     └─────────┘└──────────┘└────────────┘└────────────┘└──────────┘

>>> ansatz = ExcitationPreserving(3, reps=1, mode='fsim', entanglement=[[0,2]],
... insert_barriers=True, flatten=True)
>>> print(ansatz.decompose())
     ┌──────────┐ ░ ┌────────────┐┌────────────┐        ░ ┌──────────┐
q_0: ┤ RZ(θ[0]) ├─░─┤0           ├┤0           ├─■──────░─┤ RZ(θ[5]) ├
     ├──────────┤ ░ │            ││            │ │      ░ ├──────────┤
q_1: ┤ RZ(θ[1]) ├─░─┤  RXX(θ[3]) ├┤  RYY(θ[3]) ├─┼──────░─┤ RZ(θ[6]) ├
     ├──────────┤ ░ │            ││            │ │θ[4]  ░ ├──────────┤
q_2: ┤ RZ(θ[2]) ├─░─┤1           ├┤1           ├─■──────░─┤ RZ(θ[7]) ├
     └──────────┘ ░ └────────────┘└────────────┘        ░ └──────────┘

Parameters

• num_qubits (int | None) – The number of qubits of the ExcitationPreserving circuit.
• mode (str) – Choose the entangler mode, can be ‘iswap’ or ‘fsim’.
• reps (int) – Specifies how often the structure of a rotation layer followed by an entanglement layer is repeated.
• entanglement (str |list[list[int]] | Callable[[int], list[int]]) – Specifies the entanglement structure. Can be a string (‘full’, ‘linear’ or ‘sca’), a list of integer-pairs specifying the indices of qubits entangled with one another, or a callable returning such a list provided with the index of the entanglement layer. See the Examples section of TwoLocal for more detail.
• initial_state (QuantumCircuit | None) – A QuantumCircuit object to prepend to the circuit.
• skip_unentangled_qubits (bool) – If True, the single qubit gates are only applied to qubits that are entangled with another qubit. If False, the single qubit gates are applied to each qubit in the Ansatz. Defaults to False.
• skip_final_rotation_layer (bool) – If True, a rotation layer is added at the end of the ansatz. If False, no rotation layer is added. Defaults to True.
• parameter_prefix (str) – The parameterized gates require a parameter to be defined, for which we use ParameterVector.
• insert_barriers (bool) – If True, barriers are inserted in between each layer. If False, no barriers are inserted.
• flatten (bool | None) – Set this to True to output a flat circuit instead of nesting it inside multiple layers of gate objects. By default currently the contents of the output circuit will be wrapped in nested objects for cleaner visualization. However, if you’re using this circuit for anything besides visualization its strongly recommended to set this flag to True to avoid a large performance overhead for parameter binding.
• name (str) –

Raises

ValueError – If the selected mode is not supported.

Attributes

parameter_bounds

name

from qiskit import QuantumCircuit
 
qc = QuantumCircuit(2, 2, name="my_circuit")
print(qc.name)

my_circuit