qiskit.circuit.library.excitation_preserving

qiskit.circuit.library.excitation_preserving(num_qubits, mode='iswap', entanglement='full', reps=3, skip_unentangled_qubits=False, skip_final_rotation_layer=False, parameter_prefix='θ', insert_barriers=False, name='ExcitationPreserving')

The heuristic excitation-preserving wave function ansatz.

The excitation_preserving 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".

Examples

With linear entanglement, this circuit is given by:

from qiskit.circuit.library import excitation_preserving
 
ansatz = excitation_preserving(3, reps=1, insert_barriers=True, entanglement="linear")
ansatz.draw("mpl")

The entanglement structure can be explicitly specified with the entanglement argument. The "fsim" mode includes an additional parameterized CPhaseGate in each block:

ansatz = excitation_preserving(3, reps=1, mode="fsim", entanglement=[[0, 2]])
ansatz.draw("mpl")

Parameters

• num_qubits (int) – The number of qubits.
• 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 (BlockEntanglement | Iterable[BlockEntanglement] | Callable[[int], BlockEntanglement | Iterable[BlockEntanglement]]) – The indices specifying on which qubits the input blocks act. See n_local() for detailed information.
• skip_final_rotation_layer (bool) – Whether a final rotation layer is added to the circuit.
• skip_unentangled_qubits (bool) – If True, the rotation gates act only on qubits that are entangled. If False, the rotation gates act on all qubits.
• parameter_prefix (str) – The name of the free parameters.
• insert_barriers (bool) – If True, barriers are inserted in between each layer. If False, no barriers are inserted.
• name (str) – The name of the circuit.

Returns

An excitation-preserving circuit.