# ExcitationPreserving

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

\begin{align}\begin{aligned}\newcommand{\th}{\theta/2}\\\begin{split}\begin{pmatrix} 1 & 0 & 0 & 0 \\ 0 & \cos\left(\th\right) & -i\sin\left(\th\right) & 0 \\ 0 & -i\sin\left(\th\right) & \cos\left(\th\right) & 0 \\ 0 & 0 & 0 & e^{-i\phi} \end{pmatrix}\end{split}\end{aligned}\end{align}

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)  # show the circuit
┌──────────┐ ░ ┌────────────┐┌────────────┐                             ░ ┌──────────┐
q_0: ┤ RZ(θ) ├─░─┤0           ├┤0           ├─────────────────────────────░─┤ RZ(θ) ├
├──────────┤ ░ │  RXX(θ) ││  RYY(θ) │┌────────────┐┌────────────┐ ░ ├──────────┤
q_1: ┤ RZ(θ) ├─░─┤1           ├┤1           ├┤0           ├┤0           ├─░─┤ RZ(θ) ├
├──────────┤ ░ └────────────┘└────────────┘│  RXX(θ) ││  RYY(θ) │ ░ ├──────────┤
q_2: ┤ RZ(θ) ├─░─────────────────────────────┤1           ├┤1           ├─░─┤ RZ(θ) ├
└──────────┘ ░                             └────────────┘└────────────┘ ░ └──────────┘
>>> ansatz = ExcitationPreserving(2, reps=1)
>>> 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 swaprz
>>> qc.draw()
┌──────────┐┌────────────┐┌────────────┐┌──────────┐
q_0: ─────■─────┤ RZ(θ) ├┤0           ├┤0           ├┤ RZ(θ) ├
┌────┴────┐├──────────┤│  RXX(θ) ││  RYY(θ) │├──────────┤
q_1: ┤ RY(0.2) ├┤ RZ(θ) ├┤1           ├┤1           ├┤ RZ(θ) ├
└─────────┘└──────────┘└────────────┘└────────────┘└──────────┘
>>> ansatz = ExcitationPreserving(3, reps=1, mode='fsim', entanglement=[[0,2]],
... insert_barriers=True)
>>> print(ansatz)
┌──────────┐ ░ ┌────────────┐┌────────────┐        ░ ┌──────────┐
q_0: ┤ RZ(θ) ├─░─┤0           ├┤0           ├─■──────░─┤ RZ(θ) ├
├──────────┤ ░ │            ││            │ │      ░ ├──────────┤
q_1: ┤ RZ(θ) ├─░─┤  RXX(θ) ├┤  RYY(θ) ├─┼──────░─┤ RZ(θ) ├
├──────────┤ ░ │            ││            │ │θ  ░ ├──────────┤
q_2: ┤ RZ(θ) ├─░─┤1           ├┤1           ├─■──────░─┤ RZ(θ) ├
└──────────┘ ░ └────────────┘└────────────┘        ░ └──────────┘

Parameters

• num_qubits (int (opens in a new tab) | None) – The number of qubits of the ExcitationPreserving circuit.
• mode (str (opens in a new tab)) – Choose the entangler mode, can be ‘iswap’ or ‘fsim’.
• reps (int (opens in a new tab)) – Specifies how often the structure of a rotation layer followed by an entanglement layer is repeated.
• entanglement (str (opens in a new tab) |list (opens in a new tab)[list (opens in a new tab)[int (opens in a new tab)]] | Callable[[int (opens in a new tab)], list (opens in a new tab)[int (opens in a new tab)]]) – 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 (opens in a new tab)) – 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_unentangled_qubits – 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 (opens in a new tab)) – 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 (opens in a new tab)) – The parameterized gates require a parameter to be defined, for which we use ParameterVector.
• insert_barriers (bool (opens in a new tab)) – If True, barriers are inserted in between each layer. If False, no barriers are inserted.
• flatten (bool (opens in a new tab) | 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.

Raises

ValueError (opens in a new tab) – If the selected mode is not supported.

## Attributes

### ancillas

Returns a list of ancilla bits in the order that the registers were added.

### calibrations

Return calibration dictionary.

The custom pulse definition of a given gate is of the form {'gate_name': {(qubits, params): schedule}}

### clbits

Returns a list of classical bits in the order that the registers were added.

### entanglement

Get the entanglement strategy.

Returns

The entanglement strategy, see get_entangler_map() for more detail on how the format is interpreted.

### entanglement_blocks

The blocks in the entanglement layers.

Returns

The blocks in the entanglement layers.

### extension_lib

= 'include "qelib1.inc";'

### flatten

Returns whether the circuit is wrapped in nested gates/instructions or flattened.

### global_phase

Return the global phase of the current circuit scope in radians.

= 'OPENQASM 2.0;'

### initial_state

Return the initial state that is added in front of the n-local circuit.

Returns

The initial state.

### insert_barriers

If barriers are inserted in between the layers or not.

Returns

True, if barriers are inserted in between the layers, False if not.

### instances

= 159

### layout

Return any associated layout information about the circuit

This attribute contains an optional TranspileLayout object. This is typically set on the output from transpile() or PassManager.run() to retain information about the permutations caused on the input circuit by transpilation.

There are two types of permutations caused by the transpile() function, an initial layout which permutes the qubits based on the selected physical qubits on the Target, and a final layout which is an output permutation caused by SwapGates inserted during routing.

The user provided metadata associated with the circuit.

The metadata for the circuit is a user provided dict of metadata for the circuit. It will not be used to influence the execution or operation of the circuit, but it is expected to be passed between all transforms of the circuit (ie transpilation) and that providers will associate any circuit metadata with the results it returns from execution of that circuit.

### num_ancillas

Return the number of ancilla qubits.

### num_clbits

Return number of classical bits.

### num_layers

Return the number of layers in the n-local circuit.

Returns

The number of layers in the circuit.

### num_parameters_settable

The number of total parameters that can be set to distinct values.

This does not change when the parameters are bound or exchanged for same parameters, and therefore is different from num_parameters which counts the number of unique Parameter objects currently in the circuit.

Returns

The number of parameters originally available in the circuit.

Note

This quantity does not require the circuit to be built yet.

### num_qubits

Returns the number of qubits in this circuit.

Returns

The number of qubits.

### op_start_times

Return a list of operation start times.

This attribute is enabled once one of scheduling analysis passes runs on the quantum circuit.

Returns

List of integers representing instruction start times. The index corresponds to the index of instruction in QuantumCircuit.data.

Raises

AttributeError (opens in a new tab) – When circuit is not scheduled.

### ordered_parameters

The parameters used in the underlying circuit.

This includes float values and duplicates.

## Examples

>>> # prepare circuit ...
>>> print(nlocal)
┌───────┐┌──────────┐┌──────────┐┌──────────┐
q_0: ┤ Ry(1) ├┤ Ry(θ) ├┤ Ry(θ) ├┤ Ry(θ) ├
└───────┘└──────────┘└──────────┘└──────────┘
>>> nlocal.parameters
{Parameter(θ), Parameter(θ)}
>>> nlocal.ordered_parameters
[1, Parameter(θ), Parameter(θ), Parameter(θ)]

Returns

The parameters objects used in the circuit.

### parameter_bounds

Return the parameter bounds.

Returns

The parameter bounds.

### preferred_init_points

The initial points for the parameters. Can be stored as initial guess in optimization.

Returns

The initial values for the parameters, or None, if none have been set.

### prefix

= 'circuit'

### qregs

list[QuantumRegister]

A list of the quantum registers associated with the circuit.

### qubits

Returns a list of quantum bits in the order that the registers were added.

### reps

The number of times rotation and entanglement block are repeated.

Returns

The number of repetitions.

### rotation_blocks

The blocks in the rotation layers.

Returns

The blocks in the rotation layers.