--- title: ExcitationPreserving (v1.0) description: API reference for qiskit.circuit.library.ExcitationPreserving in qiskit v1.0 source: https://quantum.cloud.ibm.com/docs/en/api/qiskit/1.0/qiskit.circuit.library.ExcitationPreserving --- # 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)` [GitHub](https://github.com/Qiskit/qiskit/tree/stable/1.0/qiskit/circuit/library/n_local/excitation_preserving.py#L24-L173) Bases: [`TwoLocal`](/docs/api/qiskit/1.0/qiskit.circuit.library.TwoLocal "qiskit.circuit.library.n_local.two_local.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 $$ \newcommand{\rotationangle}{\theta/2} \begin{pmatrix} 1 & 0 & 0 & 0 \\ 0 & \cos\left(\rotationangle\right) & -i\sin\left(\rotationangle\right) & 0 \\ 0 & -i\sin\left(\rotationangle\right) & \cos\left(\rotationangle\right) & 0 \\ 0 & 0 & 0 & e^{-i\phi} \end{pmatrix} $$ 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`](/docs/api/qiskit/1.0/qiskit.circuit.library.RealAmplitudes "qiskit.circuit.library.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** ```python >>> ansatz = ExcitationPreserving(3, reps=1, insert_barriers=True, entanglement='linear') >>> print(ansatz) # 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]) ├ └──────────┘ ░ └────────────┘└────────────┘ ░ └──────────┘ ``` ```python >>> 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 ├┤0 ├┤ RZ(θ[3]) ├ ┌────┴────┐├──────────┤│ RXX(θ[2]) ││ RYY(θ[2]) │├──────────┤ q_1: ┤ RY(0.2) ├┤ RZ(θ[1]) ├┤1 ├┤1 ├┤ RZ(θ[4]) ├ └─────────┘└──────────┘└────────────┘└────────────┘└──────────┘ ``` ```python >>> ansatz = ExcitationPreserving(3, reps=1, mode='fsim', entanglement=[[0,2]], ... insert_barriers=True) >>> print(ansatz) ┌──────────┐ ░ ┌────────────┐┌────────────┐ ░ ┌──────────┐ 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*](https://docs.python.org/3/library/functions.html#int) *| None*) – The number of qubits of the ExcitationPreserving circuit. - **mode** ([*str*](https://docs.python.org/3/library/stdtypes.html#str)) – Choose the entangler mode, can be ‘iswap’ or ‘fsim’. - **reps** ([*int*](https://docs.python.org/3/library/functions.html#int)) – Specifies how often the structure of a rotation layer followed by an entanglement layer is repeated. - **entanglement** ([*str*](https://docs.python.org/3/library/stdtypes.html#str) *|*[*list*](https://docs.python.org/3/library/stdtypes.html#list)*\[*[*list*](https://docs.python.org/3/library/stdtypes.html#list)*\[*[*int*](https://docs.python.org/3/library/functions.html#int)*]] | Callable\[\[*[*int*](https://docs.python.org/3/library/functions.html#int)*],* [*list*](https://docs.python.org/3/library/stdtypes.html#list)*\[*[*int*](https://docs.python.org/3/library/functions.html#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`](/docs/api/qiskit/1.0/qiskit.circuit.library.TwoLocal "qiskit.circuit.library.TwoLocal") for more detail. - **initial\_state** ([*QuantumCircuit*](/docs/api/qiskit/1.0/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") *| None*) – A QuantumCircuit object to prepend to the circuit. - **skip\_unentangled\_qubits** ([*bool*](https://docs.python.org/3/library/functions.html#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*](https://docs.python.org/3/library/functions.html#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*](https://docs.python.org/3/library/stdtypes.html#str)) – The parameterized gates require a parameter to be defined, for which we use [`ParameterVector`](/docs/api/qiskit/1.0/qiskit.circuit.ParameterVector "qiskit.circuit.ParameterVector"). - **insert\_barriers** ([*bool*](https://docs.python.org/3/library/functions.html#bool)) – If True, barriers are inserted in between each layer. If False, no barriers are inserted. - **flatten** ([*bool*](https://docs.python.org/3/library/functions.html#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. **Raises** [**ValueError**](https://docs.python.org/3/library/exceptions.html#ValueError) – 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. ### data ### 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. ### 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. ### 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 Default value: `166` ### layout Return any associated layout information about the circuit This attribute contains an optional [`TranspileLayout`](/docs/api/qiskit/1.0/qiskit.transpiler.TranspileLayout "qiskit.transpiler.TranspileLayout") object. This is typically set on the output from [`transpile()`](/docs/api/qiskit/1.0/compiler#qiskit.compiler.transpile "qiskit.compiler.transpile") or [`PassManager.run()`](/docs/api/qiskit/1.0/qiskit.transpiler.PassManager#run "qiskit.transpiler.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()`](/docs/api/qiskit/1.0/compiler#qiskit.compiler.transpile "qiskit.compiler.transpile") function, an initial layout which permutes the qubits based on the selected physical qubits on the [`Target`](/docs/api/qiskit/1.0/qiskit.transpiler.Target "qiskit.transpiler.Target"), and a final layout which is an output permutation caused by [`SwapGate`](/docs/api/qiskit/1.0/qiskit.circuit.library.SwapGate "qiskit.circuit.library.SwapGate")s inserted during routing. ### metadata 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 ### 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`](/docs/api/qiskit/1.0/qiskit.circuit.Parameter "qiskit.circuit.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**](https://docs.python.org/3/library/exceptions.html#AttributeError) – When circuit is not scheduled. ### ordered\_parameters The parameters used in the underlying circuit. This includes float values and duplicates. **Examples** ```python >>> # prepare circuit ... >>> print(nlocal) ┌───────┐┌──────────┐┌──────────┐┌──────────┐ q_0: ┤ Ry(1) ├┤ Ry(θ[1]) ├┤ Ry(θ[1]) ├┤ Ry(θ[3]) ├ └───────┘└──────────┘└──────────┘└──────────┘ >>> nlocal.parameters {Parameter(θ[1]), Parameter(θ[3])} >>> nlocal.ordered_parameters [1, Parameter(θ[1]), Parameter(θ[1]), Parameter(θ[3])] ``` **Returns** The parameters objects used in the circuit. ### parameter\_bounds Return the parameter bounds. **Returns** The parameter bounds. ### parameters ### 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 Default value: `'circuit'` ### qregs Type: `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.