VQE

class VQE(ansatz=None, optimizer=None, initial_point=None, gradient=None, expectation=None, include_custom=False, max_evals_grouped=1, callback=None, quantum_instance=None, sort_parameters_by_name=None)

Bases: qiskit.algorithms.variational_algorithm.VariationalAlgorithm, qiskit.algorithms.minimum_eigen_solvers.minimum_eigen_solver.MinimumEigensolver

The Variational Quantum Eigensolver algorithm.

VQE is a quantum algorithm that uses a variational technique to find the minimum eigenvalue of the Hamiltonian $H$ of a given system.

An instance of VQE requires defining two algorithmic sub-components: a trial state (a.k.a. ansatz) which is a QuantumCircuit, and one of the classical optimizers. The ansatz is varied, via its set of parameters, by the optimizer, such that it works towards a state, as determined by the parameters applied to the ansatz, that will result in the minimum expectation value being measured of the input operator (Hamiltonian).

An optional array of parameter values, via the initial_point, may be provided as the starting point for the search of the minimum eigenvalue. This feature is particularly useful such as when there are reasons to believe that the solution point is close to a particular point. As an example, when building the dissociation profile of a molecule, it is likely that using the previous computed optimal solution as the starting initial point for the next interatomic distance is going to reduce the number of iterations necessary for the variational algorithm to converge. It provides an initial point tutorial detailing this use case.

The length of the initial_point list value must match the number of the parameters expected by the ansatz being used. If the initial_point is left at the default of None, then VQE will look to the ansatz for a preferred value, based on its given initial state. If the ansatz returns None, then a random point will be generated within the parameter bounds set, as per above. If the ansatz provides None as the lower bound, then VQE will default it to $-2\pi$; similarly, if the ansatz returns None as the upper bound, the default value will be $2\pi$.

Parameters

• ansatz (Optional[QuantumCircuit]) – A parameterized circuit used as Ansatz for the wave function.

• optimizer (Optional[Optimizer]) – A classical optimizer.

• initial_point (Optional[ndarray]) – An optional initial point (i.e. initial parameter values) for the optimizer. If None then VQE will look to the ansatz for a preferred point and if not will simply compute a random one.

• gradient (Union[GradientBase, Callable, None]) – An optional gradient function or operator for optimizer.

• expectation (Optional[ExpectationBase]) – The Expectation converter for taking the average value of the Observable over the ansatz state function. When None (the default) an ExpectationFactory is used to select an appropriate expectation based on the operator and backend. When using Aer qasm_simulator backend, with paulis, it is however much faster to leverage custom Aer function for the computation but, although VQE performs much faster with it, the outcome is ideal, with no shot noise, like using a state vector simulator. If you are just looking for the quickest performance when choosing Aer qasm_simulator and the lack of shot noise is not an issue then set include_custom parameter here to True (defaults to False).

• include_custom (bool) – When expectation parameter here is None setting this to True will allow the factory to include the custom Aer pauli expectation.

• max_evals_grouped (int) – Max number of evaluations performed simultaneously. Signals the given optimizer that more than one set of parameters can be supplied so that potentially the expectation values can be computed in parallel. Typically this is possible when a finite difference gradient is used by the optimizer such that multiple points to compute the gradient can be passed and if computed in parallel improve overall execution time. Deprecated if a gradient operator or function is given.

• callback (Optional[Callable[[int, ndarray, float, float], None]]) – a callback that can access the intermediate data during the optimization. Four parameter values are passed to the callback as follows during each evaluation by the optimizer for its current set of parameters as it works towards the minimum. These are: the evaluation count, the optimizer parameters for the ansatz, the evaluated mean and the evaluated standard deviation.`

• quantum_instance (Union[Backend, BaseBackend, QuantumInstance, None]) – Quantum Instance or Backend

• sort_parameters_by_name (Optional[bool]) – Deprecated. If True, the initial point is bound to the ansatz parameters strictly sorted by name instead of the default circuit order. That means that the ansatz parameters are e.g. sorted as x[0] x[1] x[10] x[2] ... instead of x[0] x[1] x[2] ... x[10]. Set this to True to obtain the behavior prior to Qiskit Terra 0.18.0.

Methods

cleanup_parameterized_circuits	set parameterized circuits to None
compute_minimum_eigenvalue	Computes minimum eigenvalue.
construct_circuit	Return the circuits used to compute the expectation value.
construct_expectation	Generate the ansatz circuit and expectation value measurement, and return their runnable composition.
find_minimum	Optimize to find the minimum cost value.
get_energy_evaluation	Returns a function handle to evaluates the energy at given parameters for the ansatz.
get_optimal_circuit	Get the circuit with the optimal parameters.
get_optimal_cost	Get the minimal cost or energy found by the VQE.
get_optimal_vector	Get the simulation outcome of the optimal circuit.
get_prob_vector_for_params	Helper function to get probability vectors for a set of params
get_probabilities_for_counts	get probabilities for counts
print_settings	Preparing the setting of VQE into a string.
supports_aux_operators	Whether computing the expectation value of auxiliary operators is supported.

Attributes

ansatz

Returns the ansatz.

Return type

Optional[QuantumCircuit]

expectation

The expectation value algorithm used to construct the expectation measurement from the observable.

Return type

Optional[ExpectationBase]

gradient

Returns the gradient.

Return type

Union[GradientBase, Callable, None]

initial_point

Returns initial point

Return type

Optional[ndarray]

optimal_params

The optimal parameters for the ansatz.

Return type

ndarray

optimizer

Returns optimizer

Return type

Optional[Optimizer]

quantum_instance

Returns quantum instance.

Return type

Optional[QuantumInstance]

setting

Prepare the setting of VQE as a string.