qiskit.aqua.operators.gradients.QFI

class QFI(qfi_method='lin_comb_full')

Compute the Quantum Fisher Information (QFI).

Computes the QFI given a pure, parametrized quantum state, where QFI is:

\[\mathrm{QFI}_{kl}= 4 \mathrm{Re}[\langle \partial_k \psi | \partial_l \psi \rangle − \langle\partial_k \psi | \psi \rangle \langle\psi | \partial_l \psi \rangle].\]

Parameters

qfi_method (Union[str, CircuitQFI]) – The method used to compute the state/probability gradient. Can be either a CircuitQFI instance or one of the following pre-defined strings 'lin_comb_full', 'overlap_diag'` or 'overlap_block_diag'`.

Raises

ValueError – if qfi_method is neither a CircuitQFI object nor one of the predefined strings.

__init__(qfi_method='lin_comb_full')

Parameters

qfi_method (Union[str, CircuitQFI]) – The method used to compute the state/probability gradient. Can be either a CircuitQFI instance or one of the following pre-defined strings 'lin_comb_full', 'overlap_diag'` or 'overlap_block_diag'`.

Raises

ValueError – if qfi_method is neither a CircuitQFI object nor one of the predefined strings.

Methods

__init__([qfi_method])	type qfi_method Union[str, CircuitQFI]
convert(operator[, params])	type operator CircuitStateFn
gradient_wrapper(operator, bind_params[, …])	Get a callable function which provides the respective gradient, Hessian or QFI for given parameter values.
parameter_expression_grad(param_expr, param)	Get the derivative of a parameter expression w.r.t.

Attributes

qfi_method

Returns CircuitQFI.

convert(operator, params=None)

Parameters

• operator (CircuitStateFn) – The operator corresponding to the quantum state |ψ(ω)〉for which we compute the QFI

• params (Union[ParameterVector, ParameterExpression, List[ParameterExpression], None]) – The parameters we are computing the QFI wrt: ω

Return type

ListOp

Returns

ListOp[ListOp] where the operator at position k,l corresponds to QFI_kl

gradient_wrapper(operator, bind_params, grad_params=None, backend=None)

Get a callable function which provides the respective gradient, Hessian or QFI for given parameter values. This callable can be used as gradient function for optimizers.

Parameters

• operator (OperatorBase) – The operator for which we want to get the gradient, Hessian or QFI.

• bind_params (Union[ParameterExpression, ParameterVector, List[ParameterExpression]]) – The operator parameters to which the parameter values are assigned.

• grad_params (Union[ParameterExpression, ParameterVector, List[ParameterExpression], Tuple[ParameterExpression, ParameterExpression], List[Tuple[ParameterExpression, ParameterExpression]], None]) – The parameters with respect to which we are taking the gradient, Hessian or QFI. If grad_params = None, then grad_params = bind_params

• backend (Union[BaseBackend, QuantumInstance, None]) – The quantum backend or QuantumInstance to use to evaluate the gradient, Hessian or QFI.

Returns

Function to compute a gradient, Hessian or QFI. The function takes an iterable as argument which holds the parameter values.

Return type

callable(param_values)

static parameter_expression_grad(param_expr, param)

Get the derivative of a parameter expression w.r.t. the given parameter.

Parameters

• param_expr (ParameterExpression) – The Parameter Expression for which we compute the derivative

• param (ParameterExpression) – Parameter w.r.t. which we want to take the derivative

Return type

Union[ParameterExpression, float]

Returns

ParameterExpression representing the gradient of param_expr w.r.t. param

property qfi_method

Returns CircuitQFI.

Return type

CircuitQFI

Returns

CircuitQFI.