qiskit.opflow.gradients.QFIBase¶

class QFIBase(qfi_method='lin_comb_full')[source]¶

Base class for Quantum Fisher Information (QFI).

Compute the Quantum Fisher Information (QFI) given a pure, parameterized quantum state.

The QFI is:

[QFI]kl= Re[〈∂kψ|∂lψ〉−〈∂kψ|ψ〉〈ψ|∂lψ〉] * 4.

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')[source]¶

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 `OperatorBase`
`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.

abstract convert(operator, params=None)¶

Parameters

operator (OperatorBase) – The operator we are taking the gradient, Hessian or QFI of
params (Union[ParameterVector, ParameterExpression, List[ParameterExpression], None]) – The parameters we are taking the gradient, Hessian or QFI with respect to.

Return type

OperatorBase

Returns

An operator whose evaluation yields the gradient, Hessian or QFI.

Raises

ValueError – If params contains a parameter not present in operator.

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.