squlearn.observables.CustomObservable

class squlearn.observables.CustomObservable(num_qubits: int, operator_string: str | list[str] | tuple[str], parameterized: bool = False)

Class for defining a custom observable.

The operator is supplied as a string of Pauli operators, e.g. operator_string='ZI' for a two qubit operator with a Z operator on the second qubit. Note that the index of the qubits is reversed, i.e. the first qubit is the last character in the string, similar to the Qiskit computational state numbering.

Multiple operators that are summed can be specified by a list of strings, e.g. operator_string=['ZZ', 'XX'].

Parameters:

• num_qubits (int) – Number of qubits.

• operator_string (Union[str, list[str], tuple[str]]) – String of operator to measure. Also list or tuples of strings are allowed for multiple operators.

• parameterized (bool) – If True, the operator is parameterized.

Attributes:

num_qubits

Number of qubits.

Type:

int

num_parameters

Number of trainable parameters in the custom operator.

Type:

int

operator_string

String of operator to measure.

Type:

Union[str, list[str], tuple[str]]

parameterized

If True, the operator is parameterized.

Type:

bool

Methods:

generate_initial_parameters(ones: bool = True, seed: int | None = None) → ndarray

Generates random parameters for the observable

Parameters:

• ones (bool) – If True, returns an array of ones (default: True)

• seed (Union[int,None]) – Seed for the random number generator

Returns:

The randomly generated parameters

get_operator(parameters: ParameterVector | ndarray) → SparsePauliOp

Returns Operator as a SparsePauliOp expression.

Parameters:

parameters (Union[ParameterVector, np.ndarray]) – Vector of parameters used in the operator

Returns:

StateFn expression of the observable.

get_params(deep: bool = True) → dict

Returns hyper-parameters and their values of the custom operator.

Parameters:

deep (bool) – If True, also the parameters for contained objects are returned (default=True).

Returns:

Dictionary with hyper-parameters and values.

get_pauli(parameters: ParameterVector | ndarray = None) → SparsePauliOp

Function for generating the SparsePauliOp expression of the custom operator.

Parameters:

parameters (Union[ParameterVector, np.ndarray]) – Parameters of the custom operator.

Returns:

SparsePauliOp expression of the specified custom operator.

get_pauli_mapped(parameters: ParameterVector | ndarray) → SparsePauliOp

Changes the operator to the physical qubits, set by set_map().

Parameters:

parameters (Union[ParameterVector, np.ndarray]) – Vector of parameters used in the operator

Returns:

Expectation operator in Qiskit’s SprasePauliOp class with qubits mapped to physical ones

set_map(qubit_map: list | dict, num_all_qubits: int)

Function for setting a qubit mapping from physical qubits to the ones of the operator.

This function is necessary whenever the number of physical qubits are different from the operator definition, as for example when running on a real backend. The number of qubits in the system has to be larger than the number of qubits in the observable.

Parameters:

• qubit_map (Union[list, dict]) – A list or dictionary specifying which of the input qubits are mapped to the output qubits.

• num_all_qubits (int) – The total number of qubits in the system.

set_params(**params) → None

Sets value of the operator hyper-parameters.

Parameters:

params – Hyper-parameters and their values, e.g. num_qubits=2.