squlearn.observables.SinglePauli

class squlearn.observables.SinglePauli(num_qubits: int, qubit: int = 0, op_str: str = 'Z', parameterized: bool = False)

Observable constructed from a single Pauli operator of a single Qubit.

Equation for Z Pauli operator:

\[\hat{H} = \hat{Z_i} \qquad \text{or} \qquad \hat{H} = \theta\hat{Z_i}~~~~ \text{ (parameterized)}\]

Can be parameterized or not, the four Pauli operators \(\hat{X},\hat{Y},\hat{Z}\) and \(\hat{I}\) are supported.

Parameters:

• num_qubits (int) – Number of qubits.

• qubit (int) – Qubit on which the Pauli operator acts.

• op_str (str) – Pauli operator to measure. Must be one of 'I', 'X', 'Y', 'Z' (default: 'Z').

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

Attributes:

num_qubits

Number of qubits.

Type:

int

num_parameters

Number of trainable parameters in the single Pauli operator.

Type:

int

qubit

Qubit on which the Pauli operator acts.

Type:

int

op_str

Pauli operator to measure.

Type:

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 Single Pauli 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) → SparsePauliOp

Function for generating the SparsePauliOp expression of the single Pauli operator.

Parameters:

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

Returns:

SparsePauliOp expression of the specified single Pauli 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.