# Copyright (C) Unitary Fund
#
# This source code is licensed under the GPL license (v3) found in the
# LICENSE file in the root directory of this source tree.
"""High-level Quantum Susbapce Expansion tools."""
from functools import wraps
from typing import Callable, Dict, List, Sequence, Union
from mitiq import QPROGRAM, Executor, Observable, PauliString, QuantumResult
from mitiq.qse.qse_utils import (
get_expectation_value_for_observable,
get_projector,
)
[docs]
def execute_with_qse(
circuit: QPROGRAM,
executor: Union[Executor, Callable[[QPROGRAM], QuantumResult]],
check_operators: Sequence[PauliString],
code_hamiltonian: Observable,
observable: Observable,
pauli_string_to_expectation_cache: Dict[PauliString, complex] = {},
) -> float:
"""Function for the calculation of an observable from some circuit of
interest to be mitigated with quantum subspace expansion (QSE).
Args:
circuit: Quantum program to execute with error mitigation.
executor: Executes a circuit and returns a `QuantumResult`.
check_operators: List of check operators that define the
stabilizer code space.
code_hamiltonian: Hamiltonian of the code space.
observable: Observable to compute the mitigated expectation value of.
pauli_string_to_expectation_cache: Cache for expectation values of
Pauli strings used to compute the projector and the observable.
Returns:
The expectation value estimated with QSE.
"""
projector = get_projector(
circuit,
executor,
check_operators,
code_hamiltonian,
pauli_string_to_expectation_cache,
)
# Compute the expectation value of the observable: <P O P>
pop = get_expectation_value_for_observable(
circuit,
executor,
projector * observable * projector,
pauli_string_to_expectation_cache,
)
# Compute the normalization factor: <P P>
pp = get_expectation_value_for_observable(
circuit,
executor,
projector * projector,
pauli_string_to_expectation_cache,
)
return pop / pp
[docs]
def mitigate_executor(
executor: Callable[[QPROGRAM], QuantumResult],
check_operators: Sequence[PauliString],
code_hamiltonian: Observable,
observable: Observable,
pauli_string_to_expectation_cache: Dict[PauliString, complex] = {},
) -> Callable[[QPROGRAM], float]:
"""Returns a modified version of the input 'executor' which is
error-mitigated with quantum subspace expansion (QSE).
Args:
executor: Executes a circuit and returns a `QuantumResult`.
check_operators: List of check operators that define the
stabilizer code space.
code_hamiltonian: Hamiltonian of the code space.
observable: Observable to compute the mitigated expectation value for.
pauli_string_to_expectation_cache: Cache for expectation values of
Pauli strings used to compute the projector and the observable.
Returns:
The error-mitigated version of the input executor.
"""
executor_obj = Executor(executor)
if not executor_obj.can_batch:
@wraps(executor)
def new_executor(circuit: QPROGRAM) -> float:
return execute_with_qse(
circuit,
executor,
check_operators,
code_hamiltonian,
observable,
pauli_string_to_expectation_cache,
)
else:
@wraps(executor)
def new_executor(circuits: List[QPROGRAM]) -> List[float]:
return [
execute_with_qse(
circuit,
executor,
check_operators,
code_hamiltonian,
observable,
pauli_string_to_expectation_cache,
)
for circuit in circuits
]
return new_executor
[docs]
def qse_decorator(
check_operators: Sequence[PauliString],
code_hamiltonian: Observable,
observable: Observable,
pauli_string_to_expectation_cache: Dict[PauliString, complex] = {},
) -> Callable[
[Callable[[QPROGRAM], QuantumResult]], Callable[[QPROGRAM], float]
]:
"""Decorator which adds an error-mitigation layer based on quantum
subspace expansion (QSE) to an executor function, i.e., a function which
executes a quantum circuit with an arbitrary backend and returns a
``QuantumResult``.
Args:
check_operators: List of check operators that define the
stabilizer code space.
code_hamiltonian: Hamiltonian of the code space.
observable: Observable to compute the mitigated expectation value of.
pauli_string_to_expectation_cache: Cache for expectation values of
Pauli strings used to compute the projector and the observable.
Returns:
The error-mitigating decorator to be applied to an executor function.
"""
def decorator(
executor: Callable[[QPROGRAM], QuantumResult],
) -> Callable[[QPROGRAM], float]:
val = mitigate_executor(
executor,
check_operators,
code_hamiltonian,
observable,
pauli_string_to_expectation_cache,
)
return val
return decorator
