# Copyright (C) 2020 Unitary Fund
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program. If not, see <https://www.gnu.org/licenses/>.
"""Utility functions for benchmarking."""
from typing import cast, Union
import numpy as np
from cirq import (
Circuit,
depolarize,
DensityMatrixSimulator,
NoiseModel,
SingleQubitGate,
)
SIMULATOR = DensityMatrixSimulator()
[docs]def noisy_simulation(circ: Circuit, noise: float, obs: np.ndarray) -> float:
"""Simulates a circuit with depolarizing noise at level NOISE.
Args:
circ: The quantum program as a cirq object.
noise: The level of depolarizing noise.
obs: The observable that the backend should measure.
Returns:
The observable's expectation value.
"""
circuit = circ.with_noise(
cast(Union[None, NoiseModel, SingleQubitGate], depolarize(p=noise))
)
result = SIMULATOR.simulate(circuit)
rho = result.final_density_matrix
# measure the expectation by taking the trace of the density matrix
expectation = np.real(np.trace(rho @ obs))
return expectation