# Source code for mitiq.benchmarks.utils

# Copyright (C) 2020 Unitary Fund
#
# This program is free software: you can redistribute it and/or modify
# 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.
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# along with this program.  If not, see <https://www.gnu.org/licenses/>.

"""Utility functions for benchmarking."""
import numpy as np

from cirq import Circuit, depolarize, DensityMatrixSimulator

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(depolarize(p=noise))
rho = SIMULATOR.simulate(circuit).final_density_matrix
# measure the expectation by taking the trace of the density matrix
expectation = np.real(np.trace(rho @ obs))
return expectation