def _circuit_to_choi(circuit: Circuit) -> np.ndarray:
"""Returns the density matrix of the Choi state associated to the
input circuit.
The density matrix completely characterizes the quantum channel induced by
the input circuit (including the effect of noise if present).
Args:
circuit: The input circuit.
Returns:
The density matrix of the Choi state associated to the input circuit.
"""
simulator = DensityMatrixSimulator()
num_qubits = len(circuit.all_qubits())
# Copy and remove all operations
full_circ = deepcopy(circuit)[0:0]
full_circ += _max_ent_state_circuit(2 * num_qubits)
full_circ += circuit
return simulator.simulate(full_circ).final_density_matrix # type: ignore
system.normalize()
circuit = qdccirq.logsweep_protocol(system, K)
simulator = DensityMatrixSimulator()
print('done in', time.time() - stopwatch, 'seconds.')
# cooling
if not cooling_out_exists:
print('\nRunning cooling simulation')
stopwatch = time.time()
# build maximally-mixed system state tensored with |0><0| fridge
init_state = np.diag(
np.tile(np.array([1 / 2**L, 0], dtype=np.complex64), 2**L))
res = simulator.simulate(
circuit,
initial_state=init_state,
qubit_order=[*system.get_qubits(), qdccirq.FRIDGE])
final_state = res.final_density_matrix[::2, ::2] # discard fridge
out_dict = dict(L=L,
J=system.J,
B=system.B,
K=K,
init_state='maximally-mixed state',
energy=system.energy_expval(final_state),
eigoccs=list(system.eigenstate_occupations(final_state)))
with open(outfile_cooling, 'wt') as f:
json.dump(out_dict, f)
print('done in', time.time() - stopwatch, 'seconds.')