ghz.py

from qiskit import Aer, IBMQ, execute, QuantumCircuit, QuantumRegister, ClassicalRegister
from qiskit.transpiler import CouplingMap
from qiskit.tools.monitor import job_monitor
from qiskit.providers.aer import noise
from qiskit.providers.aer.noise.errors import depolarizing_error

# Construct quantum circuit
qr = QuantumRegister(3, 'qr')
cr = ClassicalRegister(3, 'cr')
circ = QuantumCircuit(qr, cr)
circ.h(qr[0])
circ.cx(qr[0], qr[1])
circ.cx(qr[1], qr[2])
circ.measure(qr, cr)

# Select the QasmSimulator from the Aer provider
simulator = Aer.get_backend('qasm_simulator')

# Execute and get counts
result = execute(circ, simulator).result()
counts = result.get_counts(circ)
print(counts);

noise_model = noise.NoiseModel(['h', 'cx'])
# print('noise_model before', noise_model.to_dict())

error2 = depolarizing_error(0.1, 2)
noise_model.add_all_qubit_quantum_error(error2, ['cx'])
print('noise_model after', noise_model.to_dict())

# Get the basis gates for the noise model
basis_gates = noise_model.basis_gates

# Select the QasmSimulator from the Aer provider
simulator = Aer.get_backend('qasm_simulator')
cm = CouplingMap([[0,1], [0,2], [1,2]])
# Execute noisy simulation and get counts
result_noise = execute(circ, simulator, 
                       noise_model=noise_model,
                       coupling_map=cm,
                       basis_gates=basis_gates).result()
counts_noise = result_noise.get_counts(circ)
print(counts_noise)

bar = [{'key': k, 'prob': v/1024} for k,v in counts_noise.items()]
print (sorted(bar, key = lambda i: i['key']))