Measuring the effect of energy relaxation with IBMQ Experience

Question

I want to investigate the effect of the decoherence time on a qubit, and I followed the tutorial here. https://quantumexperience.ng.bluemix.net/proxy/tutorial/full-user-guide/002-The_Weird_and_Wonderful_World_of_the_Qubit/006-Decoherence.html But my results don't really match up with the expected values where more gates would result in a higher P(0). Does anyone know why that is?

    # t1.py
import numpy as np
import matplotlib.pyplot as plt

from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister, execute

from qiskit import IBMQ
IBMQ.load_accounts()
print("Available backends:")
IBMQ.backends()
from qiskit.providers.ibmq import least_busy

large_enough_devices = IBMQ.backends(filters=lambda x: x.configuration().n_qubits > 3 and not x.configuration().simulator)
backend = least_busy(large_enough_devices)
print("The best backend is " + backend.name())
# Define the Quantum and Classical Registers
q = QuantumRegister(1)
c = ClassicalRegister(1)

# Build the circuits
pre = QuantumCircuit(q, c)
pre.x(q)
pre.barrier()
meas = QuantumCircuit(q, c)
meas.measure(q, c)
circuits = []
exp_vector = range(1,61)
for exp_index in exp_vector:
    middle = QuantumCircuit(q, c)
    for i in range(45*exp_index):
        middle.iden(q)
    circuits.append(pre + middle + meas)

# Execute the circuits


shots = 1024
job = execute(circuits, backend, shots=shots, max_credits=10)
result = job.result()

# Plot the result
exp_data = []
exp_error = []
for exp_index in exp_vector:
    data = result.get_counts(circuits[exp_index-1])
    try:
        p0 = data['0']/shots
    except KeyError:
        p0 = 0
    exp_data.append(p0)
    exp_error.append(np.sqrt(p0*(1-p0)/shots))

plt.errorbar(exp_vector, exp_data, exp_error)
plt.xlabel('time [45*gate time]')
plt.ylabel('Pr(0)')
plt.grid(True)
plt.show()

Answer 1

The devices are imperfect and also are periodically recalibrated. Any textual examples characterizing decoherence will probably never match exactly what you encounter in running the same code live.

Answer 2

You may want to try this tutorial: https://github.com/Qiskit/qiskit-tutorials/blob/master/qiskit/ignis/relaxation_and_decoherence.ipynb