# Conditional reset on IBM Quantum systems¶

The reset instruction is now supported in Qiskit on IBM Quantum systems. This allows users to reset qubits into the ground state (|0>) with high-fidelity. These instructions are comprised of a not-gate conditioned on the measurement outcome of the qubit, and demonstrate the real-time compute power of the IBM Quantum systems. They can be explored for the reuse of qubits in a circuit, to yield an increase in system throughput, and provide fidelity improvements over standard cavity-cooling methods. Reset provides an initial foray into real-time computation on IBM Quantum systems, and will be followed by a release of mid-circuit measurements in the near future. Here we provide a number of examples demonstrating how to use the reset instruction.

We can now define the delay time between programs in a parameter called rep_delay. Reducing rep_delay enables faster executions and finer control over jobs. Previously, a user could only control this parameter with a finite list of rep_times (the overall time of program execution) for pulse jobs. Now, users can choose from a continuous backend rep_delay_range for both circuit and pulse jobs. We will review these flags in this tutorial.

Qubit initialization can now be turned off by users (previously qubits were always initialized). Here we demonstrate how this manifests itself through poor thermalization of the qubit to the ground state in conjunction with a small rep_delay.

import numpy as np
import matplotlib.pyplot as plt

import qiskit
from qiskit import IBMQ, QuantumCircuit, execute, assemble, schedule, transpile
from qiskit.providers.ibmq.job import job_monitor
from qiskit.tools.visualization import plot_histogram


IBMQ.load_account()

provider = IBMQ.get_provider(group='deployed')
# ibmq_paris supports conditional reset.
backend = provider.get_backend('ibmq_paris')

config = backend.configuration()


here config is the backend.configuration() that will be referenced throughout this tutorial.

## Reset instruction¶

Note: These features require qiskit > 0.22.0 as they rely on qiskit-terra > 0.16.0.

We begin by demonstrating the reset instruction. Note that reset is currently only enabled for quantum circuits (not pulse schedules).

First, we check that the reset instruction is supported on the requested backend. This information is found in the backend configuration:

config.supported_instructions

['acquire',
'u2',
'x',
'measure',
'shiftf',
'play',
'cx',
'u3',
'id',
'setf',
'reset',
'u1',
'delay']


## Single-qubit reset experiment¶

We perform the following sequences of one-qubit operations: h -> x -> measure and h -> reset -> x -> measure. Without reset, the expected result of the first circuit is and a uniform distribution of 0 and 1 bits at the output. However, when the reset is included, the superposition prepared by the Hadamard gate is reset to and the X-gate performs a bit flip to .

Below we define both of these circuits, and allow for a variable number of reset operations to be performed.

h_x_circ = QuantumCircuit(1, 1)
h_x_circ.h(0)
h_x_circ.x(0)
h_x_circ.measure(0, 0)

def h_reset_x_circ(n_resets):
"""Do h, then n_resets reset instructions, then x on a single qubit."""
qc = QuantumCircuit(1, 1)
qc.h(0)
qc.reset([0]*n_resets)
qc.x(0)
qc.measure(0,0)
return qc


We run this experiment on qubit 0, with 0, 1, 3, and 5 reset operations. We see clear improvement after a single reset and further improvement with 3 resets. By 5 resets the improvement in the qubit state preparation has mostly saturated, and the final fidelity is close to the measurement fidelity of the qubit under consideration.

circs0 = [h_x_circ, h_reset_x_circ(1), h_reset_x_circ(3), h_reset_x_circ(5)]
job1q_0 = execute(circs0, backend, initial_layout=[0])
job_monitor(job1q_0)

Job Status: job has successfully run

counts1 = job1q_0.result().get_counts()

legend_1q = ['h -> x', 'h -> reset -> x', 'h -> 3*reset -> x', 'h -> 5*reset -> x' ]
plot_histogram(counts1, legend=legend_1q, figsize=(9, 6))


## Multi-qubit reset experiment¶

The next experiment tests multi-qubit circuits. A complex circuit is considered with the possibility of multiple reset instructions in the middle. The expected result is given by the Aer qasm simulator (it can also be verified analytically).

Without any reset instructions, we expect a unifrom distribution across 4 of the computational basis states (, , , ). With reset added, we expect nearly all counts in the state.

Once again, we run the circuit with 0, 1, and 3 reset instructions. We see an improvement for 1 reset and further improvement for 3 resets.

**Note:**

The reset operation can be time intensive, so when working with multiple qubits it is important to be careful how reset instructions are done.

The backend will attempt to merge resets following standard commutation rules. However, if you attempt many resets on each individual qubit and they cannot be merged, it may lead to some states being favored as much of the qubit’s T1 time is being used up. If you see results tending towards a single state as the number of resets increases, consider reducing the number of resets or performing the resets adjacent to one another.

def multiq_custom_circ(n_resets):
"""Multi-qubit circuit on qubits 0, 1, 2 with n_resets reset instructions used."""
qc = QuantumCircuit(3, 3)
qc.h(0)
qc.h(1)
qc.h(2)
qc.reset([0]*n_resets)
qc.cx(0, 2)
qc.reset([1]*n_resets)
qc.x(1)
qc.measure(range(2), range(2))
return qc

multiq_circs = [multiq_custom_circ(0), multiq_custom_circ(1), multiq_custom_circ(3)]

# verify expected output
from qiskit import Aer
job_multiq_sim = execute(multiq_circs,
backend=Aer.get_backend('qasm_simulator'))
counts_multiq_sim = job_multiq_sim.result().get_counts()
print("No reset sim counts, custom circ: ", counts_multiq_sim[0])
print("1 Reset sim counts, custom circ: ", counts_multiq_sim[1])

No reset sim counts, custom circ:  {'000': 241, '001': 258, '010': 279, '011': 246}
1 Reset sim counts, custom circ:  {'010': 1024}

job_multiq = execute(multiq_circs, backend)
job_monitor(job_multiq)

Job Status: job has successfully run

counts_multiq = job_multiq.result().get_counts()
legend_multiq = ['no reset', '1 reset', '3 resets']
plot_histogram(counts_multiq, legend=legend_multiq, figsize=(9, 6))


## Repetition delay and removing qubit initialization at the start of programs¶

This section covers two new features: Setting the repetition delay, and removing qubit initialization at the start of circuits.

The repetition delay defines the delay time between circuit executions. For instance, if you run a job with 5 circuits, the repetition delay (rep_delay for short) defines how long the backend is idle for between the conclusion of one circuit and start of the next. This parameter provides finer grained control over the execution of your quantum circuits on IBM Quantum systems.

Shortening rep_delay can reduce execution time, and therefore reduce the sensitivity of your experiment to long term drifts in hardware parameter variations. However, setting too short a time may yield poorly initialized qubits.

With the smaller rep_delay, users may prefer to initialize circuits themselves via the reset instruction introduced above. For this reason, we have added a flag init_qubits which determines whether the default backend initialization sequence will be inserted. This allows users to work with “dirty qubits” and perform experiments with non-initialized states.

Note: If init_qubits is set to false and you want qubits to start in the ground state, you must insert resets at the start of your circuit. Otherwise, qubits will begin with their state from the previous shot (after undergoing some thermalization process for the interval between shots as set by rep_delay). Note that the first shot of the first circuit may still be in the thermal ground state due to a longer delay existing during the loading of your job to hardware.

In order to use rep_delay, we must check if the backend supports it. This is controlled by a flag in the backend configuration dynamic_reprate_enabled.

config.dynamic_reprate_enabled

True


Since dynamic rep rates are enabled on this device, we can proceed with specifying the delay time.

The backend specifies an allowed range of delay times, given by rep_delay_range. Having a continuous range provides users with fine-grained control of the delays. A default is also returned by the backend as default_rep_delay.

print(f"Range of delay times: {config.rep_delay_range}s")
print(f"Default delay time: {config.default_rep_delay}s")

Range of delay times: [0.0, 0.0005]s
Default delay time: 0.00045999999999999996s


We conduct a simple experiment to demonstrate the effect of rep_delay as follows:

1. Turn off qubit initialization using init_qubits=False.

2. Set rep_delay to something small (we use rep_delay=0.005e-6 s).

3. Define a dirty_q_circ() method which puts the qubits in an unpredictable state.

4. Run the dirty circuit and then run some known circuit with a defined result.

The dirty circuit puts the qubits in an unknown state and then, since rep_delay is small and qubits are not initialized, we get an unpredictable (and often bad) result for our known circuit.

We “fix” this by modifying the known circuit to add some number of resets to the front of the circuit, thus initializing our qubits manually.

The circuits we run are (some number of resets), measure and x, (some number of resets), measure.

When no resets are inserted, we see a poor result in each case. However, with resets inserted we observe results closer to what we expect (all 0’s for the measure and all 1’s for the x, measure).

def dirty_q_circ():
"""Circuit to give bad qubit initialization."""
# does random x rotation
qc = QuantumCircuit(1, 1)
theta = np.random.uniform(0, 2*np.pi)
qc.rx(theta, 0)
qc.measure(0, 0)
return qc

def reset_x_circ(n_resets):
"""Reset, x circuit."""
qc = QuantumCircuit(1, 1)
for _ in range(n_resets):
qc.reset(0)
qc.x(0)
qc.measure(0, 0)
return qc

def reset_meas_circ(n_resets):
"""Reset, measure circuit."""
qc = QuantumCircuit(1, 1)
for _ in range(n_resets):
qc.reset(0)
qc.measure(0, 0)
return qc

circs_no_init = [dirty_q_circ(), reset_meas_circ(0),  # 1
dirty_q_circ(), reset_x_circ(0),  # 3
dirty_q_circ(), reset_meas_circ(1),  # 5
dirty_q_circ(), reset_x_circ(1),  # 7
dirty_q_circ(), reset_meas_circ(3),  # 9
dirty_q_circ(), reset_x_circ(3)] # 11


rep_delay can be specified in execute() or assemble(). We choose assemble() so that we can print and verify our rep_delay in the qobj.config. Also note that init_qubits=False.

job_no_init = execute(circs_no_init, backend, init_qubits=False, optimization_level=0, rep_delay=0.005e-6)
job_monitor(job_no_init)

Job Status: job has successfully run


Like rep_delay, init_qubits can be set in execute() or assemble().

counts_no_init = job_no_init.result().get_counts()
legend_no_init = ['0 resets', '1 reset', '3 resets']

plot_histogram([counts_no_init[1], counts_no_init[5], counts_no_init[9]],
legend=legend_no_init, title='meas w/ dirty qubits', figsize=(9,6))

plot_histogram([counts_no_init[3], counts_no_init[7], counts_no_init[11]],
legend=legend_no_init, title='x w/ dirty qubits', figsize=(9,6))


Next, we consider how measurement fidelity varies with rep_delay when we add a reset at the beginning versus when we don’t.

We replace dirty_q_circ() with an x gate as this is as far as we can get from . We then run either measure or reset, measure. The rep_delay is varied from 0.005-350 us, with 10 data points (you can run more, but it will take longer).

Qubit initialization is again turned off.

With a reset, the measurement fidelity is above 90% for all rep_delays (the fidelity here is defined by the probability of measuring a 0). However, without the reset, the fidelity begins low and steadily rises as rep_delay increases. This is because increasing rep_delay gives the qubit more time to cool back to its ground state.

def x_circ():
qc = QuantumCircuit(1, 1)
qc.x(0)
qc.measure(0, 0)
return qc

circs_no_reset = [x_circ(), reset_meas_circ(0)]
circs_reset = [x_circ(), reset_meas_circ(1)]
circs_no_reset = qiskit.transpile(circs_no_reset, backend=backend, optimization_level=0)
circs_reset = qiskit.transpile(circs_reset, backend=backend, optimization_level=0)


We have used optimization_level=0 to avoid the RemoveResetInZeroState optimization pass from Qiskit (see here). There is an open Qiskit issue discussing how best to align this pass with the new reset capabilities (see here).

# NOTE: If you run into an issue running more than 5 concurrent jobs, simply split up the loop.
no_reset_jobs = []
reset_jobs = []
rep_delays = np.linspace(0.005e-6, 350e-6, 10)  # rep delays to use in sec
print(f"rep_delays (us): {rep_delays}s")

for rep_delay in rep_delays:
qobj_no_reset = assemble(circs_no_reset, backend, init_qubits=False, optimization_level=0, rep_delay=rep_delay)
qobj_reset = assemble(circs_reset, backend, init_qubits=False, optimization_level=0, rep_delay=rep_delay)
job_no_reset = backend.run(qobj_no_reset)
job_reset = backend.run(qobj_reset)
no_reset_jobs.append(job_no_reset)
reset_jobs.append(job_reset)

rep_delays (us): [5.00000000e-09 3.88933333e-05 7.77816667e-05 1.16670000e-04
1.55558333e-04 1.94446667e-04 2.33335000e-04 2.72223333e-04
3.11111667e-04 3.50000000e-04]s

def prob0(counts):
"""Probability of 0 from a counts dict of the form {'0': int, '1': int}."""
zero_counts = counts.get('0', 0)
total_counts = zero_counts + counts.get('1', 0)
return zero_counts / total_counts

# this could take few minutes
no_reset_counts = [no_reset_jobs[i].result().get_counts()[1] for i in range(10)]
reset_counts = [reset_jobs[i].result().get_counts()[1] for i in range(10)]

no_reset_prob0 = [prob0(counts) for counts in no_reset_counts]
reset_prob0 = [prob0(counts) for counts in reset_counts]

plt.plot(rep_delays*1e6, no_reset_prob0, '.', label='no reset')
plt.plot(rep_delays*1e6, reset_prob0, '.', label='reset')
plt.legend()
plt.xlabel('rep_delay (us)')
plt.ylabel('Success probability ($\Pr(|0>)$)')
plt.title('Reset vs. no reset measurement fidelity as function of program delay')
plt.show()

import qiskit.tools.jupyter
%qiskit_version_table


### Version Information

Qiskit SoftwareVersion
Qiskit0.23.0
Terra0.16.0
Aer0.7.0
Ignis0.5.0
Aqua0.8.0
IBM Q Provider0.11.0
System information
Python3.7.9 (default, Aug 31 2020, 07:22:35) [Clang 10.0.0 ]
OSDarwin
CPUs4
Memory (Gb)16.0
Wed Oct 28 12:25:11 2020 EDT
%qiskit_copyright