qiskit.pulse.builder.barrier

barrier(*channels_or_qubits)

Barrier directive for a set of channels and qubits.

This directive prevents the compiler from moving instructions across the barrier. Consider the case where we want to enforce that one pulse happens after another on separate channels, this can be done with:

from qiskit import pulse
from qiskit.test.mock import FakeOpenPulse2Q
 
backend = FakeOpenPulse2Q()
 
d0 = pulse.DriveChannel(0)
d1 = pulse.DriveChannel(1)
 
with pulse.build(backend) as barrier_pulse_prog:
    pulse.play(pulse.Constant(10, 1.0), d0)
    pulse.barrier(d0, d1)
    pulse.play(pulse.Constant(10, 1.0), d1)

Of course this could have been accomplished with:

from qiskit.pulse import transforms
 
with pulse.build(backend) as aligned_pulse_prog:
    with pulse.align_sequential():
        pulse.play(pulse.Constant(10, 1.0), d0)
        pulse.play(pulse.Constant(10, 1.0), d1)
 
barrier_pulse_prog = transforms.remove_directives(barrier_pulse_prog)
assert barrier_pulse_prog == aligned_pulse_prog

The barrier allows the pulse compiler to take care of more advanced scheduling aligment operations across channels. For example in the case where we are calling an outside circuit or schedule and want to align a pulse at the end of one call:

import math
 
d0 = pulse.DriveChannel(0)
 
with pulse.build(backend) as pulse_prog:
    with pulse.align_right():
        pulse.x(1)
        # Barrier qubit 1 and d0.
        pulse.barrier(1, d0)
        # Due to barrier this will play before the gate on qubit 1.
        pulse.play(pulse.Constant(10, 1.0), d0)
        # This will end at the same time as the pulse above due to
        # the barrier.
        pulse.x(1)

Note

Requires the active builder context to have a backend set if qubits are barriered on.

Parameters

channels_or_qubits (Union[Channel, int]) – Channels or qubits to barrier.

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