Measuring qubit readout weights

Measure the integration weights for a qubit readout by using gaussian flat top pulses.

Requirements:

• Instruments: 1 x SHFQA Instrument

• Loopback configuration between input and output of channel 0

from zhinst.qcodes import SHFQA, SHFQAChannelMode, Waveforms

device = SHFQA("DEVXXXX", "localhost")

Configure channel inputs and outputs

CHANNEL_INDEX = 0

device.qachannels[CHANNEL_INDEX].configure_channel(
    center_frequency=5e9,
    input_range=0,
    output_range=-5,
    mode=SHFQAChannelMode.READOUT,
)
device.qachannels[CHANNEL_INDEX].input.on(1)
device.qachannels[CHANNEL_INDEX].output.on(1)

Configure scope

SCOPE_CHANNEL = 0
NUM_QUBITS = device.max_qubits_per_channel
READOUT_DURATION = 600e-9
NUM_SEGMENTS = 2
NUM_AVERAGES = 50
NUM_MEASUREMENTS = NUM_SEGMENTS * NUM_AVERAGES
SHFQA_SAMPLING_FREQUENCY = 2e9

device.scopes[SCOPE_CHANNEL].configure(
    input_select={SCOPE_CHANNEL: f"channel{CHANNEL_INDEX}_signal_input"},
    num_samples=int(READOUT_DURATION * SHFQA_SAMPLING_FREQUENCY),
    trigger_input=f"channel{CHANNEL_INDEX}_sequencer_monitor0",
    num_segments=NUM_SEGMENTS,
    num_averages=NUM_AVERAGES,
    trigger_delay=200e-9,
)

Generate waveforms

from scipy.signal import gaussian
import numpy as np

RISE_FALL_TIME = 10e-9
PULSE_DURATION = 500e-9

rise_fall_len = int(RISE_FALL_TIME * SHFQA_SAMPLING_FREQUENCY)
pulse_len = int(PULSE_DURATION * SHFQA_SAMPLING_FREQUENCY)
std_dev = rise_fall_len // 10

gauss = gaussian(2 * rise_fall_len, std_dev)
flat_top_gaussian = np.ones(pulse_len)
flat_top_gaussian[0:rise_fall_len] = gauss[0:rise_fall_len]
flat_top_gaussian[-rise_fall_len:] = gauss[-rise_fall_len:]
# Scaling
flat_top_gaussian *= 0.9

readout_pulses = Waveforms()
time_vec = np.linspace(0, PULSE_DURATION, pulse_len)

for i, f in enumerate(np.linspace(2e6, 32e6, NUM_QUBITS)):
    readout_pulses.assign_waveform(
        slot=i,
        wave1=flat_top_gaussian * np.exp(2j * np.pi * f * time_vec)
    )

device.qachannels[CHANNEL_INDEX].generator.write_to_waveform_memory(readout_pulses)

Configure sequencer

device.qachannels[CHANNEL_INDEX].generator.configure_sequencer_triggering(
    aux_trigger="software_trigger0",
    play_pulse_delay=0
)

Run the measurement

results = []

for i in range(NUM_QUBITS):
    qubit_result = {
        'weights': None,
        'ground_states': None,
        'excited_states': None
    }
    print(f"Measuring qubit {i}.")

    # upload sequencer program
    seqc_program = f"""
        repeat({NUM_MEASUREMENTS}) {{
            waitDigTrigger(1);
            startQA(QA_GEN_{i}, 0x0, true,  0, 0x0);
        }}
    """
    device.qachannels[CHANNEL_INDEX].generator.load_sequencer_program(seqc_program)

    # Start a measurement
    device.scopes[SCOPE_CHANNEL].run(single=True)
    device.qachannels[CHANNEL_INDEX].generator.enable_sequencer(single=True)
    device.start_continuous_sw_trigger(
        num_triggers=NUM_MEASUREMENTS, wait_time=READOUT_DURATION
    )

    # get results to calculate weights and plot data
    scope_data, *_ = device.scopes[0].read()

    # Calculates the weights from scope measurements
    # for the excited and ground states
    split_data = np.split(scope_data[SCOPE_CHANNEL], 2)
    ground_state_data = split_data[0]
    excited_state_data = split_data[1]
    qubit_result['ground_state_data'] = ground_state_data
    qubit_result['excited_state_data'] = excited_state_data
    qubit_result['weights'] = np.conj(excited_state_data - ground_state_data)
    results.append(qubit_result)

Plot results

import matplotlib.pyplot as plt

QUBIT = 0

qubit_result = results[QUBIT]
ground_state_data = qubit_result['ground_state_data']
excited_state_data = qubit_result['excited_state_data']
weights = qubit_result['weights']

# Plot the data
# Plot the data measured with the scope for weight calculation
time_ticks = np.array(range(len(ground_state_data))) / SHFQA_SAMPLING_FREQUENCY

fig = plt.figure(1)
fig.suptitle("Scope measurement for readout weights")

ax1 = plt.subplot(211)
ax1.plot(time_ticks, np.real(ground_state_data))
ax1.plot(time_ticks, np.imag(ground_state_data))
ax1.set_title("ground state")
ax1.set_ylabel("voltage [V]")
plt.setp(ax1.get_xticklabels(), visible=False)
ax1.xaxis.get_offset_text().set_visible(False)
ax1.grid()
plt.legend(["real", "imag"])

ax2 = plt.subplot(212, sharex=ax1)
ax2.plot(time_ticks, np.real(excited_state_data))
ax2.plot(time_ticks, np.imag(excited_state_data))
ax2.set_title("excited state")
ax2.set_xlabel("t [s]")
ax2.set_ylabel("voltage [V]")
ax2.grid()
plt.show()

# Plot the qubit readout weights
time_ticks = np.array(range(len(weights))) / SHFQA_SAMPLING_FREQUENCY
plt.plot(time_ticks, np.real(weights))
plt.plot(time_ticks, np.imag(weights))
plt.title("Qubit readout weights")
plt.xlabel("t [s]")
plt.ylabel("weights [V]")
plt.legend(["real", "imag"])
plt.grid()
plt.show()