Ramsey Spectroscopy#

Here we demonstrate Ramsey Spectroscopy, which is used to tune the \(|0\rangle \rightarrow |1\rangle\) drive frequency more precisely. Ramsey spectroscopy is also used to find \(T_2^*\).

Setup#

In this section we configure the hardware configuration which specifies the connectivity of our system.

The experiments of this tutorial are meant to be executed with a Qblox Cluster controlling a transmon system. The experiments can also be executed using a dummy Qblox device that is created via an instance of the Cluster class, and is initialized with a dummy configuration. When using a dummy device, the analysis will not work because the experiments will return np.nan values.

Configuration file#

This is a template hardware configuration file for a 2-qubit system with a flux-control line which can be used to tune the qubit frequency. We will only work with qubit 0.

The hardware connectivity is as follows, by cluster slot:

QCM (Slot 2)
- \(\text{O}^{1}\): Flux line for q0.
- \(\text{O}^{2}\): Flux line for q1.
QCM-RF (Slot 6)
- \(\text{O}^{1}\): Drive line for q0 using fixed 80 MHz IF.
- \(\text{O}^{2}\): Drive line for q1 using fixed 80 MHz IF.
QRM-RF (Slot 8)
- \(\text{O}^{1}\) and \(\text{I}^{1}\): Shared readout line for q0/q1 using a fixed LO set at 7.5 GHz.

Note that in the hardware configuration below the mixers are uncorrected, but for high fidelity experiments this should also be done for all the modules.

Quantum device settings#

Here we initialize our QuantumDevice and our qubit parameters, checkout this tutorial for further details.

In short, a QuantumDevice contains device elements where we save our found parameters. Here we are loading a template for 2 qubits, but we will only use qubit 0.

Ramsey oscillations#

[5]:

# This schedule can also be imported from from quantify_scheduler.schedules


def ramsey_sched(
    times: np.ndarray | float,
    qubit: str,
    artificial_detuning: float = 0,
    repetitions: int = 1,
) -> Schedule:
    r"""
    Generate a schedule for performing a Ramsey experiment.

    This measures the dephasing time :math:`T_2^{\star}`.

    Schedule sequence
        .. centered:: Reset -- pi/2 -- Idle(tau) -- pi/2 -- Measure

    See section III.B.2. of :cite:t:`krantz_quantum_2019` for an explanation of the Bloch-Redfield
    model of decoherence and the Ramsey experiment.

    Parameters
    ----------
    times
        an array of wait times tau between the start of the first pi/2 pulse and
        the start of the second pi/2 pulse.
    artificial_detuning
        frequency in Hz of the software emulated, or ``artificial`` qubit detuning, which is
        implemented by changing the phase of the second pi/2 (recovery) pulse. The
        artificial detuning changes the observed frequency of the Ramsey oscillation,
        which can be useful to distinguish a slow oscillation due to a small physical
        detuning from the decay of the dephasing noise.
    qubit
        the name of the device element e.g., :code:`"q0"` to perform the Ramsey experiment on.
    repetitions
        The amount of times the Schedule will be repeated.

    Returns
    -------
    :
        An experiment schedule.

    """
    device_element = qubit
    # ensure times is an iterable when passing floats.
    times = np.asarray(times)
    times = times.reshape(times.shape or (1,))

    schedule = Schedule("Ramsey", repetitions)

    if isinstance(times, float):
        times = [times]

    for i, tau in enumerate(times):
        schedule.add(Reset(device_element), label=f"Reset {i}")
        schedule.add(X90(device_element))

        # the phase of the second pi/2 phase progresses to propagate
        recovery_phase = np.rad2deg(2 * np.pi * artificial_detuning * tau)
        schedule.add(
            Rxy(theta=90, phi=recovery_phase, qubit=device_element), ref_pt="start", rel_time=tau
        )
        schedule.add(Measure(device_element, acq_index=i), label=f"Measurement {i}")
    return schedule

[6]:

tau = ManualParameter(name="tau", unit="s", label="Time")
tau.batched = True

ramsey_sched_kwargs = {
    "qubit": qubit.name,
    "times": tau,
    "artificial_detuning": 0.0,
}

gettable = ScheduleGettable(
    quantum_device,
    schedule_function=ramsey_sched,
    schedule_kwargs=ramsey_sched_kwargs,
    real_imag=False,
    batched=True,
)
meas_ctrl.gettables(gettable)

[7]:

tau_setpoints = np.arange(20e-9, 4e-6, 32e-9)

meas_ctrl.settables(tau)
meas_ctrl.setpoints(tau_setpoints)

quantum_device.cfg_sched_repetitions(500)
ramsey_ds = meas_ctrl.run("ramsey")
ramsey_ds

Starting batched measurement...
Iterative settable(s) [outer loop(s)]:
         --- (None) ---
Batched settable(s):
         tau
Batch size limit: 125

[8]:

if cluster_ip is None:
    notebook_dir = Path.cwd()
    dh.set_datadir(notebook_dir / "data")
    ramsey_ds = load_dataset("20250206-230423-264-3c48f3")
ramsey_analysis = RamseyAnalysis(ramsey_ds)
ramsey_analysis.run(
    artificial_detuning=ramsey_sched_kwargs["artificial_detuning"]
).display_figs_mpl()

../../../_images/applications_quantify_transmon_ramsey_13_0.png

[9]:

detuning = ramsey_analysis.quantities_of_interest["detuning"].nominal_value
qubit.clock_freqs.f01(qubit.clock_freqs.f01() - detuning)
print(f"New qubit frequency is {qubit.clock_freqs.f01} Hz")

New qubit frequency is q0_clock_freqs_f01 Hz

[10]:

inst_coord.last_schedule.compiled_instructions

[11]:

display_dict(quantum_device.hardware_config())

[11]:

[12]:

quantum_device.to_json_file("devices/")

[12]:

'devices/device_2q_2025-03-12_18-00-17_UTC.json'