Ramsey experiments#

In this section we show how to run Ramsey experiments using Qibocal

Ramsey#

To measure the decoherence time of a qubit it is possible to perform a Ramsey experiment. In this protocol two short \(\pi /2\) are sent separated by a waiting time \(\tau\). The first \(\pi /2\) pulse will project the qubit from \(\ket{0}\) onto the xy plane of the Bloch sphere. During the waiting time \(\tau\) the qubit will decohere around the z axis, after the second \(\pi /2\) we peform a measurement. By changing the waiting time \(\tau\) we should observe an exponential decay in the probability of measuring \(\ket{1}\) which will give us an estimate of \(T_2*\). In the case where there is a difference between the qubit frequency and the frequency of the drive pulse the final state will show a sinusoidal dependence from which we can extract a correction on the qubit frequency [2]:

\[p_e = \frac{1}{2} + \frac{1}{2} e^{-\tau/T_2} \cos(\Delta \omega \pm \pi/2)\]

In order to obtain the correction on the qubit frequency reliably usually the drive pulse it is detuned on purpose to generate oscillations.

Parameters#

class qibocal.protocols.ramsey.ramsey.RamseyParameters(delay_between_pulses_start: int, delay_between_pulses_end: int, delay_between_pulses_step: int, detuning: Optional[int] = None, unrolling: bool = False)[source]#

Ramsey runcard inputs.

detuning: Optional[int] = None#: Frequency detuning [Hz] (optional). If 0 standard Ramsey experiment is performed.

hardware_average: bool = False#: By default hardware average will be performed.

unrolling: bool = False#: If True it uses sequence unrolling to deploy multiple sequences in a single instrument call. Defaults to False.

delay_between_pulses_start: int#: Initial delay between RX(pi/2) pulses in ns.

delay_between_pulses_end: int#: Final delay between RX(pi/2) pulses in ns.

delay_between_pulses_step: int#: Step delay between RX(pi/2) pulses in ns.

nshots: int#: Number of executions on hardware.

relaxation_time: float#: Wait time for the qubit to decohere back to the gnd state.

Example#

A possible runcard to launch a Ramsey experiment could be the following:

- id: ramsey
  operation: ramsey
  parameters:
    delay_between_pulses_end: 2000
    delay_between_pulses_start: 50
    delay_between_pulses_step: 50
    detuning: -1000000
    nshots: 1024

The expected output is the following:

\(T_2\) and \(\Delta \omega\) are determined by fitting the output signal using the formula presented above.

If the protocols is successful the drive frequency will be updated only if a non-zero detuning is provided. \(T_2\) is updated only in the case where detuning is not specified.

Requirements#

Single Shot Experiments

Ramsey with raw signal#

A standard Ramsey experiment will try to compute the probability of the qubit being measured in state \(\ket{1}\). It is possible to perform a Ramsey experiment without performing single shot classification by running an experiment called ramsey_signal.

The acquisition and the fitting procedure are exactly the same, the only difference being that on the y axis it will be displayed the raw measurements from the instruments.

Parameters#

class qibocal.protocols.ramsey.ramsey_signal.RamseySignalParameters(delay_between_pulses_start: int, delay_between_pulses_end: int, delay_between_pulses_step: int, detuning: Optional[int] = None, unrolling: bool = False)[source]#

Ramsey runcard inputs.

delay_between_pulses_start: int#: Initial delay between RX(pi/2) pulses in ns.

delay_between_pulses_end: int#: Final delay between RX(pi/2) pulses in ns.

delay_between_pulses_step: int#: Step delay between RX(pi/2) pulses in ns.

detuning: Optional[int] = None#: Frequency detuning [Hz] (optional). If 0 standard Ramsey experiment is performed.

unrolling: bool = False#: If True it uses sequence unrolling to deploy multiple sequences in a single instrument call. Defaults to False.

hardware_average: bool = False#: By default hardware average will be performed.

nshots: int#: Number of executions on hardware.

relaxation_time: float#: Wait time for the qubit to decohere back to the gnd state.

Example#

- id: ramsey with signal
  operation: ramsey_signal
  parameters:
    delay_between_pulses_end: 25000
    delay_between_pulses_start: 50
    delay_between_pulses_step: 1000
    nshots: 1024

Note that in this case error bars will not be provided.

Requirements#

Rabi experiments