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.

Measuring the ZZ coupling with a Ramsey experiment#

By repeating the Ramsey experiment while putting one of the neighbor qubits in state \(\ket{1}\) we can have an estimate on the ZZ coupling \(\zeta\). The ZZ coupling \(\zeta\) is a residual interaction which leads to shifts in the frequency of the qubit when one of the neighbor is in the excited state. Given that through a Ramsey experiment we can measure carefully the frequency of the qubit, by comparing the outcome of a standard Ramsey experiment with the outcome when one of the neighbor qubit is excited we can infer the ZZ coupling term \(\zeta\). For superconducting platforms without tunable couplers such terms is expected to be of the order of a few hundred kHz.

Parameters#

class qibocal.protocols.ramsey.ramsey_zz.RamseyZZParameters(delay_between_pulses_start: int, delay_between_pulses_end: int, delay_between_pulses_step: int, detuning: Optional[int] = None, unrolling: bool = False, target_qubit: Optional[Union[int, str]] = None)[source]

RamseyZZ runcard inputs.

target_qubit: Optional[Union[int, str]] = None: Target qubit that will be excited.

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#

- id: ramsey zz
  operation: ramsey_zz
  parameters:
    delay_between_pulses_end: 2000
    delay_between_pulses_start: 10
    delay_between_pulses_step: 50
    detuning: 500000
    nshots: 1024
    target_qubit: D1

Requirements#

Rabi experiments