Coherence Times

scqubits implements coherence time calculations for estimating depolarization and pure-dephasing times for a majority of the qubits implemented. For each qubit, a variety of noise channels can be considered, ranging from ubiquitous channels affecting most circuits to more specialized ones applicable only to the qubit of interest.

Note

The coherence calculations in scqubits use standard approximation techniques discussed in the listed references. It is up to the user to ensure the assumptions are consistent with their parameter choices.

A list of the channels supported by a given qubit can be obtained by calling the supported_noise_channels() method. Each entry in the returned list represents a method that can be directly called on the qubit object. This example illustrates the use for the case of a TunableTransmon qubit:

tune_tmon = scq.TunableTransmon(
    EJmax=20.0,
    EC=0.5,
    d=0.00,
    flux=0.0,
    ng=0.3,
    ncut=150
)
transmon.supported_noise_channels()
transmon.tphi_1_over_f_flux()

The last call to the method tphi_1_over_f_flux() returns an estimate of the pure dephasing time due to 1/f flux noise.

By default, all noise methods return a decoherence time (\(T_1\) in the case of a depolarizing channel or \(T_\phi\) in case of a dephasing channel). However, by supplying the option get_rate=True, one can obtain the corresponding rate instead.

The units of the returned decoherence times reflect the global units settings. By default, the global units are set to GHz, and the resulting depolarization and dephasing times are returned in units of 1/GHz = ns. See the Units section for more information on how to change the global units.

By default, all coherence time calculations assume that the qubit Hilbert space is reduced to the lowest two energy levels. If needed, the user can provide different energy levels as arguments in order to extend the relevant subspace.

A set of examples showing how to perform a variety of coherence-times estimates is presented in this jupyter notebook .

More detailed discussions of the individual supported noise channels can be found in the following subsections.