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SQUBIT-2 Résumé de rapport

Project ID: IST-2001-39083
Financé au titre de: FP5-IST
Pays: Sweden

Dynamics and phonon-induced decoherence of the Andreev level qubit

Chalmers theory group has investigated the problem of decoherence of Andreev levels in super-conducting junctions due to interaction with the quantum fluctuations of the super-conducting phase across the junction. We have derived a general kinetic equation for the Andreev level de-phasing and relaxation including quasiparticle exchange with the bands.

Our main finding is that the fermionic nature of the Andreev levels considerably affect the decoherence terms in the kinetic equation leading to a substantial suppression of the decoherence rate. This many-body effect even leads to a non-exponential relaxation of Andreev levels at small temperature. Recently we have specifically considered the case of a large Josephson junction inserted in the qubit loop forming an oscillator for dispersive readout of the qubit. The junction is permanently connected via small capacitor to a line providing measurement rf signal.

We have applied the derived kinetic equation to the present case of interaction of the Andreev levels with EM fluctuations in the line and found that the decoherence rate is very small. Namely, under a generic for qubit operation condition that the plasma frequency of the measurement junction exceeds the qubit frequency, the decoherence time may be in a milliseconds range for properly chosen circuit parameters.


Vitaly SHUMEIKO, (Head of Laboratory)
Tél.: +46-31-7723144
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