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Non-destructive SQUID readout

We have implemented a non-destructive method for the readout of a persistent current flux qubit. The detection is based on the measurement of the Josephson inductance of a DC super-conducting quantum interference device (DC-SQUID) inductively coupled to the qubit. The SQUID is included in a resonant circuit, allowing for high flux sensitivity. The measurement strength can be tuned with the amplitude of the RF-signal used to drive the resonant circuit. The method eliminates the disadvantages of the strongly dissipative state associated with the switching of a Josephson junction to the voltage state.

Using this method, we have measured the spectroscopy and the relaxation time of a flux qubit, obtaining relaxation times of the order of 80 microseconds. The measurement has high efficiency and further improvement is possible, by optimising the measurement circuit. This detector can be used for experimental studies of the relation between quantum measurement and decoherence, as well as for correlation measurements on two qubits.

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Technical University of Delft, Kavli Institute of Nanoscience
Lorentzweg 1
SE-41296 Delft
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