Dresden 2011 – wissenschaftliches Programm

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TT: Fachverband Tiefe Temperaturen

TT 29: Poster Session: Quantum Information Systems, Quantum Coherence (jointly with SAMOP)

TT 29.5: Poster

Dienstag, 15. März 2011, 18:00–21:00, P1

Dispersive readout scheme for a Josephson phase qubit — •Tobias Wirth, Jürgen Lisenfeld, Alexander Lukashenko, and Alexey V. Ustinov — Physikalisches Institut, Karlsruhe Institute of Technology and DFG Center for Functional Nanostructures, 76128 Karlsruhe, Germany

A vital ingredient of experiments on quantum bits is a detection tool to efficiently read out the state of a qubit. This detector must introduce as little back-action as possible, while showing a large measurement contrast; it should have negligible dissipation and offer fast operation.

The standard method to read out a Josephson phase qubit is to record the dc bias current at which the SQUID switches to its non-superconducting state. This process generates heat directly on the chip and quasi-particles in the circuitry. Both effects are responsible for a relatively long cool-down time of about 1-2 ms required after each switching event. Together with the time needed to ramp up the bias current of the SQUID, this limits the repetition rate of the experiment.

We present experimental results on a dispersive scheme for reading out a Josephson phase qubit. A capacitively shunted dc-SQUID is used as a nonlinear resonator which is inductively coupled to the qubit. The flux state of the qubit is detected by measuring the amplitude and phase of a microwave pulse reflected from the SQUID resonator. By this low-dissipative method, the qubit state measurement time is reduced to 25 µs, which is much faster than the conventional readout.