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Bulletin of the American Physical Society

cross-resonance-based readout scheme of a superconducting flux qubit

Fumiki Yoshihara, Sahel Ashhab, Tomoko Fuse, Kouichi Semba

Published Mar 5, 2019• 0 citations
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arXiv:cond-mat/0609561•Physical Review B•Apr 11, 2007•21 citations
A flux qubit readout scheme involving a dispersive technique coupled to a microstrip superconducting quantum interference device (SQUID) amplifier has been tested experimentally. Thanks to the almost quantum limited noise of this amplifier at low temperature, this readout device is very promising for a design with actual qubits. Its proof of principle and low noise performance have been tested by simulating the qubit presence by applying a tiny flux change in the input SQUID. The resonant frequency of the amplifier is adjusted in situ with a varactor diode to approach the frequency of the tank circuit. Two operating modes (crossing or anticrossing regime) were identified.
We report on electron spin resonance spectroscopy measurements using a superconducting flux qubit with a sensing volume of 6 fl. The qubit is read out using a frequency-tunable Josephson bifurcation amplifier, which leads to an inferred measurement sensitivity of about 20 spins in a 1 s measurement. This sensitivity represents an order of magnitude improvement when compared to flux-qubit schemes using a direct current-superconducting quantum interference device switching readout. Furthermore, noise spectroscopy reveals that the sensitivity is limited by flicker (1/f) flux noise.
A nonlinear resonant circuit comprising a SQUID magnetometer and a shunting capacitor is studied as a readout scheme for a persistent-current qubit. The flux state of the qubit is detected as a change in the Josephson inductance of the SQUID magnetometer, which in turn mediates a shift in the resonant frequency of the readout circuit. The nonlinearity and resulting hysteresis in the resonant behavior are characterized as a function of the power of both the input drive and the associated resonance-peak response. Numerical simulations based on a nonlinear circuit model shows that the observed nonlinearity is dominated by the effect due to an ac flux rather than current bias through the Josephson inductor.