QDev Seminar: William D. Oliver

The Flux Qubit Revisited

William D. Oliver
MIT Lincoln Laboratory and the Research Laboratory of Electronics

We revisit the design and fabrication of the persistent-current flux qubit [1]. By adding a high-Q capacitor, we dramatically improve its reproducibility and coherence times while retaining 800 MHz anharmonicity in the longest lived devices [2]. We discuss in a detail a device with T₁ = 55 us. We identify quasiparticles as causing temporal variability in the T₁, and we demonstrate the ability to pump these quasiparticles away [3]. The Hahn echo time T_2E = 40 us does not reach the 2T₁ limit, as is often observed with transmons coupled to resonators. We demonstrate that this is due to dephasing caused by the shot noise of residual thermal photons in the readout resonator. We use noise spectroscopy techniques to measure the lorentzian noise spectrum of the photon noise, and we then use CPMG dynamical decoupling to recover T_2CPMG ~ 2T₁ in a manner consistent with the noise spectrum. If time allows, we present recent results on a new type of traveling-wave parametric amplifier which features near-quantum-limited noise performance over multi-GHz bandwidth [4].

[1] W.D. Oliver and P.B. Welander, MRS Bulletin, 38, 816 (2013)

[2] F. Yan et al., in preparation (2015).

[3] S. Gustavsson et al., in preparation (2015).

[4] C. Macklin et al., under review (2015).