Decoherence And Defects In Cooper-Pair Boxes

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This dissertation describes my detailed investigation of decoherence and defects in two Al/AlOx/Al Cooper-pair box (CPB) charge qubits. Both devices were coupled to thin-film lumped-element superconducting aluminum LC resonators at a temperature of 25 mK. Device 1 was previously found to have an exceptionally long energy relaxation time of T1=205 μs and a strong correlation between the lifetime T1 and the decoupling from the microwave drive line dVg,rms/dΩR,0.

I determined the dephasing properties of this CPB though a series of experiments. I measured Ramsey fringes, extracted dephasing times Tφ in the range200-500 ns, and determined a corresponding bound of Sq(f=1 Hz)≤(3×10-3)2 e2/Hz on the amplitude of the 1/f charge noise affecting the qubit. I then carried out a spin echo experiment and found echo decay times Techo in the 2.4-3.3 μs range, implying a high frequency 1/f charge noise cutoff of ωc/2π≈0.2 MHz.

I followed this up by fabricating and characterizing a nearly identical Device 2. This CPB had a reasonably long relaxation time T1≈4-30 μs and again the lifetime T1 and decoupling dVg,rms/dΩR,0 were correlated. Although the lifetime of Device 2 was shorter than that of Device 1, the results suggest that the exceptional relaxation time was somewhat reproducible and that this approach may lead to further improvements in qubit coherence.

During my initial characterization of Device 2, I discovered that it displayed an anomalously twinned transition spectrum. I studied this feature in detail in parallel with my decoherence experiments. I found that above the resonator resonance at ω/2π=5.472 GHz the system spectrum was twinned but below it was quadrupled. This behavior was consistent with a pair of two-level systems (TLS) coupled non-resonantly to the CPB via both charge and critical current. I developed a model that matched this scenario and successfully fit the predicted spectrum to my data.

Both the coherent non-resonant interaction and joint charge and critical current CPB-TLS coupling are novel observations. From the fits I extracted microscopic parameters of the fluctuators including the well asymmetry, tunneling rate, and a minimum hopping distance of 0.2-0.45 Å. I also found a large fractional change of the Josephson energy ΔEJ,k/EJ≈30-40%, consistent with a non-uniform tunnel barrier containing a few dominant conduction channels and a defect that modulates one of them.