Z. Kim

Decoupling a Cooper-Pair Box to Enhance the Lifetime to 0.2 ms

We present results on a circuit QED experiment in which a separate transmission line is used to address a quasilumped element superconducting microwave resonator which is in turn coupled to an Al/AlO(x)/Al Cooper-pair box charge qubit. With our device, we find a strong correlation between the lifetime of the qubit and the inverse of the coupling between the qubit and the transmission line. At the smallest coupling we measured, the lifetime of the Cooper-pair box was T₁=200 μs, which represents more than a twentyfold improvement in the lifetime of the Cooper-pair box compared with previous results. These results imply that the loss tangent in the AlO(x) junction barrier must be less than about 4×10⁻⁸ at 4.5 GHz, about 4 orders of magnitude less than reported in larger area Al/AlO(x)/Al tunnel junctions.

Anomalous avoided level crossings in a Cooper-pair box spectrum

We have observed a few distinct anomalous avoided level crossings and voltage dependent transitions in the excited state spectrum of an Al/AlOx/Al Cooper-pair box (CPB). The device was measured at 40 mK in the 15 - 50 GHz frequency range. We find that a given level crosses the CPB spectrum at two different gate voltages; the frequency and splitting size of the two crossings differ and the splitting size depends on the Josephson energy of the CPB. We show that this behavior is not only consistent with the CPB being coupled to discrete charged two-level quantum systems which move atomic distances in the CPB junctions but that the spectra provide new information about the fluctuators, which is not available from phase qubit spectra of anomalous avoided levels. In particular by fitting a model Hamiltonian to our data, we extract microscopic parameters for each fluctuator, including well asymmetry, tunneling amplitude, and the minimum hopping distance for each fluctuator. The tunneling rates range from less than 3.5 to 13 GHz, which represent values between 5% and 150% of the well asymmetry, and the dipole moments give a minimum hopping distance of 0.3 to 0.8 Anstrom. We have also found that these discrete two-level systems have a pronounced effect on the relaxation time (T1) of the quantum states of the CPB and hence can be a source of dissipation for superconducting quantum bits.

Decoherence in a pair of long-lived Cooper-pair boxes

We have investigated the decoherence of quantum states in two Al/AlOx/Al Cooper-pair boxes coupled to lumped element superconducting inductor-capacitor resonators. At 25u2009mK, the first qubit had an energy relaxation time T1 that varied from 30u2009μs to 200u2009μs between 4 and 8u2009GHz and displayed an inverse correlation between T1 and the coupling to the microwave drive line. The Ramsey fringe decay times T2* were in the [200–500] ns range while the spin echo envelope decay times Techo varied from 2.4–3.3u2009μs, consistent with 1/f charge noise with a high frequency cutoff of 0.2 MHz. A second Cooper-pair box qubit with similar parameters showed T1=4−30u2009μs between 4 and 7.3u2009GHz, and that the T1 and the coupling were again inversely correlated. Although the lifetime of the second device was shorter than that of the first device, the dependence on coupling in both devices suggests that further reduction in coupling should lead to improved qubit performance.