Decoherence in dc SQUID phase qubits

Hanhee Paik; S. K. Dutta; R. M. Lewis; T. A. Palomaki; B. K. Cooper; R. C. Ramos; H. Xu; A. J. Dragt; J. R. Anderson; C. J. Lobb; F. C. Wellstood

doi:10.1103/PhysRevB.77.214510

Decoherence in dc SQUID phase qubits

Hanhee Paik, S. K. Dutta, R. M. Lewis, T. A. Palomaki, B. K. Cooper, R. C. Ramos, H. Xu, A. J. Dragt, J. R. Anderson, C. J. Lobb, F. C. Wellstood

DOI 10.1103/PhysRevB.77.214510 · Physical Review B

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Abstract

We report measurements of Rabi oscillations and spectroscopic coherence times in an Al/AlOx/Al and three Nb/AlOx/Nb dc SQUID phase qubits. One junction of the SQUID acts as a phase qubit and the other junction acts as a current-controlled nonlinear isolating inductor, allowing us to change the coupling to the current bias leads in situ by an order of magnitude. We found that for the Al qubit, a spectroscopic coherence time T2∗ varied from 3 to 7 ns and the decay envelope of Rabi oscillations had a time constant T′=25ns on average at 80 mK. The three Nb devices also showed T2∗ in the range of 3–8 ns, but T′ was 9–15 ns, just about 1/2 the value we found in the Al device. For all the devices, the time constants were roughly independent of the isolation from the bias lines, implying that noise and dissipation from the bias leads were not the principal sources of dephasing and inhomogeneous broadening.

Source-reported materials — not catalogue approval

Formula	Reported Tc (K)	Pressure (GPa)	Type
Al Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	—	Pressure not reported	unknown
Nb Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula.	—	Pressure not reported	unknown