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Condensation and Coherence in Condensed Matter

Ramsey Fringe Measurement of Decoherence in a Novel Superconducting Quantum Bit Based on the Cooper Pair Box

D. Vion, A. Aassime, Audrey Cottet, P. Joyez, H. Pothier, C. Urbina, D. Esteve, Michel Devoret

Published Mar 1, 2003• 0 citations
Publisher
DOI
10.1142/9789812791269_0026

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We have designed and operated a novel superconducting tunnel junction circuit which behaves as a controllable atom, with a ground and first excited state forming an effective spin 1/2. These two states are the symmetric and antisymmetric combinations of two charge configurations of a Cooper pair box. Any spin orientation corresponding to an arbitrary superposition of these two states can be prepared by applying NMR-like microwave pulses to the gate of the box. The spin state is readout by measuring the voltage response to a probe current pulse applied to the bridge formed by the two junctions of the box and an additional large, shunting junction. In a sample whose transition period is 60 ps, coherent superpositions decay with a 0.5 μs time constant, as measured by a Ramsey fringes experiment. This result is a step towards the realization of a solid-state quantum information processor.
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 25 mK, the first qubit had an energy relaxation time T1 that varied from 30 μs to 200 μs between 4 and 8 GHz 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 2.4–3.3 μs, 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=4−30 μs between 4 and 7.3 GHz, 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.