Decoherence in a Josephson junction qubit

• Main Authors: Berkley, A. J, Xu, H, Gubrud, M. A, Ramos, R. C, Anderson, J. R, Lobb, C. J, Wellstood, F. C
• Format: Journal Article
• Language: English
• Published: 28-02-2003
• Subjects: Physics - Quantum Physics
• DOI: 10.48550/arxiv.quant-ph/0303001

The zero-voltage state of a Josephson junction biased with constant current
consists of a set of metastable quantum energy levels. We probe the spacings of
these levels by using microwave spectroscopy to enhance the escape rate to the
voltage state. The widths of the resonances give a measurement of the coherence
time of the two states involved in the transitions. We observe a decoherence
time shorter than that expected from dissipation alone in resonantly isolated
20 um x 5 um Al/AlOx/Al junctions at 60 mK. The data is well fit by a model
including dephasing effects of both low-frequency current noise and the escape
rate to the continuum voltage states. We discuss implications for quantum
computation using current-biased Josephson junction qubits, including the
minimum number of levels needed in the well to obtain an acceptable error limit
per gate.