Strategies for Measuring the Decoherence Time of a Superconducting Qubit

The major roadblock to the development of a useful superconducting quantum coherent bit is decoherence. Integrating a flux-based qubit with Rapid Single Flux Quantum (RSFQ) electronics on a monolithic circuit provides a means to quickly prepare and measure the state of the qubit, thus enabling a number of qubit manipulations and interactions to take place within the qubit coherence time. However, the magnetic fluctuations arising from active RSFQ circuitry in proximity to the qubit are a possible source of decoherence. We present an explicit calculation of the coherence time of a rf-SQUID qubit in response to a random ambient magnetic field. An experimental strategy for measuring the spectral density of the fluctuating magnetic field in the vicinity of active RSFQ circuits also is described. These measurements along with our model will enable prediction of the coherence time of a superconducting qubit surrounded by RSFQ control electronics.