Direct spectral analysis of environmental noise in SQUID and SQUBIT type circuits using a Josephson bifurcation amplifier

The Josephson Bifurcation Amplifier is a threshold detector that may be viewed as a 1-bit analog-to-digital converter. Such a device is subject to two random processes, firstly the bifurcation itself is a dynamical process that is subject to quantum or thermal fluctuations that broaden the threshold, secondly the device is immersed in an environment that induces low frequency parametric noise. We have developed a new measurement technique that quantitatively and directly links the environmental noise spectrum to the spectrum calculated from the repetitively acquired binary output of the Josephson Bifurcation Amplifier. The technique has considerable advantages over the NMR techniques presently in common use for studies of qubit decoherence and the techniques used for SQUIDs. It is non-dissipative, enabling operation in the milliKelvin range, it has a wide bandwidth, it is operated at low photon numbers, and its sensitivity approaches the shot noise limit of a weak continuous measurement.