Laboratoire Pierre Aigrain - UMR 8551

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Andrew FEFFERMAN (UC Berkeley & LPS)

There has recently been a resurgence of interest in low frequency magnetic flux noise because of its important role in limiting the coherence time of superconducting qubits. The magnitude of this flux noise is quite similar in dc-SQUIDs and qubits, despite the considerable difference in the dimensions of these devices. Based on this observation, we assume that the flux noise in qubits and SQUIDs has a common origin. In particular, the flux noise may be due to metal-induced gap states (MIGS) localized by disorder at, e.g., the interface between the superconducting film and the insulating substrate. In order to test this hypothesis and probe the interactions between the MIGS, we have measured the flux noise in six conventionally fabricated SQUIDs with washer widths ranging from 5 to 160 microns, four identical conventionally fabricated SQUIDs, and six identical epitaxially grown SQUIDs. In preliminary results, we find that the flux noise in the epitaxially grown SQUIDs, which are thought to have fewer noise-producing MIGS due to the ordered metal/insluator interface, is lower than that of the conventional SQUIDs at the lowest temperatures. All SQUIDs exhibit a minimum in flux noise at 1 Hz near 1.5 K, with a significant increase in flux noise as the temperature is further decreased. The frequency dependence of the flux noise power spectrum is not strictly 1/f ; instead, the slope ranges from 0.6 to 1.2 as the temperature is varied, with a minimum again near 1 K. Remarkably, the slopes of the power spectra converge to 0.8 at the lowest temperatures in all of the conventional SQUIDs. We expect these observations to provide clues regarding the nature of the correlations between the fluctuators that give rise to the flux noise.