We benchmark the decoherence of superconducting transmon qubits to examine the temporal stability of energy relaxation, dephasing, and qubit transition frequency. By collecting statistics during measurements spanning multiple days, we find the mean parameters T1 = 49 µs and T * 2 = 95 µs; however, both of these quantities fluctuate, explaining the need for frequent re-calibration in qubit setups. Our main finding is that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems (TLS). Through statistical analysis, we determine sub-millihertz switching rates of these TLS and observe the coherent coupling between an individual TLS and a transmon qubit. Finally, we find evidence that the qubit's frequency stability produces a 0.8 ms limit on the pure dephasing which we also observe. These findings raise the need for performing qubit metrology to examine the reproducibility of qubit parameters, where these fluctuations could affect qubit gate fidelity. * Present address: National Physical Laboratory, Hampton road, Teddington, UK, TW11 0LW † These two authors contributed equally ‡ bylander@chalmers.se arXiv:1901.04417v2 [cond-mat.supr-con]

We demonstrate that a high kinetic inductance disordered superconductor can realize a low microwave loss, non-dissipative circuit element with an impedance greater than the quantum resistance (RQ = h/4e 2 6.5 kΩ). This element, known as a superinductor, can produce a quantum circuit where charge fluctuations are suppressed. The superinductor consists of a 40 nm wide niobium nitride nanowire and exhibits a single photon quality factor of 2.5 × 10 4 . Furthermore, by examining loss rates, we demonstrate that the dissipation of our nanowire devices can be fully understood in the framework of two-level system loss. * david.niepce@chalmers.se

The loss and noise mechanisms of superconducting resonators are useful tools for understanding decoherence in superconducting circuits. While the loss mechanisms have been heavily studied, noise in superconducting resonators has only recently been investigated. In particular, there is an absence of literature on noise in the single photon limit. Here, we measure the loss and noise of an aluminium on silicon quarter-wavelength (λ/4) resonator in the single photon regime. arXiv:1801.10204v1 [cond-mat.mes-hall]