We have used Ramsey tomography to characterize charge noise in a weakly charge-sensitive superconducting qubit. We find a charge noise that scales with frequency as 1/f α over 5 decades with α = 1.93 and a magnitude Sq(1 Hz) = 2.9 × 10 −4 e 2 /Hz. The noise exponent and magnitude of the low-frequency noise are much larger than those seen in prior work on single electron transistors, yet are consistent with reports of frequency noise in other superconducting qubits. Moreover, we observe frequent large-amplitude jumps in offset charge exceeding 0.1e; these large discrete charge jumps are incompatible with a picture of localized dipole-like two-level fluctuators. The data reveal an unexpected dependence of charge noise on device scale and suggest models involving either charge drift or fluctuating patch potentials.

We have investigated the vortex dynamics in superconducting thin film devices with non-uniform patterns of artificial pinning centers (APCs). The magneto-transport properties of a conformal crystal and a randomly diluted APC pattern are compared with that of a triangular reference lattice. We have found that in both cases the magneto-resistance below the first matching field of the triangular reference lattice is significantly reduced. For the conformal crystal, the magnetoresistance is below the noise floor indicating highly effective vortex pinning over a wide magnetic field range. Further, we have discovered that for asymmetric patterns the R vs. H curves are mostly symmetric. This implies that the enhanced vortex pinning is due to the commensurability with a stripe in the non-uniform APC pattern and not due to a rearrangement and compression of the whole vortex lattice. (submitted on 04/29/13 to APL) Vortex dynamics in superconducting thin films with artificial pinning centers (APCs) have been extensively studied in recent years. A central problem in this field is how to improve the critical current density of a superconducting device by choosing a suitable APC distribution. If the APCs are arranged in a hexagonal or a rectangular lattice, the critical current is increased for certain magnetic matching fields due to the commensurability with the Abrikosov vortex lattice [1][2][3][4][5][6][7]. In order to increase the field range for vortex pinning, different APC distributions like quasiperiodic [8,9] or random [10,11] lattices have been investigated. Recent theoretical papers are focusing on non-uniform APC distributions like hyperbolic-tessellation arrays [12] and conformal crystals [13]. A conformal crystal can be obtained by conformally mapping a semiannular section of a regular hexagonal lattice on a rectangle (see [13][14][15]). In this transformation the angles are preserved, but a vertex density gradient along one side of the rectangle is introduced. Hence, a conformal crystal is a non-uniform pattern, in which on a small scale the vertices are arranged triangularly. On a large scale however, the vertex density changes continuously. A recent theoretical study [13] has shown that the pinning in conformal crystals is significant stronger over a much wider field range than that found for other APCs with an equivalent number of pinning sites. These promising results have not yet been confirmed experimentally. In our experimental study we have investigated the vortex dynamics in superconducting thin film devices with non-uniform APC patterns. We compare the magnetotransport properties of conformal crystals and corresponding randomly diluted APC patterns with the properties of a triangular reference lattice. Additionally, we investigate whether an electric current applied to a superconducting micro bridge with a non-uniform APC pattern produces enhanced pinning due to vortex distribution rearrangement and compression. Because the cur- rent pushes the vortices against the edge of the sample, an asymmetric AP...

Atomic-vapor density change due to light induced atomic desorption (LIAD) is studied in paraffincoated rubidium, cesium, sodium and potassium cells. In the present experiment, low-intensity probe light is used to obtain an absorption spectrum and measure the vapor density, while light from an argon-ion laser, array of light emitting diodes, or discharge lamp is used for desorption. Potassium is found to exhibit significantly weaker LIAD from paraffin compared to Rb and Cs, and we were unable to observe LIAD with sodium. A simple LIAD model is applied to describe the observed vapor-density dynamics, and the role of the cell's stem is explored through the use of cells with lockable stems. Stabilization of Cs vapor density above its equilibrium value over 25 minutes is demonstrated. The results of this work could be used to assess the use of LIAD for vapor-density control in magnetometers, clocks, and gyroscopes utilizing coated cells.

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Triangular arrays of Ni nanotriangles embedded in superconducting Nb films exhibit unexpected dynamical vortex effects. Collective pinning with a vortex-lattice configuration different from the expected fundamental triangular "Abrikosov state" is found. The vortex motion, which prevails against the triangular periodic potential, is produced by channeling effects between triangles. Interstitial vortices coexisting with pinned vortices in this asymmetric potential lead to ratchet reversal, i.e., a dc output voltage that changes sign with the amplitude of an applied alternating drive current. In this landscape, ratchet reversal is always observed at all magnetic fields (all numbers of vortices) and at different temperatures. The ratchet reversal is unambiguously connected to the presence of two locations for the vortices: interstitial and above the artificial pinning sites.