Quantum capacitive phase detector

We discuss how a single Cooper-pair transistor may be used to detect the superconducting phase difference by using the phase dependence of the input capacitance from gate to the ground. The proposed device has a low power dissipation because its operation is in principle free from quasiparticle generation. According to the sensitivity estimates the device may be used for efficient qubit readout in a galvanically isolated and symmetrized circuit.Comment: 5 pages, published for

The Inductive Single-Electron Transistor (L-SET)

We demonstrate a sensitive method of charge detection based on radio-frequency readout of the Josephson inductance of a superconducting single-electron transistor. Charge sensitivity $1.4 \times 10^{-4}e/\sqrt{Hz}$, limited by preamplifier, is achieved in an operation mode which takes advantage of the nonlinearity of the Josephson potential. Owing to reactive readout, our setup has more than two orders of magnitude lower dissipation than the existing method of radio-frequency electrometry. With an optimized sample, we expect uncoupled energy sensitivity below $\hbar$ in the same experimental scheme.Comment: 10 page

Self heating and nonlinear current-voltage characteristics in bilayer graphene

We demonstrate by experiments and numerical simulations that the low-temperature current-voltage characteristics in diffusive bilayer graphene (BLG) exhibit a strong superlinearity at finite bias voltages. The superlinearity is weakly dependent on doping and on the length of the graphene sample. This effect can be understood as a result of Joule heating. It is stronger in BLG than in monolayer graphene (MLG), since the conductivity of BLG is more sensitive to temperature due to the higher density of electronic states at the Dirac point.Comment: 9 pages, 7 figures, REVTeX 4.

Charge sensitivity of the Inductive Single-Electron Transistor

We calculate the charge sensitivity of a recently demonstrated device where the Josephson inductance of a single Cooper-pair transistor is measured. We find that the intrinsic limit to detector performance is set by oscillator quantum noise. Sensitivity better than $10^{-6}$e$/\sqrt{\mathrm{Hz}}$ is possible with a high $Q$-value $\sim 10^3$, or using a SQUID amplifier. The model is compared to experiment, where charge sensitivity $3 \times 10^{-5}$e$/\sqrt{\mathrm{Hz}}$ and bandwidth 100 MHz are achieved.Comment: 3 page

Accessing nanomechanical resonators via a fast microwave circuit

The measurement of micron-sized mechanical resonators by electrical techniques is difficult, because of the combination of a high frequency and a small mechanical displacement which together suppress the electromechanical coupling. The only electromagnetic technique proven up to the range of several hundred MHz requires the usage of heavy magnetic fields and cryogenic conditions. Here we show how, without the need of either of them, to fully electrically detect the vibrations of conductive nanomechanical resonators up to the microwave regime. We use the electrically actuated vibrations to modulate an LC tank circuit which blocks the stray capacitance, and detect the created sideband voltage by a microwave analyzer. We show the novel technique up to mechanical frequencies of 200 MHz. Finally, we estimate how one could approach the quantum limit of mechanical systems

Pseudo-contact angle due to superfluid vortices in 4^{4}He

We have investigated spreading of superfluid $^{4}$He on top of polished MgF$_2$ and evaporated SiO$_{2}$ substrates. Our results show strongly varying contact angles of 0 - 15 mrad on the evaporated layers. According to our theoretical calculations, these contact angles can be explained by a spatially varying distribution of vortex lines, the unpinning velocity of which is inversely proportional to the liquid depth.Comment: 10 pages, 4 figure