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

Gyrotropic Zener tunneling and nonlinear IV curves in the zero-energy Landau level of graphene in a strong magnetic field

We have investigated tunneling current through a suspended graphene Corbino disk in high magnetic fields at the Dirac point, i.e. at filling factor $\nu$ = 0. At the onset of the dielectric breakdown the current through the disk grows exponentially before ohmic behaviour, but in a manner distinct from thermal activation. We find that Zener tunneling between Landau sublevels dominates, facilitated by tilting of the source-drain bias potential. According to our analytic modelling, the Zener tunneling is strongly affected by the gyrotropic force (Lorentz force) due to the high magnetic fieldComment: 7 pages, 4 figure

Band Engineering in Cooper-Pair Box: Dispersive Measurements of Charge and Phase

Low-frequency susceptibility of the split Cooper-pair box (SCPB) is investigated for use in sensitive measurements of external phase or charge. Depending on the coupling scheme, the box appears as either inductive or capacitive reactance which depends on external phase and charge. While coupling to the source-drain phase, we review how the SCPB looks like a tunable inductance, which property we used to build a novel radio-frequency electrometer. In the dual mode of operation, that is, while observed at the gate input, the SCPB looks like a capacitance. We concentrate on discussing the latter scheme, and we show how to do studies of fast phase fluctuations at a sensitivity of 1 mrad/$\sqrt{Hz}$ by measuring the input capacitance of the box.Comment: LT24 invited paper, 4 page

Noise properties of the Bloch oscillating transistor

We have measured the current noise spectral density of the Bloch oscillating transistor as a function of current gain. We find, as expected from theory and simulations, that the equivalent input noise that shows up in the output is less than the shot noise of the normal-insulating-superconductor tunnel junction (base junction). At the optimal operating point we find a reduced input current noise of 1.0fA/√Hz and a corresponding noise temperature of 0.4 K. The differential current gain at the same point is as large as 30 and the power gain amounts to 35.Peer reviewe

Design of cryogenic 700 MHz HEMT amplifier

We present a way to design a high-frequency low-temperature pHEMT-based balanced amplifier. The design is based on measured cryogenic S-parameters combined with a small-signal noise model. Using the design process we constructed an amplifier that was measured to have a gain of 16 dB and a noise temperature of ∼3 K when cooled to 4.2 K.Non Peer reviewe

Electron-beam welded Cu-to-Ag joints for thermal contact at low temperatures

Electrical resistance measurements at 4.2 K on an electron‐beam welded Cu‐Ag joint and a simple screw‐fastened Cu‐Ag joint are reported. It was found that the welded joint, when annealed, gives a contact resistance that is about three times smaller than the resistances for the best screw‐fastened joints.Peer reviewe

Nuclear spin relaxation at ultralow temperatures

Nuclear spin relaxation induced by hyperfine coupling is studied theoretically at positive and negative submicrokelvin temperatures. By avoiding the assumption of the high-temperature limit, adopted in conventional theories, we derive a formula in which the relaxation rate is expressed in terms of thermal averages of nuclear spin energies. The exchange interaction induces an asymmetry in the energy spectrum, which leads to relaxation rates that depend on whether the nuclear spin temperature is positive or negative. High-temperature expansion methods and Monte Carlo simulations are applied to explain the anomalous results by Hakonen et al. in rhodium qualitatively.Peer reviewe