Observation of metastable states in a superconducting Josephson circuit using an Andreev interferometer

We report on measurements of macroscopic quantum states in superconducting Josephson circuits using a highly sensitive hybrid quantum interferometer as the readout probe. The investigated Josephson circuit is one of the leading candidates as solid-state qubits (persistent current qubit), which are known to exhibit macroscopic quantum states with atomic-like properties. The readout device is a modified Andreev interferometer with semi-metallic normal segment in a “folded" geometry, and is designed to reduce the back action during measurement, as well as minimising the electromagnetic coupling between the circuit and the environment. A pulsed lock-in technique has been developed to perform continuous readout of the superconductor phase difference using pulse lengths down to 10 ns. The technique enables to control the energy of the probing quasiparticles in the normal segment of the interferometer, which in turn allows to control of the supercurrent owing in the SNS junction and prevents electron heating of the normal segment. An experimental set-up was designed and installed in a dilution fridge consisting of shielded wiring, magnetic screens, RF tight sample holder and printed circuit board to allow the injection of high frequency excitation signals, while minimising the environment effect on the qubit through careful electrical filtering. The effect of strong RF irradiation on Andreev interferometers allowed us to estimate the response time of the readout device to be less than 40 ps. The measurements show that two macroscopically distinct metastable states exist when the device is biased at the qubit degeneracy point, between which the system makes transitions that can be continuously monitored. Real time kinetics of the system has been investigated at different magnetic fluxes, pulse parameters, temperature and RF radiation. Based on statistical analysis of the transitions, we argue that the metastability is connected with macroscopic quantum tunnelling effects rather than thermal excitation. The experimental data support the hypothesis of a large low frequency noise causing low transition rates

Scanning Gate Imaging of quantum point contacts and the origin of the 0.7 Anomaly

The origin of the anomalous transport feature appearing at conductance G \approx 0.7 x (2e2/h) in quasi-1D ballistic devices - the so-called 0.7 anomaly - represents a long standing puzzle. Several mechanisms were proposed to explain it, but a general consensus has not been achieved. Proposed explanations are based on quantum interference, Kondo effect, Wigner crystallization, and more. A key open issue is whether point defects that can occur in these low-dimensional devices are the physical cause behind this conductance anomaly. Here we adopt a scanning gate microscopy technique to map individual impurity positions in several quasi-1D constrictions and correlate these with conductance characteristics. Our data demonstrate that the 0.7 anomaly can be observed irrespective of the presence of localized defects, and we conclude that the 0.7 anomaly is a fundamental property of low-dimensional systems

Andreev Interferometers in a Strong Radio-Frequency Field

We experimentally study the influence of 1-40 GHz radiation on the resistance of normal (N) mesoscopic conductors coupled to superconducting (S) loops (Andreev interferometers). At low RF amplitudes we observe the usual h/2e superconducting-phase-periodic resistance oscillations as a function of applied magnetic flux. We find that the oscillations acquire a pi-shift with increasing RF amplitude, and consistent with this result the resistance at fixed phase is an oscillating function of the RF amplitude. The results are explained qualitatively as a consequence of two processes. The first is the modulation of the phase difference between the N/S interfaces by the RF field, with the resistance adiabatically following the phase. The second process is the change in the electron temperature caused by the RF field. From the data the response time of the Andreev interferometer is estimated to be <40ps. However there are a number of experimental features which remain unexplained; these include the drastic difference in the behaviour of the resistance at different phases as a function of RF frequency and amplitude, and the existence of a "window of transparency" where heating effects are weak enough to allow for the pi-shift. A microscopic theory describing the influence of RF radiation on Andreev interferometers is required