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arXiv:cond-mat/0602408 · Physical Review B

Photoassisted Andreev reflection as a probe of quantum noise

T. K. T. Nguyen, Thibaut Jonckheere, Adeline Crépieux C, Ai Viet Nguyen, Thierry Martin

Published Jul 19, 2007• 3 citations
arXivHTMLPDF

Abstract

Andreev reflection, which corresponds to the tunneling of two electrons from a metallic lead to a superconductor lead as a Cooper pair (or vice versa), can be exploited to measure high frequency noise. A detector is proposed, which consists of a normal lead--superconductor circuit, which is capacitively coupled to a mesoscopic circuit where noise is to be measured. We discuss two detector circuits: a single normal metal-superconductor tunnel junction and a normal metal separated from a superconductor by a quantum dot operating in the Coulomb blockade regime. A substantial dc current flows in the detector circuit when an appropriate photon is provided or absorbed by the mesoscopic circuit, which plays the role of an environment for the junction to which it couples. Results for the current can be cast in all cases in the form of a frequency integral of the excess noise of the environment weighted by a kernel, which is specific to the transport process (quasiparticle tunneling, Andreev reflection, etc) considered. We apply these ideas to the measurement of the excess noise of a quantum point contact, and we provide numerical estimates of the detector current.

DOI
10.1103/physrevb.76.035421

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In this work four different mesoscopic superconducting devices have been experimentally studied: an ammeter based on a hysteretic Josephson junction switching from the superconducting state to the normal state, a conventional Cooper pair pump (CPP) based on two superconducting islands separated by tunnel junctions, a novel flux assisted Cooper pair pump and a thermometer based on a tunnel junction between a superconductor and a normal metal. These devices make use of phenomena related to superconductivity and are also benefiting from some properties of small structures, e.g., of quantum mechanical tunneling and of transport of single electrons. The present research focused on investigating performance limitations of these devices and the aim was to improve their performance. A hysteretic Josephson junction switching can be used as a sensitive current threshold detector. For many purposes, a smaller critical current Ic of the detecting junction would yield a higher current sensitivity. It was found in this Thesis that the main limitation of the ammeter is that with increasing sensitivity the negative effects due to dissipation start to dominate. In this Thesis the Josephson junction was also demonstrated as a shot-noise detector, and it was concluded that this kind of a device could probably be made into an absolute on-chip detector of Fano-factors and noise in general. A charge pump is a device which can periodically transfer a discrete amount of charge Q through an electrical circuit. The measured double island CPP introduced a frequency dependent current with a value close to the expected one at low pumping frequencies. The drawback in the measurements was that the direction of the pumped current was set by the bias voltage. The results of the measurement on flux assisted pumping were indeed very promising and the studied device was able to produce sufficiently high pumped currents of around 0.1 nA with reasonable accuracy. Thermometer based on NIS (Normal metal - Insulator - Superconductor) tunnel junctions is a very promising temperature sensor for bolometric radiation sensor applications. This work showed that it can be used as a sensitive thermometer up to MHz frequencies range and that its response time is limited by the electron-phonon scattering rate.
We study the interplay between Coulomb blockade and superconductivity in a tunable superconductor-superconductor-normal-metal single-electron transistor. The device is realized by connecting the superconducting island via an oxide barrier to the normal-metal lead and with a break junction to the superconducting lead. The latter enables Cooper pair transport and (multiple) Andreev reflection. We show that these processes are relevant also far above the superconducting gap and that signatures of Coulomb blockade may reoccur at high bias while they are absent for small bias in the strong-coupling regime. Our experimental findings agree with simulations using a rate equation approach in combination with the full counting statistics of multiple Andreev reflection.