RUI: Classical and Quantum Ratchets in Josephson Arrays

Colgate University, Hamilton NY

Investigators

Abstract

*****NON-TECHNICAL ABSTRACT***** Noise and randomness in nature are not always undesirable. In recent years it has been learned that many physical systems have the capability to use noise and randomness to their advantage. Such a system is called a ratchet, indicating a system that only moves in one direction regardless of which direction it is pushed. An everyday example of a ratchet is a windmill, where regardless of which way the wind blows, net positive energy is produced. Ratchets can be realized in many different chemical, biological, optical and electronic systems. The open questions that exist relate to how much net motion is produced for different amounts and different types of noise. Scientists seek out different systems to try to quantify the answers to these questions. In this research, the ratchet effect is being studied in a superconducting circuit. Lithographic techniques, similar to ones used in the computer industry, can be used to fabricate tiny microscopic circuits made of superconducting metals. When these circuits are cooled to ultra-low temperatures, small bits of magnetic field called ?fluxons? can be trapped inside them. If the circuit layout has been designed correctly, these fluxons will be able to move in only one direction, thus exhibiting ratchet behavior. Studying the ratchet effect in a superconducting circuit is advantageous because many different circuit architectures can be engineered, each one operating slightly differently from the next. By measuring many such circuits, one can work toward more general ideas about how ratchets work. The broader impact of this research includes the training of undergraduate physics majors, who will be involved with much of the proposed studies. *****TECHNICAL ABSTRACT***** This individual investigator award supports an experimental study of the Ratchet Effect in arrays of superconducting Josephson junctions. The Ratchet Effect characterizes physical systems in which random noise and fluctuations can cause motion in a preferred direction. A physical system where the Ratchet Effect can be realized is an array of superconducting Josephson junctions, where applied electrical current can shuttle quanta of magnetic flux called fluxons. The motion of these fluxons can be ascertained by so-called switching current measurements, where the current-voltage characteristics are measured multiple times under the same conditions. Of particular interest are the different modes of transport for fluxons to ?depin? and move through the array. At low temperatures, the fluxon is expected to depin via quantum tunneling, although that has yet to be observed. At moderate temperatures the fluxon depins via thermal activation, characterized by Kramers? law type of behavior. At higher temperatures the fluxon can retrap again after being thermally activated, and move through in a series of depinning and retrapping events; this is known as fluxon diffusion. Our main objective is to observe these three domains and identify the ?crossover? regions in temperature. The broader impact of this work includes the training of undergraduate physics majors, who will perform much of the proposed research.