*** 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. This Research at an Undergraduate Institution (RUI) award supports studies of the ratchet effect in a superconducting circuit. Lithographic techniques, similar to the 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 only be able to move in 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 different than 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.
This Research at an Undergraduate Institution (RUI) 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 measuring voltages in the array. A series of low temperature measurements will be performed on previously fabricated Josephson arrays which have been designed to display the Ratchet Effect. Of particular interest is how different types of fluctuations in the arrays cause different types of ratchet behavior. Classical thermal fluctuations, always present in varying degrees, cause the so-called rocking ratchet effect. At low enough temperatures and dissipation, quantum fluctuations can cause additional transport; this is known as the quantum ratchet effect. Finally, types of 1/f fluctuations often present in Josephson junctions manifest themselves in yet another way, in a kind of ratchet known as a flashing ratchet. Many of these effects have not yet been observed, and this research aims to shed more light on these issues. The broader impact of this work includes the training of undergraduate physics majors, who will perform much of the proposed research.
