RUI: Classical and Quantum Ratchets in Josephson Arrays

***自然界中的噪音和随机性并不总是不受欢迎的。近年来，人们已经了解到许多物理系统具有利用噪声和随机性的能力。这样的系统被称为棘轮，表明系统只在一个方向上运动，而不管它被推向哪个方向。棘轮的一个日常例子是风车，不管风朝哪个方向吹，都能产生净正能量。棘轮可以在许多不同的化学、生物、光学和电子系统中实现。存在的悬而未决的问题与不同数量和不同类型的噪音产生多少净运动有关。科学家们寻找不同的系统来试图量化这些问题的答案。这项本科院校研究（RUI）奖支持对超导电路中棘轮效应的研究。光刻技术，类似于计算机工业中使用的技术，可以用来制造由超导金属制成的微型电路。当这些电路冷却到超低温时，被称为“通量子”的小磁场就会被困在其中。如果电路布局设计正确，这些通量子将只能在一个方向上移动，从而表现出棘轮行为。研究超导电路中的棘轮效应是有利的，因为可以设计许多不同的电路结构，每个电路的工作原理都与下一个略有不同。通过测量许多这样的电路，人们可以对棘轮的工作原理有更一般的了解。这项研究更广泛的影响包括训练物理专业的本科生，他们将参与许多拟议的研究。****技术摘要****本研究在一个本科机构（RUI）奖支持在超导约瑟夫森结阵列棘轮效应的实验研究。棘轮效应描述的是随机噪声和波动能导致运动朝偏好方向运动的物理系统。一个可以实现棘轮效应的物理系统是一组超导约瑟夫森结，在那里施加的电流可以穿梭被称为通量子的磁通量量子。这些通量子的运动可以通过测量阵列中的电压来确定。一系列低温测量将在先前制造的约瑟夫森阵列上进行，该阵列设计用于显示棘轮效应。特别有趣的是阵列中不同类型的波动如何导致不同类型的棘轮行为。经典的热波动，总是以不同的程度存在，导致所谓的摇摆棘轮效应。在足够低的温度和耗散下，量子涨落会导致额外的输运；这就是所谓的量子棘轮效应。最后，约瑟夫森结中经常出现的1/f波动类型还以另一种方式表现出来，即一种被称为闪烁棘轮的棘轮。这些影响中有许多尚未被观察到，而这项研究旨在为这些问题提供更多的线索。这项工作的更广泛的影响包括训练物理专业的本科生，他们将执行大部分拟议的研究。

*** 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. **** TECHNICAL ABSTRACT ****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.