RUI: Classical and Quantum Ratchets in Josephson Arrays

*非技术抽象*本质上的噪声和随机性并不总是不受欢迎的。近年来，人们了解到，许多物理系统都有能力利用噪声和随机性来发挥其优势。这样的系统被称为棘轮，表示一个系统只向一个方向移动，无论它被推向哪个方向。棘轮的一个日常例子是风车，在那里，无论风向哪个方向吹，都会产生净正能量。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是，对于不同数量和不同类型的噪音，会产生多少净运动。科学家们寻找不同的系统，试图量化这些问题的答案。在这项研究中，我们研究了超导电路中的棘轮效应。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，被称为磁通子的小块磁场？可能会被困在它们里面。如果电路布局设计正确，这些磁通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架构，每种架构的运行方式都略有不同。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全面的了解。这项研究的更广泛影响包括对本科物理专业学生的培训，他们将参与大部分拟议的研究。*技术摘要*本研究人员个人奖支持超导约瑟夫森结阵列中棘轮效应的实验研究。棘轮效应是物理系统的特征，在这些物理系统中，随机噪声和波动可以导致沿首选方向的运动。一个可以实现棘轮效应的物理系统是超导约瑟夫森结阵列，在那里施加的电流可以穿梭被称为磁通子的量子磁通量。这些磁通子的运动可以通过所谓的开关电流测量来确定，即在相同条件下多次测量电流-电压特性。特别令人感兴趣的是不同的运输方式。并在阵列中移动。在低温下，弗克森预计将通过量子隧道传播，尽管这一点尚未被观察到。在中等温度下，氟松子依赖于热激活，由Kramers？法律行为类型。在较高的温度下，在被热激活后，通量可以再次被捕获，并在一系列脱钉和再捕获事件中移动；这被称为通量扩散。我们的主要目标是观察这三个领域，并确定？交叉？温度的不同区域。这项工作的更广泛影响包括对本科生物理专业的培训，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. 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.