Quantum Phase Slip Nanowires for Current Standards

伏特目前的定义精度为一万万亿分之一，使用约瑟夫森结。目前，利用量子霍尔效应，欧姆的定义精度为100亿分之一。电学单位制计量三角中最薄弱的环节是安培。目前，这是通过单电子泵中的库仑封锁效应来定义的。然而，这里的精度比一亿分之一差，即比约瑟夫森电压标准差八个数量级。本提案的目的是对量子相滑（QPS）纳米线作为基本电流标准的使用进行可行性研究，其精度可能超过一亿分之一。量子相滑移（QPS）纳米线最初是由Mooij及其同事提出的，用于量子计算实验。随后表明，电压偏置QPS纳米线是电流偏置约瑟夫森结的精确对偶。约瑟夫森电压标准是基于夏皮罗阶跃的测量-即当微波在（通常）70 GHz耦合到结时，在电流-电压特性中恒定电压的阶跃。通过适当的转换，当微波耦合到QPS纳米线上时，可以预期在恒定电流下的阶跃会出现在其电流-电压特性中。如果微波频率为f，则步骤出现在等间隔电流i = 2nef，其中e是电子电荷，n是整数。因此，电流-频率关系只取决于量子数e，因此，宇宙中任何地方的测量都应该得到一个电流标准，而这个标准最终只受微波频率测量精度的限制。上述讨论必然以超导纳米线中量子相滑移的存在为前提。虽然它们的存在似乎是可信的，但迄今为止还没有令人信服的实验证据。迄今为止，大多数发表的关于超导纳米线的实验都集中在测量超导体体转变温度以下的电阻，该电阻比热激活相滑移的标准LAMH模型预测的电阻要大。然而，Rogachev等人对这些数据的解释提出了质疑，他们表明，仅通过使用修改的LAMH模型，热活化也可以解释这种增强的抗性。由于电阻测量本身对这种解释的模糊性是开放的，在我们的实验中，我们将尝试通过实验解决QPS纳米线核心的基本物理问题：是否存在正弦电压-电荷关系？（这是第一约瑟夫森方程的对偶，该方程指出约瑟夫森结中存在正弦电流-相位关系。）为此，我们将在越来越复杂的QPS器件上进行一系列实验，并在三年计划结束时尝试测量QPS纳米线中微波诱导的恒流步骤。

The Volt is currently defined with accuracy of one part in ten-thousand trillion using the Josephson junction. The Ohm is currently defined with accuracy of one part in ten billion using the quantum Hall effect. The weakest link in the metrological triangle of electrical S. I. units is the Ampre. This is currently defined by means of Coulomb blockade effects in a single-electron pump. The accuracy here however is worse than one part in a hundred million, i.e. more than eight orders of magnitude worse than the Josephson voltage standard. The aim of this proposal is to conduct feasibility studies on the use of quantum phase-slip (QPS) nanowires as a fundamental current standard with accuracy potentially exceeding one part in a hundred million.The quantum phase-slip (QPS) nanowire was proposed by Mooij and colleagues initially for experiments in quantum computation. It was subsequently shown that a voltage-biased QPS nanowire is the precise dual of the current-biased Josephson junction. The Josephson voltage standard is based upon measurement of Shapiro steps - i.e. steps at constant voltage in the current-voltage characteristics when the microwaves at (typically) 70 GHz are coupled to the junction. Making appropriate transformations one would expect that steps at constant current would appear in the current-voltage characteristics of a QPS nanowire when microwaves are coupled to it. If the microwave frequency is f then the steps appear at equally-spaced currents i = 2nef, where e is the electronic charge and n an integer. Hence the current-frequency relationship depends only upon the quantum number e, and measurements anywhere in this universe should therefore yield a current standard which is ultimately limited only by the accuracy of the measurement of the microwave frequency.The above discussion necessarily pre-supposes the existence of quantum phase slips in superconducting nanowires. While their existence seems plausible, as yet there has been no compelling experimental evidence for this. Most of the published experiments on superconducting nanowires to date focus on the measurement of a resistance below the bulk transition temperature of the superconductor, this resistance being larger than that predicted by the standard LAMH model of thermally-activated phase slips. The interpretation of these data has however been called into question by Rogachev et al. who showed that this enhanced resistance could also be accounted for by thermal activation simply by using a modified LAMH model. Since resistance measurements alone are open to this ambiguity of interpretation, in our experiments we will attempt to experimentally address the fundamental physics question at the heart of the QPS nanowire: Is there a sinusoidal voltage-charge relationship? (This is the dual of the first Josephson equation which states that there is a sinusoidal current-phase relationship in a Josephson junction.) We will do this by a series of experiments on QPS devices of increasing complexity, culminating at the end of the three-year programme in attempts to measure microwave-induced constant-current steps in QPS nanowires.