宏观共振隧穿的新进展及其在量子信息处理中的应用

从量子力学诞生以来,关于宏观物体的运动是否遵循量子力学的辩论就一直没有停止过.上世纪八十年代初期以来,一系列在约瑟夫森结和超导最子干涉器件(SQUID)中观测到的实验结果,包括相位和磁通的宏观量子隧穿,能级量子化,宏观共振隧穿,和在微波驱动下的相干动力学过程对认为宏观物体的运动在满足一定条件下同样遵循量子力学规律的观点提供了强有力的实验证据.在众多已观察到的宏观量子现象中,宏观共振隧穿结合了能级分立和隧穿这两个最具特征的量子现象.由于宏观共振隧穿的观测无需使用高频电磁波激发,这就避免了实验结果也可以用经典物理解释的可能,所以在一个系统中观测到宏观共振隧穿可以说是展示该系统的量子属性的最有力证据.本文讨论近年来从理论和实验两方面理解耗散和磁通噪声对类似SQUID的双势阱系统宏观共振隧穿率和谱线形状的影响.评述宏观共振隧穿谱的测量在探寻、理解、克服超导磁通量子比特中的退相干机制并最终实现规模化量子计算方面的应用.

RECENT PROGRESS IN MACROSCOPIC RESONANT TUNNELING AND ITS APPLICATIONS TO QUANTUM INFORMATION PROCESSING

The question of whether macroscopic variables obey the laws of quantum mechanics has been debated ever since the earliest days of quantum mechanics. Since early 1980s results from a series of experiments in Josephson junctions and superconducting interference devices (SQUIDs), including macro-scopic quantum tunneling, energy level quantization, macroscopic resonant tunneling (MRT), and ac driv-en coherent dynamics of the phase and flux, strongly support the notion that under proper conditions mo-tions of macroscopic variables are indeed governed by the laws of quantum mechanics. Among many macro-scopic quantum phenomena, MRT, which combines two of the most distinctive quantum phenomena-tun-neling and energy level-quantization together and does not involve excitation by high frequency fields, is perhaps the strongest evidence demonstrating quantum nature of macroscopic variables. In this article, we review recent development in theory and experiment of MRT. In particular, we show that how dissipation and flux noise affect the rate and line shape of MRT in the double-well of rf SQUID flux qubits and appli-cations of MRT in the ongoing efforts of identifying and understanding mechanisms of decoherence in su-perconducting qubits for scalable quantum computing.