The quantum dynamics of Josephson junctions with controlled current-phase relationships

由于电子电路具有一组定义良好的量子态，国际上正在迅速发展可作为人造原子的电子电路。这种电路被称为量子比特（qubits），其应用范围从量子计算到测量标准。这些量子态非常脆弱（容易退相干），最容易在极低温度（<1K）下建立，并且可以被环境中的电子噪声源或与材料本身相关的源破坏。超导材料已被证明是非常成功的量子比特制造和进展迅速，近年来证明了许多基于超导环路中的约瑟夫森结的方法。量子比特设计显著提高抗退相干性对该领域非常重要。基于被称为π结的非常规约瑟夫森器件，已经有了许多环境解耦量子比特的设计。其中有两种主要类型：超导隧道结，在隧道势垒和一个电极之间包含铁磁层（SFIS结构），以及以传统超导体作为一个电极和d波超导体作为另一个电极的结（SND结构）。本提案的目的是进行关键实验，以测试它们对量子比特应用的适用性。这两个系统有许多共同的特征，因此通过并行研究，我们将能够采用共同的实验方法，评估它们的相对优点和差异，并确定依赖于量子比特设计的总体可行性？连接。实验将涉及优化量子比特应用的两种技术，确定什么结设计提供最合适的pi结行为，研究与设备相关的潜在固有噪声源，研究结本身是否在低温下发展良好定义的量子态，最后，使用先前的实验结果选择合适的量子比特设计并研究其特性。申请人是剑桥大学和伯明翰大学的研究人员，他们在金属和氧化物异质结构超导器件的制造以及温度低于1K的测量方面有着良好的记录，因此他们很有可能在这一领域产生重大影响。

There is rapidly expanding international activity on the development of electronic circuits which can be used as artificial atoms in that they possess a set of well defined quantum states. Such circuits are known as quantum bits (qubits) with applications ranging from quantum computing to measurement standards. These quantum states are extremely fragile (decohere readily), are most easily established at very low temperatures (<1K) and can be destroyed by sources of electronic noise in the environment or associated with the materials themselves. Superconducting materials have proved to be very sucessful for qubit fabrication and progress has been rapid with a number of approaches being demonstrated in recent years based upon Josephson junctions in superconducting loops.Qubit designs with significantly improved immunity to decoherence would extremely important to this field. There have been a number of designs for environmentally decoupled qubits based upon unconventional Josephson devices known as pi junctions. There are two main types of these: superconducting tunnel junctions which contain a ferromagnetic layer between the tunnel barrier and one electrode (SFIS structures) and junctions with a conventional superconductor as one electrode and a d-wave superconductor as the other (SND structures). The aim of this proposal is to carry out the key experiments which test their suitability for qubit applications. The two systems share many features common and so by investigating both in parallel, we will be able employ common experimental approaches, evaluate their relative merits and differences and to determine the overall feasibility of qubit designs which rely on ? junctions. The experiments will involve optimising both technologies for qubit applications, determining what junction designs give the most suitable pi junction behaviour, investigating potential intrinsic noise sources associated with the devices, investigating whether the junctions themselves develop well defined quantum states at low temperature and finally, using the results of the previous experiments to select a suitable qubit design and investigate its properties.The applicants are researchers at the Universities of Cambridge and Birmingham with a strong track record of the fabrication of superconducting devices from metallic and oxide heterostructures and measurements at temperatures below 1K, who are thus well placed to make a major impact in this area.