Hybrid quantum and classical computation: exploiting the best of both paradigms

数字电子计算已经在非常快的时间尺度上变得无处不在：比以往任何时候都更需要更多更快的计算。量子计算承诺了比我们传统计算能力更多的原始计算能力：将这一承诺变为现实是我研究的首要目标。我将解决使我们能够充分利用量子计算的关键理论问题：如何将量子和经典计算联合收割机结合起来，以获得最大的计算能力和效率。现在是加紧发展量子计算的关键时刻：谷歌最近购买了他们的第一台“量子计算机”。这个来自D-Wave（加拿大伯纳比）的设备正在为商业应用解决真实的问题，尽管我们还不知道它是否真的利用量子力学来实现经典机器无法实现的高效计算。量子计算显然已经成熟：为了确保英国在这些发展的前沿占有一席之地，我们需要我们的理论家和实验学家在领导这场计算革命中发挥作用。我的研究是D-Wave量子计算机挑战我们的关键问题的核心。我们究竟如何说服量子系统有效地为我们解决困难的经典问题？我们已经知道如何解决量子问题：Feynman在1982年首次描述了量子计算机如何有效地模拟量子系统，并且可以做到这一点的实验正在世界各地的实验室中进行。经典的问题更难，有相对较少的算法承诺的速度。使用量子计算机解决经典问题，例如分解大量数字，或搜索随机数据集，或在一组复杂的约束条件下找到最佳解决方案，或对大型系统建模（例如气候或蛋白质），我们需要一个混合的经典-量子设备，可以从经典问题开始，将其转换为量子表示，解决它，然后将解作为经典数据返回。现有的计算理论模型是简单、优雅的单范式模型，在分析复杂性和可计算性方面表现良好-解决它有多难，以及所需的最低资源是多少-但将不同模型组合成更接近的混合复合材料的方法匹配真实的计算设备还没有。即使是最简单的实验量子处理器也是一种混合设备，通常将经典控制硬件与两个或多个不同的量子系统相结合，通过精确指定的操作序列进行交互。混合量子系统可以实现更实用的实验和更有效的量子计算机程序，这两者对于减少噪声至关重要，否则会使量子设备变得无用。但我们还不知道什么是最好的计算模型，我们应该使用物理构建有用的计算机。基于硅的数字技术为我们提供了很好的服务，但是经典计算能力的每两年翻一番，在基本组件的尺寸方面达到了量子极限，并且各种不太传统的设备正在入侵我们的日常生产力和娱乐市场。利基市场正在为许多特殊用途类型的计算机开放，其中量子是一个重要的例子。我将通过发展对复合量子经典计算设备的理论理解并应用现实世界的约束来解决我们知识中的这些关键差距，并通过对混合量子经典系统的详细理论和计算建模来验证它们的属性，计算能力和有效操作所需的条件。这将使我能够提供科学和领导力，使英国在量子计算的新时代处于生产和利用技术的首要地位。

Digital electronic computation has become ubiquitous on a very rapid timescale: more and faster computation is in greater demand than ever. Quantum computing promises more raw computing power than we can achieve classically: turning this promise into reality is the overarching goal of my research. I will address the key theoretical issue that will enable us to fully exploit quantum computation: how to combine quantum and classical computation to gain maximum computational power and efficiency.It is a crucial time to step up the development of quantum computing: Google recently bought their first "quantum computer". This device, from D-Wave (Burnaby, Canada), is solving real problems for commercial applications, even though we don't yet know whether it is actually exploiting quantum mechanics to achieve efficient computation beyond the reach of classical machines. Quantum computing is clearly coming of age: to ensure the UK has a place in the forefront of these developments we need our theorists and experimentalists to play their part in leading this computing revolution.My research is central to the key questions the D-Wave quantum computer challenges us with.How, exactly, do we persuade quantum systems to solve hard classical problems efficiently for us? We are part of the way there, we already know how to solve quantum problems: Feynman in 1982 first described how a quantum computer could efficiently simulate quantum systems, and experiments that can do this are well under way in labs around the world. Classical problems are tougher, there are relatively few algorithms promising a speed up. To use a quantum computer to solve a classical problem, such as factoring large numbers, or searching a random data set, or finding the best solution under a complex set of constraints, or modelling a large system (climate or proteins for example), we need a hybrid classical-quantum device that can start with the classical problem, convert it into a quantum representation, solve it, and then return the solution as classical data. Existing theoretical models of computation are simple, elegant, single paradigm models that perform well for analysis of complexity and computability - how hard it is to solve, and what are the minimum resources required - but methods of combining different models into hybrid composites that more closely match real computational devices are missing. Even the simplest experimental quantum processor is a hybrid device, typically combining classical controling hardware with two or more different quantum systems interacting through precisely specified sequences of operations. Hybrid quantum systems enable more practical experiments and more efficient quantum computer programs, both of which are essential to reduce the noise that would otherwise render quantum devices useless. But we don't yet know what is the best model of computation we should use to physically build useful computers. Silicon-based digital technology is serving us well, but the bienniel doubling of classical computing power is reaching quantum limitations in how small the elemental components can be made, and a diversity of less conventional devices are invading the marketplace for our daily productivity and entertainment. Niches are opening up for many special purpose types of computer, of which quantum is one important example. I will address these key gaps in our knowledge by developing a theoretical understanding of composite quantum-classical computational devices with real-world constraints applied, and by detailed theoretical and computational modelling of hybrid quantum-classical systems to characterise their properties, computational power and the conditions required for their efficient operation. This will enable me to provide the science and leadership that will place the UK in a prime position to produce and exploit the technology in the new era of quantum computation.