Holographic Quantum Processing

该项目旨在为量子处理器的实验实现开发一种新的范式。实现可扩展的量子处理器一直是当前国际研究的长期目标。近年来，实验和理论研究取得了令人印象深刻的成功，并且已经证明了对少量量子比特的卓越控制。可能是最先进的方法已经实现了与捕获的离子，和计算与完整的量子字节是可能的。然而，量子计算机离日常使用还很远，将此类设备扩展到大量量子位并因此实现技术相关性仍然是一个重大挑战。到目前为止，每增加一个比特都需要大量的高端研究技术。这个项目将探索一种潜在的方法来规避这种资源的规模化。量子比特“传统上”由物质的单个成分表示，例如离子，原子或光子。量子位之间的逻辑操作，需要一个通用的编程语言，已经实现了这些成分之间或多或少的直接相互作用。我们采用不同的方法，用全息图表示量子比特。通过大量的原子，在这种情况下是激光冷却原子的气体，有可能将量子信息编码在所谓的自旋波中，这是气体的集体激发。在这里，信息不再被本地存储和处理，而是在集合的傅立叶表示或动量空间中存储和处理。我们希望通过实验证明，用这些波进行逻辑运算是可能的。我们将评估通用量子计算所需的一整套操作的性能，从而研究在大量量子比特上运行任意量子算法的可能性。我们将使用超冷原子系综，并采用已建立的量子存储技术，通过产生自旋波的单次激发来制备两类量子比特。正如最近演示的梯度回波量子存储器一样，激发将在傅立叶空间中移动，有效地产生两个线性量子比特寄存器，就像一个双磁带图灵机。然后将四个元素结合起来以实现通用计算能力：A）与单光子探测器相结合的光学散射过程允许用量子比特填充虚拟图灵磁带。B）状态相关的相位印记引入了实现对虚拟磁带位置的独立控制的灵活性。C）相位匹配条件使得能够选择性地光学读出固定寄存器位置。D）微波驱动耦合虚拟磁带并实现寄存器之间的分束器类型操作。这些元素的结合导致了Knill，Laflamme和Milburn的着名建议的非传统实现，即使用线性光学进行通用量子计算。但在这里，不需要增加更多的材料资源与更多的量子位。最终，我们设想一个高度集成的设备，直接与光纤连接的访问和通信。因此，它将与量子通信中的应用兼容，这可能是这种技术的首批用途之一。

This project aims at developing a new paradigm for the experimental realisation of a quantum processor. Realising a scalable quantum processor has been a long-standing goal of current international research. Experimental and theoretical research efforts have seen impressive success over recent years and superb control over small numbers of quantum bits has been demonstrated. The probably most advanced approach has been implemented with trapped ions, and calculations with a full quantum byte are possible. Quantum computers are, however, still far from everyday use and it remains a major challenge to scale such devices to large numbers of qubits and thus to technological relevance. Up to now, large amounts of high-end research technology are required for every added bit.This project will explore a potential way of circumventing this scaling of resources. Quantum bits are "traditionally" represented by individual constituents of matter, such as ions, atoms, or photons. Logical operations between qubits, required for a universal programming language, have been implemented by more or less direct interactions between these constituents. We take a different approach and represent quantum bits holographically. By taking a large amount of atoms, in this case a gas of laser cooled atoms, it is possible to encode quantum information in so-called spin waves, which are collective excitations of the gas. Here, the information is no longer stored and processed locally, but in the Fourier representation or momentum space of an ensemble. We want to experimentally demonstrate that it is possible to perform logical operations with these waves. We will assess the performance of a complete set of operations required for universal quantum computing, and thus investigate the possibility to run an arbitrary quantum algorithm on a very large number of quantum bits.We will use ensembles of ultracold atoms and employ established quantum memory techniques to prepare two classes of qubits by generating single excitations of spin waves. As in the recently demonstrated gradient echo quantum memory, excitations will be shifted in Fourier space, effectively generating two linear qubit registers like a two-taped Turing machine. Four elements are then combined to achieve universal computing capability: A) Optical scattering processes combined with single photon detectors allow for the population of the virtual Turing tapes with qubits. B) State-dependent phase-imprinting introduces the flexibility to achieve independent control over the virtual tape positions. C) Phase matching conditions enable selective optical read-out of fixed register positions. D) Microwave driving couples the virtual tapes and realises beam splitter type operations between registers. The combination of these elements leads to an unconventional implementation of the celebrated proposal by Knill, Laflamme, and Milburn for universal quantum computing with linear optics. But here, no increasing material resources are required with greater number of qubits.Ultimately, we envision a highly integrated device that links directly with fibre optics for access and communication. It will thus be compatible with applications in quantum communication which might be among the first uses of such a technology.