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arXiv:1701.03306 · New Journal of Physics

Relative multiplexing for minimising switching in linear-optical quantum computing

Mercedes Gimeno-Segovia, Hugo V. Cable, Gabriel J. Mendoza, Pete Shadbolt, Joshua W. Silverstone, Jacques Carolan, Mark G. Thompson, Jeremy L. O’Brien, Terry Rudolph

Published May 2, 2017• 44 citations
arXivHTMLPDF

Abstract

Many existing schemes for linear-optical quantum computing (LOQC) depend on multiplexing (MUX), which uses dynamic routing to enable near-deterministic gates and sources to be constructed using heralded, probabilistic primitives. MUXing accounts for the overwhelming majority of active switching demands in current LOQC architectures. In this manuscript we introduce relative multiplexing (RMUX), a general-purpose optimisation which can dramatically reduce the active switching requirements for MUX in LOQC, and thereby reduce hardware complexity and energy consumption, as well as relaxing demands on performance for various photonic components. We discuss the application of RMUX to the generation of entangled states from probabilistic single-photon sources, and argue that an order of magnitude improvement in the rate of generation of Bell states can be achieved. In addition, we apply RMUX to the proposal for percolation of a 3D cluster state by Gimeno-Segovia et al (2015 Phys. Rev. Lett. 115 020502), and we find that RMUX allows an 2.4 × increase in loss tolerance for this architecture.

DOI
10.1088/1367-2630/aa7095

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Many existing schemes for linear-optical quantum computing (LOQC) depend on multiplexing (MUX), which uses dynamic routing to enable near-deterministic gates and sources to be constructed using heralded, probabilistic primitives. MUXing accounts for the overwhelming majority of active switching demands in current LOQC architectures. In this manuscript we introduce relative multiplexing (RMUX), a general-purpose optimisation which can dramatically reduce the active switching requirements for MUX in LOQC, and thereby reduce hardware complexity and energy consumption, as well as relaxing demands on performance for various photonic components. We discuss the application of RMUX to the generation of entangled states from probabilistic single-photon sources, and argue that an order of magnitude improvement in the rate of generation of Bell states can be achieved. In addition, we apply RMUX to the proposal for percolation of a 3D cluster state by Gimeno-Segovia et al (2015 Phys. Rev. Lett. 115 020502), and we find that RMUX allows an 2.4 × increase in loss tolerance for this architecture.
Interference between single photons lies at the heart of linear optical quantum computing (LOQC). However, the lack of deterministic single photon sources hinders the large-scale experimental demonstration and its practical applications, as the probability to generate N single photons simultaneously (denoted by N-photon hereafter) decreases exponentially with the increasing number of single photon sources [1]. Multiplexing is the leading proposal to overcome this challenge by relocating single photon from different modes to one fixed mode. The theoretical work [2] indicates that the relative time multiplexing (RMUX) scheme could significantly enhance this probability and meanwhile reduce the requirements of multiplexing system, such as switching speed and system complexity. In this paper, RMUX of two heralded single photon sources is demonstrated for the first time. We observe around 90% enhancement on the heralded photon pair rate, compared to the standard time multiplexing (SMUX) scheme [3].
Fusion-based quantum computing (FBQC) offers a powerful approach to building a fault-tolerant universal quantum computer using photonic components -- single-photon sources, linear-optical circuits, single-photon detectors, and optical switching with feedforward control. Both individual optical switches and sophisticated switch networks are needed where it is necessary to perform operations conditionally, using feedforward of previous photon-detection outcomes, within the lifetime of remaining photons. Most visibly, feedforward switching is required for fault-tolerant operations at the level of logical qubits, which are needed in turn for useful quantum algorithms. However, switch networks are also required for multiplexing ("muxing") stages that are needed for generating specific small entangled resource states, where it is used to boost the probabilities for allocating quantum states to fusion gates and other operations -- a task which dominates the footprint of photonic FBQC. Despite their importance, limited attention has been paid to exploring possible designs of switch networks in this setting. Here we present a wide range of new techniques and schemes which enable major improvements in terms of muxing efficiency and reductions in hardware requirements. Since the use of photonic switching heavily impacts qubit losses and errors, our schemes are constructed with low switch depth. They also exploit specific features of linear-optical circuits which are commonly used to generate entanglement in proposed quantum computing and quantum network schemes.
Using only linear optical elements, the creation of dual-rail photonic entangled states is inherently probabilistic. Known entanglement generation schemes have low success probabilities, requiring large-scale multiplexing to achieve near-deterministic operation of quantum information processing protocols. In this paper, we introduce multiple techniques and methods to generate photonic entangled states with high probability, which have the potential to reduce the footprint of Linear Optical Quantum Computing (LOQC) architectures drastically. Most notably, we are showing how to improve Bell state preparation from four single photons to up to p=2/3, boost Type-I fusion to 75% with a dual-rail Bell state ancilla and improve Type-II fusion beyond the limits of Bell state discrimination.
Quantum computing promises a new paradigm of computation where information is processed in a way that has no classical analogue. There are a number of physical platforms conducive to quantum computation, each with a number of advantages and challenges. Single photons, manipulated using integrated linear optics, constitute a promising platform for universal quantum computation. Their low decoherence rates make them particularly favourable, however the inability to perform deterministic two-qubit gates and the issue of photon loss are challenges that need to be overcome. In this thesis we explore the construction of a linear optical quantum computer based on the cluster state model. We identify the different necessary stages: state preparation, cluster state construction and implementation of quantum error correcting codes, and address the challenges that arise in each of these stages. For the state preparation, we propose a series of linear optical circuits for the generation of small entangled states, assessing their performance under different scenarios. For the cluster state construction, we introduce a ballistic scheme which not only consumes an order of magnitude fewer resources than previously proposed schemes, but also benefits from a natural loss tolerance. Based on this scheme, we propose a full architectural blueprint with fixed physical depth. We make investigations into the resource efficiency of this architecture and propose a new multiplexing scheme which optimises the use of resources. Finally, we study the integration of quantum error-correcting codes in the linear optical scheme proposed and suggest three ways in which the linear optical scheme can be made fault-tolerant.
An on-demand single-photon source is a key requirement for scaling many optical quantum technologies. A promising approach to realize an on-demand single-photon source is to multiplex an array of heralded single-photon sources using an active optical switching network. However, the performance of multiplexed sources is degraded by photon loss in the optical components and the non-unit detection efficiency of the heralding detectors. We provide a theoretical description of a general multiplexed single-photon source with lossy components and derive expressions for the output probabilities of single-photon emission and multi-photon contamination. We apply these expressions to three specific multiplexing source architectures and consider their tradeoffs in design and performance. To assess the effect of lossy components on near- and long-term experimental goals, we simulate the multiplexed sources when used for many-photon state generation under various amounts of component loss. We find that with a multiplexed source composed of switches with ∼ 0.2 − 0.4 dB loss and high efficiency number-resolving detectors, a single-photon source capable of efficiently producing 20–40 photon states with low multi-photon contamination is possible, offering the possibility of unlocking new classes of experiments and technologies.
Linear optical quantum computing (LOQC) seems attractively simple: Information is borne entirely by light and processed by components such as beam splitters, phase shifters, and detectors.However, this very simplicity leads to limitations, such as the lack of deterministic entangling operations, which are compensated for by using substantial hardware overheads.Here, we quantify the resource costs for full-scale LOQC by proposing a specific protocol based on the surface code.With the caveat that our protocol can be further optimized, we report that the required number of physical components is at least 5 orders of magnitude greater than in comparable matter-based systems.Moreover, the resource requirements grow further if the per-component photon-loss rate is worse than 10 -3 or the per-component noise rate is worse than 10 -5 .We identify the performance of switches in the network as the single most influential factor influencing resource scaling.