This topical issue features optical quantum computer, OQC and its related technology developments. Every article describes fundamental aspects of OQC and quantum phenomena that OQC is based on. These include universal quantum computer based on quantum circuit model, quantum simulator, oneway quantum computer, fault tolerance, Boson sampling, squeezed state of photon, superposition, and entanglement. Much emphasis is laid on Linear Optical Quantum Computing for non-linear phase coupling between two photons which makes the technology very unique and gives advantages over other kinds of physical realization rather than optical. Alternative method of non-linear coupling has been realized by circuit QED with microwave photons.
This topical issue features optical quantum computer, OQC and its related technology developments. Every article describes fundamental aspects of OQC and quantum phenomena that OQC is based on. These include universal quantum computer based on quantum circuit model, quantum simulator, oneway quantum computer, fault tolerance, Boson sampling, squeezed state of photon, superposition, and entanglement. Much emphasis is laid on Linear Optical Quantum Computing for non-linear phase coupling between two photons which makes the technology very unique and gives advantages over other kinds of physical realization rather than optical. Alternative method of non-linear coupling has been realized by circuit QED with microwave photons.
Quantum computing leverages quantum mechanical concepts, including superposition, entanglement, and interference, to solve problems that traditional computers cannot handle efficiently. This study presents a comprehensive overview of the field, covering both the theoretical foundations and the latest developments in quantum hardware and algorithms. This study covers fundamental elements, qubits, gates, and circuits while emphasizing quantum parallelism as a distinctive feature. To illustrate efficiency gains, we explore notable algorithms like Shor’s factorization procedure and Grover’s search algorithm. Furthermore, we analyze recent advancements in quantum hardware, focusing on superconducting qubits, trapped ions, and photonic quantum computers, with an emphasis on implementations by IBM, Google, and other industry leaders. The discussion also addresses major challenges such as decoherence, noise, and the need for quantum error correction to achieve fault tolerance. Finally, we explore emerging applications in cryptography, optimization, quantum machine learning, and material simulation, and identify critical open problems that must be addressed to realize scalable, universal quantum computing. Our review underscores the transformative potential of quantum technologies across science and industry.
Compared with other physical platforms for Quantum Information Processing (QIP), photons have the unique advantages of travelling over free-space links or optical fiber with relatively low decoherence and of propagating with minimal coupling to the environment, while also being easily coupled to existing telecommunications technology. The R&D of various single photon and entangled photon generation technologies and the linear-optical implementations of quantum computation and communication are now surveyed and analyzed in a structured manner. We discuss photonic qubit degrees of freedom (polarization, time-bin, path, frequency and Orbital angular momentum), determine sources such as deterministic quantum-dot based sources and probabilistic sources based on spontaneous parametric down conversion (SPDC), and review architectures for Linear Optical Quantum Computation (LOQC), such as the Knill–Laflamme–Milburn (KLM) protocol and measurement-based quantum computation. Building on the density-matrix treatment of biexciton–exciton coherent dynamics in semiconductor quantum dots, we recapitulate the development of two-pulse coherent control schemes for generation and readout of photonic qubits in these systems and how they have been stretched towards optical control of logic gates. A comparative performance framework is established along the lines of source brightness, entanglement fidelity, photon indistinguishability, channel loss and scalability, which is illustrated by simulated performance plots and tabulated against literature numbers. Remaining hurdles in the field regarding issues of photon loss, scalable multiplexing, deterministic two-qubit interaction, and its integration with a quantum memory are highlighted and a perspective is provided on hybrid photonic–matter architectures for fault-tolerant quantum networks.
Under the label of optical quantum computing, there are a variety of protocols and experiments that use the quantum properties of light to achieve a computational advantage over classical computing machines. In this review, we describe some of the main implementations, which differ in the type of encoding and in how the computation is performed, whether using gates or cluster states. For each protocol, we describe advantages and challenges with an overview of the experimental results obtained, summarized in tables. Because of the great relevance achieved in this field, there is a section dedicated to non-universal quantum computation with photons, where boson sampling, variational quantum eigensolvers, and quantum machine learning applications are described. The aim is to give the reader the broadest overview of the applications where photons and their quantum properties play a key role in computation.
Photons are very promising resources for the whole field of quantum information processing. Bulk optics are currently not suitable for the realization of complex quantum optical schemes due to several drawbacks. The ongoing miniaturization of photonic structures due to the availability of sophisticated nanofabrication has provided huge opportunities for physical research of novel phenomena in nanophotonic systems and quantum technological applications. These include quantum information processing such as quantum computing, quantum communication, quantum metrology, and quantum sensing. Quantum communication is already quite advanced with several quantum key distribution (QKD) systems commercially available offering absolute security. Photonic quantum technologies are enabling breakthroughs in experimental tests of quantum physics and are close to practical applications of quantum technologies in imaging, secure communications, ultrasensitive metrology and quantum computing. There are several advantages to encoding a quantum bit onto a photon: the preparation and manipulation of single photons is easy, and the photonic qubit is a natural candidate for quantum imaging and communications. Furthermore, the qubit can be encoded in various degrees of freedom such as on the position, phase, time-bin, energy, angular momentum, polarization of the photon—or a combination of these. This flexibility leads to enormous opportunities for scientific exploration and in practical applications. This special issue on “Photonic Quantum Technologies” presents selected papers on photonic quantum systems from leading groups in the world. The articles represent a flavor of the cutting-edge research that is going on in this area. The studied material systems range from molecules to semiconductor quantum emitters. A major challenge is the design of telecom photon emitters with high efficiency, good yield, spectral purity, stability and the possibility of mass manufacture. We believe that this Special Issue will provide an overview of recent exciting developments in the field of photonic quantum technologies, serving to keep researchers up-to-date with the latest progress and spurring on new concepts in this field.
The last two decades have witnessed the rapid growth of quantum information processing as an active field of multidisciplinary science and technology. Tremendous theoretical and experimental progress has been made in quantum communication and quantum computation, and it seems clear that some of this progress will soon result in practical applications outside the laboratory. Quantum computation aims to build a quantum computer and to explore the origins and limits of quantum computing power. The original circuit model of a quantum computer, due to Deutsch, has a strong formal similarity with the circuit model of a classical computer. Both models depict a computation as processing via the wiring of a number of fiducial gates, each of which dynamically couples a small number of input (qu)bits in a controlled manner. In other words, unitary evolution is the basic mechanism of information processing in the circuit model of quantum computers. Starting with a given fixed state of many qubits, measurement based quantum computation (MBQC) processes information by applying a sequence of measurements to designated qubits in designated bases. This is remarkable, given the probabilistic nature of quantum measurement and the murky physical status of the dynamics of 'wavefunction collapse'. The final result of the computation is determined from the classical data of all the measurement outcomes. There exist two principle models: the teleportation based model and the 'one-way quantum computer' of 'cluster state computation'. The formalism of cluster states has proven to be a powerful way of describing the essential entanglement resources needed to perform quantum information processing. Unlike the circuit model, the MBQC models have no natural classical counterpart. There are a number of physical architectures for which measurements on the relevant systems are more easily performed than controlled unitary evolution. Moreover, it appears the MBQC models can offer novel computational benefits, especially regarding issues of fault-tolerance and parallelizability of algorithms. As a result, the theory of MBQC is now shaping the latest experimental proposals across the full spectrum of quantum information processing technologies. Naturally, understanding the scope of measurement based quantum computation is also yielding many insights into what is necessary (as opposed to merely sufficient) for us to harness the power of quantum computers. Quantum communication aims to offer efficient and secure ways of information exchange, directly based on quantum mechanics. Photons serve as natural information carriers, owing to their robustness against decoherence. A major problem for photonic communication systems is due to photon loss. It turns out that this difficulty can be overcome by the quantum repeater, a measurement-based technique. The basic idea is to divide the transmission channel into many segments and perform some quantum measurement on each segment. Additional measurement-based protocols and technologies have also been demonstrated to improve the quality of quantum communication, such as entanglement swapping and purification. The articles in this invited Focus Issue cover topics both in measurement-based quantum communication and computation, but the emphasis is given to quantum computation. A significant number of articles deal with the preparation of cluster states, the experimental realization of a one-way quantum computer, and the associated efficiency and benefits. We hope that you will find that some of these new results are of interest to you, and further hope that the issue will increase your interest in this interdisciplinary research field if you are a non-expert. Focus on Measurement-Based Quantum Information Processing Contents A proof-of-principle experiment of eliminating photon-loss errors in cluster states Wei-Bo Gao, Xiao-Qi Zhou, Jin Zhang, Tao Yang and Jian-Wei Pan Continuous variable multipartite entanglement and optical implementations of quantum communication networks Yimin Lian, Changde Xie and Kunchi Peng Generalized flow and determinism in measurement-based quantum computation Daniel E Browne, Elham Kashefi, Mehdi Mhalla and Simon Perdrix The entanglement of the four-photon cluster state C Schmid, N Kiesel, W Wieczorek and H Weinfurter Optical zeno gate: bounds for fault tolerant operation Patrick M Leung and Timothy C Ralph Towards experimental entanglement connection with atomic ensembles in the single excitation regime Julien Laurat, Chin-wen Chou, Hui Deng, Kyung Soo Choi, Daniel Felinto, Hugues de Riedmatten and H J Kimble Towards minimal resources of measurement-based quantum computation Simon Perdrix Experimental realization of a quantum game on a one-way quantum computer Robert Prevedel, Andre Stefanov, Philip Walther and Anton Zeilinger Fundamentals of universality in one-way quantum computation M Van den Nest, W Dur, A Miyake and H J Briegel Loss tolerant linear optical quantum memory by measurement-based quantum computing Michael Varnava, Daniel E Browne and Terry Rudolph Graph state generation with noisy mirror-inverting spin chains Stephen R Clark, Alexander Klein, Martin Bruderer and Dieter Jaksch One-way quantum computing in adecoherence-free subspace M S Tame, M Paternostro and M S Kim Cluster state preparation using gates operating at arbitrary success probabilities K Kieling, D Gross and J Eisert Topological fault-tolerance in cluster state quantum computation R Raussendorf, J Harrington and K Goyal Strategies for the preparation of large cluster states using non-deterministic gates Peter P Rohde and Sean D Barrett A repeat-until-success quantum computing scheme A Beige, Y L Lim and L C Kwek Efficient growth of complex graph states via imperfect path erasure Earl T Campbell, Joseph Fitzsimons, Simon C Benjamin and Pieter Kok Investigation of a single-photon source based on quantum interference T B Pittman, J D Franson and B C Jacobs Entanglement and local information access for graph states Damian Markham, Akimasa Miyake and Shashank Virmani The efficiencies of generating cluster states with weak nonlinearities Sebastien G R Louis, Kae Nemoto, W J Munro and T P Spiller A direct approach to fault-tolerance in measurement-based quantum computation via teleportation Marcus Silva, Vincent Danos, Elham Kashefi and Harold Ollivier Experimental ancilla-assisted qubit transmission against correlated noise T Yamamoto, R Nagase, J Shimamura, S K Ozdemir, M Koashi and N Imoto Terry Rudolph, Imperial College, London, UK Jian-Wei Pan, University of Science and Technology of China, Hefei, People's Republic of China and Universitat Heidelberg, Germany
This special issue is composed mainly of extended versions of talks and papers presented at the Seventh International Conference on Squeezed States and Uncertainty Relations held in Boston, Massachusetts, USA, on 4-8 June 2001. The conference was hosted by Boston University. The first meeting of this kind took place in College Park, Maryland, USA in 1991 as a small workshop. The second and third meetings were held in Moscow, Russia and Baltimore, USA in 1992 and 1993, respectively. The fourth meeting took place in Taiyuan, Shanxi Province, China in 1995 as an international conference sponsored by the International Union of Pure and Applied Physics. The fifth and sixth meetings were held in Balatonfüred, Hungary and Naples, Italy in 1997 and 1999, respectively. The eighth conference will take place in Puebla, Mexico in 2003. The contributions to this special issue address, in the first instance, the foundations of quantum mechanics and quantum optics, and several modern topics which have been intensively developed during the last decade. Quantum fluctuations, squeezing phenomena and different kinds of nonclassical states of light represent some of the most intensively discussed subjects. In particular, several papers deal with the properties of different kinds of `Schrödinger cat states' modelled in the simplest case by the even and odd coherent states. This paradigm exhibits in the most distinct form such cornerstone properties of quantum mechanics as the superposition principle, which manifests itself in quantum interference and entanglement. The quantum entanglement phenomenon has been studied intensively for the past few years due to its importance, for example, in quantum computer projects. In spite of the fact that this concept was known from the beginning of quantum mechanics, its properties, for example, the most appropriate entanglement measures, are still areas of discussion. Another related aspect of quantum nature is teleportation, which is also a subject of intensive study. The phenomena of dissipations and decoherence of the initial pure states are also very important, because fast decoherence can destroy all the advantages of quantum processes in quantum computing, teleportation and image processing. Due to these aspects, papers concerning various methods of controlling the decoherence, for example, by means of using different kinds of nonlinearities and deformations, are also presented here. From the very beginning of quantum mechanics, the uncertainty relations were basic inequalities distinguishing the classical and quantum worlds. Developing a deeper understanding of the well-known uncertainty relations and striving to find new ones (connected with purity parameters of states, entropy, etc.) is an important aspect of modern research. Recently developed tomographic methods of measuring quantum states (in terms of the Wigner function or other phase-space characteristics) have resulted in the formulation of quantum mechanics in which the standard probability density describes the quantum states in an alternative way to the density matrix or wave function. Some papers devoted to tomographic methods of measuring quantum states and studying different sorts of uncertainty relations are presented here. Group-theoretical methods of studying quantum-mechanical and quantum-optical phenomena, including the group-representation description of light polarization, also attract ongoing interest in the conference series discussions. We hope that this special issue will help readers to obtain an overview of the current trends in modern quantum optics, both experimental and theoretical. Young S Kim University of Maryland, College Park, USA Margarita A Man'ko P.N. Lebedev Physical Institute, Moscow, Russia Alexander Sergienko Boston University, MA, USA Guest Editors