Quantum memory is essential for the development of many devices in quantum information processing, including a synchronization tool that matches various processes within a quantum computer, an identity quantum gate that leaves any state unchanged, and a mechanism to convert heralded photons to on-demand photons. In addition to quantum computing, quantum memory will be instrumental for implementing long-distance quantum communication using quantum repeaters. The importance of this basic quantum gate is exemplified by the multitude of optical quantum memory mechanisms being studied, such as optical delay lines, cavities and electromagnetically induced transparency, as well as schemes that rely on photon echoes and the off-resonant Faraday interaction. Here, we report on state-of-the-art developments in the field of optical quantum memory, establish criteria for successful quantum memory and detail current performance levels.

Optical quantum memory

The distribution of quantum states over long distances is limited by photon loss. Straightforward amplification as in classical telecommunications is not an option in quantum communication because of the no-cloning theorem. This problem could be overcome by implementing quantum repeater protocols, which create long-distance entanglement from shorter-distance entanglement via entanglement swapping. Such protocols require the capacity to create entanglement in a heralded fashion, to store it in quantum memories, and to swap it. One attractive general strategy for realizing quantum repeaters is based on the use of atomic ensembles as quantum memories, in combination with linear optical techniques and photon counting to perform all required operations. Here we review the theoretical and experimental status quo of this very active field. We compare the potential of different approaches quantitatively, with a focus on the most immediate goal of outperforming the direct transmission of photons.

Quantum repeaters based on atomic ensembles and linear optics

Quantum memory is important to quantum information processing in many ways: a synchronization device to match various processes within a quantum computer, an identity quantum gate that leaves any state unchanged, and a tool to convert heralded photons to photons-on-demand. In addition to quantum computing, quantum memory would be instrumental for the implementation of long-distance quantum communication using quantum repeaters. The importance of this basic quantum gate is exemplified by the multitude of optical quantum memory mechanisms being studied: optical delay lines, cavities, electromagnetically-induced transparency, photon-echo, and off-resonant Faraday interaction. Here we report on the state-of-the-art in the field of optical quantum memory, including criteria for successful quantum memory and current performance levels.

We perform a review of various approaches to the implementation of quantum memories, with an emphasis on activities within the quantum memory sub-project of the EU integrated project “Qubit Applications”. We begin with a brief overview over different applications for quantum memories and different types of quantum memories. We discuss the most important criteria for assessing quantum memory performance and the most important physical requirements. Then we review the different approaches represented in “Qubit Applications” in some detail. They include solid-state atomic ensembles, NV centers, quantum dots, single atoms, atomic gases and optical phonons in diamond. We compare the different approaches using the discussed criteria.

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Quantum memories - A review based on the European integrated project “Qubit Applications (QAP)”

This is the fifth edition of H Dieter Zeh's classic text on the physical foundations of time-irreversibility in the phenomena. A forerunner of this book was the 1984 German text 'Die Physik der Zeitrichtung' of about 80 pages, which appeared as volume 200 in the Springer series Lecture Notes in Physics. It was soon followed by a largely revised and extended English edition of about twice the length. Since then each new edition has been thoroughly revised and, edition by edition, new topics and chapters have been added. As the author says in the introduction: 'The prime intention of this book is to discuss the relations between various arrows of time, and to search for a universal master arrow'. Correspondingly, after a short chapter on 'the physical concept of time', the author systematically discusses in the remaining five chapters the time arrows in electromagnetic radiation theory, in thermodynamics, in quantum mechanics, in black-hole physics and cosmology, and in quantum cosmology. The chapters on thermodynamics and quantum mechanics slightly outweigh the others in terms of length. The fifth edition now includes two new section on 'cosmic probabilities and history' and 'quantum computers', and the section on the 'expansion of the universe' has been restructured and extended. Other changes concentrate on the sections on radiation damping, decoherence, interpretation of quantum theory, and quantum cosmology. It should also be mentioned that the author maintains a regularly updated website for the book at www.time-direction.de. The reading is always highly stimulating and uses results and ideas from a very broad range of physics, with interspersed historical and philosophical comments. Somehow outstanding and of particular interest is the chapter on quantum cosmology, which raises novel interpretational issues that cannot be found in any other textbook I know of on time asymmetry. As regards the mathematical prerequisites, the reader is assumed to have some knowledge on Green functions, special-relativistic electrodynamics, Hamiltonian mechanics, the formalisms of phenomenological and statistical thermodynamics, quantum mechanics, and general relativity. At some rare places the author avowedly shows a certain impatience with mathematical details, in particular if they do not fit with his expectations based on personal intuition. This stands in peculiar contrast to the conceptual depth and thoroughness that otherwise characterizes most of the book. Quite generally, the reader should be prepared to encounter provoking and partially controversial statements, but this should not be too surprising in such a complex and partially highly speculative field. All this certainly sets high standards for the reader's intellectual independence and maturity, but this is definitely in accord with the philosophy of Springer's 'Frontiers Collection'. On the other hand, from personal experience I can say that already the original German text has been very popular with beginning graduate students who had a serious interest in foundational issues. (I obtained my first personal copy as a birthday present from a fellow student.) This is essentially due to the fact that the author's genuine urge to understand, rather than just describe, gives the text a characteristic flair of freshness and authenticity to which beginning researchers are particularly susceptible. Being provocative at places is just part of that. This spirit has essentially survived the various editions, thanks to the author's constant efforts to improve the existing presentation, and also by adding modern topics. However, the negative side of this constant streamlining of presentation according to the author's evolving understanding is that it tends to amplify the already existing idiosyncratic tendencies, sometimes resulting in cryptic remarks which fail their intended clarifying purpose. This already applies to the fairly steep introduction, where the author attempts to clarify in a few lines the central issue of what it means (structurally) to say that a particular dynamical law is (a)symmetric under time reversal. Here I think it would definitely be necessary to give more detailed explanations, e.g., by elaborating on the author's own discussion in 'Note on the time reversal asymmetry of equations of motion' (1999 Found. Phys. Lett. 12 193?96). Also, the structural relations between the operation of time reversal and other space-time symmetries are hardly mentioned. This may be excused insofar as T symmetry in quantum-field theory is not generally addressed in this book (which itself may be regretted), but as part of a structural characterization of the operation of time reversal it would certainly have been useful. The books provides an extensive bibliography which seems (as far as I can tell) fairly complete, though sometimes and for no obvious reason, preprints posted on web-archives are cited instead of the corresponding journal articles. Each reference comes with the page numbers of where it is cited in the text, which is very useful indeed. The positive aspects by far outweigh the critical ones. The new edition of H Dieter Zeh's classic text is highly recommended to anybody interested in foundational issues and ready to take the challenge to follow the sometimes intuitive approach of someone who has spent much time and effort to understand the conceptual intricacies and variations of this indisputably difficult and demanding subject.

https://iopscience.iop.org/article/10.1088/0264-9381/25/20/209003/pdf

The Physical Basis of the Direction of Time

We examine a quantum memory scheme based on controllable dephasing of atomic coherence of a non-resonant, inhomogeneously broadened Raman transition. We show that it generalizes the physical conditions for time-reversible interaction between light and atomic ensembles from weak to strong fields and from linear to non-linear interactions. We also develop a unified framework for different realizations exploiting either controlled reversible inhomogeneous broadening or atomic frequency combs, and discuss new aspects related to storage and manipulation of quantum states.

S.A. Moiseev

Papers

Optical quantum memory with generalized time-reversible atom-light interactions