Search

We present a commentary on the famous 1964 paper of John Bell that rules out the entire class of underlying hidden variable theories for quantum mechanics that are local., Comment: Commentary, 4 pages, to appear in the November issue of Resonance

Quantum physics modeling is technically complex and often non-descriptive. This article presents some approaches how quantum physical ideas can be represented by haptic models. For this purpose, models made from 3D printers, models made from paper strips, and models made from textiles are compared. A novelty is the use of zippers instead of paper strips, which can be easily ''cut'' and ''glued'' together. The models have been developed primarily with the aim of conveying and visualizing topological ideas with little basic mathematical knowledge., Comment: Translation, original title: Quantenphysik zum Anfassen - Von Papierstreifen zu Rei{\ss}verschl\"ussen. 4 pages, 7 figures

QBism has long recognized quantum states, POVM elements, Kraus operators, and even unitary operations to be cut from the same cloth: They express aspects of an agent's belief system concerning the consequences (for her) of actions she might take upon her external world. Such action-consequence pairs have conventionally been called "quantum measurements." The calculus of quantum theory is then viewed as an empirically motivated addition to Bayesian decision theory when brought to this notion of measurement. This radical approach has allowed QBism to eliminate conceptual problems that plague other interpretations of quantum mechanics. However, one issue has remained elusive: If a QBist does not believe in the existence of an ontic (agent-independent) dynamical variable evolving over time, why would there be any constraints on her quantum-state assignment in the absence of performing a measurement? Why would she introduce unitary or open-system quantum dynamics at all? Here, we present a representation theorem based on van Fraassen's reflection principle to answer these questions. Simply put, an agent's assignment of quantum dynamics represents her belief that a measurement action she is contemplating would not change her current odds for future gambles. A corollary to this approach is that one can make sense of "open-system dynamics" without the need to introduce an "environment with a measurement record," as is common in decoherence accounts of quantum measurement. QBism's understanding instead rests more fundamentally on an agent's beliefs about the system of interest (not system plus environment) and her judgments about measurements she might perform on that system. More broadly, this result establishes QBism's contention that measurement itself is the central concept of quantum theory and thus the framework upon which any future QBist ontology must hang., Comment: 32 pages, 2 figures. v2: Substantial additions to existing sections and a new section added. v3: Significant revisions and addition of material motivated by a referee. v4: This version is as it was re-submitted to Physical Review A before their editors deleted important historical information without permission and made other changes

We present the white paper developed during the QEYSSat 2.0 study, which was undertaken between June 2021 and March 2022. The study objective was to establish a technology road-map for a Canada-wide quantum network enabled by satellites. We survey the state-of-art in quantum communication technologies, identify the main applications and architectures, review the technical readiness levels and technology bottlenecks and identify a future mission scenario. We report the findings of a dedicated one-day workshop that included Canadian stakeholders from government, industry and academia to gather inputs and insights for the applications and technical road-map. We also provide an overview of the Quantum EncrYption and Science Satellite (QEYSSat) mission expected to launch in 2024-2025 and its anticipated outcomes. One of the main outcomes of this study is that developing the main elements for a Canada-wide quantum internet will have the highest level of impact, which includes Canada-wide entanglement distribution and teleportation. We present and analyze a possible future mission ('QEYSSat 2.0') that would enable a long range quantum teleportation across Canada as an important step towards this vision., Comment: 108 pages, 38 figures, white paper to be submitted to CJP

NEWFOCUS is an EU COST Action targeted at exploring radical solutions that could influence the design of future wireless networks. The project aims to address some of the challenges associated with optical wireless communication (OWC) and to establish it as a complementary technology to the radio frequency (RF)-based wireless systems in order to meet the demanding requirements of the fifth generation (5G) and the future sixth generation (6G) backhaul and access networks. Only 6G will be able to widely serve the exponential growth in connected devices (i.e., more than 500 billion) in 2030, real-time holographic communication, future virtual reality, etc. Space is emerging as the new frontier in 5 and 6G and beyond communication networks, where it offers high-speed wireless coverage to remote areas both in lands and sees. This activity is supported by the recent development of low-altitude Earth orbit satellite mega-constellations. The focus of this 2nd White Paper is on the use of OWC as an enabling technology for medium- and long-range links for deployment in (i) smart-cities and intelligent transportation systems; (ii) first- and last-mile access and backhaul/fronthaul wireless networks; (iii) hybrid free-space optics/RF adaptive wireless connections; (iv) space-to-ground, inter-satellite, ground-to-air, and air-to-air communications; and (v) underwater communications.

During the last two decades many new insights into the dynamics of strongly coupled quantum many-body systems have been obtained using gauge/gravity duality, with black holes often playing a universal role. In this white paper we summarize the results obtained and offer some outlook for future developments, including the ongoing mutually beneficial feedback loop with the study of more general, not necessarily holographic, quantum many-body systems., Comment: 18+24 pages

Black holes provide a window into the microscopic structure of spacetime in quantum gravity. Recently the quantum information contained in Hawking radiation has been calculated, verifying a key aspect of the consistency of black hole evaporation with quantum mechanical unitarity. This calculation relied crucially on recent progress in understanding the emergence of bulk spacetime from a boundary holographic description. Spacetime wormholes have played an important role in understanding the underpinnings of this result, and the precision study of such wormholes, in this and other contexts, has been enabled by the development of low-dimensional models of holography. In this white paper we review these developments and describe some of the deep open questions in this subject. These include the nature of the black hole interior, potential applications to quantum cosmology, the gravitational explanation of the fine structure of black holes, and the development of further connections to quantum information and laboratory quantum simulation., Comment: 16 + 17 pages. v2: references added

It has recently been theoretically shown that Quantum Memories (QM) could enable truly global quantum networking when deployed in space thereby surpassing the limited range of land-based quantum repeaters. Furthermore, QM in space could enable novel protocols and long-range entanglement and teleportation applications suitable for Deep-Space links and extended scenarios for fundamental physics tests. In this white paper we will make the case for the importance of deploying QMs to space, and also discuss the major technical milestones and development stages that will need to be considered., Comment: Toptical white paper submitted to National Academies of Sciences, Engineering and Medicine's Decadal Survey on Biological and Physical Sciences Research in Space 2023-2032

Progress in the quantum readout and control of mechanical devices from single atoms to large masses may enable a first generation of experiments probing the gravitational interaction in the quantum regime, conceivably within the next decade. In this Snowmass whitepaper, we briefly outline the possibilities and challenges facing the realization of these experiments. In particular, we emphasize the need for detailed theories of modifications to the usual effective QFT of gravitons in the infrared regime $E/L^3 \ll m_{\rm Pl}/\ell_{\rm Pl}^3$ in which these experiments operate, and relations to possible UV completions., Comment: Short contribution to Snowmass 2021. 5 pages, 1 figure. v2: typo fixed

We present a summary of recent progress and remaining challenges in applying the methods and ideas of quantum information theory to the study of quantum field theory and quantum gravity. Important topics and themes include: entanglement entropy in QFTs and what it reveals about RG flows, symmetries, and phases; scrambling, information spreading, and chaos; state preparation and complexity; classical and quantum simulation of QFTs; and the role of information in holographic dualities. We also highlight the ways in which quantum information science benefits from the synergy between the fields., Comment: 24 + 21 pages; contribution to Snowmass 2021; v2: references added

Quantum computing offers a new paradigm for advancing high-energy physics research by enabling novel methods for representing and reasoning about fundamental quantum mechanical phenomena. Realizing these ideals will require the development of novel computational tools for modeling and simulation, detection and classification, data analysis, and forecasting of high-energy physics (HEP) experiments. While the emerging hardware, software, and applications of quantum computing are exciting opportunities, significant gaps remain in integrating such techniques into the HEP community research programs. Here we identify both the challenges and opportunities for developing quantum computing systems and software to advance HEP discovery science. We describe opportunities for the focused development of algorithms, applications, software, hardware, and infrastructure to support both practical and theoretical applications of quantum computing to HEP problems within the next 10 years., Comment: A contribution to the 2021 Snowmass Proceedings, Computational Frontier, Topical Group on Quantum Computing

In the paper "Quantum theory cannot consistently describe the use of itself" by D. Frauchiger and R. Renner an atempt is made at proving a "no-go theorem" that states that either quantum theory cannot be universally applied, even to macroscopic systems, or some very intuitive properties concerning recursive reasoning and uniquenes of physical values must be false. In this paper, we give a concise description of the paper's result, and expose a detail in the proof., Comment: 3 pages

We present an English translation of Erwin Schr\"odinger's paper on "On the Reversal of the Laws of Nature". In this paper Schr\"odinger analyses the idea of time reversal of a diffusion process. Schr\"odinger's paper acted as a prominent source of inspiration for the works of Bernstein on reciprocal processes and of Kolmogorov on time reversal properties of Markov processes and detailed balance. The ideas outlined by Schr\"odinger also inspired the development of probabilistic interpretations of quantum mechanics by F\'enyes, Nelson and others as well as the notion of "Euclidean Quantum Mechanics" as probabilistic analogue of quantization. In the second part of the paper Schr\"odinger discusses the relation between time reversal and statistical laws of physics. We emphasize in our commentary the relevance of Schr\"odinger's intuitions for contemporary developments in statistical nano-physics., Comment: Revised version. 24 pages, 2 figures. Introduction 2 pages, Translation of the german original pag. 3-10, Commentary pag 10-22

On the seminal paper written by Einstein, Podolsky and Rosen [1], a critique to the completeness of quantum mechanics was posed. Part of the critique consisted in the following argument: if quantum mechanics is complete, then, two physical quantities, with non-commuting operators, can have simultaneous reality. In this paper I aim to provide a pedagogical approach to the notions used in the EPR's argument., Comment: 12 pages, 2 figures

The single-mode bosonic channel is addressed with classical interference in the modulation and with side information at the transmitter. This model can viewed as the quantum counterpart of the classical random-parameter Gaussian channel. Based on Costa's writing-on-dirty-paper result (1983), the effect of the channel parameter can be canceled even when the decoder has no side information, and regardless of the input power constraint. For both homodyne and heterodyne detection with a coherent-state protocol, the model reduces to a classical channel with either real or complex-valued Gaussian noise. Thereby, by applying Costa's dirty paper coding strategy, we observe that the effect of the classical interference can be canceled for those channels as well. Then, we consider the bosonic channel with joint detection, for which the classical results do not apply, and derive a dirty-paper coding lower bound. Furthermore, considering the special case of a pure-loss bosonic channel, we demonstrate that the optimal coefficient for dirty paper coding is not necessarily the MMSE estimator coefficient as in the classical setting.

We have discovered several severe errors in the recent paper M. Hu, K. Guo, Q. Yu, Z. Zhang [Superlattices and Microstructures, 122 (2018) 538-547]. Specifically, we demonstrate that both the solution of the Schr\"odinger equation and the bound-state wave functions used in the paper are incorrect.

We present a translation of the 1933 paper by R. F\"urth in which a profound analogy between quantum fluctuations and Brownian motion is pointed out. This paper opened in some sense the way to the stochastic methods of quantization developed almost 30 years later by Edward Nelson and others., Comment: 23 pages, 4 figures

An attempt is made to avoid the difficulty of the infinite reaction of the electron on itself, which occurs in quantum electrodynamics, by introducing difference equations instead of differential equations. This vision allows the difficulty of the relativistic wave equation emphasised by Klein, for example, to be essentially eliminated., Comment: 4 pages, translated from: V. Ambarzumian and D. Ivanenko (1930). Zur Frage nach Vermeidung der unendlichen Selbstr\"uckwirkung des Elektrons. Zeitschrift f\"ur Physik, Vol.64: p.563--567

The Gedankenexperiment advanced by Frauchiger and Renner in their "Nature" paper was based on an implicit assumption that one can synchronize stochastic measurement intervals between two non-interacting systems. This hypothesis, the author demonstrates, is equivalent to the complete entanglement of these systems. Consequently, Frauchiger and Renner's postulate Q is meaningless and needs to be narrowed. Accurate reformulation of the postulate Q1 does not entail any paradoxes with measurement. This paper is agnostic concerning particular interpretations of quantum mechanics. Nor does it refer to the "collapse of the wavefunction."

Quantum Teleportation is the key communication functionality of the Quantum Internet, allowing the "transmission' of qubits without either the physical transfer of the particle storing the qubit or the violation of the quantum mechanical principles. Quantum teleportation is facilitated by the action of quantum entanglement, a somewhat counter-intuitive physical phenomenon with no direct counterpart in the classical word. As a consequence, the very concept of the classical communication system model has to be redesigned to account for the peculiarities of quantum teleportation. This re-design is a crucial prerequisite for constructing any effective quantum communication protocol. The aim of this manuscript is to shed light on this key concept, with the objective of allowing the reader: i) to appreciate the fundamental differences between the transmission of classical information versus the teleportation of quantum information; ii) to understand the communications functionalities underlying quantum teleportation, and to grasp the challenges in the design and practical employment of these functionalities; iii) to acknowledge that quantum information is subject to the deleterious effects of a noise process termed as quantum decoherence. This impairment has no direct counterpart in the classical world; iv) to recognize how to contribute to the design and employment of the Quantum Internet., Comment: Invited Paper, 53 pages, 18 figures, 2 tables, double column

Recently Dzhafarov and Kon published the paper advertising the possibility to use the coupling technique of classical probability theory to model incompatible observables in quantum physics and quantum-like models of psychology. Here I present comments on this paper by stressing advantages and disadvantages.

In May of 1935, Einstein published with two co-authors the famous EPR-paper about entangled particles, which questioned the completeness of Quantum Mechanics by means of a gedankenexperiment. Only one month later, he published a work that seems unconnected to the EPR-paper at first, the so called Einstein-Rosen-paper, that presented a solution of the field equations for particles in the framework of general relativity. Both papers ask for the conception of completeness in a theory and, from a modern perspective, it is easy to believe that there is a connection between these topics. We question whether Einstein might have considered that a correlation between nonlocal features of Quantum Mechanics and the Einstein-Rosen bridge can be used to explain entanglement. We analyse this question by discussing the used conceptions of "completeness," "atomistic structure of matter," and "quantum phenomena." We discuss the historical embedding of the two works and the context to modern research. Recent approaches are presented that formulate an EPR=ER principle and claim an equivalence of the basic principles of these two papers., Comment: 43 pages, typos corrected

This is a brief comment on the paper "Quantum mechanics needs no consciousness" by Shan Yu and Danko Nikolic [1]. Yu and Nikolic argue that the "consciousness causes collapse hypothesis" interpretation of quantum mechanics, or CCCH, can be falsified by a particular experimental setup. This claim is incorrect and the cause of the error appears to be a confusion over where and when a collapse can be assumed to occur., Comment: Dr Yu and Dr Nikolic have both seen this article. Shan Yu has said he is happy for the comment to be posted in its current form but wishes it to be known that he does not agree with my conclusions

This volume covers one of the most thrilling two-year periods in twentieth-century physics, as matrix mechanics - developed chiefly by W. Heisenberg, M. Born, and P. Jordan - and wave mechanics - developed by E. Schr\"odinger - supplanted the earlier quantum theory. The almost one hundred writings by Einstein, of which a third have never been published, and the more than thirteen hundred letters show Einstein's immense productivity and hectic pace of life. Einstein quickly grasps the conceptual peculiarities involved in the new quantum mechanics, such as the difference between Schr\"odinger's wave function and a field defined in spacetime, or the emerging statistical interpretation of both matrix and wave mechanics. Inspired by correspondence with G. Y. Rainich, he investigates with Jakob Grommer the problem of motion in general relativity, hoping for a hint at a new avenue to unified field theory. Einstein falls victim to scientific fraud when, in a collaboration with E. Rupp, he becomes convinced that the latter's experiments, aimed at deciding whether excited atoms emit light instantaneously (in quanta) or in a finite time (in waves), confirm a wave-theoretic explanation. While it was known that the teenage Einstein had been romantically involved with Marie Winteler in 1895, newly discovered documents reveal that his love for Marie was rekindled in 1909-10 while he was still married to Mileva Mari\'c. The 1925 Locarno Treaties renew Einstein's optimism in European reconciliation. He backs the `International manifesto against compulsory military service' and continues his participation in the League of Nations' International Committee on Intellectual Cooperation. He remains intensely committed to the shaping of the Hebrew University in Jerusalem, although his enthusiasm for this cause is sorely tested., Comment: Corresponding Editor: Dennis Lehmkuhl (lehmkuhl@caltech.edu). The complete volume is available from Princeton University Press: https://press.princeton.edu/titles/11327.html

We address Gillis' recent criticism [arXiv:1506.05795] of a series of papers (by different combinations of the present authors) on formulations of Bell's theorem. Those papers intended to address an unfortunate gap of communication between two broad camps in the quantum foundations community that we identify as "operationalists" and "realists". Here, we once again urge the readers to approach the question from an unbiased standpoint, and explain that Gillis' criticism draws too heavily on the philosophical inclinations of one side of that debate -- the realist camp. As part of that explanation we discuss intuition versus proof, look again at Bell's formalizations of locality, and correct misstatements by Gillis of our views, and those of Bell and Einstein., Comment: 10 pages. To be published in International Journal of Quantum Foundations

Wiseman has claimed that Bell was wrong in stating that determinism was inferred rather than assumed in the summary of the EPR argument in his 1964 paper. The reply of Wiseman and his co-authors to my comment misstates my reasons for disputing this point, and fails to address the central criticism that their claim is based on a seriously flawed formalization of Bell's argument deriving from an unreasonably strong interpretation of the the terms, 'influence', 'affect', and 'depend on'., Comment: 6 pages

In a recent series of papers Wiseman, Cavalcanti, and Rieffel have outlined and contrasted two different views about what we now call Bell's theorem. They also assert that Bell presented these two different versions at different times. This view is clearly at odds with the detailed explanation that Bell himself gave in his later writings. A careful examination of the historic 1964 paper in context shows clearly that Bell's own later interpretation is the correct one., Comment: 16 pages. Revised. Published in International Journal of Quantum Foundations

Confined quantum systems involving $N$ identical interacting fermions are found in many areas of physics, including condensed matter, atomic, nuclear and chemical physics. In a previous series of papers, a manybody perturbation method that is applicable to both weakly and strongly-interacting systems of bosons has been set forth by the author and coworkers. A symmetry invariant perturbation theory was developed which uses group theory coupled with the dimension of space as the perturbation parameter to obtain an analytic correlated wave function through first order for a system under spherical confinement with a general two-body interaction. In the present paper, we extend this formalism to large systems of fermions, circumventing the numerical demands of applying the Pauli principle by enforcing the Pauli principle on paper. The method does not scale in complexity with $N$ and has minimal numerical cost. We apply the method to a unitary Fermi gas and compare to recent Monte Carlo values., Comment: arXiv admin note: text overlap with arXiv:cond-mat/0607600

When the magnetic vector potential is expressed in terms of the magnetic field it, is found to be explicitly non-local in space. This gives support to the conclusions of Aharonov et al. in a recent comment, that the Aharonov-Bohm effect may be interpreted as being either due to a local gauge potential or else due to non-local gauge-invariant fields but not due to local gauge-invariant fields., Comment: 6 pages pdf

In the aforementioned paper, the authors claim that my results concerning the consequent application of Garrett's method for obtaining approximate expressions of the bound states energy of a particle in a finite rectangular well are incorrect. I shall show hat this is not the case, and demonstrate that both their and my results lead to the same conclusion, i.e. that the consequent application of Garrett's method is equivalent to Barker's approximation., Comment: 3 pages, no figure

We present a summary for non-specialists of loop quantum gravity as part of the modern legacy of the series of papers by Arnowitt, Deser and Misner circa 1960., Comment: 7 pages, prepared for Classical and Quantum Gravity for its "Milestones of General Relativity" focus issue to be published during the Centenary Year of GR

The authors of the recent paper [1] boldly claim to discover a new fully quantum approach to foundation of statistical mechanics: "Our conceptually novel approach is free of mathematically ambiguous notions such as probability, ensemble, randomness, etc." The aim of this note is to show that this approach is neither specific for quantum systems nor really conceptually different from the standard textbook arguments supporting microcanonical or canonical ensembles in statistical mechanics., Comment: 2 pages, no figures

Yes. That is my polemical reply to the titular question in Travis Norsen's self-styled "polemical response to Howard Wiseman's recent paper." Less polemically, I am pleased to see that on two of my positions --- that Bell's 1964 theorem is different from Bell's 1976 theorem, and that the former does not include Bell's one-paragraph heuristic presentation of the EPR argument --- Norsen has made significant concessions. In his response, Norsen admits that "Bell's recapitulation of the EPR argument in [the relevant] paragraph leaves something to be desired," that it "disappoints" and is "problematic". Moreover, Norsen makes other statements that imply, on the face of it, that he should have no objections to the title of my recent paper ("The Two Bell's Theorems of John Bell"). My principle aim in writing that paper was to try to bridge the gap between two interpretational camps, whom I call 'operationalists' and 'realists', by pointing out that they use the phrase "Bell's theorem" to mean different things: his 1964 theorem (assuming locality and determinism) and his 1976 theorem (assuming local causality), respectively. Thus, it is heartening that at least one person from one side has taken one step on my bridge. That said, there are several issues of contention with Norsen, which we (the two authors) address after discussing the extent of our agreement with Norsen. The most significant issues are: the indefiniteness of the word 'locality' prior to 1964; and the assumptions Einstein made in the paper quoted by Bell in 1964 and their relation to Bell's theorem., Comment: 13 pages (arXiv version) in http://www.ijqf.org/archives/2093

This note analyzes the physical basis of J.R. England's paper "Statistical physics of self-replication." [J. Chem. Phys. {\bf 139}, 121923(2013)]. We follow England's use of time-reversal symmetry but replace stochastic by deterministic dynamics, and introduce a definition of metastable states based on equilibrium statistical mechanics. We rederive England's detailed balance relation and obtain another similar relation which appears more natural and remains valid for quantum systems. The detailed balance relations are based on serious physical ideas, and either of them can be used for England's biological discussion. This biological discussion does of course deserve further scrutiny., Comment: 11 pages, 1figure

We review in this article the role which the work of Pauli and Weisskopf played in formulating a quantum field theory of spinless particles. To make our computations as transparent as possible, we offer a physicist's derivation of the Klein-Gordon-Fock equation. Since invariant functions play a significant part in our paper, we will discuss them in great detail. We emphasize Pauli's and Weisskopf's view that Dirac's hole theory is totally obsolete in formulating a consistent quantum field theory, be it for scalar or spinor particles. As an important example we present the calculation for producing charged scalar particles in an external electric field.

The authors of a recent paper [Phys. Rev. Lett. 113, 120404 (2014)] suggest that "weak values are not inherently quantum but rather a purely statistical feature of pre- and postselection with disturbance". We argue that this claim is erroneous, since such values require averaging with distributions which change sign. This type of averaging arises naturally in quantum mechanics, but may not occur in classical statistics., Comment: 3 pages, no figures

I introduce the concept of *entanglement entropy* (as it's now called) and point out that it follows an *area law* which renders it a suitable source of black hole entropy. I also suggest to conceive the latter as residing on the horizon at approximately one bit per "Planckian plaquette"., Comment: This is a verbatim copy of a paper published originally in 1983, before the existence of "arXiv". Also available at http://www.pitp.ca/personal/rsorkin/some.papers/31.padova.entropy.pdf

We criticize the thesis exposed the paper 'Seeded quantum FEL at 478 keV' AIP Conf. Proc. 'Light at Extreme Intensities,' 1462 173-176 (2012), which presents the possibility of producing gamma rays at 478 KeV by means of a seeded quantum FEL driven by an electron beam at 125 MeV, current I=40 A, interacting with an infrared laser. We show that, in the case analyzed, the FEL Pierce parameter has a value two orders of magnitude less than what claimed in the paper in question, overturning the conclusions of the analysis.

In [A.W. Harrow and R.A. Low, Commun. Math. Phys. 291, 257-302 (2009)], it was shown that a quantum circuit composed of random 2-qubit gates converges to an approximate quantum 2-design in polynomial time. We point out and correct a flaw in one of the paper's main arguments. Our alternative argument highlights the role played by transpositions induced by the random gates in achieving convergence.

According to Richard Feynman, the adventure of our science of physics is a perpetual attempt to recognize that the different aspects of nature are really different aspects of the same thing. It is therefore interesting to combine some, if not all, of Feynman's papers into one. The first of his three papers is on the ``rest of the universe'' contained in his 1972 book on statistical mechanics. The second idea is Feynman's parton picture which he presented in 1969 at the Stony Brook conference on high-energy physics. The third idea is contained in the 1971 paper he published with his students, where they show that the hadronic spectra on Regge trajectories are manifestations of harmonic-oscillator degeneracies. In this report, we formulate these three ideas using the mathematics of two coupled oscillators. It is shown that the idea of entanglement is contained in his rest of the universe, and can be extended to a space-time entanglement. It is shown also that his parton model and the static quark model can be combined into one Lorentz-covariant entity. Furthermore, Einstein's special relativity, based on the Lorentz group, can also be formulated within the mathematical framework of two coupled oscillators., Comment: 31 pages, 6 figures, based on the concluding talk at the 3rd Feynman Festival (Collage Park, Maryland, U.S.A., August 2006), minor corrections

In the preceding paper quant-ph/0312060 we considered a general model of an atom with n energy levels interacting with n-1 external laser fields and constructed a Rabi oscillation in the case of n =3, 4 and 5. In the paper we present a systematic method getting along with computer to construct a Rabi oscillation in the general case., Comment: Latex ; 10 pages ; 2 figures

The laws of physics play a crucial role in the security of quantum key distribution (QKD). This fact has often been misunderstood as if the security of QKD would be based only on the laws of physics. As the experts know well, things are more subtle. We review the progresses in practical QKD focusing on (I) the elements of trust that are common to classical and quantum implementations of key distribution; and (II) some threats to security that have been highlighted recently, none of which is unredeemable (i.e., in principle QKD can be made secure). This leads us to guess that the field, similar to non-quantum modern cryptography, is going to split in two directions: those who pursue practical devices may have to moderate their security claims; those who pursue ultimate security may have to suspend their claims of usefulness., Comment: Small changes and clarifications. This paper still describes the situation of QKD in June 2009, when its first version was sent out; we have mentioned a few developments in a Note added in proof. Accepted in a special issue of "Theoretical Computer Science" (T. Mor and R. Renner, editors)

In contemporary cryptographic systems, secret keys are usually exchanged by means of methods, which suffer from mathematical and technology inherent drawbacks. That could lead to unnoticed complete compromise of cryptographic systems, without a chance of control by its legitimate owners. Therefore a need for innovative solutions exists when truly and reliably secure transmission of secrets is required for dealing with critical data and applications. Quantum Cryptography (QC), in particular Quantum Key Distribution (QKD) can answer that need. The business white paper (BWP) summarizes how secret key establishment and distribution problems can be solved by quantum cryptography. It deals with several considerations related to how the quantum cryptography innovation could contribute to provide business effectiveness. It addresses advantages and also limitations of quantum cryptography, proposes a scenario case study, and invokes standardization related issues. In addition, it answers most frequently asked questions about quantum cryptography.

In July 1925 Heisenberg published a paper [Z. Phys. 33, 879-893 (1925)] which ended the period of `the Old Quantum Theory' and ushered in the new era of Quantum Mechanics. This epoch-making paper is generally regarded as being difficult to follow, perhaps partly because Heisenberg provided few clues as to how he arrived at the results which he reported. Here we give details of calculations of the type which, we suggest, Heisenberg may have performed. We take as a specific example one of the anharmonic oscillator problems considered by Heisenberg, and use our reconstruction of his approach to solve it up to second order in perturbation theory. We emphasize that the results are precisely those obtained in standard quantum mechanics, and suggest that some discussion of the approach - based on the direct computation of transition amplitudes - could usefully be included in undergraduate courses in quantum mechanics., Comment: 24 pages, no figures, Latex, submitted to Am. J. Phys

This paper has been withdrawn by the author. My counter-example to the Koashi-Imoto decomposition can be avoided by setting j=1 for all states, at fixed l., Comment: This paper has been withdrawn by the author

In a recent paper [quant-ph/0607008] Lapaire and Sipe argue that one can discuss interference experiments using entangled photons in terms of single photon wave functions. Furthermore, they argue that contrary to the claim of the authors of the postponed compensation experiment [2], the single photon wave functions overlap on the beam splitter when interference is observed. In this comment, we show that the claim in [2] is correct and we argue that the idea of single photon wave functions in entangled states is misleading.

Schulz claims to have constructed an actively local stochastic theory which violates Bell's inequality. This is false.

These are the commentaries for a volume of reprints of my selected papers with commentaries that I am preparing for publication by World Scientific. Contents: Preface; (1)Early Years, and Condensed Matter Physics; (2) High Energy Neutrino Reactions, PCAC Relations, and Sum Rules; (3) Anomalies: Chiral Anomalies and Their Nonrenormalization, Perturbative Corrections to Scaling, and Trace Anomalies to All Orders; (4) Quantum Electrodynamics; (5) Particle Phenomenology and Neutral Currents; (6) Gravitation; (7) Non-Abelian Monopoles, Confinement Models, and Chiral Symmetry Breaking; (8) Overrelaxation for Monte-Carlo and Other Algorithms; (9) Quaternionic Quantum Mechanics, Trace Dynamics, and Emergent Quantum Theory; (10) Where Next?, Comment: Latex 115p; Final version. Book version will differ in reference format and indexing; version 3 differs from version 2 by minor copy-editing corrections

In Bohm's version of the EPR gedanken experiment, the spin of the second particle along any vector is minus the spin of the other particle along the same vector. It seems that either the choice of vector along which one projects the spin of the first particle influences at superluminal speed the state of the second particle, or naive realism holds true i.e., the projections of the spin of any EPR particle along all the vectors are determined before any measurement occurs). Naive realism is negated by Bell's theory that originated and is still most often presented as related to non-locality, a relation whose necessity has recently been proven to be false. I advocate here that the solution of the apparent paradox lies in the fact that the spin of the second particle is determined along any vector, but not along all vectors. Such an any-all distinction was already present in quantum mechanics, for instance in the fact that the spin can be measured along any vector but not at once along all vectors, as a result of the Uncertainty Principle. The time symmetry of the any-all distinction defended here is in fact reminiscent of (and I claim, due to) the time symmetry of the Uncertainty Principle described by Einstein, Tolman, and Podolsky in 1931, in a paper entitled ``Knowledge of Past and Future in Quantum Mechanics" that is enough to negate naive realism and to hint at the any-all distinction. A simple classical model is next built, which captures aspects of the any-all distinction: the goal is of course not to have a classical exact model, but to provide a caricature that might help some people., Comment: 4 pages, no figure. The text has been changed to reflect comments and suggestions that I got during lectures and further comments and suggestions made by colleagues. The wording has been changed, as the one in the new version of quant-ph/0501030

The concept of $\e$-randomizing quantum channels has been introduced by Hayden, Leung, Shor and Winter in connection with approximately encrypting quantum states. They proved using a discretization argument that sets of roughly $d \log d$ random unitary operators provide examples of such channels on $\C^d$. We show that a simple trick improves the efficiency of the argument and reduces the number of unitary operators to roughly $d$.