Quantum technology: the second quantum revolution

We are currently in the midst of a second quantum revolution. The first quantum revolution gave us new rules that govern physical reality. The second quantum revolution will take these rules and use them to develop new technologies. In this review we discuss the principles upon which quantum technology is based and the tools required to develop it. We discuss a number of examples of research programs that could deliver quantum technologies in coming decades including: quantum information technology, quantum electromechanical systems, coherent quantum electronics, quantum optics and coherent matter technology.     

Quantum statistical mechanics of vortices in type II superconductors

We critically review recent developments in the quantum statistical mechanics and in the quantum dynamics of the vortex system in high temperature- and in conventional high-resistivity thin-film superconductors.     

Quantum Griffiths Effects and Smeared Phase Transitions in Metals: Theory and Experiment

In this paper, we review theoretical and experimental research on rare region effects at quantum phase transitions in disordered itinerant electron systems. After summarizing a few basic concepts about phase transitions in the presence of quenched randomness, we introduce the idea of rare regions and discuss their importance. We then analyze in detail the different phenomena that can arise at magnetic quantum phase transitions in disordered metals, including quantum Griffiths singularities, smeared phase transitions, and cluster-glass formation. For each scenario, we discuss the resulting phase diagram and summarize the behavior of various observables. We then review several recent experiments that provide examples of these rare region phenomena. We conclude by discussing limitations of current approaches and open questions.     

Cold trapped ions as quantum information processors

In this tutorial we review the physical implementation of quantum computing using a system of cold trapped ions. We discuss systematically all the aspects for making the implementation possible. Firstly, we go through the loading and confining of atomic ions in the linear Paul trap, then we describe the collective vibrational motion of trapped ions. Further, we discuss interactions of the ions with a laser beam. We treat the interactions in the travelling-wave and standing-wave configuration for dipole and quadrupole transitions. We review different types of laser cooling techniques associated with trapped ions. We address Doppler cooling, sideband cooling in and beyond the Lamb-Dicke limit, sympathetic cooling and laser cooling using electromagnetically induced transparency. After that we discuss the problem of state detection using the electron shelving method. Then quantum gates are described. We introduce single-qubit rotations, two-qubit controlled-NOT and multi-qubit controlled-NOT gates. We also comment on more advanced multiple-qubit logic gates. We describe how quantum logic networks may be used for the synthesis of arbitrary pure quantum states. Finally, we discuss the speed of quantum gates and we also give some numerical estimations for them. A discussion of dynamics on off-resonance transitions associated with a qualitative estimation of the weak-coupling regime is included in Appendix A and of the Lamb-Dicke regime in Appendix B.     

How events come into being: EEQT,particle tracks,quantum chaos and tunnelling time

Abstract

We review Event Enhanced Quantum Theory (EEQT), discuss applications of EEQT to tunnelling time, and compare its quantitative predictions with other approaches, in particular with phase time and the Büttiker-Larmor approach. We discuss quantum chaos and quantum fractals resulting from simultaneous continuous monitoring of several non-commuting observables. In particular we show self-similar, nonlinear, iterated function system type, patterns arising from quantum jumps and from the associated Markov operator.     

Bioconjugated quantum dots as fluorescent probes for biomedical imaging

Luminescent semiconductor quantum dots have become an important class of fluorescent labels for biological and biomedical imaging. In comparison with conventional organic dyes and fluorescent proteins, quantum dots have extraordinary fluorescent properties including high brightness, high resistance to photobleaching and tunable wavelengths. In this review, we briefly discuss the properties and modification of quantum dots. We focus on the applications of quantum dots in biomedical imaging, including molecular detection, live cell imaging and in vivo imaging. The toxicity of the quantum dots to cells and animals is also discussed.     

Quantum Relaxation of Ensembles of Nanomagnets

Recent theory and experiment in crystals of molecular magnets suggest that fundamental tests of the decoherence mechanisms of macroscopic quantum phenomena may be feasible in these systems (which are also almost ideal quantum spin glasses). We review these, and suggest new experiments.     

Quantum entanglement criteria

Abstract

We review the main criteria used to detect entanglement in quantum systems. The main properties of each criteria are summarized depending on whether the criteria provides a sufficient or necessary condition, whether it involves density matrix or operators, or if the criteria is phase sensitive. We show that several criteria have much in common and they could be related mathematically. We also discuss the features of entanglement which are useful in quantum information technology.     

Squeezed Light

Abstract

In this paper we review the current state of progress in research on squeezed light. We discuss the basic theory of squeezing and the nature of quantum noise in optical fields. We examine various atomic sources of squeezed light and discuss phase-sensitive detectors of quantum noise including homodyne and heterodyne detectors. Various successful nonlinear optical methods for generating squeezed light are reviewed. We conclude with a discussion of the possible applications of squeezed light.     

Photosynthetic light harvesting: excitons and coherence

Photosynthesis begins with light harvesting, where specialized pigment–protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques.     

Quantum Random-number Generation and Key Sharing

Abstract

We review the status of interferometry-based quantum cryptography and compare photon-pair and faint-pulse schemes. The key technical limitations in both cases are the propagation losses and detector performance. We also discuss a simple approach to generating the random measurement bases used in quantum cryptography systems which exploits random partition at a beam splitter. This removes the need for active components in the receiver, reducing system complexity and losses.     

Fractal fluctuations in quantum mechanics

In quantum open systems, transport coejcients such as crosssections or conductance fluctuate as functions of the relevant parameters. These fluctuations give rise to complicated patterns. An important question is their possible fractality. In this paper we present a historical review of fractal fluctuations in quantum systems and their relation to the properties of their classical counterparts. We discuss the original semiclassical predictions and various numerical and experimental results. Some numerical simulations claim explanations based on pure quantum grounds. We thus report an argument that traces fractality back to the statistical properties of the S -matrix poles. Although full understanding of the reasons for fractal fluctuations might seema secondary problem, it could lead to a deeper comprehension of quantum mechanics and its interplay with classical mechanics.     

Synthesis and application of quantum dot-tagged fluorescent microbeads

Fluorescent quantum dots have been used in biological applications as desirable fluorescent labels instead of traditional fluorophores. Incorporation into microspheres enhances many features of quantum dots that make them ideal for biological detection, such as photostability, multi-target, and improved brightness. Quantum dot-tagged microbeads are emerging as a new class of fluorescent labels and are expected to open new opportunities in nanotechnology and biology. In this review, we describe different approaches for the synthesis of quantum dot-tagged microbeads, surface modification methods that make microbeads suitable for bioconjugation, and the biological applications of the quantum dot-tagged fluorescent microbeads with their desired features in recent research. We also discuss the limitations of some kinds of quantum dot-tagged microbeads and the developments that will enhance their abilities in biological applications.     

Modeling and simulations of a single-spin measurement using MRFM

We review the quantum theory of a single-spin magnetic resonance force microscopy (MRFM). We concentrate on the novel technique called oscillating cantilever-driven adiabatic reversals (OSCARs), which has been used for a single-spin detection. First we describe the quantum dynamics of the cantilever-spin system using simple estimates in the spirit of the mean field approximation. Then we present the results of our computer simulations of the Schro/spl uml/dinger equation for the wave function of the cantilever-spin system and of the master equation for the density matrix of the system. We demonstrate that the cantilever behaves like a quasi-classical measurement device which detects the spin projection along the effective magnetic field. We show that the OSCAR technique provides continuous monitoring of the single spin, which could be used to detect the mysterious quantum collapses of the wave function of the cantilever-spin system.     

Numerical Studies of Quantum Turbulence

We review numerical studies of quantum turbulence. Quantum turbulence is currently one of the most important problems in low temperature physics and is actively studied for superfluid helium and atomic Bose–Einstein condensates. A key aspect of quantum turbulence is the dynamics of condensates and quantized vortices. The dynamics of quantized vortices in superfluid helium are described by the vortex filament model, while the dynamics of condensates are described by the Gross–Pitaevskii model. Both of these models are nonlinear, and the quantum turbulent states of interest are far from equilibrium. Hence, numerical studies have been indispensable for studying quantum turbulence. In fact, numerical studies have contributed to revealing the various problems of quantum turbulence. This article reviews the recent developments in numerical studies of quantum turbulence. We start with the motivation and the basics of quantum turbulence and invite readers to the frontier of this research. Though there are many important topics in the quantum turbulence of superfluid helium, this article focuses on inhomogeneous quantum turbulence in a channel, which has been motivated by recent visualization experiments. Atomic Bose–Einstein condensates are a modern issue in quantum turbulence, and this article reviews a variety of topics in the quantum turbulence of condensates, e.g., two-dimensional quantum turbulence, weak wave turbulence, turbulence in a spinor condensate, some of which have not been addressed in superfluid helium and paves the novel way for quantum turbulence researches. Finally, we discuss open problems.     

Self-assembly of colloidal nanocrystals as route to novel classes of nanostructured materials

Colloidal crystallisation is the only way to obtain three-dimensional ordered materials in which semiconductor, metallic, and magnetic nanocrystals are in close contact. It is expected that the quantum mechanical and dipolar interactions between the nanocrystal units can lead to unseen physical phenomena and materials. Here we review the development of this new and exciting field. We first compare nanocrystal superlattices with regular atomic solids regarding their mechanical strength and opto-electronic properties. We describe how nanocrystal superlattices have been obtained from colloid suspensions in several ways. The thermodynamic driving force for colloidal crystallisation is discussed in terms of inter-particle interactions in a good solvent and entropy. We compare the binary superlattices that have been obtained by solvent evaporation with the predictions of the hard-sphere model and show that semiconductor nanocrystals in a good solvent can behave as hard spheres. Finally, we discuss the quantum mechanical and dipolar interactions in nanocrystal superlattices and review recent studies of the opto-electronic and magnetic properties of novel superlattice materials.     

Hollow-core photonic bandgap fibre: new light guidance for new science and technology

We review the progress made on the fabrication and applications of hollow-core photonic crystal fibres (HC-PCFs). The mechanism of the light guidance in these fibres is described along with their dispersion properties. We review the HC-PCF fabrication, the different results achieved in the fields of laser-induced particle guidance, low-threshold stimulated Raman scattering in hydrogen (vibrational and rotational), laser frequency metrology and quantum optics. Finally, we show the different new prospects opened up by these fibres.     

Prospective applications of optical quantum memories

An optical quantum memory can be broadly defined as a system capable of storing a quantum state through interaction with light at optical frequencies. During the last decade, intense research was devoted to their development, mostly with the aim of fulfilling the requirements of their first two applications, namely quantum repeaters and linear-optical quantum computation. A better understanding of those requirements then motivated several different experimental approaches. Along the way, other exciting applications emerged, such as as quantum metrology, single-photon detection, tests of the foundations of quantum physics, device-independent quantum information processing and nonlinear processing of quantum information. Here we review several prospective applications of optical quantum memories, as well as recent experimental achievements pertaining to these applications. This review highlights that optical quantum memories have become essential for the development of optical quantum information processing.     

Microscopic quantum coherence in a photosynthetic-light-harvesting antenna

We briefly review the coherent quantum beats observed in recent two-dimensional electronic spectroscopy experiments in a photosynthetic-light-harvesting antenna. We emphasize that the decay of the quantum beats in these experiments is limited by ensemble averaging. The in vivo dynamics of energy transport depends upon the local fluctuations of a single photosynthetic complex during the energy transfer time (a few picoseconds). Recent analyses suggest that it remains possible that the quantum-coherent motion may be robust under individual realizations of the environment-induced fluctuations contrary to intuition obtained from condensed phase spectroscopic measurements and reduced density matrices. This result indicates that the decay of the observed quantum coherence can be understood as ensemble dephasing. We propose a fluorescence-detected single-molecule experiment with phase-locked excitation pulses to investigate the coherent dynamics at the level of a single molecule without hindrance by ensemble averaging. We discuss the advantages and limitations of this method. We report our initial results on bulk fluorescence-detected coherent spectroscopy of the Fenna-Mathews-Olson complex.     

Quantum correlations in cavity QED networks

We present a review of the dynamical features such as generation, propagation, distribution, sudden transition and freezing of the various quantum correlation measures, as Concurrence, Entanglement of Formation, Quantum Discord, as well their geometrical measure counterparts within the models of Cavity Quantum Electrodynamics Networks. Dissipation and thermal effects are discussed both in the generation of quantum correlations as well as their effect on the sudden changes and freezing of the classical and quantum correlations in a cavity quantum electrodynamical network. For certain initial conditions, double transitions in the Bures geometrical discord are found. One of these transitions tends to disappear at a critical temperature.