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Recent experiments to test Bell's inequality using entangled photons and ions aimed at tests of basic quantum mechanical principles. Interesting results have been obtained and many loopholes could be closed. In this paper we want to point out that tests of Bell's inequality also play an important role in verifying atom entanglement schemes. We describe as an example a scheme to prepare arbitrary entangled states of N two-level atoms using a leaky optical cavity and a scheme to entangle atoms inside a photonic crystal. During the state preparation no photons are emitted and observing a violation of Bell's inequality is the only way to test whether a scheme works with a high precision or not., Proceedings for the conference Garda 2000, to appear in Zeitschrift fuer Naturforschung, 7 pages, 7 figures

We present some applications of high efficiency quantum interrogation ("interaction free measurement") for the creation of entangled states of separate atoms and of separate photons. The quantum interrogation of a quantum object in a superposition of object-in and object-out leaves the object and probe in an entangled state. The probe can then be further entangled with other objects in subsequent quantum interrogations. By then projecting out those cases were the probe is left in a particular final state, the quantum objects can themselves be left in various entangled states. In this way we show how to generate two-, three-, and higher qubit entanglement between atoms and between photons. The effect of finite efficiency for the quantum interrogation is delineated for the various schemes., Comment: 7 pages, 13 figures, Submitted to PRA

We describe the construction of a conditional quantum control-not (CNOT) gate from linear optical elements following the program of Knill, Laflamme and Milburn [Nature {\bf 409}, 46 (2001)]. We show that the basic operation of this gate can be tested using current technology. We then simplify the scheme significantly., Comment: Problems with PDF figures corrected

Quantum entanglement and its paradoxical properties hold the key to an information processing revolution. Much attention has focused recently on the challenging problem of characterizing entanglement. Entanglement for a two qubit system is reasonably well understood, however, the nature and properties of multiple qubit systems are largely unexplored. Motivated by the importance of such systems in quantum computing, we show that typical pure states of N qubits are highly entangled but have decreasing amounts of pairwise entanglement (measured using the Wootter's concurrence formula) as N increases. Above six qubits very few states have any pairwise entanglement, and generally, for a typical pure state of N qubits there is a sharp cut-off where its subsystems of size m become PPT (positive partial transpose i.e., separable or only bound entangled) around N >~ 2m + 3, based on numerical analysis up to N=13., 4 pages, 5 embedded eps figures, RevTeX4, minor revisions, to appear in J. Mod. Optics

The simplest model of three coupled Bose-Einstein Condensates (BEC) is investigated using a group theoretical method. The stationary solutions are determined using the SU(3) group under the mean field approximation. This semiclassical analysis using the system symmetries shows a transition in the dynamics of the system from self trapping to delocalization at a critical value for the coupling between the condensates. The global dynamics are investigated by examination of the stable points and our analysis shows the structure of the stable points depends on the ratio of the condensate coupling to the particle-particle interaction, undergoes bifurcations as this ratio is varied. This semiclassical model is compared to a full quantum treatment, which also displays the dynamical transition. The quantum case has collapse and revival sequences superposed on the semiclassical dynamics reflecting the underlying discreteness of the spectrum. Non-zero circular current states are also demonstrated as one of the higher dimensional effects displayed in this system., Comment: Accepted to PRA

While it is known that copying a quantum system does not increase the amount of information obtainable about the originals, it may increase the amount available in practice, when one is restricted to imperfect measurements. We present a detection scheme which using imperfect detectors, and possibly noisy quantum copying machines (that entangle the copies), allows one to extract more information from an incoming signal, than with the imperfect detectors alone. The case of single-photon detection with noisy, inefficient detectors and copiers (single controlled-NOT gates in this case) is investigated in detail. The improvement in distinguishability between a photon and vacuum is found to occur for a wide range of parameters, and to be quite robust to random noise. The properties that a quantum copying device must have to be useful in this scheme are investigated., 10 pages, 6 figures, accepted PRA

We find that very different quantum copying machines are optimal depending on the indicator used to assess their performance. Several quantum copying machine models acting on non-orthogonal input states are investigated, and assessed according to two types of criteria: Transfer of (Shannon) information encoded in the initial states to the copies, and fidelity between the copies and the initial states. Transformations which optimise information transfer for messages encoded in qubits are found. If the message is decoded one symbol at-a-time, information is best copied by a Wootters-Zurek copier., 14 pages, 3 Figures, REVTeX. Accepted Physical Review A. Corrected minor grammatical errors

We analyse the fidelity of teleportation protocols, as a function of resource entanglement, for three kinds of two mode oscillator states: states with fixed total photon number, number states entangled at a beam splitter, and the two-mode squeezed vacuum state. We define corresponding teleportation protocols for each case including phase noise to model degraded entanglement of each resource., 21 pages REVTeX, manuscript format, 7 figures postscript, many changes to paper

Intracavity and external third order correlations in the damped nondegenerate parametric oscillator are calculated for quantum mechanics and stochastic electrodynamics (SED), a semiclassical theory. The two theories yield greatly different results, with the correlations of quantum mechanics being cubic in the system's nonlinear coupling constant and those of SED being linear in the same constant. In particular, differences between the two theories are present in at least a mesoscopic regime. They also exist when realistic damping is included. Such differences illustrate distinctions between quantum mechanics and a hidden variable theory for continuous variables., accepted by PRA

We present a formulation of the Bell inequalities using simple correlated photon number states and phase measurements. Such tests generally require binning of the information, and this effect is closely examined. Our proposal opens up the opportunity for a new novel test of quantum mechanics versus local realism. Some insight in entanglement in such systems may be achieved.

We identify what ideal correlated photon number states are to required to maximize the discrepancy between local realism and quantum mechanics when a quadrature homodyne phase measurement is used. Various Bell inequality tests are considered., 6 pages, 5 Figures

We present a strategy to engineer a simple cavity-QED two-bit universal quantum gate using mesoscopic distinct quantum superposition states. The dissipative effect on decoherence and amplitude damping of the quantum bits are analyzed and the critical parameters are presented., 9 pages, 5 Postscript and 1 Encapsulated Postscript figures. To be published in Phys. Rev. A

We show theoretically that Bell-type correlations can be observed between continuous variable measurements performed on a parametric source. An auxiliary measurement, performed on the detection environment, negates the possibility of constructing a local realistic description of these correlations.

Previous work on Bell's inequality realised in the laboratory has used entangled photons. Here we describe how entangled atoms can violate Bell's inequality, and how these violations can be measured with a very high detection efficiency. We first discuss a simple scheme based on two-level atoms inside a cavity to prepare the entangled state. We then discuss a scheme using four-level atoms, which requires a parameter regime much easier to access experimentally using current technology. As opposed to other schemes, our proposal relies on the presence of finite decay rates and its implementation should therefore be much less demanding., Comment: 9 pages, 5 figures, Section III describing how to suppress spontaneous emission during the state preparation has been replaced with a revised version to correct an error

A lower bound on the amount of noise that must be added to a GHZ-like entangled state to make it separable (also called the random robustness) is found using the transposition condition. The bound is applicable to arbitrary numbers of subsystems, and dimensions of Hilbert space, and is shown to be exact for qubits. The new bound is compared to previous such bounds on this quantity, and found to be stronger in all cases. It implies that increasing the number of subsystems, rather than increasing their Hilbert space dimension is a more effective way of increasing entanglement. An explicit decomposition into an ensemble of separable states, when the state is not entangled,is given for the case of qubits., Comment: 2 figures. accepted J. Opt. B: Quantum Semiclass. Opt. (2000)

We present a detection scheme which using imperfect detectors, and imperfect quantum copying machines (which entangle the copies), allows one to extract more information from an incoming signal, than with the imperfect detectors alone., 4 pages, 2 figures, REVTeX, to be published in Phys. Rev. A

We show how entangled qubits can be encoded as entangled coherent states of two-dimensional centre-of-mass vibrational motion for two ions in an ion trap. The entangled qubit state is equivalent to the canonical Bell state, and we introduce a proposal for entanglement transfer from the two vibrational modes to the electronic states of the two ions in order for the Bell state to be detected by resonance fluorescence shelving methods., 4 pages, No figures, accepted to PRA, minor change

We present a potential realization of the Greenberger, Horne and Zeilinger “all or nothing” contradiction of quantum mechanics with local realism using phase measurement techniques in a simple photon number triplet.

We show how macroscopically distinct quantum superposition states (Schroedinger cat states) may be used as logical qubit encodings for the correction of spontaneous emission errors. Spontaneous emission causes a bit flip error which is easily corrected by a standard error correction circuit. The method works arbitrarily well as the distance between the amplitudes of the superposed coherent states increases., Comment: 4 pages, 2 postscript figures, LaTeX2e, RevTeX, minor changes, 1 reference added

We show that various experimental configurations using correlated photon-number states are predicted to violate many forms of Bells inequality. This test is potentially one of macroscopic quantum mechanics in that violations are predicted for situations where large numbers of photons are detected at a single detector at a time. The configurations also allow higher-spin tests of quantum mechanics. We indicate how the violations of Bell inequalities might be achieved with both parametric amplification and parametric oscillation. The effect of photodetection inefficiencies is analyzed. In order to investigate the potential for Bell-inequality tests in the high-intensity regime of recent experiments where detection efficiencies may be very high, we introduce a simple model for the effect of electronic noise that introduces a poor resolution of the photon number detected.

We study a non-linear quantum mechanical oscillator, acting as a measurement device. Candidate systems for realising such apparatus range from superconducting devices through to nano-mechanical resonators. The measurement device comprises an oscillator circuit where the dynamics of expectation values, in its correspondence limit, are either chaotic-like or periodic depending on the measured state of the quantum object – in this case a qubit. In a previous work we showed how the classical like trajectories of such a quantum system can act as a model of a projective measurement process. Here we investigate the quantum to classical transition of the measurement device and postulate criteria for realisation of an effective implementation of such a device.

The architecture scalability afforded by recent proposals of a large scale photonic based quantum computer, utilizing the theoretical developments of topological cluster states and the photonic chip, allows us to move on to a discussion of massively scaled Quantum Information Processing (QIP). In this letter we introduce the model for a secure and unsecured topological cluster mainframe. We consider the quantum analogue of High Performance Computing, where a dedicated server farm is utilized by many users to run algorithms and share quantum data. The scaling structure of photonics based topological cluster computing leads to an attractive future for server based QIP, where dedicated mainframes can be constructed and/or expanded to serve an increasingly hungry user base with the ideal resource for individual quantum information processing., 4 Pages, 2 Figures (figures are a little low-res due to arXiv file size limits)

Diamond has recently become one of the leading candidates for applications in quantum communication and quantum computing. Diamond color centers are ideal as single photon sources. In this article we give an overview of the various techniques that can be used to improve the collection of photons from these emitters. These range from solid immersion lenses to Purcell-enhancement in microstructures and microcavities.

It is shown that the recent Greenberger-Horne-Zeilinger «all or nothing» contradiction of quantum mechanics with local realism can be exhibited on a macroscopic scale where a large number of particles are incident on each analyzer. We present a formulation of the all or nothing paradox in terms of boson fields, and suggest how the paradox might be realized using a correlated photon triplet. The suggested experiment might be readily extended to test, for the first time to our knowledge, quantum mechanics against local realism for situations of more than one quanta per wave packet indicent on each measurement apparatus

We present the design of a novel free-space quantum cryptography system, complete with purpose-built software, that can operate in daylight conditions. The transmitter and receiver modules are built using inexpensive off-the-shelf components. Both modules are compact allowing the generation of renewed shared secrets on demand over a short range of a few metres. An analysis of the software is shown as well as results of error rates and therefore shared secret yields at varying background light levels. As the system is designed to eventually work in short-range consumer applications, we also present a use scenario where the consumer can regularly 'top up' a store of secrets for use in a variety of one-time-pad and authentication protocols.

Maximally entangled mixed states are those states that, for a given mixedness, achieve the greatest possible entanglement. For two-qubit systems and for various combinations of entanglement and mixedness measures, the form of the corresponding maximally entangled mixed states is determined primarily analytically. As measures of entanglement, we consider entanglement of formation, relative entropy of entanglement, and negativity; as measures of mixedness, we consider linear and von Neumann entropies. We show that the forms of the maximally entangled mixed states can vary with the combination of (entanglement and mixedness) measures chosen. Moreover, for certain combinations, the forms of the maximally entangled mixed states can change discontinuously at a specific value of the entropy., 13 pages, 12 figures

We present a simple design of a quantum repeater design build from single NV- centers embedded in an optical cavity. We compare different quantum networks from a simple linear chain to a fully fault-tolerant quantum internet.

In recent years, NV centers in diamond has attracted significant attention as a candidate for quantum information devices. The negatively charged NV center, in particular, has been intensely investigated [1-3]. NV− centers host both an electron spin qubit and a nitrogen nuclear spin, in our case a nuclear spin-1/2 of 15N is imbedded. The ground state of the electron spin qubit has a long coherence time and an optical transition at 637nm. The nuclear spins may be considered as a long-lived quantum memory, which has been experimentally demonstrated [2]. Because of these preferable quantum properties, NV centers have been considered as a good candidate for implementation of quantum information processing and there has been a number of theoretically propose designs for quantum information processing [3].

The research into quantum science and technology has reached an interesting stage where we can now in principle engineer composite systems to exploit the best properties of these individual systems. Such hybridization techniques have been demonstrated for instance with superconducting circuits and electron spin ensembles where strong coupling and the coherent transfer & storage of quantum information has been achieved. However in these cases, generally the hybrid system properties have not exceeded those of the individual systems. Recently we have observed a remarkably sharp resonance in the spectrum of flux qubit NV- diamond hybrid quantum system with no external magnetic field applied to the diamond. What is surprising is that its measured line-width is much narrower than that of both the flux-qubit and spin ensemble. We show this resonance is evidence of a collective & long-lived dark state.

We generate a multi-partite d-level cluster state via two photons simultaneously entangled in time and frequency, and validate its entanglement by developing witness operators that enable the practical detection of any complex photon state.

We develop witness operators enabling the practical detection of arbitrary complex photon states and apply them to validate the entanglement of the first multipartite d-level cluster state achieved through simultaneous time- and frequency-entanglement.

The development of quantum information enabled technologies has reached an interesting stage where we can now in principle engineer composite systems to exploit the best properties of these individual systems. Hybridization potentially allows us to undertake operations and tasks that may otherwise prove difficult. Such an example occurs where we couple a superconducting circuit to an electron spin (or ensemble). We show in this presentation how the coherence properties of single electron spins (or ensembles) can be dramatically increased just by it coupling with the superconducting qubit, that is two systems with limited coherence properties can form a combined system with a much longer coherence time.

Electron-spin nitrogen-vacancy color centers in diamond are a natural candidate to act as a quantum memory for superconducting qubits because of their large collective coupling and long coherence times. We report here the first demonstration of strong coupling and coherent exchange of a single quantum of energy between a flux-qubit and an ensemble of nitrogen-vacancy color centers., Comment: 5 pages, 4 figures, already published in the 13th October 2011 issue of Nature journal

The generation of two-photon multi-correlated states over four modes and their pairwise entanglement is demonstrated by superimposing two different spontaneous four-wave mixing processes inside a bi-modally pumped CMOS-compatible microring resonator.

By superimposing two different spontaneous four-wave mixing processes inside a bi-modally pumped CMOS-compatible mircoring resonator, we report the generation of four-mode two-photon multi-correlated states and measure two-photon entanglement.

Cluster states, a specific class of multi-partite entangled states, are of particular importance for quantum science, as such systems are equivalent to the realization of one-way (or measurement-based) quantum computers [1]. In this scheme, algorithms are implemented through high-fidelity measurements on the parties of the state [2]. While two-level (i.e. qubit) cluster states have been realized so far, increasing the number of particles to boost the computational resource comes at the price of significantly reduced coherence time and detection rates, as well as increased sensitivity to noise, restricting the realization of discrete cluster states to a record of eight qubits. In contrast, the demonstration of d-level (i.e. qudit) cluster states has the potential to i) increase quantum resources without modifying the number of particles; ii) enable the implementation of highly efficient computational protocols; iii) reduce the noise sensitivity of the states. Up till now, the realization of discrete d-level cluster states has not been shown in any quantum platform. We here demonstrate the realization of d-level cluster states, perform d-level one-way quantum processing operations on the states, and show that higher-dimensional forms of cluster states are more noise tolerant than lower dimensional realizations.

Entanglement is an essential resource in quantum information science [1] and its presence in any quantum system can be experimentally detected through entanglement witness operators [2]. In particular, measuring a negative expectation value of a witness with high statistical confidence provides a necessary and sufficient condition to confirm the generation of a genuine multipartite [3] and/or d-level entangled state [4]. In recent years, the experimental generation of complex quantum states has intensified the need for witnesses that are capable of detecting such systems and are experimentally optimal at the same time. This means that the witness should require the least measurement effort (in terms of number and complexity of the measurement settings), include only projections on single qudits, while at the same time possessing a high noise tolerance (Fig. 1a). However, "experimentally-friendly" witnesses capable of accomplishing these tasks have not been derived yet.