arXiv subjects

Hugo Cable

Publications and source records attributed to Hugo Cable.

30 records · Page 2Linked to original sources

arXiv2011

Quantum Correlations in Mixed-State Metrology

We analyze the effects of quantum correlations, such as entanglement and discord, on the efficiency of phase estimation by studying four quantum circuits that can be readily implemented using NMR techniques. These circuits define a standard strategy of repeated single-qubit measurements, a classical strategy where only classical correlations are allowed, and two quantum strategies where nonclassical correlations are allowed. In addition to counting space (number of qubits) and time (number of gates) requirements, we introduce mixedness as a key constraint of the experiment. We compare the efficiency of the four strategies as a function of the mixedness parameter. We find that the quantum strategy gives square-root of N enhancement over the standard strategy for the same amount of mixedness. This result applies even for highly mixed states that have nonclassical correlations but no entanglement.

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arXiv2010

Bipartite Entanglement in Continuous-Variable Cluster States

We present a study of the entanglement properties of Gaussian cluster states, proposed as a universal resource for continuous-variable quantum computing. A central aim is to compare mathematically-idealized cluster states defined using quadrature eigenstates, which have infinite squeezing and cannot exist in nature, with Gaussian approximations which are experimentally accessible. Adopting widely-used definitions, we first review the key concepts, by analysing a process of teleportation along a continuous-variable quantum wire in the language of matrix product states. Next we consider the bipartite entanglement properties of the wire, providing analytic results. We proceed to grid cluster states, which are universal for the qubit case. To extend our analysis of the bipartite entanglement, we adopt the entropic-entanglement width, a specialized entanglement measure introduced recently by Van den Nest M et al., Phys. Rev. Lett. 97 150504 (2006), adapting their definition to the continuous-variable context. Finally we add the effects of photonic loss, extending our arguments to mixed states. Cumulatively our results point to key differences in the properties of idealized and Gaussian cluster states. Even modest loss rates are found to strongly limit the amount of entanglement. We discuss the implications for the potential of continuous-variable analogues of measurement-based quantum computation.

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arXiv2010

Quantum Phase Estimation with Entangled Photons produced by Parametric Down Conversion

We explore the advantages offered by twin light beams produced in parametric down-conversion for precision measurement. The symmetry of these bipartite quantum states, even under losses, suggests that monitoring correlations between the divergent beams permits a high-precision inference of any symmetry-breaking effect, e.g. fiber birefringence. We show that the quantity of entanglement is not the key feature for such an instrument. In a lossless setting, scaling of precision at the ultimate `Heisenberg' limit is possible with photon counting alone. Even as photon losses approach 100% the precision is shot-noise limited, and we identify the crossover point between quantum and classical precision as a function of detected flux. The predicted hypersensitivity is demonstrated with a Bayesian simulation.

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arXiv2009

A non-degenerate optical parametric oscillator as a high-flux source for quantum lithography

We investigate the use of a non-degenerate parametric oscillator (NDPO) as a source for quantum lithography, for which the light can have high-flux and strong non-classical features. This builds on the proposal of Boto, et al. [A. N. Boto, et al., PRL (85), 2733 (2000)], for etching simple patterns on multi-photon absorbing materials with sub-Rayleigh resolution, using special two-mode entangled states of light. An NDPO has two outgoing modes differentiated by polarization or direction of propagation, but sharing the same optical frequency. We derive analytical expressions for the multi-photon absorption rates when the NDPO is operated below, near, and above its threshold. The resulting interference patterns are characterized by an effective wavelength half that for the illuminating modes. We compare our results with those for the case of a high-gain optical amplifier source, and discuss the relative merit of the NDPO.

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arXiv2008

Entanglement-Seeded-Dual Optical Parametric Amplification: Applications to Quantum Communication, Imaging, and Metrology

The study of optical parametric amplifiers (OPAs) has been successful in describing and creating nonclassical light for use in fields such as quantum metrology and quantum lithography [Agarwal, et al., J. Opt. Soc. Am. B, 24, 2 (2007)]. In this paper we present the theory of an OPA scheme utilizing an entangled state input. The scheme involves two identical OPAs seeded with the maximally path-entangled N00N state (|2,0>+|0,2>)/sqrt{2}. The stimulated amplification results in output state probability amplitudes that have a dependence on the number of photons in each mode, which differs greatly from two-mode squeezed vacuum. The output contains a family of entangled states directly applicable to quantum key distribution. Specific output states allow for the heralded creation of N=4 N00N states, which may be used for quantum lithography, to write sub-Rayleigh fringe patterns, and for quantum interferometry, to achieve Heisenberg-limited phase measurement sensitivity.

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arXiv2008

Experimental sub-Rayleigh resolution by an unseeded high-gain optical parametric amplifier for quantum lithography

Quantum lithography proposes to adopt entangled quantum states in order to increase resolution in interferometry. In the present paper we experimentally demonstrate that the output of a high-gain optical parametric amplifier can be intense yet exhibits quantum features, namely, sub-Rayleigh fringes, as proposed by Agarwal et al. (Phys. Rev. Lett. 86, 1389 (2001)). We investigate multiphoton states generated by a high-gain optical parametric amplifier operating with a quantum vacuum input for a gain values up to 2.5. The visibility has then been increased by means of three-photon absorption. The present article opens interesting perspectives for the implementation of such an advanced interferometrical setup.

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arXiv2007

Generation of Large Number-Path Entanglement Using Linear Optics and Feed-Forward

We show how an idealised measurement procedure can condense photons from two modes into one, and how, by feeding forward the results of the measurement, it is possible to generate efficiently superpositions of components for which only one mode is populated, commonly called ``N00N states''. For the basic procedure, sources of number states leak onto a beam splitter, and the output ports are monitored by photodetectors. We find that detecting a fixed fraction of the input at one output port suffices to direct the remainder to the same port with high probability, however large the initial state. When instead photons are detected at both ports, Schrödinger cat states are produced. We describe a circuit for making the components of such a state orthogonal, and another for subsequent conversion to a N00N state. Our approach scales exponentially better than existing proposals. Important applications include quantum imaging and metrology.

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arXiv2006

Quantum States of Light Produced by a High-Gain Optical Parametric Amplifier for Use in Quantum Lithography

We present a theoretical analysis of the properties of an unseeded optical parametic amplifier (OPA) used as the source of entangled photons for applications in quantum lithography. We first study the dependence of the excitation rate of a two-photon absorber on the intensity of the light leaving the OPA. We find that the rate depends linearly on intensity only for output beams so weak that they contain fewer than one photon per mode. We also study the use of an N-photon absorber for arbitrary N as the recording medium to be used with such a light source. We find that the contrast of the interference pattern and the sharpness of the fringe maxima tend to increase with increasing values of N, but that the density of fringes and thus the limiting resolution does not increase with N. We conclude that the output of an unseeded OPA exciting an N-photon absorber provides an attractive system in which to perform quantum lithography.

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arXiv2006

Localising Relational Degrees of Freedom in Quantum Mechanics

This thesis presents a wide-ranging study of localising relational degrees of freedom. Three physical systems are studied in depth, each built upon a simple measurement-based process. For each physical system - light from independent sources leaking onto a beam splitter monitored by photodetectors, spatially interfering Bose-Einstein condensates extracted from separate preparation procedures, and delocalised massive particles or mirrors scattering light which is detected in the far-field - the thesis investigates the key features of the underlying process of localisation, and explores the properties of the induced post-measurement states of the system. A range of analytical and numerical methods are used. Many new results are presented - for example, cases of mixed initial states are considered in addition to the more commonly considered pure states. Up to now there has been little attempt to develop in detail the themes common to studies concerned with specific physical examples. This thesis addresses this, and sets out a "modus operandi" that can be applied widely. A variety of specific applications are considered - both in the context of controlled laboratory experiments, and with a view to understanding processes occurring in nature. The thesis is introduced and summarised in Chapter 1. Chapter 2 studies localising relative optical phase in the canonical interference process. This discussion is extended and applied in Chapter 3. Chapter 4 discusses localising relative atomic phase in interference experiments with Bose-Einstein condensates. The emergence of relative positions between particles scattering light is explored in Chapter 5. The thesis concludes with an Outlook, Chapter 6.

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arXiv2006

Measurement Induced Localization of Relative Degrees of Freedom

We present a comprehensive study, using both analytical and numerical methods, of measurement-induced localization of relational degrees of freedom. Looking first at the interference of two optical modes, we find that the localization of the relative phase can be as good for mixed states - in particular for two initially Poissonian or thermal states - as for the well-known case of two Fock states. In a realistic setup the localization for mixed states is robust and experimentally accessible, and we discuss applications to superselection rules. For an ideal setup and initial Fock states we show how a relational Schrödinger cat state emerges, and investigate circumstances under which such a state is destroyed. In our second example we consider the localization of relative atomic phase between two Bose Einstein condensates, looking particularly at the build up of spatial interference patterns, an area which has attracted much attention since the work of Javanainen and Yoo. We show that the relative phase localizes much faster than was intimated in previous studies focusing on the emerging interference pattern itself. Finally, we explore the localization of relative spatial parameters discussed in recent work by Rau, Dunningham and Burnett. We retain their models of indistinguishable scattering but make different assumptions. In particular we consider the case of a real distant observer monitoring light scattering off two particles, who records events only from a narrow field of view. The localization is only partial regardless of the number of observations. This paper contributes to the wider debate on relationism in quantum mechanics, which treats fundamental concepts - reference frames and conservation laws - from a fully quantum and operational perspective.

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arXiv2004

Speeding up gate operations through dissipation

It is commonly believed that decoherence is the main obstacle to quantum information processing. In contrast to this, we show how decoherence in the form of dissipation can improve the performance of certain quantum gates. As an example we consider the realisations of a controlled phase gate and a two-qubit SWAP operation with the help of a single laser pulse in atom-cavity systems. In the presence of spontaneous decay rates, the speed of the gates can be improved by a factor 2 without sacrificing high fidelity and robustness against parameter fluctuations. Even though this leads to finite gate failure rates, the scheme is comparable with other quantum computing proposals.

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arXiv2003

Dissipation-assisted quantum computation in atom-cavity systems

The principal obstacle to quantum information processing with many qubits is decoherence. One source of decoherence is spontaneous emission which causes loss of energy and information. Inability to control system parameters with high precision is another possible source of error. Strategies aimed at overcoming one kind of error typically increase sensitivity to others. As a solution we propose quantum computing with dissipation-assisted quantum gates. These can be run relatively fast while achieving fidelities close to one. The success rate of each gate operation can, at least in principle, be arbitrary close to one.

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