Publications (214)653.89 Total impact
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ABSTRACT: It is a topic of fundamental and practical importance how a quantum correlated state can be reliably distributed through a noisy channel for quantum information processing. The concept of quantum steering recently defined in a rigorous manner is relevant to study it under certain circumstances and we here address quantum steerability of Gaussian states to this aim. In particular, we attempt to reformulate the criterion for Gaussian steering in terms of local and global purities and show that it is sufficient and necessary for the case of steering a 1mode system by a $N$mode system. It subsequently enables us to reinforce a strong monogamy relation under which only one party can steer a local system of 1mode. Moreover, we show that only a negative partialtranspose state can manifest quantum steerability by Gaussian measurements in relation to the Peres conjecture. We also discuss our formulation for the case of distributing a twomode squeezed state via oneway quantum channels making dissipation and amplification effects, respectively.11/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Phase estimation, at the heart of many quantum metrology and communication schemes, can be strongly affected by noise, whose amplitude may not be known, or might be subject to drift. Here we investigate the joint estimation of a phase shift and the amplitude of phase diffusion at the quantum limit. For several relevant instances, this multiparameter estimation problem can be effectively reshaped as a twodimensional Hilbert space model, encompassing the description of an interferometer phase probed with relevant quantum statessplit singlephotons, coherent states or N00N states. For these cases, we obtain a tradeoff bound on the statistical variances for the joint estimation of phase and phase diffusion, as well as optimum measurement schemes. We use this bound to quantify the effectiveness of an actual experimental setup for joint parameter estimation for polarimetry. We conclude by discussing the form of the tradeoff relations for more general states and measurements.Nature Communications 10/2014; 5:3532. · 10.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study quantum nonMarkovianity in the early stage of the emission process of a twolevel atom coupled to a semiinfinite waveguide, where the waveguide termination behaves as a perfect mirror. Specifically, we restrict to the analysis of the process for times shorter than twice the time delay t_d, where t_d is the duration of a round trip along the atommirror path. We show the emergence of a threshold in the parameters space separating the Markovian and nonMarkovian regions.Physica Scripta 09/2014; T160. · 1.30 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Wheeler's delayedchoice experiment illustrates vividly that the observer plays a central role in quantum physics by demonstrating that complementarity or waveparticle duality can be enforced even after the photon has already entered the interferometer. The delayedchoice quantum eraser experiment further demonstrates that complementarity can be enforced even after detection of a quantum system, elucidating the foundational nature of complementarity in quantum physics. However, the applicability of the delayedchoice method for practical quantum information protocols continues to be an open question. Here, we introduce and experimentally demonstrate the delayedchoice decoherence suppression protocol, in which the decision to suppress decoherence on an entangled twoqubit state is delayed until after the decoherence and even after the detection of a qubit. Our result suggests a new way to tackle Markovian decoherence in a delayed manner, applicable for practical entanglement distribution over a dissipative channel.Nature Communications 07/2014; 5:4522. · 10.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the emission of a polaritonic system, where the coupling between a large number of twolevel emitters and a singlemode cavity field is nonadiabatically switched on. Counterrotating terms as well as the socalled $A^2$ term are included in the lightmatter interaction, where ${\bf A }$ is the vector potential. We find that the ThomasReicheKuhn sum rule enforces qualitative constraints on the quantum statistics of the system radiation, which consists of two spectrally resolved output modes. For ideal twolevel emitters the populations of the two modes are always found equal. This result cannot be recovered if $A^2$ is neglected, or even if it is included perturbatively via renormalization of the cavity frequency. We then extend our study to imperfect twolevel emitters, featuring residual couplings to higher levels, and find that a naive application of the twolevel approximation alters these predictions incorrectly. We discuss how a refined twolevel approximation may be obtained by rescaling the $A^2$ term.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We address the problem of continuousvariable quantum phase estimation in the presence of linear disturbance at the Hamiltonian level, by means of Gaussian probe states. In particular we discuss both unitary and random disturbance, by considering the parameter which characterizes the unwanted linear term present in the Hamiltonian as fixed (unitary disturbance) or random with a given probability distribution (random disturbance). We derive the optimal input Gaussian states at fixed energy, maximizing the quantum Fisher information over the squeezing angle and the squeezing energy fraction, and we discuss the scaling of the quantum Fisher information in terms of the output number of photons $n_{out}$. We observe that in the case of unitary disturbance the optimal state is a squeezed vacuum state and the quadratic scaling is conserved. As regards the random disturbance, we observe that the optimal squeezing fraction may not be equal to one, and, for any nonzero value of the noise parameter, the quantum Fisher information scales linearly with the average number of photons. We finally discuss the performance of homodyne measurement, comparing the achievable precision with the ultimate limit posed by the quantum Cram\'erRao bound.Physical Review A 07/2014; 90(4). · 2.99 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The superposition principle is at the heart of quantum mechanics and at the root of many paradoxes arising when trying to extend its predictions to our everyday world. Schroedinger's cat is the prototype of such paradoxes and here, in contrast to many others, we choose to investigate it from the operational point of view. We experimentally demonstrate a universal strategy for producing an unambiguously distinguishable type of superposition, that of an arbitrary pure state and its orthogonal. It relies on only a limited amount of information about the input state to first generate its orthogonal one. Then, a simple change in the experimental parameters is used to produce arbitrary superpositions of the mutually orthogonal states. Constituting a sort of Schroedinger's black box, able to turn a whole zoo of input states into coherent superpositions, our scheme can produce arbitrary continuousvariable optical qubits, which may prove practical for implementing quantum technologies and measurement tasks.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Utilizing the tools of quantum optics to prepare and manipulate quantum states of motion of a mechanical resonator is currently one of the most promising routes to explore nonclassicality at a macroscopic scale. An important quantum optomechanical tool yet to be experimentally demonstrated is the ability to perform complete quantum state reconstruction. Here, after providing a brief introduction to quantum states in phase space, we review and contrast the current proposals for state reconstruction of mechanical motional states and discuss experimental progress. Furthermore, we show that mechanical quadrature tomography using backactionevading interactions gives an $s$parameterized Wigner function where the numerical parameter $s$ is directly related to the optomechanical measurement strength. We also discuss the effects of classical noise in the optical probe for both state reconstruction and state preparation by measurement.Annalen der Physik 06/2014; · 1.51 Impact Factor 
Article: Coherently opening a highq cavity.
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ABSTRACT: We propose a general framework to effectively "open" a highQ resonator, that is, to release the quantum state initially prepared in it in the form of a traveling electromagnetic wave. This is achieved by employing a mediating mode that scatters coherently the radiation from the resonator into a onedimensional continuum of modes such as a waveguide. The same mechanism may be used to "feed" a desired quantum field to an initially empty cavity. Switching between an open and "closed" resonator may then be obtained by controlling either the detuning of the scatterer or the amount of time it spends in the resonator. First, we introduce the model in its general form, identifying (i) the traveling mode that optimally retains the full quantum information of the resonator field and (ii) a suitable figure of merit that we study analytically in terms of the system parameters. Then, we discuss two feasible implementations based on ensembles of twolevel atoms interacting with cavity fields. In addition, we discuss how to integrate traditional cavity QED in our proposal using threelevel atoms.Physical Review Letters 04/2014; 112(13):133605. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We address detection of quantum nonGaussian states, i.e. nonclassical states that cannot be expressed as a convex mixture of Gaussian states, and present a method to derive a new family of criteria based on generic linear functionals. We then specialise this method to derive witnesses based on $s$parametrized quasiprobability functions, generalising previous criteria based on the Wigner function. In particular we discuss in detail and analyse the properties of Husimi Qfunction based witnesses and prove that they are often more effective than previous criteria in detecting quantum nonGaussianity of various kinds of nonGaussian states evolving in a lossy channel.Physical Review A 03/2014; 90(1). · 2.99 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report direct evidence of the bosonic nature of surface plasmon polaritons (SPPs) in a scatteringbased beamsplitter. A parametric downconversion source is used to produce two indistinguishable photons, each of which is converted into a SPP on a metalstripe waveguide and then made to interact through a semitransparent Bragg mirror. In this plasmonic analog of the HongOuMandel experiment, we measure a coincidence dip with a visibility of 72%, a key signature that SPPs are bosons and that quantum interference is clearly involved.02/2014; 1(3).  [Show abstract] [Hide abstract]
ABSTRACT: We consider the integration of quantum emitters into a negative permeability metamaterial design in order to introduce tunability as well as nonlinear behavior. The unit cell of our metamaterial is a ring of metamolecules, each consisting of a metal nanoparticle and a twolevel semiconductor quantum dot (QD). Without the QDs, the ring of the unit cell is known to act as an artificial optical magnetic resonator. By adding the QDs we show that a Fano interference profile is introduced into the magnetic field scattered from the ring. This induced interference is shown to cause an appreciable effect in the collective magnetic resonance of the unit cell. We find that the interference provides a means to tune the response of the negative permeability metamaterial. The exploitation of the QD's inherent nonlinearity is proposed to modulate the metamaterial's magnetic response with a separate control field.Physical Review A 01/2014; 89(1). · 2.99 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We consider a quantum emitter ("atom") radiating in a onedimensional (1D) photonic waveguide in the presence of a single mirror. This setup can be implemented in a variety of platforms, such as quantum dots emitting in a terminated 1D photonic crystal/nanowire, or even in free space by means of a standard mirror and an ion trapped at the focus of highnumericalaperture lenses. It is known that the feedback mechanism introduced by the mirror gives rise to a complex emission process that can feature significant memory effects. Here, we carry out a systematic analysis of the nonMarkovian (NM) character of such process in terms of refined, recently developed notions of quantum nonMarkovianity such as indivisibility and information backflow. NM effects are quantified as a function of the time delay and phase shift associated with the atommirror optical path. We find, in particular, that unless an atomphoton bound state is formed a finite time delay is always required in order for NM effects to be exhibited. This identifies a finite threshold in parameter space, separating the Markovian and nonMarkovian regimes.Physical Review A 12/2013; 90(1). · 2.99 Impact Factor 
Article: Matterwave interferometry of a levitated thermal nanooscillator induced and probed by a spin.
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ABSTRACT: We show how the interference between spatially separated states of the center of mass (c.m.) of a mesoscopic harmonic oscillator can be evidenced by coupling it to a spin and performing solely spin manipulations and measurements (Ramsey interferometry). We propose to use an optically levitated diamond bead containing a nitrogenvacancy center spin. The nanoscale size of the bead makes the motional decoherence due to levitation negligible. The form of the spinmotion coupling ensures that the scheme works for thermal states so that moderate feedback cooling suffices. No separate control or observation of the c.m. state is required and thereby one dispenses with cavities, spatially resolved detection, and lowmassdispersion ensembles. The controllable relative phase in the Ramsey interferometry stems from a gravitational potential difference so that it uniquely evidences coherence between states which involve the whole nanocrystal being in spatially distinct locations.Physical Review Letters 11/2013; 111(18):180403. · 7.73 Impact Factor  Physical Review Letters 09/2013; 111(12). · 7.73 Impact Factor
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ABSTRACT: Highly quantum nonlinear interactions between different bosonic modes lead to the generation of quantum nonGaussian states, i.e. states that cannot be written as mixtures of Gaussian states. A paradigmatic example is given by Schr\"odinger's cat states, that is coherent superpositions of coherent states with opposite amplitude. We here consider a novel quantum nonGaussianity criterion recently proposed in the literature and prove its effectiveness on Schr\"odinger cat states evolving in a lossy bosonic channel. We prove that quantum nonGaussianity can be effectively detected for high values of losses and for large coherent amplitudes of the cat states.Physica Scripta 09/2013; T160. · 1.30 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Many paradoxes of quantum mechanics come from the fact that quantum systems can possess different features simultaneously, such as in waveparticle duality or quantum superposition. In recent delayedchoice experiments, a quantum system can be observed to manifest one feature such as the wave or particle nature, depending on the measurement setup, which is chosen after the system itself has already entered the measuring device; hence its behaviour is not predetermined. Here we adapt this paradigmatic scheme to multidimensional quantum walks. In our experiment, the way in which a photon interferes with itself in a strongly nontrivial pattern depends on its polarization, which is determined after the photon has already been detected. This is the first experiment realizing a multidimensional quantum walk with a single photon source and we present also the first experimental simulation of the Grover walk, a model that can be used to implement the Grover quantum search algorithm.Nature Communications 09/2013; 4:2471. · 10.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Enforcing a nonclassical behavior in mesoscopic systems is important for the study of the boundaries between quantum and classical world. Recent experiments have shown that optomechanical devices are promising candidates to pursue such investigations. Here we consider two different setups where the indirect coupling between a threelevel atom and the movable mirrors of a cavity is achieved. The resulting dynamics is able to conditionally prepare a nonclassical state of the mirrors by means of projective measurements operated over a pure state of the atomic system. The nonclassical features are persistent against incoherent thermal preparation of the mechanical systems and their dissipative dynamics.Physical Review A 09/2013; 88(1). · 2.99 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: For many open quantum systems, a master equation approach employing the Markov approximation cannot reliably describe the dynamical behaviour. This is the case, for example, in a number of solid state or biological systems, and it has motivated a line of research aimed at quantifying the amount of nonMarkovian behaviour in a given model. Within this framework, we investigate the dynamics of a quantum harmonic oscillator coupled to a bosonic bath with linear coupling Hamiltonians. We focus on Gaussian states, which are suitably treated using a covariance matrix approach. Concentrating on an entanglement based nonMarkovian behaviour quantifier (NMBQ) proposed by Rivas et. al. [1], we consider the role that resonant and offresonant modes play in affecting the NMBQ. By using a large but finite bath of oscillators for both Ohmic and super Ohmic spectral densities we find, by systematically increasing the coupling strength, initially the resonant modes provide the most significant nonMarkovian effects, while after a certain threshold of coupling strength the offresonant modes play the dominant role. We also consider the NMBQ for two other models where we add a single strongly coupled oscillator to the model in extra bath mode and 'buffer' configurations, which affects the modes that determine nonMarkovian behaviour.Journal of Physics B Atomic Molecular and Optical Physics 08/2013; 47(1). · 1.92 Impact Factor 
Article: Quantum plasmonics
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ABSTRACT: Quantum plasmonics is a rapidly growing field of research that involves the study of the quantum properties of light and its interaction with matter at the nanoscale. Here, surface plasmons—electromagnetic excitations coupled to electron charge density waves on metal–dielectric interfaces or localized on metallic nanostructures—enable the confinement of light to scales far below that of conventional optics. We review recent progress in the experimental and theoretical investigation of the quantum properties of surface plasmons, their role in controlling light–matter interactions at the quantum level and potential applications. Quantum plasmonics opens up a new frontier in the study of the fundamental physics of surface plasmons and the realization of quantumcontrolled devices, including singlephoton sources, transistors and ultracompact circuitry at the nanoscale. P lasmonics provides a unique setting for the manipulation of light via the confinement of the electromagnetic field to regions well below the diffraction limit 1,2 . This has opened up a wide range of applications based on extreme light concentration 3 , including nanophotonic lasers and amplifiers 4,5 , optical metamaterials 6 , biochemical sensing 7 and antennas transmitting and receiving light signals at the nanoscale 8 . These applications and their rapid development have been made possible by the large array of experimental tools that have become available in recent years for nanoscale fabrication and theory tools in the form of powerful electromagnetic simulation methods. At the same time, and completely parallel to this remarkable progress, there has been a growing excitement about the prospects for exploring quantum properties of surface plasmons and building plasmonic devices that operate faithfully at the quantum level 9 . The hybrid nature of surface plasmon polaritons (SPPs) as 'quasiparticles' makes them intriguing from a fundamental point of view, with many of their quantum properties still largely unknown. In addition, their potential for providing strong coupling of light to emitter systems, such as quantum dots 10,11 and nitrogen–vacancy (NV) centres 12 , via highly confined fields offers new opportunities for the quantum control of light, enabling devices such as efficient singlephoton sources 13–16 and transistors 17–19 to be realized. Although surface plasmons are well known to suffer from large losses, there are also attractive prospects for building devices that can exploit this lossy nature for controlling dissipative quantum dynamics 20 . This new field of research combining modern plasmonics with quantum optics has become known as 'quantum plasmonics'. In this Review, we describe the wide range of research activities being pursued in the field of quantum plasmonics. We begin with a short description of SPPs and their quantization. Then, we discuss one of the major strengths of plasmonic systems: the ability to provide highly confined electromagnetic fields. We describe how this enables the enhancement of light–matter interactions and the progress that has been made so far in demonstrating a variety of schemes that take advantage of it in the quantum regime. We also review key experiments that have probed fundamental quantum properties of surface plasmons and their potential for building compact nanophotonic circuitry. We conclude by providing anNature Physics 06/2013; 9:329340. · 19.35 Impact Factor
Publication Stats
4k  Citations  
653.89  Total Impact Points  
Top Journals
Institutions

2013–2014

University College London
 Department of Physics and Astronomy
Londinium, England, United Kingdom


1987–2014

Imperial College London
 Section of Statistics
Londinium, England, United Kingdom


2008–2013

University of Vienna
 Faculty of Physics
Vienna, Vienna, Austria 
Università degli studi di Palermo
 Dipartimento di Fisica e Chimica
Palermo, Sicily, Italy


2011–2012

Texas A&M University at Qatar
Ad Dawḩah, Ad Dawḩah, Qatar


2000–2011

Queen's University Belfast
 School of Mathematics and Physics
Béal Feirste, N Ireland, United Kingdom


2006–2008

Sungkyunkwan University
 School of Advanced Materials Science and Engineering (AMSE)
Sŏul, Seoul, South Korea 
Kyungpook National University
Daikyū, Daegu, South Korea 
University of Seoul
Sŏul, Seoul, South Korea


2005–2006

Hanyang University
 Department of Physics
Seoul, Seoul, South Korea


2003

Hannam University
 Department of Polymer Science and Engineering
Daiden, Daejeon, South Korea


1994–2000

Sogang University
 Department of Physics
Seoul, Seoul, South Korea 
Slovak Academy of Sciences
 Institute of Physics
Presburg, Bratislavský, Slovakia


1991–1996

Korea Institute of Science and Technology
 Center for OptoElectronic Convergence Systems
Sŏul, Seoul, South Korea
