M S Kim

University College London, Londinium, England, United Kingdom

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Publications (214)653.89 Total impact

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    Se-Wan Ji, M. S. Kim, Hyunchul Nha
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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 1-mode 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 1-mode. Moreover, we show that only a negative partial-transpose 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 two-mode squeezed state via one-way quantum channels making dissipation and amplification effects, respectively.
    11/2014;
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    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 two-dimensional Hilbert space model, encompassing the description of an interferometer phase probed with relevant quantum states-split single-photons, coherent states or N00N states. For these cases, we obtain a trade-off 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 set-up for joint parameter estimation for polarimetry. We conclude by discussing the form of the trade-off relations for more general states and measurements.
    Nature Communications 10/2014; 5:3532. · 10.74 Impact Factor
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    ABSTRACT: We study quantum non-Markovianity in the early stage of the emission process of a two-level atom coupled to a semi-infinite 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 atom-mirror path. We show the emergence of a threshold in the parameters space separating the Markovian and non-Markovian regions.
    Physica Scripta 09/2014; T160. · 1.30 Impact Factor
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    ABSTRACT: Wheeler's delayed-choice experiment illustrates vividly that the observer plays a central role in quantum physics by demonstrating that complementarity or wave-particle duality can be enforced even after the photon has already entered the interferometer. The delayed-choice 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 delayed-choice method for practical quantum information protocols continues to be an open question. Here, we introduce and experimentally demonstrate the delayed-choice decoherence suppression protocol, in which the decision to suppress decoherence on an entangled two-qubit 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
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    ABSTRACT: We investigate the emission of a polaritonic system, where the coupling between a large number of two-level emitters and a single-mode cavity field is non-adiabatically switched on. Counter-rotating terms as well as the so-called $A^2$ term are included in the light-matter interaction, where ${\bf A }$ is the vector potential. We find that the Thomas-Reiche-Kuhn sum rule enforces qualitative constraints on the quantum statistics of the system radiation, which consists of two spectrally resolved output modes. For ideal two-level 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 two-level emitters, featuring residual couplings to higher levels, and find that a naive application of the two-level approximation alters these predictions incorrectly. We discuss how a refined two-level approximation may be obtained by rescaling the $A^2$ term.
    07/2014;
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    ABSTRACT: We address the problem of continuous-variable 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 non-zero 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\'er-Rao bound.
    Physical Review A 07/2014; 90(4). · 2.99 Impact Factor
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    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 continuous-variable optical qubits, which may prove practical for implementing quantum technologies and measurement tasks.
    07/2014;
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    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 non-classicality 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 back-action-evading 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
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    ABSTRACT: We propose a general framework to effectively "open" a high-Q 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 one-dimensional 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 two-level atoms interacting with cavity fields. In addition, we discuss how to integrate traditional cavity QED in our proposal using three-level atoms.
    Physical Review Letters 04/2014; 112(13):133605. · 7.73 Impact Factor
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    ABSTRACT: We address detection of quantum non-Gaussian 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 Q-function based witnesses and prove that they are often more effective than previous criteria in detecting quantum non-Gaussianity of various kinds of non-Gaussian states evolving in a lossy channel.
    Physical Review A 03/2014; 90(1). · 2.99 Impact Factor
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    ABSTRACT: We report direct evidence of the bosonic nature of surface plasmon polaritons (SPPs) in a scattering-based beamsplitter. A parametric down-conversion source is used to produce two indistinguishable photons, each of which is converted into a SPP on a metal-stripe waveguide and then made to interact through a semi-transparent Bragg mirror. In this plasmonic analog of the Hong-Ou-Mandel 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).
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    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 two-level 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
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    ABSTRACT: We consider a quantum emitter ("atom") radiating in a one-dimensional (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 high-numerical-aperture 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 non-Markovian (NM) character of such process in terms of refined, recently developed notions of quantum non-Markovianity such as indivisibility and information back-flow. NM effects are quantified as a function of the time delay and phase shift associated with the atom-mirror optical path. We find, in particular, that unless an atom-photon 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 non-Markovian regimes.
    Physical Review A 12/2013; 90(1). · 2.99 Impact Factor
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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 nitrogen-vacancy center spin. The nanoscale size of the bead makes the motional decoherence due to levitation negligible. The form of the spin-motion 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 low-mass-dispersion 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 non-linear interactions between different bosonic modes lead to the generation of quantum non-Gaussian 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 non-Gaussianity 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 non-Gaussianity 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
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    ABSTRACT: Many paradoxes of quantum mechanics come from the fact that quantum systems can possess different features simultaneously, such as in wave-particle duality or quantum superposition. In recent delayed-choice 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 multi-dimensional quantum walks. In our experiment, the way in which a photon interferes with itself in a strongly non-trivial pattern depends on its polarization, which is determined after the photon has already been detected. This is the first experiment realizing a multi-dimensional 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
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    ABSTRACT: Enforcing a non-classical 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 three-level atom and the movable mirrors of a cavity is achieved. The resulting dynamics is able to conditionally prepare a non-classical state of the mirrors by means of projective measurements operated over a pure state of the atomic system. The non-classical 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
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    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 non-Markovian 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 non-Markovian behaviour quantifier (NMBQ) proposed by Rivas et. al. [1], we consider the role that resonant and off-resonant 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 non-Markovian effects, while after a certain threshold of coupling strength the off-resonant 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 non-Markovian behaviour.
    Journal of Physics B Atomic Molecular and Optical Physics 08/2013; 47(1). · 1.92 Impact Factor
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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 quantum-controlled devices, including single-photon sources, transistors and ultra-compact 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 trans-mitting and receiving light signals at the nanoscale 8 . These appli-cations 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 'quasi-particles' 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 single-photon 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 an
    Nature Physics 06/2013; 9:329-340. · 19.35 Impact Factor

Publication Stats

4k Citations
653.89 Total Impact Points

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 Opto-Electronic Convergence Systems
      Sŏul, Seoul, South Korea