## Publications

213 results found

Pikovski I, Vanner MR, Aspelmeyer M,
et al., 2012, Probing Planck-scale physics with quantum optics (vol 11, pg 1643, 2002), *NATURE PHYSICS*, Vol: 8, ISSN: 1745-2473

Pikovski I, Vanner MR, Aspelmeyer M,
et al., 2012, Probing Planck-scale physics with quantum optics (vol 11, pg 1643, 2002), *NATURE PHYSICS*, Vol: 8, ISSN: 1745-2473

Sudhir V, Genoni MG, Lee J,
et al., 2012, Critical behavior in ultrastrong-coupled oscillators, *PHYSICAL REVIEW A*, Vol: 86, ISSN: 1050-2947

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- Citations: 13

Tufarelli T, Ferraro A, Kim MS,
et al., 2012, Reconstructing the quantum state of oscillator networks with a single qubit, *PHYSICAL REVIEW A*, Vol: 85, ISSN: 2469-9926

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- Citations: 11

Vacanti G, Fazio R, Kim MS,
et al., 2012, Geometric-phase backaction in a mesoscopic qubit-oscillator system, *PHYSICAL REVIEW A*, Vol: 85, ISSN: 1050-2947

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- Citations: 20

BUZEK V, GANTSOG T, KIM M, 2011, PHASE PROPERTIES OF SCHRODINGER CAT STATES OF LIGHT DECAYING IN PHASE-SENSITIVE RESERVOIRS, *PHYSICA SCRIPTA*, Vol: T48, Pages: {131-142}-{131-142}, ISSN: 0281-1847

We study phase properties of quantum mechanical superposition states decaying into phase-sensitive reservoirs. We show that the Wigner phase quasiprobability distribution reflects the quantum interference effects very well while the Pegg-Barnett and the Husimi phase probability distributions are quite insensitive with respect to the character of the quantum interference between component states. We study in detail phase properties of the even coherent state which decays into the thermal and the squeezed reservoirs. In thermal reservoirs the phase of the superposition state becomes randomized under the influence of thermal fluctuations. The higher the number of thermal photons the faster the randomization process is. On the other hand we show that depending on the relative phase between the superposition state and the phase-sensitive reservoir the process of phase randomization can be either enhanced or completely s suppressed.

BUZEK V, KIM M, 2011, DECAY WITHOUT LOSSES - MEAN-PHOTON-NUMBER DECAY AND SHANNON ENTROPY DECAY OF MICROMASER FIELDS VIA CONDITIONAL MEASUREMENTS, *JOURNAL OF THE KOREAN PHYSICAL SOCIETY*, Vol: 27, Pages: {215-220}-{215-220}, ISSN: 0374-4884

We investigate the dynamics of a lossless micromaser when a single-mode cavity field is resonantly coupled via one-photon transitions to two-level atoms crossing the cavity in a sequence. We analyze the statistical properties of the cavity field when the atoms after the interaction are conditionally measured, provided the interaction time is short. Namely, we select only those sequences wherein each atom after the interaction is measured in its initial state. We assume that the total sequence duration is smaller than the cavity decay time so that cavity losses can be neglected. We show that even though there is no direct transfer of the energy from the atoms to the field and vice versa, the mean photon number of the cavity field is reduced due to the conditional measurement. This energy ‘’decay” is accompanied by a reduction of the Shannon entropy of the field. We show that there exists an attractor state of the cavity field which is the Fock state with the smallest photon number which contributes to the initial state of the field. Finally, we find the remarkable result that the time evolution of a field initially prepared in a coherent state which is conditionally measured by the sequence of atoms is exactly the same as the time evolution of a coherent field decaying into a zero-temperature heat bath.

BUZEK V, KIM M, GANTSOG T, 2011, QUANTUM PHASE DISTRIBUTIONS OF AMPLIFIED SCHRODINGER-CAT STATES OF LIGHT, *PHYSICAL REVIEW A*, Vol: 48, Pages: {3394-3397}-{3394-3397}, ISSN: 1050-2947

We study the phase properties of quantum superpositions of two coherent states (Schrodinger-cat states) amplified by phase-sensitive (squeezed) amplifiers. We show that a phase-sensitive amplifier with a properly chosen phase can preserve the phase distribution of the Schrodinger-cat-state input.

BUZEK V, KIM M, KIM M, 2011, ENTROPIC UNCERTAINTY RELATIONS FOR OPERATIONAL PHASE-SPACE MEASUREMENT, *JOURNAL OF THE KOREAN PHYSICAL SOCIETY*, Vol: 28, Pages: {529-533}-{529-533}, ISSN: 0374-4884

We introduce entropic measures of uncertainty related to operational phase-space probability density distributions. With the help of these entropies, we find the entropic uncertainty relation for the position ((q) over cap) and the momentum ((p) over cap) operators of a quantum-mechanical system in the form S-Psi Phi((q)) + S-Psi Phi((p)) greater than or equal to S-Psi Phi((q,p)) greater than or equal to k(B). The fundamental lower bound on the entropic uncertainty relation is equal to the Boltzmann constant k(B).

BUZEK V, KIM M, KIM M, 2011, DESCRIPTION OF THE INFLUENCE OF A RESERVOIR VIA COARSE-GRAINING IN PHASE-SPACE, *JOURNAL OF THE KOREAN PHYSICAL SOCIETY*, Vol: 28, Pages: {123-127}-{123-127}, ISSN: 0374-4884

We show that the Q-function of a single-mode field which is attenuated or amplified by a phase-sensitive environment can be expressed through a coarse-grained Q-function with time-dependent coarse-graining parameters. This fact can be utilized for simulation of the decay of quantum systems into various environments. In particular, we have derived an analytical expression describing the time-evolution of the Q-function of the initial Fock state which decays into a phase-sensitive reservoir.

Ballester D, Cho J, Kim MS, 2011, Generation of graph-state streams, *PHYSICAL REVIEW A*, Vol: 83, ISSN: 1050-2947

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- Citations: 2

Cho J, Bose S, Kim MS, 2011, Optical Pumping into Many-Body Entanglement, *PHYSICAL REVIEW LETTERS*, Vol: 106, ISSN: 0031-9007

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- Citations: 52

Cho J, Kim MS, 2011, Two-Dimensional Imaging of Gauge Fields in Optical Lattices, *PHYSICAL REVIEW LETTERS*, Vol: 107, ISSN: 0031-9007

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- Citations: 2

Di Franco C, Paternostro M, Kim MS, 2011, Bypassing State Initialisation in Perfect State Transfer Protocols on Spin-Chains, 5th Conference on Theory of Quantum Computation, Communication and Cryptography, Publisher: SPRINGER, Pages: 168-+, ISSN: 0302-9743

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- Citations: 1

Janszky J, Kim MG, Kim MS, 2011, Quasiprobabilities and the nonclassicality of fields, *PHYSICAL REVIEW A*, Vol: 53, Pages: {502-506}-{502-506}, ISSN: 1050-2947

The existence of positive well-defined quasiprobabilities is known to be related to the measure of nonclassicality for a given field state. We test the coincidence of the loss of well-known nonclassical properties with the appearance of the positive well-defined quasiprobabilities for various fields influenced by phase-insensitive reservoirs. For Gaussian fields the appearance of a positive Glauber-Sudarshan P function coincides with the loss of nonclassical properties in quadrature fluctuations and in photon statistics. We find that when an initial Fock state dissipates or is amplified the initial field loses its nonclassicality in photon statistics before the P function becomes positive, which is different from the Gaussian field. The Wigner function may be positive even when the decaying Fock state is sub-Poissonian. For the amplification of the Pock state, however, the Wigner function is positive only when the field is super-Poissonian. For the dissipation and amplification of the Fock state the point when a certain quasiprobability function becomes positive definite depends only on the average number of photons N of the heat bath.

KIM M, BUZEK V, 2011, PHOTON STATISTICS OF SUPERPOSITION STATES IN PHASE-SENSITIVE RESERVOIRS, *PHYSICAL REVIEW A*, Vol: 47, Pages: {610-619}-{610-619}, ISSN: 1050-2947

Using the Wiper function formalism in phase space, we analyze the decay of quantum coherences in phase-sensitive reservoirs. We show that the decay rate of quantum coherences in phase-sensitive reservoirs can be significantly modified compared to the decay rate in ordinary (phase-insensitive) thermal reservoirs. Depending on the phases of the quantum system (field mode) and the squeezed reservoir, the decay rate of the quantum coherence can be either enhanced or significantly suppressed, which is in agreement with the results obtained recently by other methods [T. A. B. Kennedy and D. F. Walls, Phys. Rev. A 37, 152 (1988)). We show that in an ideally squeezed reservoir with a high degree of squeezing, the decay rate of the quantum coherence (i.e., the decay rate of off-diagonal terms of the density matrix in the coherent-state basis) can be equal to the decay rate of the energy of the system (i.e., the decay rate of diagonal terms of the density matrix). Suppression of the decay rate of the quantum coherence leads to preservation of nonclassical effects such as the oscillations in the photon number distribution. Moreover, we find that some initial superposition states of light exhibiting super-Poissonian photon statistics can be transformed into intermediate sub-Poissonian states under the influence of phase-sensitive reservoirs.

KIM M, BUZEK V, 2011, SCHRODINGER-CAT STATES AT FINITE TEMPERATURE - INFLUENCE OF A FINITE-TEMPERATURE HEAT BATH ON QUANTUM INTERFERENCES, *PHYSICAL REVIEW A*, Vol: 46, Pages: {4239-4251}-{4239-4251}, ISSN: 1050-2947

Recently several methods have been proposed for generation of superposition (Schrodinger-cat) states in microwave cavities. At microwave frequencies, thermal photons can significantly affect statistical properties of superposition states. In the present paper we study the influence of a thermal heat bath on nonclassical properties of quantum superposition states. We show that at nonzero temperature the loss of coherences is much faster than at zero temperature. Using the formalism of quasiprobability distributions and solving the corresponding Fokker-Planck equations, we describe the time evolution of the super-position states in phase space and derive the rate of the decay of quantum coherence. This decay rate depends on the separation between the component states and on the temperature of the heat bath. Moreover, we discuss in detail how the interaction with a nonzero-temperature heat bath leads to a transformation of a nonclassical state to a classical state. We show that the sensitivity of the quantum coherence to the presence of thermal photons can lead to some difficulties in the preparation of Schrodinger-cat states in microwave cavities unless the temperature of the microwave cavity is sufficiently low.

KIM M, BUZEK V, 2011, DECAY OF QUANTUM COHERENCES UNDER THE INFLUENCE OF A THERMAL HEATBATH - SCHRODINGER CAT STATES AT FINITE TEMPERATURE, *JOURNAL OF MODERN OPTICS*, Vol: 39, Pages: {1609-1614}-{1609-1614}, ISSN: 0950-0340

Recently several methods have been proposed for generation of superposition (Schrodinger cat) states in microwave cavities. At microwave frequencies thermal photons can significantly affect statistical properties of superposition states. In the present letter we study the influence of a thermal heatbath on non-classical properties of quantum superposition states. We show that at non-zero temperatures the loss of quantum coherences is much faster than at zero temperature and that the sensitivity of the quantum coherence to the presence of thermal photons can lead to some difficulties in the preparation of Schrodinger cat states in microwave cavities unless the temperature of the microwave cavity is sufficiently low.

KIM M, IMOTO N, 2011, PHASE-SENSITIVE RESERVOIR MODELED BY BEAM-SPLITTERS, *PHYSICAL REVIEW A*, Vol: 52, Pages: {2401-2410}-{2401-2410}, ISSN: 1050-2947

The superposition of input fields in a lossless beam splitter is studied in the Schrodinger picture by using the convolution of the positive P representations, and the convolution law for these representations is extended to other quasiprobability functions such as the Wigner and Q functions. We show that the reservoir can be modeled by an infinite array of beam splitters, and we use the convolution law and this model to derive the Fokker-Planck equation for a system coupled with a phase-sensitive reservoir. Solving this equation shows that a phase-sensitive attenuation and amplification can be described by the superposition of two independent quantum fields, one of which is the initial signal field and the other the squeezed thermal noise field representing the reservoir.

KIM M, LEE K, BUZEK V, 2011, AMPLIFICATION OF SUPERPOSITION STATES IN PHASE-SENSITIVE AMPLIFIERS, *PHYSICAL REVIEW A*, Vol: 47, Pages: {4302-4307}-{4302-4307}, ISSN: 1050-2947

We study statistical properties of quantum superposition states (Schrodinger-cat states) amplified by phase-sensitive (squeezed) amplifiers. We show that the phase-sensitive amplifier with a properly chosen phase can preserve quantum coherences and nonclassical behavior of the Schrodinger-cat-state input even for a gain factor G larger than 2. In particular, we show that for an even coherent state (CS) phase-sensitive amplifiers can preserve squeezing for G > 2 but simultaneously in the process of amplification the noise added by the amplifier leads to a rapid increase of fluctuations in the photon number. Because of the finite maximum degree of squeezing obtainable for the even CS the maximum gain factor G(m) for which squeezing can still be observed in the output state is finite. The phase-sensitive amplifier with a properly chosen phase can also reduce fluctuations in the photon number of the initial even CS. Nevertheless, one cannot amplify the initial even CS with super-Poissonian photon statistics into the state with sub-Poissonian photon statistics.

Kim MG, Imoto N, Cho K,
et al., 2011, Quantum noise in optical beam propagation in distributed amplifiers, *OPTICS COMMUNICATIONS*, Vol: 130, Pages: {377-384}-{377-384}, ISSN: 0030-4018

Quantum effects on optical beam propagation in a gain medium having distributed loss are analyzed using an amplifier-attenuator array model. For the amplifier-attenuator set, the Wigner function of the output field is related to that of the input field by a convolution law. Using this, we derive the Fokker-Planck equation for the beam propagation in the distributed amplifier with loss, and the added quantum noise is analyzed by solving this equation. When the amplification just compensates the attenuation, the mean amplitude of the signal does not change but the mean energy increases due to the added noise. Even in this case, the photon number variance can be reduced when a proper phase-sensitive reservoir is incorporated.

Kim MS, Antesberger G, Bodendorf CT,
et al., 2011, Scheme for direct observation of the Wigner characteristic function in cavity QED, *PHYSICAL REVIEW A*, Vol: 58, Pages: {R65-R68}-{R65-R68}, ISSN: 1050-2947

We suggest a quantum state reconstruction scheme. Our proposal is applicable to the field inside a microwave cavity as well as to the harmonic motion of a trapped atom. It will be shown that the inversion of two-level atoms after a resonant interaction with a coherently displaced quantum state is directly related to the Wigner characteristic function of the initial state. This method neither requires the preparation of atoms in a quantum superposition of the upper and lower energy levels nor heavy numerical processing of the measured data. We demonstrate the reconstruction scheme for the example of a Schrodinger-cat superposition state.

Kim MS, Sanders BC, 2011, Squeezing and antisqueezing in homodyne measurements, *PHYSICAL REVIEW A*, Vol: 53, Pages: {3694-3697}-{3694-3697}, ISSN: 1050-2947

Squeezing the signal field or the local oscillator can improve the accuracy of homodyne detection. We show that an equivalence exists between antisqueezing the signal field and squeezing the local oscillator state. The importance of this result is illustrated in two contexts: a homodyne measurement of squeezing and the reconstruction of the wave function.

LEE K, KIM M, LEE S,
et al., 2011, SQUEEZING PROPERTIES OF MULTICOMPONENT SUPERPOSITION STATES OF LIGHT, *JOURNAL OF THE KOREAN PHYSICAL SOCIETY*, Vol: 26, Pages: {197-204}-{197-204}, ISSN: 0374-4884

Interacting with a Kerr medium, an input coherent field evolves into a generalized coherent state. We show that at the interaction time lambdat = pi/N, where lambda is the nonlinear coupling coefficient, the generalized coherent state is transformed into a superposition of N coherent component states. Using the Wigner-function phase-space formalism, we show that the noise reduction takes place along rotated quadrature axes for the superposition states and for the generalized coherent states. The influence of a dissipative environment (vacuum reservoir) on the squeezing properties of these multicomponent superposition states is investigated. In the stationary limit, the noise is again reduced to the vacuum level. Deterioration of squeezing is related with the rapid decay of quantum coherences under the influence of the reservoir.

Lee J, Kim MS, Nha H, 2011, Comment on "Role of Initial Entanglement and Non-Gaussianity in the Decoherence of Photon-Number Entangled States Evolving in a Noisy Channel", *PHYSICAL REVIEW LETTERS*, Vol: 107, ISSN: 0031-9007

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- Citations: 11

SANDERS B, LEE K, KIM M, 2011, OPTICAL HOMODYNE MEASUREMENTS AND ENTANGLED COHERENT STATES, *PHYSICAL REVIEW A*, Vol: 52, Pages: {735-741}-{735-741}, ISSN: 1050-2947

Tufarelli T, Kim MS, Bose S, 2011, Oscillator state reconstruction via tunable qubit coupling in Markovian environments, *PHYSICAL REVIEW A*, Vol: 83, ISSN: 1050-2947

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- Citations: 10

Vanner MR, Pikovski I, Cole GD,
et al., 2011, Pulsed quantum optomechanics, *PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA*, Vol: 108, Pages: 16182-16187, ISSN: 0027-8424

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- Citations: 134

Campbell S, Paternostro M, Bose S,
et al., 2010, Probing the environment of an inaccessible system by a qubit ancilla, *PHYSICAL REVIEW A*, Vol: 81, ISSN: 1050-2947

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- Citations: 13

Cho J, Kim MS, 2010, Emergence of Canonical Ensembles from Pure Quantum States, *PHYSICAL REVIEW LETTERS*, Vol: 104, ISSN: 0031-9007

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- Citations: 23

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