Publications
274 results found
Rigovacca L, Kato G, Bauml S, et al., 2018, Versatile relative entropy bounds for quantum networks, New Journal of Physics, Vol: 20, ISSN: 1367-2630
We provide a versatile upper bound on the number of maximally entangled qubits, or private bits, shared by two parties via a generic adaptive communication protocol over a quantum network when the use of classical communication is not restricted. Although our result follows the idea of Azuma et al (2016 Nat. Commun. 7 13523) of splitting the network into two parts, our approach relaxes their strong restriction, consisting of the use of a single entanglement measure in the quantification of the maximum amount of entanglement generated by the channels. In particular, in our bound the measure can be chosen on a channel-by-channel basis, in order to make it as tight as possible. This enables us to apply the relative entropy of entanglement, which often gives a state-of-the-art upper bound, on every Choi-simulable channel in the network, even when the other channels do not satisfy this property. We also develop tools to compute, or bound, the max-relative entropy of entanglement for channels that are invariant under phase rotations. In particular, we present an analytical formula for the max-relative entropy of entanglement of the qubit amplitude damping channel.
Bose S, Mazumdar A, Morley GW, et al., 2017, Spin Entanglement Witness for Quantum Gravity, Physical Review Letters, Vol: 119, ISSN: 0031-9007
Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. However, the lack of empirical evidence has lead to a debate on whether gravity is a quantum entity. Despite varied proposed probes for quantum gravity, it is fair to say that there are no feasible ideas yet to test its quantum coherent behavior directly in a laboratory experiment. Here, we introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. We show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. We provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple spin correlation measurements.
Dieleman F, Tame MS, Sonnefraud Y, et al., 2017, Experimental Verification of Entanglement Generated in a Plasmonic System., Nano Letters, Vol: 17, Pages: 7455-7461, ISSN: 1530-6984
A core process in many quantum tasks is the generation of entanglement. It is being actively studied in a variety of physical settings-from simple bipartite systems to complex multipartite systems. In this work we experimentally study the generation of bipartite entanglement in a nanophotonic system. Entanglement is generated via the quantum interference of two surface plasmon polaritons in a beamsplitter structure, i.e., utilizing the Hong-Ou-Mandel (HOM) effect, and its presence is verified using quantum state tomography. The amount of entanglement is quantified by the concurrence and we find values of up to 0.77 ± 0.04. Verifying entanglement in the output state from HOM interference is a nontrivial task and cannot be inferred from the visibility alone. The techniques we use to verify entanglement could be applied to other types of photonic system and therefore may be useful for the characterization of a range of different nanophotonic quantum devices.
Armata F, Calajo G, Jaako T, et al., 2017, Harvesting Multiqubit Entanglement from Ultrastrong Interactions in Circuit Quantum Electrodynamics, Physical Review Letters, Vol: 119, ISSN: 0031-9007
We analyze a multi-qubit circuit QED system in the regime where the qubit-photon couplingdominates over the system’s bare energy scales. Under such conditions a manifold of low-energystates with a high degree of entanglement emerges. Here we describe a time-dependent protocol forextracting these quantum correlations and converting them into well-defined multi-partite entangledstates of non-interacting qubits. Based on a combination of various ultrastrong-coupling effects theprotocol can be operated in a fast and robust manner, while still being consistent with experimentalconstraints on switching times and typical energy scales encountered in superconducting circuits.Therefore, our scheme can serve as a probe for otherwise inaccessible correlations in strongly-coupledcircuit QED systems. It also shows how such correlations can potentially be exploited as a resourcefor entanglement-based applications.
Armata F, Latmiral L, Plato ADK, et al., 2017, Quantum limits to gravity estimation with optomechanics, Physical Review A, Vol: 96, ISSN: 1050-2947
We present a table-top quantum estimation protocol to measure the gravitational acceleration g by using an optomechanical cavity. In particular, we exploit the nonlinear quantum light-matter interaction between an optical field and a massive mirror acting as mechanical oscillator. The gravitational field influences the system dynamics affecting the phase of the cavity field during the interaction. Reading out such a phase carried by the radiation leaking from the cavity, we provide an estimate of the gravitational acceleration through interference measurements. Contrary to previous studies, having adopted a fully quantum description, we are able to propose a quantum analysis proving the ultimate bound to the estimability of the gravitational acceleration and verifying optimality of homodyne detection. Noticeably, thanks to the light-matter decoupling at the measurement time, no initial cooling of the mechanical oscillator is demanded in principle.
Wan KH, Dahlsten O, Kristjansson H, et al., 2017, Quantum generalisation of feedforward neural networks, npj Quantum Information, Vol: 3, ISSN: 2056-6387
We propose a quantum generalisation of a classical neural network. The classical neurons are firstly rendered reversible by adding ancillary bits. Then they are generalised to being quantum reversible, i.e., unitary (the classical networks we generalise are called feedforward, and have step-function activation functions). The quantum network can be trained efficiently using gradient descent on a cost function to perform quantum generalisations of classical tasks. We demonstrate numerically that it can: (i) compress quantum states onto a minimal number of qubits, creating a quantum autoencoder, and (ii) discover quantum communication protocols such as teleportation. Our general recipe is theoretical and implementation-independent. The quantum neuron module can naturally be implemented photonically.
Barnett SM, Beige A, Ekert A, et al., 2017, Journeys from quantum optics to quantum technology, PROGRESS IN QUANTUM ELECTRONICS, Vol: 54, Pages: 19-45, ISSN: 0079-6727
Sir Peter Knight is a pioneer in quantum optics which has now grown to an important branch of modern physics to study the foundations and applications of quantum physics. He is leading an effort to develop new technologies from quantum mechanics. In this collection of essays, we recall the time we were working with him as a postdoc or a PhD student and look at how the time with him has influenced our research.
Browne D, Bose S, Mintert F, et al., 2017, From quantum optics to quantum technologies, Progress in Quantum Electronics, Vol: 54, Pages: 2-18, ISSN: 0079-6727
Quantum optics is the study of the intrinsically quantum properties of light. During the second part of the 20th century experimental and theoretical progress developed together; nowadays quantum optics provides a testbed of many fundamental aspects of quantum mechanics such as coherence and quantum entanglement. Quantum optics helped trigger, both directly and indirectly, the birth of quantum technologies, whose aim is to harness non-classical quantum effects in applications from quantum key distribution to quantum computing. Quantum light remains at the heart of many of the most promising and potentially transformative quantum technologies. In this review, we celebrate the work of Sir Peter Knight and present an overview of the development of quantum optics and its impact on quantum technologies research. We describe the core theoretical tools developed to express and study the quantum properties of light, the key experimental approaches used to control, manipulate and measure such properties and their application in quantum simulation, and quantum computing.
Lv D, An S, Um M, et al., 2017, Reconstruction of the Jaynes-Cummings field state of ionic motion in a harmonic trap, PHYSICAL REVIEW A, Vol: 95, ISSN: 2469-9926
A quantum state is fully characterized by its density matrix or equivalently by its quasiprobabilities in phase space. A scheme to identify the quasiprobabilities of a quantum state is an important tool in the recent development of quantum technologies. One of the most fundamental interaction models in quantum optics is the so-called Jaynes-Cummings model (JCM), which has been massively studied theoretically and experimentally. However, the expected essential dynamics of the field states under the resonant JCM has not been observed experimentally due to the lack of a proper reconstruction scheme. In this paper, we further develop a highly efficient vacuum measurement scheme and study the JCM dynamics in a trapped ion system with the capability of the vacuum measurement to reconstruct its quasiprobability Q function, which is a preferred choice to study the core of the dynamics of a quantum state in phase space. During the JCM dynamics, the Gaussian peak of the initial coherent state bifurcates and rotates around the origin of phase space. They merge at the so-called revival time at the other side of phase space. The measured Q function agrees with the theoretical prediction. Moreover, we reconstruct the Wigner function by deconvoluting the Q function and observe the quantum interference in the Wigner function at half of the revival time, where the vibrational state becomes nearly disentangled from the internal energy states and forms a superposition of two composite states. The scheme can be applied to other physical setups including cavity or circuit-QED and optomechanical systems.
Bose S, Wan C, Scala M, et al., 2017, Comment on "Matter-Wave Interferometry of a Levitated Thermal Nano-Oscillator Induced and Probed by a Spin" Reply, PHYSICAL REVIEW LETTERS, Vol: 118, ISSN: 0031-9007
Rashid M, Tufarelli T, Bateman J, et al., 2016, Experimental realization of a thermal squeezed state of levitated optomechanics, Physical Review Letters, Vol: 117, ISSN: 1079-7114
We experimentally squeeze the thermal motional state of an optically levitated nanosphere by fast switching between two trapping frequencies. The measured phase-space distribution of the center of mass of our particle shows the typical shape of a squeezed thermal state, from which we infer up to 2.7 dB of squeezing along one motional direction. In these experiments the average thermal occupancy is high and, even after squeezing, the motional state remains in the remit of classical statistical mechanics. Nevertheless, we argue that the manipulation scheme described here could be used to achieve squeezing in the quantum regime if preceded by cooling of the levitated mechanical oscillator. Additionally, a higher degree of squeezing could, in principle, be achieved by repeating the frequency-switching protocol multiple times.
Rigovacca L, Franco CD, Metcalf BJ, et al., 2016, Non-Classicality Criteria in Multi-port Interferometry, Physical Review Letters, Vol: 117, ISSN: 1079-7114
Quantum interference lies at the basis of fundamental differences betweenquantum and classical behaviors. It is thus crucial to understand theboundaries between what interference patterns can be described by classicalwave mechanics and what, on the other hand, can only be understood with aproper quantum mechanical description. While a lot of work has already beendone for the simple case of two-mode interference, the multi-mode case has notbeen fully explored yet. Here we derive bounds for classical models of lightfields in a general scenario of intensity interferometry, and we show how theycan be violated in a quantum framework. As a consequence, this violation actsas a non-classicality witness, able to detect the presence of sources withsub-Poissonian photon-number statistics. We also derive a criterion forcertifying the indivisibility of a quantum interferometer and obtain a methodto simultaneously measure the average pairwise distinguishability of the inputsources.
Facchi P, Kim MS, Pascazio S, et al., 2016, Bound states and entanglement generation in waveguide quantum electrodynamics, PHYSICAL REVIEW A, Vol: 94, ISSN: 2469-9926
Kim MS, Wan C, Scala M, et al., 2016, Free nano-object Ramsey interferometry for large quantum superpositions, Physical Review Letters, Vol: 117, ISSN: 1079-7114
We propose an interferometric scheme based on an untrapped nano-object subjected to gravity.The motion of the center of mass (c.m.) of the free object is coupled to its internal spin systemmagnetically, and a free ight scheme is developed based on coherent spin control. The wavepacketof the test object, under a spin-dependent force, may then be delocalized to a macroscopic scale.A gravity induced dynamical phase (accrued solely on the spin state, and measured through aRamsey scheme) is used to reveal the above spatially delocalised superposition of the spin-nano-object composite system that arises during our scheme. We nd a remarkable immunity to themotional noise in the c.m. (initially in a thermal state with moderate cooling), and also a dynamicaldecoupling nature of the scheme itself. Together they secure a high visibility of the resulting Ramseyfringes. The mass independence of our scheme makes it viable for a nano-object selected from anensemble with a high mass variability. Given these advantages, a quantum superposition with 100nm spatial separation for a massive object of 10⁹ amu is achievable experimentally, providing a routeto test postulated modi cations of quantum theory such as continuous spontaneous localisation.
Boutari J, Feizpour A, Barz S, et al., 2016, Large scale quantum walks by means of optical fiber cavities, Journal of Optics, Vol: 18, ISSN: 2040-8986
We demonstrate a platform for implementing quantum walks that overcomes many of the barriers associated with photonic implementations. We use coupled fiber-optic cavities to implement time-bin encoded walks in an integrated system. We show that this platform can achieve very low losses combined with high-fidelity operations, enabling an unprecedented large number of steps in a passive system, as required for scenarios with multiple walkers. Furthermore the platform is reconfigurable, enabling variation of the coin, and readily extends to multidimensional lattices. We demonstrate variation of the coin bias experimentally for three different values.
Armata F, Latmiral L, Pikovski I, et al., 2016, Quantum and Classical Phases in Optomechanics, Physical Review A, Vol: 93, ISSN: 1094-1622
The control of quantum systems requires the ability to change and read-outthe phase of a system. The non-commutativity of canonical conjugate operatorscan induce phases on quantum systems, which can be employed for implementingphase gates and for precision measurements. Here we study the phase acquired bya radiation field after its radiation pressure interaction with a mechanicaloscillator, and compare the classical and quantum contributions. The classicaldescription can reproduce the nonlinearity induced by the mechanical oscillatorand the loss of correlations between mechanics and optical field at certaininteraction times. Such features alone are therefore insufficient for probingthe quantum nature of the interaction. Our results thus isolate genuine quantumcontributions of the optomechanical interaction that could be probed in currentexperiments.
Milburn TJ, Kim MS, Vanner MR, 2016, Nonclassical-state generation in macroscopic systems via hybrid discrete-continuous quantum measurements, Physical Review A, Vol: 93, ISSN: 1094-1622
Nonclassical-state generation is an important component throughout experimental quantum science for quantum information applications and probing the fundamentals of physics. Here, we investigate permutations of quantum nondemolition quadrature measurements and single quanta addition or subtraction to prepare quantum superposition states in bosonic systems. The performance of each permutation is quantified and compared using several different nonclassicality criteria including Wigner negativity, nonclassical depth, and optimal fidelity with a coherent-state superposition. We also compare the performance of our protocol using squeezing instead of a quadrature measurement and find that the purification provided by the quadrature measurement can significantly increase the nonclassicality generated. Our approach is ideally suited for implementation in light-matter systems such as quantum optomechanics and atomic spin ensembles, and offers considerable robustness to initial thermal occupation.
Latmiral L, Armata F, Genoni MG, et al., 2016, Probing anharmonicity of a quantum oscillator in an optomechanical cavity, Physical Review A, Vol: 93, ISSN: 1094-1622
We present a way of measuring with high precision the anharmonicity of a quantum oscillator coupled to an optical field via radiation pressure. Our protocol uses a sequence of pulsed interactions to perform a loop in the phase space of the mechanical oscillator, which is prepared in a thermal state. We show how the optical field acquires a phase depending on the anharmonicity. Remarkably, one only needs small initial cooling of the mechanical motion to probe even small anharmonicities. Finally, by applying tools from quantum estimation theory, we calculate the ultimate bound on the estimation precision posed by quantum mechanics and compare it with the precision obtainable with feasible measurements such as homodyne and heterodyne detection on the cavity field. In particular we demonstrate that homodyne detection is nearly optimal in the limit of a large number of photons of the field and we discuss the estimation precision of small anharmonicities in terms of its signal-to-noise ratio.
Kim MS, Wan C, Scala M, et al., 2016, Tolerance in the Ramsey interference of a trapped nanodiamond, Physical Review A, Vol: 93, ISSN: 1094-1622
The scheme recently proposed in [M. Scala et al., Phys Rev Lett 111, 180403 (2013)], where agravity-dependent phase shift is induced on the spin of a nitrogen-vacancy (NV) center in a trappednanodiamond by the interaction between its magnetic moment and the quantized motion of theparticle, provides a way to detect spatial quantum superpositions by means of spin measurementsonly. Here, the effect of unwanted coupling with other motional degrees of freedom is considered andwe show that it does not affect the validity of the scheme. Both this coupling and the additionalerror source due to misalignment between the quantization axis of the NV center spin and thetrapping axis are shown not to change the qualitative behavior of the system, so that a proof-ofprincipleexperiment can be neatly performed. Our analysis, which shows that the scheme retainsthe important features of not requiring ground state cooling and of being resistant to thermalfluctuations, can be useful for the several schemes which have been proposed recently for testingmacroscopic superpositions in trapped microsystems.
Kim MS, Um M, Zhang J, et al., 2016, Phonon arithmetic in a trapped ion system, Nature Communications, Vol: 7, ISSN: 2041-1723
Single-quantum level operations are important tools to manipulate a quantum state. Annihilation or creation of single particles translates a quantum state to another by adding or subtracting a particle, depending on how many are already in the given state. The operations are probabilistic and the success rate has yet been low in their experimental realization. Here we experimentally demonstrate (near) deterministic addition and subtraction of a bosonic particle, in particular a phonon of ionic motion in a harmonic potential. We realize the operations by coupling phonons to an auxiliary two-level system and applying transitionless adiabatic passage. We show handy repetition of the operations on various initial states and demonstrate by the reconstruction of the density matrices that the operations preserve coherences. We observe the transformation of a classical state to a highly non-classical one and a Gaussian state to a non-Gaussian one by applying a sequence of operations deterministically.
Hwang M-J, Kim MS, Choi M-S, 2016, Recurrent Delocalization and Quasiequilibration of Photons in Coupled Systems in Circuit Quantum Electrodynamics, Physical Review Letters, Vol: 116, ISSN: 1079-7114
We explore the photon population dynamics in two coupled circuit QED systems. For a sufficiently weakintercavity photon hopping, as the photon-cavity coupling increases, the dynamics undergoes doubletransitions first from a delocalized to a localized phase and then from the localized to another delocalizedphase. The latter delocalized phase is distinguished from the former one; instead of oscillating between thetwo cavities, the photons rapidly quasiequilibrate over the two cavities. These intriguing features areattributed to an interplay between two qualitatively distinctive nonlinear behaviors of the circuit QEDsystems in the utrastrong coupling regime, whose distinction has been widely overlooked.
Coehlho AS, Costanzo LS, Zavatta A, et al., 2016, Universal continuous-variable state orthogonalizer and qubit generator, Physical Review Letters, Vol: 116, ISSN: 1079-7114
We experimentally demonstrate a universal strategy for producing a quantum state that is orthogonalto an arbitrary, infinite-dimensional, pure input one, even if only a limited amount ofinformation about the latter is available. Arbitrary coherent superpositions of the two mutuallyorthogonal states are then produced by a simple change in the experimental parameters. We useinput coherent states of light to illustrate two variations of the method. However, we show that thescheme works equally well for arbitrary input fields and constitutes a universal procedure, whichmay thus prove a useful building block for quantum state engineering and quantum informationprocessing with continuous-variable qubits.
Plato ADK, Hughes CN, Kim MS, 2016, Gravitational effects in quantum mechanics, Contemporary Physics, Vol: 57, Pages: 477-495, ISSN: 1366-5812
To date, both quantum theory and Einstein’s theory of general relativity have passed every experimental test in their respective regimes. Nevertheless, almost since their inception, there has been debate surrounding whether they should be unified, and by now, there exists strong theoretical arguments pointing to the necessity of quantising the gravitational field. In recent years, a number of experiments have been proposed which, if successful, should give insight into features at the Planck scale. Here, we review some of the motivations, from the perspective of semi-classical arguments, to expect new physical effects at the overlap of quantum theory and general relativity. We conclude with a short introduction to some of the proposals being made to facilitate empirical verification.
Rahman ATMA, Frangeskou AC, Kim MS, et al., 2016, Burning and graphitization of optically levitated nanodiamonds in vacuum, Scientific Reports, Vol: 6, ISSN: 2045-2322
A nitrogen-vacancy (NV−) centre in a nanodiamond, levitated in high vacuum, has recently beenproposed as a probe for demonstrating mesoscopic centre-of-mass superpositions and for testingquantum gravity. Here, we study the behaviour of optically levitated nanodiamonds containing NV−centres at sub-atmospheric pressures and show that while they burn in air, this can be prevented byreplacing the air with nitrogen. However, in nitrogen the nanodiamonds graphitize below ≈10 mB.Exploiting the Brownian motion of a levitated nanodiamond, we extract its internal temperature(Ti) and find thatit would be detrimentalto the NV− centre’s spin coherence time. These values of Timake it clear that the diamond is not melting, contradicting a recent suggestion. Additionally, usingthe measured damping rate of a levitated nanoparticle at a given pressure, we propose a new way ofdetermining its size.
Vidrighin MD, Dahlsten O, Barbieri M, et al., 2016, Photonic Maxwell's demon, Physical Review Letters, Vol: 116, ISSN: 1079-7114
We report an experimental realization of Maxwell’s demon in a photonic setup. We show that ameasurement at the few-photons level followed by a feed-forward operation allows the extraction of workfrom intense thermal light into an electric circuit. The interpretation of the experiment stimulates thederivation of an equality relating work extraction to information acquired by measurement. We derive abound using this relation and show that it is in agreement with the experimental results. Our work putsforward photonic systems as a platform for experiments related to information in thermodynamics.
Latmiral L, Di Franco C, Mennea PL, et al., 2015, State-transfer simulation in integrated waveguide circuits, Physical Review A, Vol: 92, ISSN: 1094-1622
Spin-chain models have been widely studied in terms of quantum information processes, for instance for the faithful transmission of quantum states. Here, we investigate the limitations of mapping this process to an equivalent one through a bosonic chain. In particular, we keep in mind experimental implementations, which the progress in integrated waveguide circuits could make possible in the very near future. We consider the feasibility of exploiting the higher dimensionality of the Hilbert space of the chain elements for the transmission of a larger amount of information, and the effects of unwanted excitations during the process. Finally, we exploit the information-flux method to provide bounds to the transfer fidelity.
Tufarelli T, McEnery KR, Maier SA, et al., 2015, Signatures of the A2 term in ultrastrongly coupled oscillators, Physical Review A, Vol: 91, ISSN: 1094-1622
We study a bosonic matter excitation coupled to a single-mode cavity field via electric dipole. Counter-rotating and A2 terms are included in the interaction model, A being the vector potential of the cavity field. In the ultrastrong coupling regime the vacuum of the bare modes is no longer the ground state of the Hamiltonian and contains a nonzero population of polaritons, the true normal modes of the system. If the parameters of the model satisfy the Thomas-Reiche-Kuhn sum rule, we find that the two polaritons are always equally populated. We show how this prediction could be tested in a quenching experiment, by rapidly switching on the coupling and analyzing the radiation emitted by the cavity. A refinement of the model based on a microscopic minimal coupling Hamiltonian is also provided, and its consequences on our results are characterized analytically.
Ji S-W, Kim MS, Nha H, 2015, Quantum steering of multimode Gaussian states by Gaussian measurements: monogamy relations and the Peres conjecture, JOURNAL OF PHYSICS A-MATHEMATICAL AND THEORETICAL, Vol: 48, ISSN: 1751-8113
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- Citations: 34
Cho J, Bose S, Kim MS, 2015, Cavity-assisted energy relaxation for quantum many-body simulations, OPTICS COMMUNICATIONS, Vol: 337, Pages: 66-70, ISSN: 0030-4018
Vanner MR, Pikovski I, Kim MS, 2015, Towards optomechanical quantum state reconstruction of mechanical motion, ANNALEN DER PHYSIK, Vol: 527, Pages: 15-26, ISSN: 0003-3804
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- Citations: 42
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