Bose-Einstein Condensation in a Plasmonic Lattice. (arXiv:1706.01528v2 [cond-mat.quant-gas] UPDATED) Quantum Physics - Tue, 2017-09-19 19:45

Bose-Einstein condensation is a remarkable manifestation of quantum statistics and macroscopic quantum coherence. Superconductivity and superfluidity have their origin in Bose-Einstein condensation. Ultracold quantum gases have provided condensates close to the original ideas of Bose and Einstein, while condensation of polaritons and magnons have introduced novel concepts of non-equilibrium condensation. Here, we demonstrate a Bose-Einstein condensate (BEC) of surface plasmon polaritons in lattice modes of a metal nanoparticle array. Interaction of the nanoscale-confined surface plasmons with a room-temperature bath of dye molecules enables thermalization and condensation in picoseconds. The ultrafast thermalization and condensation dynamics are revealed by an experiment that exploits thermalization under propagation and the open cavity character of the system. A crossover from BEC to usual lasing is realized by tailoring the band structure. This new condensate of surface plasmon lattice excitations has promise for future technologies due to its ultrafast, room-temperature and on-chip nature.

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Two-color phase-of-the-phase spectroscopy in the multiphoton regime. (arXiv:1706.01565v2 [physics.atom-ph] UPDATED) Quantum Physics - Tue, 2017-09-19 19:45

Momentum-resolved photoelectron emission from xenon in colinearly polarized two-color laser fields at above-threshold ionization conditions is studied both experimentally and theoretically. We utilize phase-of-the-phase spectroscopy as recently introduced by Skruszewicz et al., Phys. Rev. Lett. 115, 043001 (2015) to analyze the dependence of the yields on the relative phase $\varphi$ between the fundamental and second harmonic laser fields. The resulting phase-of-phase spectra feature a characteristic checkerboard pattern, which can analytically be described within the strong-field approximation.

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Internal clock formulation of quantum mechanics. (arXiv:1706.00743v2 [gr-qc] UPDATED) Quantum Physics - Tue, 2017-09-19 19:45

The basic tenet of the present work is the assumption of the lack of external and fixed time in the Universe. This assumption is best embodied by general relativity, which replaces the fixed space-time structure with the gravitational field, which is subject to dynamics. The lack of time does not imply the lack of evolution but rather brings to the forefront the role of internal clocks which are some largely arbitrary internal degrees of freedom with respect to which the evolution of timeless systems can be described. We take this idea seriously and try to understand what it implies for quantum mechanics when the fixed external time is replaced by an arbitrary internal clock. We put the issue in a solid, mathematically rigorous framework. We find that the dynamical interpretation of a quantum state of a timeless system depends on the employed internal clock. In particular, we find that the continuous spectra of well-known dynamical observables like the position of a free particle on the real line may turn discrete if measured in unusual clocks. We discuss the meaning of our result for attempts at quantization of global gravitational degrees of freedom.

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Reexamination of the Abraham-Minkowski dilemma

Author(s): Mário G. Silveirinha

Here the Abraham-Minkowski controversy on the correct definition of the light momentum in a macroscopic medium is revisited with the purpose to highlight that an effective medium formalism necessarily restricts the available information on the internal state of a system, and that this is ultimately ...

[Phys. Rev. A 96, 033831] Published Tue Sep 19, 2017

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Force-induced transparency and conversion between slow and fast light in optomechanics

Author(s): Zhen Wu, Ren-Hua Luo, Jian-Qi Zhang, Yu-Hua Wang, Wen Yang, and Mang Feng

The optomechanics can generate fantastic effects of optics due to appropriate mechanical control. Here we theoretically study effects of slow and fast lights in a single-sided optomechanical cavity with an external force. The force-induced transparency of slow and fast lights and the force-dependent...

[Phys. Rev. A 96, 033832] Published Tue Sep 19, 2017

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Quantum noise reduction in intensity-sensitive surface-plasmon-resonance sensors

Author(s): Joong-Sung Lee, Trung Huynh, Su-Yong Lee, Kwang-Geol Lee, Jinhyoung Lee, Mark Tame, Carsten Rockstuhl, and Changhyoup Lee

We investigate the use of twin-mode quantum states of light with symmetric statistical features in their photon number for improving intensity-sensitive surface plasmon resonance (SPR) sensors. For this purpose, one of the modes is sent into a prism setup where the Kretschmann configuration is emplo...

[Phys. Rev. A 96, 033833] Published Tue Sep 19, 2017

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Time-dependent spectrum of a single photon and its positive-operator-valued measure

Author(s): S. J. van Enk

Suppose we measure the time-dependent spectrum of a single photon. That is, we first send the photon through a set of frequency filters (which we assume to have different filter frequencies but the same finite bandwidth Γ) and then record at what time (with some finite precision Δt and some finite e...

[Phys. Rev. A 96, 033834] Published Tue Sep 19, 2017

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Quantum Correlations in Nonlocal Boson Sampling

Author(s): Farid Shahandeh, Austin P. Lund, and Timothy C. Ralph

Determination of the quantum nature of correlations between two spatially separated systems plays a crucial role in quantum information science. Of particular interest is the questions of if and how these correlations enable quantum information protocols to be more powerful. Here, we report on a dis...

[Phys. Rev. Lett. 119, 120502] Published Tue Sep 19, 2017

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Synopsis: The Softness of an Atom’s Touch

APS Physics - Tue, 2017-09-19 12:00

Two spectroscopic probes are combined to measure the “softness” of collisions between cold atoms.

[Physics] Published Tue Sep 19, 2017

Categories: Physics

Graph-associated entanglement cost of a multipartite state in exact and finite-block-length approximate constructions

PRA: Quantum information - Tue, 2017-09-19 12:00

Author(s): Hayata Yamasaki, Akihito Soeda, and Mio Murao

We introduce and analyze graph-associated entanglement cost, a generalization of the entanglement cost of quantum states to multipartite settings. We identify a necessary and sufficient condition for any multipartite entangled state to be constructible when quantum communication between the multiple...

[Phys. Rev. A 96, 032330] Published Tue Sep 19, 2017

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Uncertainty relations with quantum memory for the Wehrl entropy. (arXiv:1709.04921v1 [quant-ph]) Quantum Physics - Mon, 2017-09-18 19:37

We prove two new fundamental uncertainty relations with quantum memory for the Wehrl entropy. These are the first entropic uncertainty relations with quantum memory ever proposed for a single measurement. The first relation applies to the bipartite memory scenario and provides a lower bound to the Wehrl entropy of a quantum state conditioned on the memory quantum system in terms of the von Neumann entropy of the same quantum state conditioned on the same memory quantum system. The second relation applies to the tripartite memory scenario and provides a lower bound to the sum of the Wehrl entropy of a quantum state conditioned on the first memory quantum system with the Wehrl entropy of the same state conditioned on the second memory quantum system. The Wehrl entropy of a quantum state is the Shannon differential entropy of the outcome of a heterodyne measurement performed on the state. The heterodyne measurement is one of the main measurements in quantum optics, and lies at the basis of one of the most promising protocols for quantum key distribution. These fundamental entropic uncertainty relations will be a valuable tool in quantum information, and will e.g. find application in security proofs of quantum key distribution protocols in the asymptotic regime and in entanglement witnessing in quantum optics.

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Measuring Entanglement Negativity. (arXiv:1709.04923v1 [quant-ph]) Quantum Physics - Mon, 2017-09-18 19:37

Entanglement not only plays a crucial role in quantum technologies, but is key to our understanding of quantum correlations in many-body systems. However, in an experiment, the only way of measuring entanglement in a generic mixed state is through reconstructive quantum tomography, requiring an exponential number of measurements in the system size. Here, we propose an operational scheme to measure the entanglement --- as given by the negativity --- between arbitrary subsystems of size $N_A$ and $N_B$, with $\mathcal{O}(N_A + N_B)$ measurements, and without any prior knowledge of the state. We propose how to experimentally measure the partially transposed moments of a density matrix, and using just the first few of these, extract the negativity via Chebyshev approximation or machine learning techniques. Our procedure will allow entanglement measurements in a wide variety of systems, including strongly interacting many body systems in both equilibrium and non-equilibrium regimes.

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Interpretation of quantum mechanics with indefinite norm. (arXiv:1709.04925v1 [quant-ph]) Quantum Physics - Mon, 2017-09-18 19:37

The Born postulate can be reduced to its deterministic content that only applies to eigenvectors of observables: the standard probabilistic interpretation of generic states then follows from algebraic properties of repeated measurements and states. Extending this reasoning suggests an interpretation of quantum mechanics generalized with indefinite quantum norm.

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Algorithms for Embedding Quantum-Dot Cellular Automata Networks onto a Quantum Annealing Processor. (arXiv:1709.04972v1 [quant-ph]) Quantum Physics - Mon, 2017-09-18 19:37

Advancements in computing based on qubit networks, and in particular the flux-qubit processor architecture developed by D-Wave System's Inc., have enabled the physical simulation of quantum-dot cellular automata (QCA) networks beyond the limit of classical methods. However, the embedding of QCA networks onto the available processor architecture is a key challenge in preparing such simulations. In this work, two approaches to embedding QCA circuits are characterized: a dense placement algorithm that uses a routing method based on negotiated congestion; and a heuristic method implemented in D-Wave's Solver API package. A set of benchmark QCA networks is used to characterise the algorithms and a stochastic circuit generator is employed to investigate the performance for different processor sizes and active flux-qubit yields.

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Integral transforms of the quantum mechanical path integral: hit function and path averaged potential. (arXiv:1709.04984v1 [quant-ph]) Quantum Physics - Mon, 2017-09-18 19:37

We introduce two new integral transforms of the quantum mechanical transition kernel that represent physical information about the path integral. These transforms can be interpreted as probability distributions on particle trajectories measuring respectively the relative contribution to the path integral from paths crossing a given spatial point (the hit function) and the likelihood of values of the line integral of the potential along a path in the ensemble (the path averaged potential).

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Entanglement Constraints on States Locally Connected to the Greenberger-Horne-Zeilinger State. (arXiv:1709.05004v1 [quant-ph]) Quantum Physics - Mon, 2017-09-18 19:37

The multi-qubit GHZ state possesses tangles with elegant transformation properties under stochastic local operations and classical communication. Since almost all pure 3-qubit states are connected to the GHZ state via SLOCC, we derive a necessary and sufficient achievability inequality on arbitrary 3-qubit tangles, which is a strictly stronger constraint than both the monogamy inequality and the marginal eigenvalue inequality. We then show that entanglement shared with any single party in the n-qubit GHZ SLOCC equivalence class is precisely accounted for by the sum of its k-tangles, recently coined the strong monogamy equality, acknowledging competing but agreeing definitions of the k-tangle on this class, one of which is then computable for arbitrary mixed states. Strong monogamy is known to not hold arbitrarily, and so we introduce a unifying outlook on entanglement constraints in light of basic real algebraic geometry.

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Quantum walks on regular uniform hypergraphs. (arXiv:1709.05015v1 [quant-ph]) Quantum Physics - Mon, 2017-09-18 19:37

Quantum walks on graphs have shown prioritized benefits and applications in wide areas. In some scenarios, however, it may be more natural and accurate to mandate high-order relationships for hypergraphs, due to the density of information stored inherently. Therefore, we can explore the potential of quantum walks on hypergraphs. In this paper, by presenting the one-to-one correspondence between regular uniform hypergraphs and bipartite graphs, we construct a model for quantum walks on bipartite graphs of regular uniform hypergraphs with Szegedy's quantum walks, which gives rise to a quadratic speed-up. Furthermore, we deliver spectral properties of the transition matrix, given that the cardinalities of the two disjoint sets are different in the bipartite graph. Our model provides the foundation for building quantum algorithms on the strength of quantum walks, suah as quantum walks search, quantized Google's PageRank and quantum machine learning, based on hypergraphs.

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Non-closure of the set of quantum correlations via graphs. (arXiv:1709.05032v1 [math.OA]) Quantum Physics - Mon, 2017-09-18 19:37

We prove that the set of quantum correlations for a bipartite system of 5 inputs and 2 outputs is not closed. Our proof relies on computing the correlation functions of a graph, which is a concept that we introduce.

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Energy spectrum of the Kronig-Penney model in the presence of the strong spin-orbit coupling. (arXiv:1709.05039v1 [cond-mat.mes-hall]) Quantum Physics - Mon, 2017-09-18 19:37

The Kronig-Penney model, an exactly solvable one-dimensional model of crystal in solid physics, shows how the allowed and forbidden bands are formed in solids. In this paper, we study this model in the presence of both the strong spin-orbit coupling and the Zeeman field. We analytically obtain four transcendental equations, each defined in the corresponding energy regime. Solving these four transcendental equations, we obtain the total band structure of the Kronig-Penney model exactly. Our results give a clear physical picture of the combined effects of the strong spin-orbit coupling and the periodic potential. Especially, the energy anticrossing can give rise to a large bad gap in this model.

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R\'enyi Entropies from Random Quenches in Atomic Hubbard and Spin Models. (arXiv:1709.05060v1 [quant-ph]) Quantum Physics - Mon, 2017-09-18 19:37

We present a scheme for measuring R\'enyi entropies in generic atomic Hubbard and spin models using single copies of a quantum state and for partitions in arbitrary spatial dimension. Our approach is based on the generation of random unitaries from random quenches, implemented using engineered time-dependent disorder potentials, and standard projective measurements, as realized by quantum gas microscopes. By analyzing the properties of the generated unitaries and the role of statistical errors, with respect to the size of the partition, we show that the protocol can be realized in exisiting AMO quantum simulators, and used to measure for instance area law scaling of entanglement in two-dimensional spin models or the entanglement growth in many-body localized systems.

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