Physics

A distinguishable single excited-impurity in a Bose-Einstein condensate. (arXiv:1802.01897v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

We investigate the properties of a distinguishable single excited state impurity pinned in the center of a trapped Bose-Einstein condensate (BEC) in a one-dimensional harmonic trapping potential by changing the bare mass of the impurity and its interspecies interaction strength with the BEC. We model our system by using two coupled differential equations for the condensate and the single excited-impurity wave function, which we solve numerically. For equilibrium, we obtain that an excited-impurity induces two bumps or dips on the condensate for the attractive- or repulsive-interspecies coupling strengths, respectively. Afterwards, we show that the excited-impurity induced imprint upon the condensate wave function remains present during a time-of-flight (TOF) expansion after having switched off the harmonic confinement. We also investigate shock-waves or gray-solitons by switching off the interspecies coupling strength in the presence of harmonic trapping potential. During this process, we found out that the generation of gray bi-soliton or gray quad-solitons (four-solitons) depends on the bare mass of the excited-impurity in a harmonic trap.

Categories: Journals, Physics

Stochasticity and Bell-type Quantum Field Theory. (arXiv:1802.01898v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

This paper critically discusses an objection proposed by H. Nikolic against the naturalness of the stochastic dynamics implemented by the Bell-type Quantum Field Theory, an extension of Bohmian Mechanics able to describe the phenomena of particles creation and annihilation. Here I present: (i) Nikolic's ideas for a pilot-wave theory accounting for QFT phenomenology evaluating the robustness of his criticism, (ii) Bell's original proposal for a Bohmian QFT with a particle ontology and (iii) the mentioned Bell-type QFT. I will argue that although Bell's model should be interpreted as a heuristic example showing the possibility to extend Bohm's pilot- wave theory to the domain of QFT, the same judgement does not hold for the Bell-type QFT, which is candidate to be a promising possible alternative proposal to the standard version of quantum field theory. Finally, contra Nikolic, I will provide arguments in order to show how a stochastic dynamics is perfectly compatible with a Bohmian quantum theory.

Categories: Journals, Physics

Programming infinite machines. (arXiv:1802.01910v1 [cs.LO])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

For infinite machines which are free from the classical Thompson's lamp paradox we show that they are not free from its inverted version. We provide a program for infinite machines and an infinite mechanism which simulate this paradox. While their finite analogs work predictably, the program and the infinite mechanism demonstrate an undefined behavior. As in the case of infinite Davies's machines, our examples are free from infinite masses, infinite velocities, infinite forces, etc. Only infinite divisibility of space and timeis assumed. Thus, the considered infinite devices are possible in a continuous Newtonian Universe and they do not conflict with continuous Newtonian mechanics. Some possible applications to the analysis of the Navier-Stokes equations are discussed.

Categories: Journals, Physics

Hyper- and hybrid nonlocality. (arXiv:1802.01917v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

The controlled generation and identification of quantum correlations, usually encoded in either qubits or continuous degrees of freedom, builds the foundation of quantum information science. Recently, more sophisticated approaches, involving a combination of two distinct degrees of freedom have been proposed to improve on the traditional strategies. Hyperentanglement describes simultaneous entanglement in more than one distinct degree of freedom, whereas hybrid entanglement refers to entanglement shared between a discrete and a continuous degree of freedom. In this work we propose a scheme that allows to combine the two approaches, and to extend them to the strongest form of quantum correlations. Specifically, we show how two identical, initially separated particles can be manipulated to produce Bell nonlocality among their spins, among their momenta, as well as across their spins and momenta. We discuss possible experimental realizations with atomic and photonic systems.

Categories: Journals, Physics

Manipulation of two quantum systems through their mutual interaction in presence of a radiation field. (arXiv:1802.01935v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

In cavity QED, the mutual interaction between natural atomic systems in presence of a radiation field was ignored due to its negligible impact compared with the coupling to the field. The newly engineered artificial atomic systems (such as quantum dots and superconducting circuits) proposed for quantum information processing enjoy strong interaction with same type of systems or even with other types in hybrid structures, which is coherently controllable and moreover they can be efficiently coupled to radiation fields. We present an exact analytic solution for the time evolution of a composite system of two interacting two-level quantum systems coupled to a single mode radiation field, which can be realized in cavity (circuit) QED. We show how the non-classical dynamical properties of the composite system are affected and can be tuned by introducing and varying the mutual coupling between the two systems. Particularly, the collapse-revival pattern shows a splitting during the revival intervals as the coupling ratio (system-system to system-field) increases, which is a sign of an interruption in the system-radiation energy exchange process. Furthermore, the time evolution of the bipartite entanglement between the two systems is found to vary significantly depending on the coupling ratio as well as the initial state of the composite system resulting in either an oscillatory behavior or a collapse-revival like pattern. Increasing the coupling ratio enhances the entanglement, raises its oscillation average value and emphasizes the collapse-revival like pattern. However, raising the coupling ratio beyond unity increases the revival time considerably. The effect of the other system parameters such as detuning and radiation field intensity on the system dynamics has been investigated as well.

Categories: Journals, Physics

Probing the Interface of a Phase-Separated State in a Repulsive Bose-Fermi Mixture. (arXiv:1802.01954v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

We probe the interface between a phase-separated Bose-Fermi mixture consisting of a small BEC of $^{41}$K residing in a large Fermi sea of $^6$Li. We quantify the residual spatial overlap between the two components by measuring three-body recombination losses for variable strength of the interspecies repulsion. A comparison with a numerical mean-field model highlights the importance of the kinetic energy term for the condensed bosons in maintaining the thin interface far into the phase-separated regime. Our results demonstrate a corresponding smoothing of the phase transition in a system of finite size.

Categories: Journals, Physics

Crystals as a Source of Gamma Quanta. (arXiv:1802.01962v1 [cond-mat.other])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

Conditions for the appearance of a nuclear exciton in a crystal consisting of excited and unexcited nuclei of a given type are determined. The total probabilities for spontaneous -emission by a single excited nucleus or by an arbitrary number of excited nuclei in the crystal are derived. It is shown that the formation of a nuclear exciton is connected with an increase of the width of the emitting level and with the concentration of the radiation in a narrow solid angle.

Categories: Journals, Physics

Revisited Swanson's Hamiltonian. (arXiv:1802.01981v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

We notice new Hermitian counterpart of Swanson's Hamiltonian.

Categories: Journals, Physics

Of Local Operations and Physical Wires. (arXiv:1802.02021v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

In this work (multipartite) entanglement, discord and coherence are unified as different aspects of a single underlying resource theory defined through simple and operationally meaningful elemental operations. This is achieved by revisiting the resource theory defining entanglement, Local Operations and Classical Communication (LOCC), placing the focus on the underlying quantum nature of the communication channels. Taking the natural elemental operations in the resulting generalization of LOCC yields a resource theory that singles out coherence in the wire connecting the spatially separated systems as an operationally useful resource. The approach naturally allows to consider a reduced setting as well, namely the one with only the wire connected to a single quantum system, which leads to discord-like resources. The general form of free operations in this latter setting is derived and presented as a closed form. We discuss in what sense the present approach defines a resource theory of quantum discord and in which situations such an interpretation is sound -- and why in general discord is not a resource. This unified and operationally meaningful approach makes transparent many features of entanglement that in LOCC might seem surprising, such as the possibility to use a particle to entangle two parties, without it ever being entangled with either of them, or that there exist different forms of multipartite entanglement.

Categories: Journals, Physics

Entanglement-ergodic quantum systems equilibrate exponentially well. (arXiv:1802.02052v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

One of the outstanding problems in non-equilibrium physics is to precisely understand when and how physically relevant observables in many-body systems equilibrate under unitary time evolution. While general equilibration results have been proven that show that equilibration is generic provided that the initial state has overlap with sufficiently many energy levels, at the same time results showing that natural initial states fulfill this condition are lacking. In this work, we present stringent results for equilibration for ergodic systems in which the amount of entanglement in energy eigenstates with finite energy density grows volume-like with the system size. Concretely, we carefully formalize notions of entanglement-ergodicity in terms of R\'enyi entropies, from which we derive that such systems equilibrate exponentially well. Our proof uses insights about R\'enyi entropies and combines them with recent results about the probability distribution of energy in lattice systems with initial states that are weakly correlated.

Categories: Journals, Physics

Effect of spin relaxations on the spin mixing conductances for a bilayer structure. (arXiv:1802.02058v1 [cond-mat.mes-hall])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

The spin current can result in a spin-transfer torque in the normal-metal(NM)|ferromagnetic-insulator(FMI) or normal-metal(NM)|ferromagnetic-metal(FMM) bilayer. In the earlier study on this issue, the spin relaxations were ignored or introduced phenomenologically. In this paper, considering the FMM or FMI with spin relaxations described by a non-Hermitian Hamiltonian, we derive an effective spin-transfer torque and an effective spin mixing conductance in the non-Hermitian bilayer. The dependence of the effective spin mixing conductance on the system parameters (such as insulating gap, \textit{s-d} coupling, and layer thickness) as well as the relations between the real part and the imaginary part of the effective spin mixing conductance are given and discussed. We find that the effective spin mixing conductance can be enhanced in the non-Hermitian system. This provides us with the possibility to enhance the spin mixing conductance.

Categories: Journals, Physics

Control of fluctuations and heavy tails for heat variation in the two-time measurement framework. (arXiv:1802.02073v1 [math-ph])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

We study heat fluctuations in the two-time measurement framework. For bounded perturbations, we give sufficient ultraviolet regularity conditions on the perturbation for the moments of the heat variation to be uniformly bounded in time, and for the Fourier transform of the heat variation distribution to be analytic and uniformly bounded in time in a complex neighborhood of 0. On a set of canonical examples, with bounded and unbounded perturbations, we show that our ultraviolet conditions are essentially necessary. If the form factor of the perturbation does not meet our assumptions, the heat variation distribution exhibits heavy tails. The tails can be as heavy as preventing the existence of a fourth moment of the heat variation.

Categories: Journals, Physics

Causal Classical Physics in Time Symmetric Quantum Mechanics. (arXiv:1802.02090v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

The letter submitted is an executive summary of our previous paper. To solve the Einstein Podolsky Rosen 'paradox' the two boundary quantum mechanics is taken as self consistent interpretation of quantum dynamics. The difficulty with this interpretation is to reconcile it with classical physics. To avoid macroscopic backward causation two 'corresponding transition rules' are formulated which specify needed properties of macroscopic observations and manipulations. The apparent classical causal decision tree requires to understand the classically unchosen options. They are taken to occur with an 'incomplete knowledge' of the boundary states typically in macroscopic considerations. The precise boundary conditions with given phases then select the actual measured path and this selection is mistaken to happen at the time of measurement. The apparent time direction of the decision tree originates in an assumed relative proximity to the initial state. Only the far away final state allows for classically distinct options to be selected from. Cosmologically the picture could correspond to a big bang initial and a hugely extended final state scenario. It is speculated that it might also hold for a big bang/big crunch world. If this would be the case the Born probability postulate could find a natural explanation if we coexist in the expanding and the correlated CPT conjugate contracting world.

Categories: Journals, Physics

Observation of the topological Anderson insulator in disordered atomic wires. (arXiv:1802.02109v1 [cond-mat.quant-gas])

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

Topology and disorder have deep connections and a rich combined influence on quantum transport. In order to probe these connections, we synthesized one-dimensional chiral symmetric wires with controllable disorder via spectroscopic Hamiltonian engineering, based on the laser-driven coupling of discrete momentum states of ultracold atoms. We characterize the system's topology through measurement of the mean chiral displacement of the bulk density extracted from quench dynamics. We find evidence for the topological Anderson insulator phase, in which the band structure of an otherwise trivial wire is driven topological by the presence of added disorder. In addition, we observed the robustness of topological wires to weak disorder and measured the transition to a trivial phase in the presence of strong disorder. Atomic interactions in this quantum simulation platform will enable future realizations of strongly interacting topological fluids.

Categories: Journals, Physics

A new quantum version of f-divergence. (arXiv:1311.4722v4 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

This paper proposes and studies new quantum version of $f$-divergences, a class of convex functionals of a pair of probability distributions including Kullback-Leibler divergence, Rnyi-type relative entropy and so on. There are several quantum versions so far, including the one by Petz. We introduce another quantum version ($\mathrm{D}_{f}^{\max}$, below), defined as the solution to an optimization problem, or the minimum classical $f$- divergence necessary to generate a given pair of quantum states. It turns out to be the largest quantum $f$-divergence. The closed formula of $\mathrm{D}_{f}^{\max}$ is given either if $f$ is operator convex, or if one of the state is a pure state. Also, concise representation of $\mathrm{D}_{f}^{\max}$ as a pointwise supremum of linear functionals is given and used for the clarification of various properties of the quality.

Using the closed formula of $\mathrm{D}_{f}^{\max}$, we show: Suppose $f$ is operator convex. Then the\ maximum $f\,$- divergence of the probability distributions of a measurement under the state $\rho$ and $\sigma$ is strictly less than $\mathrm{D}_{f}^{\max}\left( \rho\Vert\sigma\right) $. This statement may seem intuitively trivial, but when $f$ is not operator convex, this is not always true. A counter example is $f\left( \lambda\right) =\left\vert 1-\lambda\right\vert $, which corresponds to total variation distance.

We mostly work on finite dimensional Hilbert space, but some results are extended to infinite dimensional case.

Categories: Journals, Physics

Testing for entanglement with periodic coarse-graining. (arXiv:1506.01095v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

Continuous variables systems find valuable applications in quantum information processing. To deal with an infinite-dimensional Hilbert space, one in general has to handle large numbers of discretized measurements in tasks such as entanglement detection. Here we employ the continuous transverse spatial variables of photon pairs to experimentally demonstrate novel entanglement criteria based on a periodic structure of coarse-grained measurements. The periodization of the measurements allows for an efficient evaluation of entanglement using spatial masks acting as mode analyzers over the entire transverse field distribution of the photons and without the need to reconstruct the probability densities of the conjugate continuous variables. Our experimental results demonstrate the utility of the derived criteria with a success rate in entanglement detection of $\sim60\%$ relative to $7344$ studied cases.

Categories: Journals, Physics

Out-of-time-order fluctuation-dissipation theorem. (arXiv:1612.08781v4 [cond-mat.stat-mech] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

We prove a generalized fluctuation-dissipation theorem for a certain class of out-of-time-ordered correlators (OTOCs) with a modified statistical average, which we call bipartite OTOCs, for general quantum systems in thermal equilibrium. The difference between the bipartite and physical OTOCs defined by the usual statistical average is quantified by a measure of quantum fluctuations known as the Wigner-Yanase skew information. Within this difference, the theorem describes a universal relation between chaotic behavior in quantum systems and a nonlinear-response function that involves a time-reversed process. We show that the theorem can be generalized to higher-order $n$-partite OTOCs as well as in the form of generalized covariance.

Categories: Journals, Physics

Equivalence of restricted Boltzmann machines and tensor network states. (arXiv:1701.04831v2 [cond-mat.str-el] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

The restricted Boltzmann machine (RBM) is one of the fundamental building blocks of deep learning. RBM finds wide applications in dimensional reduction, feature extraction, and recommender systems via modeling the probability distributions of a variety of input data including natural images, speech signals, and customer ratings, etc. We build a bridge between RBM and tensor network states (TNS) widely used in quantum many-body physics research. We devise efficient algorithms to translate an RBM into the commonly used TNS. Conversely, we give sufficient and necessary conditions to determine whether a TNS can be transformed into an RBM of given architectures. Revealing these general and constructive connections can cross-fertilize both deep learning and quantum many-body physics. Notably, by exploiting the entanglement entropy bound of TNS, we can rigorously quantify the expressive power of RBM on complex data sets. Insights into TNS and its entanglement capacity can guide the design of more powerful deep learning architectures. On the other hand, RBM can represent quantum many-body states with fewer parameters compared to TNS, which may allow more efficient classical simulations.

Categories: Journals, Physics

Majorana representation, qutrit Hilbert space and NMR implementation of qutrit gates. (arXiv:1703.06102v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

We report a study of the Majorana geometrical representation of a qutrit, where a pair of points on a unit sphere represents its quantum states. A canonical form for qutrit states is presented, where every state can be obtained from a one-parameter family of states via $SO(3)$ action. The notion of spin-1 magnetization which is invariant under $SO(3)$ is geometrically interpreted on the Majorana sphere. Furthermore, we describe the action of several quantum gates in the Majorana picture and experimentally implement these gates on a spin-1 system (an NMR qutrit) oriented in a liquid crystalline environment. We study the dynamics of the pair of points representing a qutrit state under various useful quantum operations and connect them to different NMR operations. Finally, using the Gell Mann matrix picture we experimentally implement a scheme for complete qutrit state tomography.

Categories: Journals, Physics

Toolbox for the design of $LiNbO_3$-based passive and active integrated quantum circuits. (arXiv:1704.03769v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-02-07 20:33

We present and discuss perspectives of current developments on advanced quantum optical circuits monolithically integrated in the lithium niobate platform. A set of basic components comprising photon pair sources based on parametric down conversion (PDC), passive routing elements and active electro-optically controllable switches and polarisation converters are building blocks of a toolbox which is the basis for a broad range of diverse quantum circuits. We review the state-of-the-art of these components and provide models that properly describe their performance in quantum circuits. As an example for applications of these models we discuss design issues for a circuit providing on-chip two-photon interference. The circuit comprises a PDC section for photon pair generation followed by an actively controllable modified Mach-Zehnder structure for observing Hong-Ou-Mandel (HOM) interference. The performance of such a chip is simulated theoretically by taking even imperfections of the properties of the individual components into account.

Categories: Journals, Physics
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