Physics

Fluctuation-induced forces on an atom near a photonic topological material. (arXiv:1711.04941v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

We theoretically study the Casimir-Polder force on an atom in a arbitrary initial state in a rather general electromagnetic environment wherein the materials may have a nonreciprocal bianisotropic dispersive response. It is shown that under the Markov approximation the force has resonant and nonresonant contributions. We obtain explicit expressions for the optical force both in terms of the system Green function and of the electromagnetic modes. We apply the theory to the particular case wherein a two-level system interacts with a topological gyrotropic material, showing that the nonreciprocity enables exotic light-matter interactions and the opportunity to sculpt and tune the Casimir-Polder forces on the nanoscale. With a quasi-static approximation, we obtain a simple analytical expression for the optical force and unveil the crucial role of surface plasmons in fluctuation induced forces. Finally, we derive the Green function for a gyrotropic material half-space in terms of a Sommerfeld integral.

Categories: Journals, Physics

Floquet states of kicked particle in a singular potential: Exponential and power-law profile. (arXiv:1711.04946v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

It is well known that, in the chaotic regime, all the floquet states of kicked rotor system display an exponential profile resulting from dynamical localization. If the kicked rotor is placed in an additional stationary infinite potential well, the floquet states of this non-KAM system are known to display power-law decay profile. It has also been suggested in general that in the case of periodically kicked systems with singularities in the potential, the floquet states would display power-law profile. In this work, we examine this question by studying the floquet states of a kicked particle in finite potential barriers. We map this system to a tight binding model and show that the occurrence of exponential or power-law profile of floquet states depends on the effective strength of singularity experienced by the floquet states in their energy band. Thus, the decay profile of the floquet state is a combination of both exponential and power-law profile depending on strength of singular potential. The signatures of this property can also be inferred from nearest neighbour spacing distribution as well in the statistics of eigenvectors. Quite unusually, this leads to a scenario in which the level spacing distribution, as a window in to the spectral correlations, is not a unique characteristic for the entire system.

Categories: Journals, Physics

Clifford Algebras and Spinorial Representation of Linear Canonical Transformations in Quantum Theory. (arXiv:1711.04975v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

This work is a continuation of previous works that we have done concerning linear canonical transformations and a phase space representation of quantum theory. It is mainly focused on the description of an approach which permits to establish spinorial representation of linear canonical transformations. It begins with an introduction section in which the reason and context of the content are discussed. The introduction section is followed by a brief recall about Clifford algebra and spin group. The description of the approach is started with the presentation of an adequate parameterization of linear canonical transformations which permits to represent them with special pseudo-orthogonal transformations in an operators space. The establishment of the spinorial representation is deduced using relation between special pseudo-orthogonal groups and spin groups. The cases of one dimension quantum mechanics and general multidimensional theory are both studied. The case of linear canonical transformation related to Minkowski space is particularly studied and it is shown that Lorentz transformation may be considered as particular case of linear canonical transformation. Some results from the spinorial representation are also exploited to define operators which may be used to establish equations for fields if one considers the possibility of envisaging a field theory which admits as main symmetry group the group constituted by linear canonical transformations.

Categories: Journals, Physics

Quantum transport senses community structure in networks. (arXiv:1711.04979v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

Quantum time evolution exhibits rich physics, attributable to the interplay between the density and phase of a wave function. However, unlike classical heat diffusion, the wave nature of quantum mechanics has not yet been extensively explored in modern data analysis. We propose that the Laplace transform of quantum transport (QT) can be used to construct a powerful ensemble of maps from a given complex network to a circle $S^1$, such that closely-related nodes on the network are grouped into sharply concentrated clusters on $S^1$. The resulting QT clustering (QTC) algorithm is shown to outperform the state-of-the-art spectral clustering method on synthetic and real data sets containing complex geometric patterns. The observed phenomenon of QTC can be interpreted as a collective behavior of the microscopic nodes that evolve as macroscopic cluster orbitals in an effective tight-binding model recapitulating the network.

Categories: Journals, Physics

Experimental test of an entropic measurement uncertainty relation for arbitrary qubit observables. (arXiv:1711.05023v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

A tight information-theoretic measurement uncertainty relation is experimentally tested with neutron spin-1/2 qubits. The noise associated to the measurement of an observable is defined via conditional Shannon entropies and a tradeoff relation between the noises for two arbitrary spin observables is demonstrated. The optimal bound of this tradeoff is experimentally obtained for various non-commuting spin observables. For some of these observables this lower bound can be reached with projective measurements, but we observe that, in other cases, the tradeoff is only saturated by general quantum measurements (i.e., positive-operator valued measures), as predicted theoretically.

Categories: Journals, Physics

Measure and detection of genuine multipartite entanglement for tripartite systems. (arXiv:1711.05041v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

It is a computationally hard task to certify genuine multipartite entanglement (GME). We investigate the relation between the norms of the correlation vectors and the detection of GME for tripartite quantum systems. A sufficient condition for GME and an effective lower bound for the GME concurrence are derived. Several examples are considered to show the effectiveness of the criterion and the lower bound of GME concurrence.

Categories: Journals, Physics

Physical models from noncommutative causality. (arXiv:1711.05057v1 [math-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

We introduced few years ago a new notion of causality for noncommutative spacetimes directly related to the Dirac operator and the concept of Lorentzian spectral triple. In this paper, we review in a non-technical way the noncommutative causal structure of many toy models as almost commutative spacetimes and the Moyal-Weyl spacetime. We show that those models present some unexpected physical interpretations as a geometrical explanation of the Zitterbewegung trembling motion of a fermion as well as some geometrical constraints on translations and energy jumps of wave packets on the Moyal spacetime.

Categories: Journals, Physics

Exploring Energy-Time Entanglement Using Geometric Phase. (arXiv:1711.05079v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

Using the signal and idler photons produced by parametric downconversion, we report an experimental observation of a violation of the Bell inequality for energy and time based purely on the geometric phases of the signal and idler photons. We thus show that energy-time entanglement between the signal and idler photons can be explored by means of their geometric phases. These results may have important practical implications for quantum information science by providing an additional means by which entanglement can be manipulated.

Categories: Journals, Physics

The Galois coaction on the electron anomalous magnetic moment. (arXiv:1711.05118v1 [math-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

Recently S. Laporta published a partial result on the fourth order QED contribution to the electron anomalous magnetic moment $g-2$. This result contains explicit polylogarithmic parts with fourth and sixth roots of unity. In this note we convert Laporta's result into the motivic `$f$ alphabet'. This provides a much shorter expression which makes the Galois structure visible. We conjecture the $Q$ vector spaces of Galois conjugates of the QED $g-2$ up to weight four. The conversion into the $f$ alphabet relies on a conjecture by D. Broadhurst that iterated integrals in certain Lyndon words provide an algebra basis for the extension of multiple zeta values by sixth roots of unity. We prove this conjecture in the motivic setup.

Categories: Journals, Physics

Efficient single-photon frequency conversion in the microwave domain using superconducting quantum circuits. (arXiv:1711.05153v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

We present an approach to achieve efficient single-photon frequency conversion in the microwave domain based on coherent control in superconducting quantum circuits, which consist of a driven artificial atom coupled to a semi-infinite transmission line. Using the full quantum-mechanical method, we analyze the single-photon scattering process in this system and find that single-photon frequency up- or down-conversion with efficiency close to unity can be achieved by adjusting the parameters of the control field applied to the artificial atom. We further show that our approach is experimentally feasible in currently available superconducting flux qubit circuits.

Categories: Journals, Physics

Classical Control, Quantum Circuits and Linear Logic in Enriched Category Theory. (arXiv:1711.05159v1 [cs.LO])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

We describe categorical models of a circuit-based (quantum) functional pro- gramming language. We show that enriched categories play a crucial role. Following earlier work on QWire by Paykin et al., we consider both a simple first-order linear language for circuits, and a more powerful host language, such that the circuit language is embedded inside the host language. Our categorical semantics for the host language is standard, and involves cartesian closed categories and monads. We interpret the circuit language not in an ordinary category, but in a category that is enriched in the host category. We show that this structure is also related to linear/non-linear models. As an extended example, we recall an earlier result that the category of W*-algebras is dcpo-enriched, and we use this model to extend the circuit language with some recursive types.

Categories: Journals, Physics

Non-Markovianity in the optimal control of an open quantum system described by hierarchical equations of motion. (arXiv:1711.05162v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

Optimal control theory is implemented with fully converged hierarchical equations of motion (HEOM) describing the time evolution of an open system density matrix strongly coupled to the bath in a spin-boson model. The populations of the two-level sub-system are taken as control objectives; namely, their revivals or exchange when switching off the field. We, in parallel, analyze how the optimal electric field consequently modifies the information back flow from the environment through different non-Markovian witnesses. Although the control field has a dipole interaction with the central sub-system only, its indirect influence on the bath collective mode dynamics is probed through HEOM auxiliary matrices, revealing a strong correlation between control and dissipation during a non-Markovian process. A heterojunction is taken as an illustrative example for modeling in a realistic way the two-level sub-system parameters and its spectral density function leading to a non-perturbative strong coupling regime with the bath. Although, due to strong system-bath couplings, control performances remain rather modest, the most important result is a noticeable increase of the non-Markovian bath response induced by the optimally driven processes.

Categories: Journals, Physics

Entanglement induced interactions in binary mixtures. (arXiv:1711.05171v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

We establish a conceptual framework for the identification and the characterization of induced interactions in binary mixtures and reveal their intricate relation to entanglement between the components or species of the mixture. Exploiting an expansion in terms of the strength of the entanglement among the two species, enables us to deduce an effective single-species description. In this way, we naturally incorporate the mutual feedback of the species and obtain induced interactions for both species which are effectively present among the particles of same type. Importantly, our approach incorporates few-body and inhomogeneous systems extending the scope of induced interactions where two particles interact via a bosonic bath-type environment. Employing the example of a one-dimensional spin-polarized ultracold Bose-Fermi mixture, we obtain induced Bose-Bose and Fermi-Fermi interactions with short-range attraction and long-range repulsion. With this, we show how beyond species mean-field physics visible in the two-body correlation functions can be understood via the induced interactions.

Categories: Journals, Physics

Violations of a Leggett-Garg inequality without signalling for a photonic qutrit probed with ambiguous measurements. (arXiv:1711.05172v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

We realise a quantum three-level system with photons distributed among three different spatial and polarization modes. Ambiguous measurement of the state of the qutrit are realised by blocking one out for the three modes at any one time. Using these measurements we construct a test of a Leggett-Garg inequality as well as tests of no-signalling-in-time for the measurements. We observe violations of the Leggett-Garg inequality that can not be accounted for in terms of signalling. Moreover, we tailor the qutrit dynamics such that both ambiguous and unambiguous measurements are simultaneously non-signalling, which is an essential step for the justification of the use of ambiguous measurements in Leggett-Garg tests.

Categories: Journals, Physics

Massive Unruh particles cannot be directly observed. (arXiv:1711.05179v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

We show that massive particles created in a relativistically accelerated reference frame, as predicted by the Unruh effect, can only be found in a tiny layer above the event horizon, whose thickness corresponds to a single Compton wavelength. This is beyond the reach of any detector and suggests that the Unruh effect may not ever be directly observed for massive fields. The case of massless particles is also examined, for which qualitatively different behaviour is observed in a low-acceleration regime, suggesting that an observation of the Unruh effect for massless particles is more promising.

Categories: Journals, Physics

Dynamics of Entanglement and the Schmidt Gap in a Driven Light-Matter System. (arXiv:1711.05182v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

The ability to modify light-matter coupling in time (e.g. using external pulses) opens up the exciting possibility of generating and probing new aspects of quantum correlations in many-body light-matter systems. Here we study the impact of such a pulsed coupling on the light-matter entanglement in the Dicke model as well as the respective subsystem quantum dynamics. Our dynamical many-body analysis exploits the natural partition between the radiation and matter degrees of freedom, allowing us to explore time-dependent intra-subsystem quantum correlations by means of squeezing parameters, and the inter-subsystem Schmidt gap for different pulse duration (i.e. ramping velocity) regimes -- from the near adiabatic to the sudden quench limits. Our results reveal that both types of quantities indicate the emergence of the superradiant phase when crossing the quantum critical point. In addition, at the end of the pulse light and matter remain entangled even though they become uncoupled, which could be exploited to generate entangled states in non-interacting systems.

Categories: Journals, Physics

Quantum parameter estimation with a neural network. (arXiv:1711.05238v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

We propose to use neural networks to estimate the rates of coherent and incoherent processes in quantum systems from continuous measurement records. In particular, we adapt an image recognition algorithm to recognize the patterns in experimental signals and link them to physical quantities. We demonstrate that the parameter estimation works unabatedly in the presence of detector imperfections which complicate or rule out Bayesian filter analyses.

Categories: Journals, Physics

The strange (hi)story of particles and waves. (arXiv:1304.1003v22 [physics.hist-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

This is a non-technical presentation (in historical context) of the quantum theory that is strictly based on global unitarity. While the first part was written for a general readership, Sect. 5 may appear a bit provocative. I argue that the single-particle wave functions of quantum mechanics have to be correctly interpreted as field modes that are "occupied once" (that is, first excited states of the corresponding quantum oscillators in the case of a boson field). Multiple excitations lead non-relativistically to apparent many-particle wave functions, while the quantum states proper are defined by wave function(al)s on the configuration space of fundamental fields, or on another, as yet elusive, fundamental basis.

Categories: Journals, Physics

Spectral Gap of the Anti-Ferromagnetic Lipkin-Meshkov-Glick Model. (arXiv:1506.00492v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

The spectral property of the supersymmetric (SUSY) antiferromagnetic Lipkin-Meshkov-Glick (LMG) model with an even number of spins is studied. The supercharges of the model are explicitly constructed. By using the exact form of the supersymmetric ground state we introduce simple trial variational states for first excited states. It is demonstrated numerically that they provide a relatively accurate upper bound for the spectral gap (the energy difference between the ground state and first excited states) in all parameter ranges. However, being an upper bound, it does not allow us to determine vigorously whether the model is gapped or gapless. Here, we provide a non-trivial lower bound for the spectral gap and thereby show that the antiferromagnetic SUSY LMG model is gapped for any even number of spins.

Categories: Journals, Physics

Quantum Horn's lemma, finite heat baths, and the third law of thermodynamics. (arXiv:1605.06092v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2017-11-15 22:45

Interactions of quantum systems with their environment play a crucial role in resource-theoretic approaches to thermodynamics in the microscopic regime. Here, we analyze the possible state transitions in the presence of "small" heat baths of bounded dimension and energy. We show that for operations on quantum systems with fully degenerate Hamiltonian (noisy operations), all possible state transitions can be realized exactly with a bath that is of the same size as the system or smaller, which proves a quantum version of Horn's lemma as conjectured by Bengtsson and Zyczkowski. On the other hand, if the system's Hamiltonian is not fully degenerate (thermal operations), we show that some possible transitions can only be performed with a heat bath that is unbounded in size and energy, which is an instance of the third law of thermodynamics. In both cases, we prove that quantum operations yield an advantage over classical ones for any given finite heat bath, by allowing a larger and more physically realistic set of state transitions.

Categories: Journals, Physics
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