Integrable time-dependent Hamiltonians, solvable Landau-Zener models and Gaudin magnets. (arXiv:1802.01571v2 [cond-mat.stat-mech] UPDATED) Quantum Physics - Thu, 2018-04-19 06:33

We solve the non-stationary Schrodinger equation for several time-dependent Hamiltonians, such as the BCS Hamiltonian with an interaction strength inversely proportional to time, periodically driven BCS and linearly driven inhomogeneous Dicke models as well as various multi-level Landau-Zener tunneling models. The latter are Demkov-Osherov, bow-tie, and generalized bow-tie models. We show that these Landau-Zener problems and their certain interacting many-body generalizations map to Gaudin magnets in a magnetic field. Moreover, we demonstrate that the time-dependent Schrodinger equation for the above models has a similar structure and is integrable with a similar technique as Knizhnikov-Zamolodchikov equations. We also discuss applications of our results to the problem of molecular production in an atomic Fermi gas swept through a Feshbach resonance and to the evaluation of the Landau-Zener transition probabilities.

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Producing superfluid circulation states using phase imprinting. (arXiv:1801.04792v2 [cond-mat.quant-gas] UPDATED) Quantum Physics - Thu, 2018-04-19 06:33

We propose a method to prepare states of given quantized circulation in annular Bose-Einstein condensates (BEC) confined in a ring trap using the method of phase imprinting without relying on a two-photon angular momentum transfer. The desired phase profile is imprinted on the atomic wave function using a short light pulse with a tailored intensity pattern generated with a Spatial Light Modulator. We demonstrate the realization of 'helicoidal' intensity profiles suitable for this purpose. Due to the diffraction limit, the theoretical steplike intensity profile is not achievable in practice. We investigate the effect of imprinting an intensity profile smoothed by a finite optical resolution onto the annular BEC with a numerical simulation of the time-dependent Gross-Pitaevskii equation. This allows us to optimize the intensity pattern for a given target circulation to compensate for the limited resolution.

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Interaction instability of localization in quasiperiodic systems. (arXiv:1801.02955v2 [cond-mat.str-el] UPDATED) Quantum Physics - Thu, 2018-04-19 06:33

Integrable models form pillars of theoretical physics because they allow for full analytical understanding. Despite being rare, many realistic systems can be described by models that are close to integrable. Therefore, an important question is how small perturbations influence the behavior of solvable models. This is particularly true for many-body interacting quantum systems where no general theorems about their stability are known. Here, we show that no such theorem can exist by providing an explicit example of a one-dimensional many-body system in a quasiperiodic potential whose transport properties discontinuously change from localization to diffusion upon switching on interaction. This demonstrates an inherent instability of a possible many-body localization in a quasiperiodic potential at small interactions. We also show how the transport properties can be strongly modified by engineering potential at only a few lattice sites.

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Relay entanglement and clusters of correlated spins. (arXiv:1712.10188v2 [quant-ph] UPDATED) Quantum Physics - Thu, 2018-04-19 06:33

Considering a spin-1/2 chain, we {suppose} that the entanglement passes from a given pair of particles to another one, thus establishing the relay transfer of entanglement along the chain. Therefore, we introduce the relay entanglement as a sum of all pairwise entanglements in a spin chain. For more detailed studying the effects of {remote} pairwise entanglements, we use the partial sums collecting entanglements between the spins separated by up to a certain number of nodes. The problem of entangled cluster formation is considered, and the geometric mean entanglement is introduced as a {characteristics} of quantum correlations in a cluster. Generally, the life-time of a cluster decreases with an increase in its size.

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Scattering processes and resonances from lattice QCD

Reviews of Modern Physics - Wed, 2018-04-18 12:00

Author(s): Raúl A. Briceño, Jozef J. Dudek, and Ross D. Young

Hadrons and their interactions arise via the coupling between quarks and gluons, as dictated by quantum chromodynamics (QCD), the theory of strong interactions. Unlike protons and neutrons, very few hadrons observed in nature are stable under the strong interaction: the majority of them appear as resonances in scattering experiments. This work reviews progress and prospects in the studies of few-hadron reactions and resonance properties using lattice QCD techniques.

[Rev. Mod. Phys. 90, 025001] Published Wed Apr 18, 2018

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Scissors Mode of Dipolar Quantum Droplets of Dysprosium Atoms

Author(s): Igor Ferrier-Barbut, Matthias Wenzel, Fabian Böttcher, Tim Langen, Mathieu Isoard, Sandro Stringari, and Tilman Pfau

We report on the observation of the scissors mode of a single dipolar quantum droplet. The existence of this mode is due to the breaking of the rotational symmetry by the dipole-dipole interaction, which is fixed along an external homogeneous magnetic field. By modulating the orientation of this mag...

[Phys. Rev. Lett. 120, 160402] Published Wed Apr 18, 2018

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Qudit-Basis Universal Quantum Computation Using ${χ}^{(2)}$ Interactions

Author(s): Murphy Yuezhen Niu, Isaac L. Chuang, and Jeffrey H. Shapiro

We prove that universal quantum computation can be realized—using only linear optics and χ(2) (three-wave mixing) interactions—in any (n+1)-dimensional qudit basis of the n-pump-photon subspace. First, we exhibit a strictly universal gate set for the qubit basis in the one-pump-photon subspace. Next...

[Phys. Rev. Lett. 120, 160502] Published Wed Apr 18, 2018

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Two-photon processes based on quantum commutators

Author(s): F. Fratini, L. Safari, P. Amaro, and J. P. Santos

We developed a method to calculate two-photon processes in quantum mechanics that replaces the infinite summation over the intermediate states by a perturbation expansion. This latter consists of a series of commutators that involve position, momentum, and Hamiltonian quantum operators. We analyzed ...

[Phys. Rev. A 97, 043842] Published Wed Apr 18, 2018

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Probing the limits of the rigid-intensity-shift model in differential-phase-contrast scanning transmission electron microscopy

Author(s): L. Clark, H. G. Brown, D. M. Paganin, M. J. Morgan, T. Matsumoto, N. Shibata, T. C. Petersen, and S. D. Findlay

The rigid-intensity-shift model of differential-phase-contrast imaging assumes that the phase gradient imposed on the transmitted probe by the sample causes the diffraction pattern intensity to shift rigidly by an amount proportional to that phase gradient. This behavior is seldom realized exactly i...

[Phys. Rev. A 97, 043843] Published Wed Apr 18, 2018

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Utilizing strongly absorbing materials for low-loss surface-wave nonlinear optics

Author(s): Nicolai B. Grosse, Philipp Franz, Jan Heckmann, Karsten Pufahl, and Ulrike Woggon

Optical media endowed with large nonlinear susceptibilities are highly prized for their employment in frequency conversion and the generation of nonclassical states of light. Although the presence of an optical resonance can greatly increase the nonlinear response (e.g., in epsilon-near-zero materia...

[Phys. Rev. A 97, 043844] Published Wed Apr 18, 2018

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Optical binding with cold atoms

Author(s): C. E. Máximo, R. Bachelard, and R. Kaiser

Dielectric spheres moving in a dissipative medium are subjected to the effects of optical binding, i.e., a net force resulting from the collective scattering of light. Optical binding is now shown to exist with frictionless cold atoms, bringing as an additional feature the existence of rotating bound states.

[Phys. Rev. A 97, 043845] Published Wed Apr 18, 2018

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Synopsis: How Diamond-Like Carbon Films Grow

APS Physics - Wed, 2018-04-18 12:00

Machine-learning-based molecular dynamics simulations explain the growth mechanism of diamond-like amorphous carbon films.

[Physics] Published Wed Apr 18, 2018

Categories: Physics

Scrambling of quantum information in quantum many-body systems

PRA: Quantum information - Wed, 2018-04-18 12:00

Author(s): Eiki Iyoda and Takahiro Sagawa

We systematically investigate scrambling (or delocalizing) processes of quantum information encoded in quantum many-body systems by using numerical exact diagonalization. As a measure of scrambling, we adopt the tripartite mutual information (TMI) that becomes negative when quantum information is de...

[Phys. Rev. A 97, 042330] Published Wed Apr 18, 2018

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Measurement-device-independent quantum key distribution with correlated source-light-intensity errors

PRA: Quantum information - Wed, 2018-04-18 12:00

Author(s): Cong Jiang, Zong-Wen Yu, and Xiang-Bin Wang

We present an analysis for measurement-device-independent quantum key distribution with correlated source-light-intensity errors. Numerical results show that the results here can greatly improve the key rate especially with large intensity fluctuations and channel attenuation compared with prior res...

[Phys. Rev. A 97, 042331] Published Wed Apr 18, 2018

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Weighted polygamy inequalities of multiparty entanglement in arbitrary-dimensional quantum systems

PRA: Quantum information - Wed, 2018-04-18 12:00

Author(s): Jeong San Kim

We provide a generalization for the polygamy constraint of multiparty entanglement in arbitrary-dimensional quantum systems. By using the βth power of entanglement of assistance for 0≤β≤1 and the Hamming weight of the binary vector related with the distribution of subsystems, we establish a class of...

[Phys. Rev. A 97, 042332] Published Wed Apr 18, 2018

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Multidimensional quantum optics of spin waves through ac-Stark modulation. (arXiv:1804.05854v1 [quant-ph]) Quantum Physics - Wed, 2018-04-18 05:45

Spatial structure of photons renders them extremely versatile carriers of quantum information, as it can be tailored with simple optical elements such as lenses, phase gratings or holograms. Substantial challenges emerge, however, when such spatially-structured photons carrying quantum information need to be stored in quantum memories or if advanced quantum information processing capability beyond linear optics is required. These quandaries are usually not shared by material systems, for which strong interaction can be engineered, leading to efforts in demonstrating quantum-interferometric properties of atoms, phonons or plasmons. Here we harness the full three-dimensional potential of material quasi-particles - collective atomic excitations known as spin waves. We demonstrate that the spatial structure of single spin waves can be manipulated via the off-resonant ac-Stark shift. Through spin-wave diffraction based beam-splitter transformation, we realize the Hanbury Brown-Twiss (HBT) type measurement at the spin-wave level, demonstrating nonclassical statistics of atomic excitations. Finally, we observe interference of two spin waves - an analogue of the Hong-Ou-Mandel (HOM) effect for photons. Thanks to the reversible photon-spin wave mapping via the Duan-Lukin-Cirac-Zoller (DLCZ) protocol, these techniques enable encoding states from a high-dimensional Hilbert space into the spatial structure of spin waves to facilitate not only new quantum communication schemes, but also high data rate classical telecommunication.

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Strategies for optimal single-shot discrimination of quantum measurements. (arXiv:1804.05856v1 [quant-ph]) Quantum Physics - Wed, 2018-04-18 05:45

In this work we study the problem of single-shot discrimination of von Neumann measurements. We associate each measurement with a measure-and-prepare channel. There are two possible approaches to this problem. The first one, which is simple, does not utilize entanglement. We focus only on discrimination of classical probability distribution, which are outputs of the channels. We find necessary and sufficient criterion for perfect discrimination in this case. A more advanced approach requires the usage and entanglement. We quantify the distance of the two measurements in terms of the diamond norm (called sometimes the completely bounded trace norm). We provide an exact expression for the optimal probability of correct distinction and relate it to the discrimination of unitary channels. We also state a necessary and sufficient condition for perfect discrimination and a semidefinite program which checks this condition. Our main result, however, is a cone program which calculates the distance of these measurements and hence provides an upper bound on the probability of their correct distinction. As a by-product the program also finds a strategy (input state) which achieves this bound. Finally, we provide a full description for the cases of Fourier matrices and mirror isometries.

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Randomized Benchmarking as Convolution: Fourier Analysis of Gate Dependent Errors. (arXiv:1804.05951v1 [quant-ph]) Quantum Physics - Wed, 2018-04-18 05:45

We provide an alternative proof of Wallman's \cite{Wallman2018} bounds on the effect of gate-dependent noise on randomized benchmarking (RB). Our primary insight is that a RB sequence is a convolution amenable to Fourier space analysis, and we adopt the mathematical framework of Fourier transforms of matrix-valued functions on groups established in recent work from Gowers and Hatami \cite{GH15}. We show explicitly that as long as our faulty gate-set is close to some representation of the Clifford group, an RB sequence is described by the exponential decay of a process that has exactly two eigenvalues close to one and the rest close to zero, even though the bounds with respect to any particular representation of the Clifford group may not tightly describe the rate of decay. This framework reveals some very provocative properties of maximally entangled states, and likely has applications in other areas of quantum information theory.

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Simple negative result for physically universal controllers with macroscopic interface. (arXiv:1804.05954v1 [quant-ph]) Quantum Physics - Wed, 2018-04-18 05:45

To study potential limitations of controllability of physical systems I have earlier proposed physically universal cellular automata and Hamiltonians. These are translation invariant interactions for which any control operation on a finite target region can be implemented by the autonomous time evolution if the complement of the target region is 'programmed' to an appropriate initial state. This provides a model of control where the cut between a system and its controller can be consistently shifted, in analogy to the Heisenberg cut defining the boundary between a quantum system and its measurement device. However, in the known physically universal CAs the implementation of microscopic transformations requires to write the 'program' into microscopic degrees of freedom, while human actions take place on the macroscopic level. I therefore ask whether there exist physically universal interactions for which any desired operation on a target region can be performed by only controlling the macroscopic state of its surrounding. A very simple argument shows that this is impossible with respect to the notion of 'macroscopic' proposed here: control devices whose position is only specified up to 'macroscopic precision' cannot operate at a precise location in space. This suggests that reasonable notions of `universal controllability' need to be tailored to the manipulation of relative coordinates, but it is not obvious how to do this. The statement that any microscopic transformation can be implemented in principle, whenever it is true in any sense, it does not seem to be true in its most obvious sense.

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Quantum Blockchain using entanglement in time. (arXiv:1804.05979v1 [quant-ph]) Quantum Physics - Wed, 2018-04-18 05:45

A conceptual design for a quantum blockchain is proposed. Our method involves encoding the blockchain into a temporal GHZ (Greenberger-Horne-Zeilinger) state of photons that do not simultaneously coexist. It is shown that the entanglement in time, as opposed to an entanglement in space, provides the crucial quantum advantage. All the subcomponents of this system have already been shown to be experimentally realized. Perhaps more shockingly, our encoding procedure can be interpreted as non-classically influencing the past; hence this decentralized quantum blockchain can be viewed as a quantum networked time machine.

Categories: Journals, Physics
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