## Physics

### Focus: New View of Cold Atoms Flowing

APS Physics - Fri, 2017-07-21 16:00

Author(s): Mark Buchanan

A new technique produces an image of the flow of cold atoms through a channel, a potentially important tool for future cold-atom technology.

[Physics 10, 84] Published Fri Jul 21, 2017

Categories: Physics

### Multiple emitters in a waveguide: Nonreciprocity and correlated photons at perfect elastic transmission

Author(s): Yao-Lung L. Fang (方耀龍) and Harold U. Baranger

We investigate interference and correlation effects when several detuned emitters are placed along a one-dimensional photonic waveguide. Such a setup allows multiple interactions between the photons and the strongly coupled emitters, and underlies proposed devices for quantum information processing....

[Phys. Rev. A 96, 013842] Published Fri Jul 21, 2017

Categories: Journals, Physics

### Pure quadratic or higher-order optical effects in anisotropic crystals induced by external dc fields and probed by a single low-intensity plane electromagnetic wave

Author(s): Mike Melnichuk and Lowell T. Wood

The determination of a clear theoretical demarcation between a true or a false quadratic or higher-order low-intensity optical effect induced by an externally applied static or quasistatic (dc) vector field in anisotropic crystals is the scope of the present work. A complete set of necessary and suf...

[Phys. Rev. A 96, 013843] Published Fri Jul 21, 2017

Categories: Journals, Physics

### Remote polarization-entanglement generation by local dephasing and frequency up-conversion

Author(s): S. Hamedani Raja, G. Karpat, E.-M. Laine, S. Maniscalco, J. Piilo, C.-F. Li, and G.-C. Guo

We introduce a scheme for remote entanglement generation for the photon polarization. The technique is based on transferring the initial frequency correlations to specific polarization-frequency correlations by local dephasing and their subsequent removal by frequency up-conversion. On fundamental l...

[Phys. Rev. A 96, 013844] Published Fri Jul 21, 2017

Categories: Journals, Physics

### Pseudo-Hermitian Landau-Zener-Stückelberg-Majorana model

Author(s): Boyan T. Torosov and Nikolay V. Vitanov

We derive the analytical solution of the model of a two-state system interacting with an external coherent field, in which the Hamiltonian is pseudo-Hermitian. We describe in detail the non-Hermitian generalization of the famed Landau-Zener-Stückelberg-Majorana model, but similar generalizations can...

[Phys. Rev. A 96, 013845] Published Fri Jul 21, 2017

Categories: Journals, Physics

### Evolution-Free Hamiltonian Parameter Estimation through Zeeman Markers

Author(s): Daniel Burgarth and Ashok Ajoy

We provide a protocol for Hamiltonian parameter estimation which relies only on the Zeeman effect. No time-dependent quantities need to be measured; it fully suffices to observe spectral shifts induced by fields applied to local “markers.” We demonstrate the idea with a simple tight-binding Hamilton...

[Phys. Rev. Lett. 119, 030402] Published Fri Jul 21, 2017

Categories: Journals, Physics

### Scanning Gate Microscope for Cold Atomic Gases

Author(s): Samuel Häusler, Shuta Nakajima, Martin Lebrat, Dominik Husmann, Sebastian Krinner, Tilman Esslinger, and Jean-Philippe Brantut

A new technique produces an image of the flow of cold atoms through a channel, a potentially important tool for future cold-atom technology.

[Phys. Rev. Lett. 119, 030403] Published Fri Jul 21, 2017

Categories: Journals, Physics

### Cat Codes with Optimal Decoherence Suppression for a Lossy Bosonic Channel

Author(s): Linshu Li, Chang-Ling Zou, Victor V. Albert, Sreraman Muralidharan, S. M. Girvin, and Liang Jiang

We investigate cat codes that can correct multiple excitation losses and identify two types of logical errors: bit-flip errors due to excessive excitation loss and dephasing errors due to quantum backaction from the environment. We show that selected choices of logical subspace and coherent amplitud...

[Phys. Rev. Lett. 119, 030502] Published Fri Jul 21, 2017

Categories: Journals, Physics

### High-Threshold Low-Overhead Fault-Tolerant Classical Computation and the Replacement of Measurements with Unitary Quantum Gates

Author(s): Benjamin Cruikshank and Kurt Jacobs

von Neumann’s classic “multiplexing” method is unique in achieving high-threshold fault-tolerant classical computation (FTCC), but has several significant barriers to implementation: (i) the extremely complex circuits required by randomized connections, (ii) the difficulty of calculating its perform...

[Phys. Rev. Lett. 119, 030503] Published Fri Jul 21, 2017

Categories: Journals, Physics

### Systematic Construction of Counterexamples to the Eigenstate Thermalization Hypothesis

Author(s): Naoto Shiraishi and Takashi Mori

We propose a general method to embed target states into the middle of the energy spectrum of a many-body Hamiltonian as its energy eigenstates. Employing this method, we construct a translationally invariant local Hamiltonian with no local conserved quantities, which does not satisfy the eigenstate ...

[Phys. Rev. Lett. 119, 030601] Published Fri Jul 21, 2017

Categories: Journals, Physics

### Kardar-Parisi-Zhang Interfaces with Inward Growth

Author(s): Yohsuke T. Fukai and Kazumasa A. Takeuchi

We study the (1+1)-dimensional Kardar-Parisi-Zhang (KPZ) interfaces growing inward from ring-shaped initial conditions, experimentally and numerically, using growth of a turbulent state in liquid-crystal electroconvection and an off-lattice Eden model, respectively. To realize the ring initial condi...

[Phys. Rev. Lett. 119, 030602] Published Fri Jul 21, 2017

Categories: Journals, Physics

### Contrasting Work Fluctuations and Distributions in Systems with Short-Range and Long-Range Correlations

Author(s): T. R. Kirkpatrick, J. K. Bhattacherjee, and J. V. Sengers

It is shown that the work fluctuations and work distribution functions are fundamentally different in systems with short-range versus long-range correlations. The two cases considered with long-range correlations are magnetic work fluctuations in an equilibrium isotropic ferromagnet and work fluctua...

[Phys. Rev. Lett. 119, 030603] Published Fri Jul 21, 2017

Categories: Journals, Physics

### One-step generation of continuous-variable quadripartite cluster states in a circuit QED system

PRA: Quantum information - Fri, 2017-07-21 16:00

Author(s): Zhi-peng Yang, Zhen Li, Sheng-li Ma, and Fu-li Li

We propose a dissipative scheme for one-step generation of continuous-variable quadripartite cluster states in a circuit QED setup consisting of four superconducting coplanar waveguide resonators and a gap-tunable superconducting flux qubit. With external driving fields to adjust the desired qubit-r...

[Phys. Rev. A 96, 012327] Published Fri Jul 21, 2017

Categories: Journals, Physics

### Rigorous free fermion entanglement renormalization from wavelet theory. (arXiv:1707.06243v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2017-07-21 14:08

We construct entanglement renormalization schemes which provably approximate the ground states of non-interacting fermion nearest-neighbor hopping Hamiltonians on the one-dimensional discrete line and the two-dimensional square lattice. These schemes give hierarchical quantum circuits which build up the states from unentangled degrees of freedom. The circuits are based on pairs of discrete wavelet transforms which are approximately related by a "half-shift": translation by half a unit cell. The presence of the Fermi surface in the two-dimensional model requires a special kind of circuit architecture to properly capture the entanglement in the ground state. We show how the error in the approximation can be controlled without ever performing a variational optimization.

Categories: Journals, Physics

### Slowing Quantum Decoherence by Squeezing in Phase Space. (arXiv:1707.06244v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2017-07-21 14:08

Non-Gaussian states, and specifically the paradigmatic Schr\"odinger cat state, are well-known to be very sensitive to losses. When propagating through damping channels, these states quickly loose their non-classical features. However, by squeezing the superposition states, the decoherence process can be qualitatively changed and substantially slowed down. Here, we experimentally observe the reduced decoherence of squeezed optical coherent-state superpositions through a lossy channel. To quantify the robustness of states, we introduce a combination of a decaying value and a rate-of-decay of the Wigner function negativity. This work, which uses squeezing as an ancillary Gaussian resource, opens new possibilities to protect and manipulate quantum superpositions in phase space.

Categories: Journals, Physics

### Multi-parameter estimation in networked quantum sensors. (arXiv:1707.06252v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2017-07-21 14:08

We introduce a general model for a network of quantum sensors, and we use this model to consider the question: When can entanglement between the sensors, and/or global measurements, enhance the precision with which the network can measure a set of unknown parameters? We rigorously answer this question by presenting precise theorems proving that for a broad class of problems there is, at most, a very limited intrinsic advantage to using entangled states or global measurements. Moreover, for many estimation problems separable states and local measurements are optimal, and can achieve the ultimate quantum limit on the estimation uncertainty. This immediately implies that there are broad conditions under which simultaneous estimation of multiple parameters cannot outperform individual, independent estimations. Our results apply to any situation in which spatially localized sensors are encoded with a priori independent parameters, such as when estimating multiple linear or non-linear optical phase shifts in quantum imaging, or when mapping out the spatial profile of an unknown magnetic field. We conclude by showing that entangling the sensors can enhance the estimation precision when the parameters of interest are global properties of the entire network.

Categories: Journals, Physics

### Lindblad dynamics of the quantum spherical model. (arXiv:1707.06273v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2017-07-21 14:08

The purely relaxational non-equilibrium dynamics of the quantum spherical model as described through a Lindblad equation is analysed. It is shown that the phenomenological requirements of reproducing the exact quantum equilibrium state as stationary solution and the associated classical Langevin equation in the classical limit $g\to 0$ fix the form of the Lindblad dissipators, up to an overall time-scale. In the semi-classical limit, the models' behaviour become effectively the one of the classical analogue, with a dynamical exponent $z=2$, and an effective temperature $T_{\rm eff}$, renormalised by the quantum coupling $g$. A distinctive behaviour is found for a quantum quench, at zero temperature, deep into the ordered phase $g\ll g_c(d)$, for $d>1$ dimensions. Only for $d=2$ dimensions, a simple scaling behaviour holds true, with a dynamical exponent $z=1$, while for dimensions $d\ne 2$, logarithmic corrections to scaling arise. The spin-spin correlator, the growing length scale and the time-dependent susceptibility show the existence of several logarithmically different length scales.

Categories: Journals, Physics

### Convex geometry of quantum resource quantification. (arXiv:1707.06298v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2017-07-21 14:08

We introduce a framework unifying the mathematical characterisation of different measures of general quantum resources and allowing for a systematic way to define a variety of faithful quantifiers for any given convex quantum resource theory. The approach allows us to describe many commonly used measures such as matrix norm-based quantifiers, robustness measures, convex roof-based measures, and witness-based quantifiers together in a common formalism based on the convex geometry of the underlying sets of resource-free states. We establish easily verifiable criteria for a measure to possess desirable properties such as faithfulness and strong monotonicity under relevant free operations, and show that many quantifiers obtained in this framework indeed satisfy them for any considered quantum resource. We derive various bounds and relations between the measures as well as their dual forms, generalising and in many cases simplifying results found in the resource theories of quantum entanglement and coherence. We also prove that the quantification of resources in this framework simplifies for pure states, allowing us to obtain more easily computable forms of the considered measures, and show that many of them are in fact equal on pure states.

We present an explicit application of the results to the resource theories of multi-level coherence, entanglement of Schmidt number k, multipartite entanglement, as well as magic states, providing insight into the quantification of the four resources and introducing new quantifiers, such as a measure of entanglement of Schmidt number k which generalises the convex roof-extended negativity, a measure of k-coherence which generalises the L1 norm of coherence, and a hierarchy of norm-based quantifiers of k-partite entanglement generalising the greatest cross norm.

Categories: Journals, Physics

### An Ontology of Nature with Local Causality, Parallel Lives, and Many Relative Worlds. (arXiv:1707.06324v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2017-07-21 14:08

Parallel Lives (PL) is an ontological model of nature in which quantum mechanics and special relativity are unified in a single universe with a single Minkowski space-time. Point-like objects called \emph{lives} are the only fundamental objects in this space-time, and they propagate at or below $c$, and interact with one another only locally at point-like events in space-time --- not unlike relativistic billiard balls. Lives are the only causal agents in the universe, and thus the causal structure of interaction events in space-time is Lorentz invariant. Each life traces a continuous world-line through space-time, and each life experiences its own \emph{relative world}, fully defined by the past events along its world-line. A quantum field comprises a continuum of lives throughout space-time, and excitations like particles are the familiar physical systems in the universe --- each comprising its own sub-continuum of lives. A pure universal quantum wavefunction tracks the collective behavior of these lives, but not their individual dynamics. There is a preferred separable basis for the Hilbert space of the universal wavefunction, and for a given physical system, each orthogonal term in this basis is a different relative world --- each containing some fraction of the lives of the system. Hidden information about entanglement correlations in the universal wavefunction is shared locally by lives at all interaction events and carried as they propagate. This hidden information governs which lives of different systems will meet during future interactions, and enforces entanglement correlations between the lives of the systems. All entanglement correlations --- including Bell violations --- are enforced by this local mechanism. These, and many other details, are explored here, but several aspects of this framework are not yet fleshed out, and work is ongoing.

Categories: Journals, Physics

### Use of global interactions in efficient quantum circuit constructions. (arXiv:1707.06356v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2017-07-21 14:08

In this paper we study the ways to use a global entangling operator to efficiently implement circuitry common to a selection of important quantum algorithms. In particular, we focus on the circuits composed with global Ising entangling gates and arbitrary addressable single-qubit gates. We show that under certain circumstances the use of global operations can substantially improve the entangling gate count.

Categories: Journals, Physics