## Feed aggregator

### Multidimensional synthetic chiral-tube lattices via nonlinear frequency conversion. (arXiv:2002.08591v1 [physics.optics])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

Geometrical dimensionality plays a fundamentally important role in the topological effects arising in discrete lattices. While direct experiments are limited by three spatial dimensions, the research topic of synthetic dimensions implemented by the frequency degree of freedom in photonics is rapidly advancing. The manipulation of light in such artificial lattices is typically realized through electro-optic modulation, yet their operating bandwidth imposes practical constraints on the range of interactions between different frequency components. Here we propose and experimentally realize all-optical synthetic dimensions involving specially tailored simultaneous short- and long-range interactions between discrete spectral lines mediated by frequency conversion in a nonlinear waveguide. We realize triangular chiral-tube lattices in three-dimensional space and explore their four-dimensional generalization. We implement a synthetic gauge field with nonzero magnetic flux and observe the associated multidimensional dynamics of frequency combs, all within one physical spatial port. We anticipate that our method will provide a new means for the fundamental study of high-dimensional physics and act as an important step towards using topological effects in optical devices operating in the time and frequency domains.

Categories: Journals, Physics

### Optical Magnetometer: Quantum Resonances at pumping repetition rate of 1/n of the Larmor frequency. (arXiv:2002.08631v1 [physics.atom-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

Quantum sub-resonances at 1/n are observed experimentally in a SERF type pulsed optical magnetometer. This is a type of synchronization phenomenon. Sequential short pump laser pulses of 70-200 $\mu$sec, circularly polarized are employed for optical pumping of the alkali vapor in 5 layers shielded magnetometer. The repetition rate of these pulses is 1/n of the Larmor frequency at specified magnetic field produced within the shield. Signal resonances are obtained at these 1/n frequencies. Mixed alkali atoms of K and Rb cells are studied, where one specie is pumped while the probe is on the other species that is polarized by spin exchange. The effect of spin destruction, spin exchange and collisions are studied, in order to account for the width of the resonances. Quantum calculations of three levels $\Lambda$ model for this phenomenon exhibit a dip at the resonance frequency in the absorption spectrum for both cases of pulsed and CW pump modes and an evidence for EIT. Such a decrease in the absorption intensity which is the result of quantum interference may cause false sensitivity in the optical magnetometer and in the determination of tiny magnetic fields. Methods in which the magnetic detection employ only frequency changes do not suffer from this effect.

Categories: Journals, Physics

### Two-dimensional confined hydrogen: An entropy and complexity approach. (arXiv:2002.08664v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

The position and momentum spreading of the electron distribution of the two-dimensional confined hydrogenic atom, which is a basic prototype of the general multidimensional confined quantum systems, is numerically studied in terms of the confinement radius for the 1s, 2s, 2p and 3d quantum states by means of the main entropy and complexity information-theoretic measures. First, the Shannon entropy and the Fisher information as well as the associated uncertainty relations are computed and discussed. Then, the Fisher-Shannon, LMC and LMC-R\'enyi complexity measures are examined and mutually compared. We have found that these entropy and complexity quantities reflect the rich properties of the electron confinement extent in the two conjugated spaces.

Categories: Journals, Physics

### Determination of the Semion Code Threshold using Neural Decoders. (arXiv:2002.08666v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

We compute the error threshold for the semion code, the companion of the Kitaev toric code with the same gauge symmetry group $\mathbb{Z}_2$. This is the first determination of an error threshold for a non-Pauli stabilizer code and the first use of machine learning methods for a non-CSS quantum error correcting code. Since the application of statistical mechanical mapping methods are highly discouraged for the semion code, we use the near-optimal performance of some neural network decoders: multilayer perceptrons (MLP) and convolutional neural networks (CNN). We find the values $p_{\text {eff}}=9.5\%$ for uncorrelated bit-flip and phase-flip noise, and $p_{\text {eff}}=10.5\%$ for depolarizing noise. We contrast these values with a similar analysis of the Kitaev toric code on a hexagonal lattice with the same methods. For convolutional neural networks, we use the ResNet architecture, which allows us to implement very deep networks and results in better performance and scalability than the multilayer perceptron approach. We analyze and compare in detail both approaches and provide a clear argument favouring CNN as the best suited numerical method for the semion code.

Categories: Journals, Physics

### Justifying Kubo's formula for gapped systems at zero temperature: a brief review and some new results. (arXiv:2002.08669v1 [math-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

We first review the problem of a rigorous justification of Kubo's formula for transport coefficients in gapped extended Hamiltonian quantum systems at zero temperature. In particular, the theoretical understanding of the quantum Hall effect rests on the validity of Kubo's formula for such systems, a connection that we review briefly as well. We then highlight an approach to linear response theory based on non-equilibrium almost-stationary states (NEASS) and on a corresponding adiabatic theorem for such systems that was recently proposed and worked out by one of us in [51] for interacting fermionic systems on finite lattices. In the second part of our paper we show how to lift the results of [51] to infinite systems by taking a thermodynamic limit.

Categories: Journals, Physics

### Identification of networking quantum teleportation on 14-qubit IBM universal quantum computer. (arXiv:2002.08671v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

Quantum teleportation enables networking participants to move an unknown quantum state between the nodes of a quantum network, and hence constitutes an essential element in constructing large-sale quantum processors with a quantum modular architecture. Herein, we propose two protocols for teleporting qubits through an N-node quantum network in a highly-entangled box-cluster state or chain-type cluster state. The proposed protocols are systematically scalable to an arbitrary finite number N and applicable to arbitrary size of modules. The protocol based on a box-cluster state is implemented on a 14-qubit IBM quantum computer for N up to 12. To identify faithful networking teleportation, namely that the elements on real devices required for the networking teleportation process are all qualified for achieving teleportation task, we quantify quantum-mechanical processes using a generic classical-process model through which any classical strategies of mimicry of teleportation can be ruled out. From the viewpoint of achieving a genuinely quantum-mechanical process, the present work provides a novel toolbox consisting of the networking teleportation protocols and the criteria for identifying faithful teleportation for universal quantum computers with modular architectures and facilitates further improvements in the reliability of quantum-information processing.

Categories: Journals, Physics

### Observation of the Crystal Hall Effect in a Collinear Antiferromagnet. (arXiv:2002.08712v1 [cond-mat.mtrl-sci])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

An electric current passing through a magnetic conductor can generate a dissipationless transversal current of topological Berry curvature origin. This anomalous Hall effect requires the breaking of spin-degeneracy of electronic bands, conventionally arising from a macroscopic moment in ferromagnets, or a non-collinear magnetic order in complex magnets. Here we report the experimental observation of a new anomalous Hall effect mechanism - the crystal Hall effect - in a system with the abundant collinear antiferromagnetic order. We detect a large crystal Hall conductivity of ~330 S/cm, consistent with our density functional theory calculations, by performing Hall measurements up to 50 T on high quality epilayers of RuO2. We demonstrate that this crystal Hall effect is an experimental manifestation of unconventional spin-splitting originating from a complex crystal structure in combination with collinear antiferromagnetism with zero net moment. This opens a previously unexplored chapter, associated with the new spin-splitting physics, of dissipationless transport and other quantum and topological phenomena in condensed matter.

Categories: Journals, Physics

### Optical Phase Measurement Using a Deterministic Source of Entangled Multi-photon States. (arXiv:2002.08715v1 [cond-mat.mes-hall])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

Precision measurements of optical phases have many applications in science and technology. Entangled multi-photon states have been suggested for performing such measurements with precision that significantly surpasses the shot-noise limit. Until recently, such states have been generated mainly using spontaneous parametric down-conversion -- a process which is intrinsically probabilistic, counteracting the advantages that the entangled photon states might have. Here, we use a semiconductor quantum dot to generate entangled multi-photon states in a deterministic manner, using periodic timed excitation of a confined spin. This way we entangle photons one-by-one at a rate which exceeds 300 MHz. We use the resulting multi-photon state to demonstrate super-resolved optical phase measurement. Our results open up a scalable way for realizing genuine quantum enhanced super-sensitive measurements in the near future.

Categories: Journals, Physics

### Reliable coherent optical memory based on a laser-written waveguide. (arXiv:2002.08780v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

$\mathrm {^{151}Eu^{3+}}$-doped yttrium silicate ($\mathrm {^{151}Eu^{3+}:Y_2SiO_5}$ ) crystal is a unique material that possesses hyperfine states with coherence time up to 6 h. Many efforts have been devoted to the development of this material as optical quantum memories based on the bulk crystals, but integrable structures (such as optical waveguides) that can promote $\mathrm {^{151}Eu^{3+}:Y_2SiO_5}$-based quantum memories to practical applications, have not been demonstrated so far. Here we report the fabrication of type 2 waveguides in a $\mathrm {^{151}Eu^{3+}:Y_2SiO_5}$ crystal using femtosecond-laser micromachining. The resulting waveguides are compatible with single-mode fibers and have the smallest insertion loss of $4.95\ dB$. On-demand light storage is demonstrated in a waveguide by employing the spin-wave atomic frequency comb (AFC) scheme and the revival of silenced echo (ROSE) scheme. We implement a series of interference experiments based on these two schemes to characterize the storage fidelity. Interference visibility of the readout pulse is $0.99\pm 0.03$ for the spin-wave AFC scheme and $0.97\pm 0.02$ for the ROSE scheme, demonstrating the reliability of the integrated optical memory.

Categories: Journals, Physics

### Heat, Work and Entropy Production in Open Quantum Systems: A Microscopic Approach Based on Observational Entropy. (arXiv:2002.08817v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

We provide an axiomatic and microscopic approach to the second law in open quantum systems based on the recently introduced notion of observational entropy. By starting with the latter as a definition for the nonequilibrium thermodynamic entropy of the `universe' (the system and the bath), various conceptual shortcomings of previous attempts are overcome. The second law as quantified by an always positive entropy production is expressed as a change of a state function (namely, observational entropy) and it obeys an integral fluctuation theorem. Furthermore, we can treat a larger class of initial states beyond the standard paradigm of canonical Gibbs ensembles. Our approach also provides an alternative perspective on the conventional definition of mechanical work in quantum systems caused by an external time-dependent field. Moreover, the entropy production in our formalism can be unambiguously divided into a quantum and a classical component. In the conventional scenario of a weakly coupled ideal thermal bath our approach is in quantitative agreement with previous ones, thus also reassuring their thermodynamic consistency.

Categories: Journals, Physics

### Instabilities in an optical black-hole laser. (arXiv:2002.08835v1 [gr-qc])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

We study the Hamiltonian of optical fields in a nonlinear dispersive fiber. Quantum field fluctuations are created spontaneously close to an optical event horizon through the analog Hawking effect. We consider the simplest model for an optical black-hole laser, where the Hawking radiation is produced and amplified inside a cavity created by two close horizons: a black hole and a white hole. We find that the resonant Hawking radiation originates from a discrete set of instabilities and can tunnel out of the horizons. Unlike other methods, we determine the most efficiently amplified instability that dominates the resonant Hawking process.

Categories: Journals, Physics

### Detecting a logarithmic nonlinearity in the Schr\"odinger equation using Bose-Einstein condensates. (arXiv:2002.08877v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

We study the effect of a logarithmic nonlinearity in the Schr\"odinger equation (SE) on the dynamics of a freely expanding Bose-Einstein condensate (BEC). The logarithmic nonlinearity was one of the first proposed nonlinear extensions to the SE which emphasized the conservation of important physical properties of the linear theory, e.g.: the separability of noninteracting states. Using this separability, we incorporate it into the description of a BEC obeying a logarithmic Gross-Pittaevskii equation. We investigate the dynamics of such BECs using variational and numerical methods and find that, using experimental techniques like delta kick collimation, experiments with extended free-fall times as available on microgravity platforms could be able to lower the bound on the strength of the logarithmic nonlinearity by at least one order of magnitude.

Categories: Journals, Physics

### Performance of real-time adaptive optics compensation in a turbulent channel with high-dimensional spatial-mode encoding. (arXiv:2002.08884v1 [eess.SP])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

The orbital angular momentum (OAM) of photons is a promising degree of freedom for high-dimensional quantum key distribution (QKD). However, effectively mitigating the adverse effects of atmospheric turbulence is a persistent challenge in OAM QKD systems operating over free-space communication channels. In contrast to previous works focusing on correcting static simulated turbulence, we investigate the performance of OAM QKD in real atmospheric turbulence with real-time adaptive optics (AO) correction. We show that, even our AO system provides a limited correction, it is possible to mitigate the errors induced by weak turbulence and establish a secure channel. The crosstalk induced by turbulence and the performance of AO systems is investigated in two configurations: a lab-scale link with controllable turbulence, and a 340 m long cross-campus link with dynamic atmospheric turbulence. Our experimental results suggest that an advanced AO system with fine beam tracking, reliable beam stabilization, precise wavefront sensing, and accurate wavefront correction is necessary to adequately correct turbulence-induced error. We also propose and demonstrate different solutions to improve the performance of OAM QKD with turbulence, which could enable the possibility of OAM encoding in strong turbulence.

Categories: Journals, Physics

### Photochemical dynamics under incoherent illumination: light harvesting in self-assembled molecular J-aggregates. (arXiv:2002.08896v1 [physics.chem-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

Transport phenomena in organic, self-assembled molecular J-aggregates have long attracted a great deal of attention due to its potential role in designing novel organic photovoltaic devices. A large number of theoretical and experimental studies have been carried out describing excitonic energy transfer in J-aggregates under the assumption that excitons are induced by a coherent laser-light source or initialized by a localized excitation on a particular chromophore. However, these assumptions may not provide an accurate description to assess the efficiency of J-aggregates, particularly as building blocks of organic solar cells. In natural conditions, J-aggregates would be subjected to an incoherent source of light (as is sunlight), which would illuminate the whole photosynthetic complex rather than a single molecule. In this work, we present the first study of the efficiency of photosynthetic energy transport in self-assembled molecular aggregates under incoherent sunlight illumination. By making use of a minimalistic model of a cyanine dye J-aggregate, we demonstrate that long-range transport efficiency is enhanced when exciting the aggregate with incoherent light. Our results thus support the conclusion that J-aggregates are indeed excellent candidates for devices where efficient long-range incoherently-induced exciton transport is desired, such as in highly efficient organic solar cells.

Categories: Journals, Physics

### Alibaba Cloud Quantum Development Platform: Surface Code Simulations with Crosstalk. (arXiv:2002.08918v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

We report, in a sequence of notes, our work on the Alibaba Cloud Quantum Development Platform (AC-QDP). AC-QDP provides a set of tools for aiding the development of both quantum computing algorithms and quantum processors, and is powered by a large-scale classical simulator deployed on Alibaba Cloud. In this note, we simulate a distance-3 logical qubit encoded in the 17-qubit surface code using experimental noise parameters for transmon qubits in a planar circuit QED architecture. Our simulation features crosstalk induced by ZZ-interactions. We show that at the current-stage noise levels, crosstalk contributes significantly to the dephasing of the logical qubit. This results in a total phase-flip probability of $\sim 0.6\%$, about $60\%$ higher than expected without considering crosstalk. This indicates that for the code considered, the current noise parameters approach, but do not yet meet, the break-even fault-tolerance regime.

Categories: Journals, Physics

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

Electrons confined in semiconductor quantum dot arrays have both charge and spin degrees of freedom. The spin provides a well-controllable and long-lived qubit implementation. The charge configuration in the dot array is influenced by Coulomb repulsion, and the same interaction enables charge sensors to probe this configuration. Here we show that the Coulomb repulsion allows an initial charge transition to induce subsequent charge transitions, inducing a cascade of electron hops, like toppling dominoes. A cascade can transmit information along a quantum dot array over a distance that extends by far the effect of the direct Coulomb repulsion. We demonstrate that a cascade of electrons can be combined with Pauli spin blockade to read out spins using a remote charge sensor. We achieve > 99.9% spin readout fidelity in 1.7 $\mathrm{\mu}$s. The cascade-based readout enables operation of a densely-packed two-dimensional quantum dot array with charge sensors placed at the periphery. The high connectivity of such arrays greatly improves the capabilities of quantum dot systems for quantum computation and simulation.

Categories: Journals, Physics

### Quantum Time-Space Tradeoffs by Recording Queries. (arXiv:2002.08944v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

We use the recording queries technique of Zhandry [Zha19] to prove lower bounds in the exponentially small success probability regime, with applications to time-space tradeoffs. We first extend the recording technique to the case of non-uniform input distributions and we describe a new simple framework for using it. Then, as an application, we prove strong direct product theorems for $K$-Search under a natural product distribution not considered in previous works, and for finding $K$ distinct collisions in a uniform random function. Finally, we use the latter result to obtain the first quantum time-space tradeoff that is not based on a reduction to $K$-Search. Namely, we demonstrate that any $T$-query algorithm using $S$ qubits of memory must satisfy a tradeoff of $T^3 S \geq \Omega(N^4)$ for finding $\Theta(N)$ collisions in a random function. We conjecture that this result can be improved to $T^2 S \geq \Omega(N^3)$, and we show that it would imply a $T^2 S \geq \tilde{\Omega}(N^2)$ tradeoff for Element Distinctness.

Categories: Journals, Physics

### Predicting Many Properties of a Quantum System from Very Few Measurements. (arXiv:2002.08953v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

Predicting properties of complex, large-scale quantum systems is essential for developing quantum technologies. We present an efficient method for constructing an approximate classical description of a quantum state using very few measurements of the state. This description, called a classical shadow, can be used to predict many different properties: order $\log M$ measurements suffice to accurately predict $M$ different functions of the state with high success probability. The number of measurements is independent of the system size, and saturates information-theoretic lower bounds. Moreover, target properties to predict can be selected after the measurements are completed. We support our theoretical findings with extensive numerical experiments. We apply classical shadows to predict quantum fidelities, entanglement entropies, two-point correlation functions, expectation values of local observables, and the energy variance of many-body local Hamiltonians, which allows applications to speedup variational quantum algorithms. The numerical results highlight the advantages of classical shadows relative to previously known methods.

Categories: Journals, Physics

### Entangling three qubits without ever touching. (arXiv:1807.05546v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

All identical particles are inherently correlated from the outset, regardless of how far apart their creation took place. In this paper, this fact is used for extraction of entanglement from independent particles unaffected by any interactions. Specifically, we are concerned with operational schemes for generation of all tripartite entangled states, essentially the GHZ state and the W state, which prevent the particles from touching one another over the entire evolution. The protocols discussed in the paper require only three particles in linear optical setups with equal efficiency for boson, fermion or anyon statistics. Within this framework indistinguishability of particles presents itself as a useful resource of entanglement accessible for practical applications.

Categories: Journals, Physics

### Continuum Schroedinger operators for sharply terminated graphene-like structures. (arXiv:1810.03497v2 [math.AP] UPDATED)

arXiv.org: Quantum Physics - Fri, 2020-02-21 14:45

We study the single electron model of a semi-infinite graphene sheet interfaced with the vacuum and terminated along a zigzag edge. The model is a Schroedinger operator acting on $L^2(\mathbb{R}^2)$: $H^\lambda_{\rm edge}=-\Delta+\lambda^2 V_\sharp$, with a potential $V_\sharp$ given by a sum of translates an atomic potential well, $V_0$, of depth $\lambda^2$, centered on a subset of the vertices of a discrete honeycomb structure with a zigzag edge. We give a complete analysis of the low-lying energy spectrum of $H^\lambda_{\rm edge}$ in the strong binding regime ($\lambda$ large). In particular, we prove scaled resolvent convergence of $H^\lambda_{\rm edge}$ acting on $L^2(\mathbb{R}^2)$, to the (appropriately conjugated) resolvent of a limiting discrete tight-binding Hamiltonian acting in $l^2(\mathbb{N}_0;\mathbb{C}^2)$. We also prove the existence of {\it edge states}: solutions of the eigenvalue problem for $H^\lambda_{\rm edge}$ which are localized transverse to the edge and pseudo-periodic (propagating or plane-wave like) parallel to the edge. These edge states arise from a "flat-band" of eigenstates the tight-binding Hamiltonian.

Categories: Journals, Physics