# Quantum Physics

## New submissions

[ total of 55 entries: 1-55 ]
[ showing up to 2000 entries per page: fewer | more ]

### New submissions for Tue, 20 Feb 18

[1]
Title: Light, the universe, and everything -- 12 Herculean tasks for quantum cowboys and black diamond skiers
Comments: A review of the leading topics in quantum optics and related areas. Accepted for publication in J. Modern Optics (42 figures, 74 pages)
Subjects: Quantum Physics (quant-ph); Other Condensed Matter (cond-mat.other); Optics (physics.optics)

The Winter Colloquium on the Physics of Quantum Electronics (PQE) has been a seminal force in quantum optics and related areas since 1971. It is rather mindboggling to recognize how the concepts presented at these conferences have transformed scientific understanding and human society. In January, 2017, the participants of PQE were asked to consider the equally important prospects for the future, and to formulate a set of questions representing some of the greatest aspirations in this broad field. The result is this multi-authored paper, in which many of the world's leading experts address the following fundamental questions: (1) What is the future of gravitational wave astronomy? (2) Are there new quantum phases of matter away from equilibrium that can be found and exploited - such as the time crystal? (3) Quantum theory in uncharted territory: What can we learn? (4) What are the ultimate limits for laser photon energies? (5) What are the ultimate limits to temporal, spatial, and optical resolution? (6) What novel roles will atoms play in technology? (7) What applications lie ahead for nitrogen-vacancy centers in diamond? (8) What is the future of quantum coherence, squeezing, and entanglement for enhanced superresolution and sensing? (9) How can we solve (some of) humanity's biggest problems through new quantum technologies? (10) What new understanding of materials and biological molecules will result from their dynamical characterization with free electron lasers? (11) What new technologies and fundamental discoveries might quantum optics achieve by the end of this century? (12) What novel topological structures can be created and employed in quantum optics?

[2]
Title: Contextuality and Single-Qubit Stabilizer Formalism
Subjects: Quantum Physics (quant-ph)

Contextuality is a fundamental non-classical property of quantum theory, which has recently been proven to be a key resource for achieving quantum speed-ups in some leading models of quantum computation. However, which of the forms of contextuality, and how much thereof, are required to obtain a speed-up in an arbitrary model of quantum computation remains unclear. In this paper, we show that the relation between contextuality and a compuational advantage is more complicated than previously thought. We achieve this by proving that generalized contextuality is present even within the simplest subset of quantum operations, the so-called single-qubit stabilizer theory, which offers no computational advantage and was previously believed to be completely non-contextual. However, the contextuality of the single-qubit stabilizer theory can be confined to transformations. Therefore our result also demonstrates that the commonly considered prepare-and-measure scenarios (which ignore transformations) do not fully capture the contextuality of quantum theory.

[3]
Title: Topological edge states in the Su-Schrieffer-Heeger model subject to balanced particle gain and loss
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)

We investigate the Su-Schrieffer-Heeger model in presence of an injection and removal of particles, introduced via a master equation in Lindblad form. It is shown that the dynamics of the density matrix follows the predictions of calculations in which the gain and loss are modeled by complex $\mathcal{PT}$-symmetric potentials. In particular it is found that there is a clear distinction in the dynamics between the topologically different cases known from the stationary eigenstates.

[4]
Title: Controlling rare-earth ions in a nanophotonic resonator using the ac Stark shift
Subjects: Quantum Physics (quant-ph)

On-chip nanophotonic cavities will advance quantum information science and measurement because they enable efficient interaction between photons and long-lived solid-state spins, such as those associated with rare-earth ions in crystals. The enhanced photon-ion interaction creates new opportunities for all-optical control using the ac Stark shift. Toward this end, we characterize the ac Stark interaction between off-resonant optical fields and Nd$^{3+}$-ion dopants in a photonic crystal resonator fabricated from yttrium orthovanadate (YVO$_4$). Using photon echo techniques, at a detuning of 160 MHz we measure a maximum ac Stark shift of 2$\pi\times$12.3 MHz per intra-cavity photon, which is large compared to both the homogeneous linewidth ($\Gamma_h =$100 kHz) and characteristic width of isolated spectral features created through optical pumping ($\Gamma_f \approx$3 MHz). The photon-ion interaction strength in the device is sufficiently large to control the frequency and phase of the ions for quantum information processing applications. In particular, we discuss and assess the use of the cavity enhanced ac Stark shift to realize all-optical quantum memory and detection protocols. Our results establish the ac Stark shift as a powerful added control in rare-earth ion quantum technologies.

[5]
Title: Classical simulation of a topological quantum computer
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)

Topological quantum computers promise a fault tolerant means to perform quantum computation. Topological quantum computers use particles with exotic exchange statistics called non-Abelian anyons, and the simplest anyon model which allows for universal quantum computation by particle exchange or braiding alone is the Fibonacci anyon model. One classically hard problem that can be solved efficiently using quantum computation is finding the value of the Jones polynomial of knots at roots of unity. We aim to provide a pedagogical, self-contained, review of topological quantum computation with Fibonacci anyons, from the braiding statistics and matrices to the layout of such a computer and the compiling of braids to perform specific operations. Then we use a simulation of a topological quantum computer to explicitly demonstrate a quantum computation using Fibonacci anyons, evaluating the Jones polynomial of a selection of simple knots.

[6]
Title: Quantum metamaterials: entanglement of spin and orbital angular momentum of a single photon
Subjects: Quantum Physics (quant-ph); Optics (physics.optics)

Metamaterials have been a major research area for more than two decades now, involving artificial structures with predesigned electromagnetic properties constructed from deep subwavelength building blocks. They have been used to demonstrate a wealth of fascinating phenomena ranging from negative refractive index and epsilon-near-zero to cloaking, emulations of general relativity effects, and super-resolution imaging, to name a few. In the past few years, metamaterials have been suggested as a new platform for quantum optics, and several pioneering experiments have already been carried out with single photons. Here, we employ a dielectric metasurface to generate entanglement between spin and orbital angular momentum of single photons. We demonstrate experimentally the generation of the four Bell states by utilizing the geometric phase arising from the photonic spin-orbit interaction. These are the first experiments with entangled states with metasurfaces, and as such they are paving the way to the new area of quantum metamaterials.

[7]
Title: Quantifying tolerance of a nonlocal multi-qudit state to any local noise
Subjects: Quantum Physics (quant-ph)

We present a general approach for quantifying tolerance of a nonlocal N-qudit state to any local noise under different classes of quantum correlation scenarios with an arbitrary number of settings and any type of measurements, generalized or projective, at each site. This allows us to derive new precise bounds in d and N on noise tolerances for: (i) an arbitrary nonlocal N-qudit state; (ii) the N-qudit Greenberger-Horne-Zeilinger (GHZ) state; (iii) the N-qubit W state and the N-qubit Dicke states, and to analyse asymptotics of these precise bounds for large N and d.

[8]
Title: Can quantum mechanics be considered consistent? a discussion of Frauchinger and Renner's argument
Authors: F. Laloë
Subjects: Quantum Physics (quant-ph)

We discuss the argument proposed in Ref.~\cite{Frauchiger-Renner}, and show that it does not particularly illustrate any inconsistency in quantum mechanics, but rather the well known difficulty often known as the \textquotedblleft shifty split\textquotedblright: the exact point at which the von Neumann reduction postulate should be applied is ill defined. The argument of Ref.~\cite{Frauchiger-Renner} is therefore just an illustration of the original Schr\"{o}dinger's cat or Wigner's friend paradoxes in another situation, and leads to the same conclusions.

[9]
Title: Semiclassical approach to finite temperature quantum annealing with trapped ions
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)

Recently it has been demonstrated that an ensemble of trapped ions may serve as a quantum annealer for the number-partitioning problem [Nature Comm. DOI: 10.1038/ncomms11524]. This hard computational problem may be addressed employing a tunable spin glass architecture. Following the proposal of the trapped ions annealer, we study here its robustness against thermal effects, that is, we investigate the role played by thermal phonons. For the efficient description of the system, we use a semiclassical approach, and benchmark it against the exact quantum evolution. The aim is to understand better and characterize how the quantum device approaches a solution of, an otherwise, difficult to solve NP-hard problem.

[10]
Title: Systematic elimination of Stokes divergences emanating from complex phase space caustics
Subjects: Quantum Physics (quant-ph)

Stokes phenomenon refers to the fact that the asymptotic expansion of complex functions can differ in different regions of the complex plane, and that beyond the so-called Stokes lines has an unphysical divergence. An important special case is when the Stokes lines emanate from phase space caustics of a complex trajectory manifold. In this case, symmetry determines that to second order there is a double coverage of the space, one portion of which is unphysical. Building on the seminal but laconic findings of Adachi, we show that the deviation from second order can be used to rigorously determine the Stokes lines and therefore the region of the space that should be removed. The method has applications to wavepacket reconstruction from complex valued classical trajectories. With a rigorous method in hand for removing unphysical divergences, we demonstrate excellent wavepacket reconstruction for the Morse, Quartic, Coulomb and Eckart systems.

[11]
Title: Deep neural decoders for near term fault-tolerant experiments
Subjects: Quantum Physics (quant-ph); Machine Learning (stat.ML)

Finding efficient decoders for quantum error correcting codes adapted to realistic experimental noise in fault-tolerant devices represents a significant challenge. In this paper we introduce several decoding algorithms complemented by deep neural decoders and apply them to analyze several fault-tolerant error correction protocols such as the surface code as well as Steane and Knill error correction. Our methods require no knowledge of the underlying noise model afflicting the quantum device making them appealing for real-world experiments. Our analysis is based on a full circuit-level noise model. It considers both distance-three and five codes, and is performed near the codes pseudo-threshold regime. Training deep neural decoders in low noise rate regimes appears to be a challenging machine learning endeavour. We provide a detailed description of our neural network architectures and training methodology. We then discuss both the advantages and limitations of deep neural decoders. Lastly, we provide a rigorous analysis of the decoding runtime of trained deep neural decoders and compare our methods with anticipated gate times in future quantum devices. Given the broad applications of our decoding schemes, we believe that the methods presented in this paper could have practical applications for near term fault-tolerant experiments.

[12]
Title: Anti-adiabatic evolution in quantum-classical hybrid system
Subjects: Quantum Physics (quant-ph)

The adiabatic theorem is an important concept in quantum mechanics, it tells that a quantum system subjected to gradually changing external conditions remains to the same instantaneous eigenstate of its Hamiltonian as it initially in. In this paper, we study the another extreme circumstance where the external conditions vary rapidly such that the quantum system can not follow the change and remains in its initial state (or wavefunction). We call this type of evolution anit-adiabatic evolution. We examine the matter-wave pressure in this situation and derive the condition for such an evolution. The study is conducted by considering a quantum particle in an infinitely deep potential, the potential width $Q$ is assumed to be change rapidly. We show that the total energy of the quantum subsystem decreases as $Q$ increases, and this rapidly change exerts a force on the wall which plays the role of boundary of the potential. For $Q<Q_{0}$ ($Q_0$ is the initial width of the potential), the force is repulsive, and for $Q>Q_{0}$, the force is positive. The condition for the anti-adiabatic evolution is given via a spin-$\frac 1 2$ in a rotating magnetic field.

[13]
Title: Quantum key distribution with an efficient countermeasure against correlated intensity fluctuations in optical pulses
Journal-ref: npj Quantum Information volume 4, Article number: 8 (2018)
Subjects: Quantum Physics (quant-ph)

Quantum key distribution (QKD) allows two distant parties to share secret keys with the proven security even in the presence of an eavesdropper with unbounded computational power. Recently, GHz-clock decoy QKD systems have been realized by employing ultrafast optical communication devices. However, security loopholes of high-speed systems have not been fully explored yet. Here we point out a security loophole at the transmitter of the GHz-clock QKD, which is a common problem in high-speed QKD systems using practical band-width limited devices. We experimentally observe the inter-pulse intensity correlation and modulation-pattern dependent intensity deviation in a practical high-speed QKD system. Such correlation violates the assumption of most security theories. We also provide its countermeasure which does not require significant changes of hardware and can generate keys secure over 100 km fiber transmission. Our countermeasure is simple, effective and applicable to wide range of high-speed QKD systems, and thus paves the way to realize ultrafast and security-certified commercial QKD systems.

[14]
Title: Non-Hermiticity and conservation of orthogonal relation in dielectric microcavity
Subjects: Quantum Physics (quant-ph); Optics (physics.optics)

Non-Hermitian properties of open quantum systems and their applications have attracted much attention in recent years. While most of the studies focus on the characteristic nature of non-Hermitian systems, one important point has been overlooked: A non-Hermitian system can be a subsystem of a Hermitian system as one can clearly see in Feshbach projective operator (FPO) formalism. In this case, the orthogonality of the eigenvectors of the total (Hermitian) system must be sustained, despite the eigenvectors of the subsystem (non-Hermitian) satisfy the bi-orthogonal condition. Therefore, one can predict that there must exist some remarkable processes that relate the non-Hermitian subsystem and the rest part, and ultimately preserve the Hermiticity of the total system. In this paper, we study such processes in open elliptical microcavities. The inner part of the cavity is a non-Hermitian system, and the outer part is the coupled bath in FPO formalism. We investigate the correlation between the inner- and the outer-part behaviors associated with the avoided resonance crossings (ARCs), and analyze the results in terms of the Lamb shift. The ARC structures we examined depend on a trade-off between the relative difference of self-energies (simply known as Lamb shifts in atomic physics) and collective Lamb shifts. While the collective Lamb shift is maximized in the region of the center of ARC, but the relative difference of self-energies is minimized, and this naturally induce a crossing of imaginary part of eigenvalues. These results come from the conservation of the orthogonality in the total Hermitian quantum system.

[15]
Subjects: Quantum Physics (quant-ph)

A discretized version of the adiabatic theorem is described with the help of a rule relating a Hermitian operator to its expectation value and variance. The simple initial operator X with known ground state is transformed in a series of N small steps into a more complicated final operator Z with unknown ground state. Each operator along the discretised path in the space of Hermitian matrices is used to measure the state, initially the ground state of X. Measurements similar to the Zeno effect or Renninger's negative measurements modify the state incrementally. This process eventually leads to an eigenstate combination of Z. In the limit of vanishing step size the state stays with overwhelming probability in the ground state of each of the N observables.

[16]
Title: Simulating and assessing boson sampling experiments with phase-space representations
Subjects: Quantum Physics (quant-ph)

The search for new, application-specific quantum computers designed to outperform any classical computer is driven by the ending of Moore's law and the quantum advantages potentially obtainable. Photonic networks are promising examples, with experimental demonstrations and potential for obtaining a quantum computer to solve problems believed classically impossible. This introduces a challenge: how does one design or understand such photonic networks? One must be able to calculate observables using general methods capable of treating arbitrary inputs, dissipation and noise. We develop novel complex phase-space software for simulating these photonic networks, and apply this to boson sampling experiments. Our techniques give sampling errors orders of magnitude lower than experimental correlation measurements for the same number of samples. We show that these techniques remove systematic errors in previous algorithms for estimating correlations, with large improvements in errors in some cases. In addition, we obtain a scalable channel-combination strategy for assessment of boson sampling devices.

[17]
Title: Cavity-enhanced spectroscopy of a few-ion ensemble in Eu3+:Y2O3
Subjects: Optics (physics.optics); Quantum Physics (quant-ph)

We report on the coupling of the emission from a single europium-doped nanocrystal to a fiber-based microcavity under cryogenic conditions. As a first step, we study the sample properties and observe a strong correlation between emission lifetime and brightness, as well as a lifetime reduction for nanocrystals embedded in a polymer film. This is explained by differences in the local density of states. We furthermore quantify the scattering loss of a nanocrystal inside the cavity and use this to deduce the crystal size. Finally, by resonantly coupling the cavity to a selected transition, we perform cavity-enhanced spectroscopy to measure the inhomogeneous linewidth, and detect the fluorescence from an ensemble of few ions in the regime of power broadening. We observe an increased fluorescence rate consistent with Purcell enhancement. The results represent an important step towards the efficient readout of single rare-earth ions with excellent optical and spin coherence properties, which is promising for applications in quantum communication and distributed quantum computation.

[18]
Title: The role of quantum coherence in the thermodynamics of energy transfer
Subjects: Quantum Physics (quant-ph)

Recent research on the thermodynamic arrow of time, at the microscopic scale, has questioned the universality of its direction. Theoretical studies showed that quantum correlations can be used to revert the natural heat flow (from the hot body to the cold one), posing an apparent challenge to the second law of thermodynamics. Such an "anomalous" heat current was observed in a recent experiment (arXiv:1711.03323), by employing two spin systems initially quantum correlated. Nevertheless, the precise relationship between this intriguing phenomenon and the initial conditions that allow it is not fully evident. Here, we address energy transfer in an extended perspective, identifying a nonclassical contribution that applies to the reversion of the heat flow as well as to more general forms of energy exchange. We derive three theorems that describe the energy transfer between two microscopic systems, for arbitrary initial bipartite states. Such a process can be optimized through a certain type of quantum coherence, outperforming incoherent states. We also introduce a class of nonequilibrium states that only allow unidirectional energy flow, thus broadening the set where the standard Clausius statement of the second law applies. Finally, we show that for two qubits, coherence boosts the energy transfer along any direction (direct or reverse). In this case, coherence is the quantum resource underpinning the reversion of the thermodynamic arrow of time.

[19]
Title: Quantum-Assisted Blockchain
Subjects: Quantum Physics (quant-ph); Cryptography and Security (cs.CR)

Bitcoin and blockchain in general is hot topic nowadays. In the paper we propose a quantum empowering of this technology and show how to speed-up the mining procedure using the modified Grover's algorithm.

### Cross-lists for Tue, 20 Feb 18

[20]  arXiv:1802.06080 (cross-list from gr-qc) [pdf, ps, other]
Title: Possibility of Entanglement at LIGO!
Comments: 6 pages, no figures; typos in eqn 35 corrected
Subjects: General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)

It is shown that a linearized classical gravity wave $\hat{a}$ {\em la} Einstein can get entangled with an array of test masses in a plane perpendicular to its direction of propagation. A Bell-CHSH inequality based on the requirement of noncontextuality for classical realism is derived, and it is shown that the entangled state produced violates this inequality.

[21]  arXiv:1802.06191 (cross-list from physics.optics) [pdf]
Title: Visible and Infrared Organic Polariton Photodiodes
Subjects: Optics (physics.optics); Applied Physics (physics.app-ph); Quantum Physics (quant-ph)

In organic photodiodes (OPDs) light is absorbed by excitons, which dissociate to generate photocurrent. Here, we demonstrate a novel type of OPD in which light is absorbed by polaritons, hybrid light-matter states. We demonstrate polariton OPDs operating in the ultra-strong coupling regime at visible and infrared energies. These devices can be engineered to show narrow responsivity with a very weak angle-dependence. More importantly, they can be tuned to operate outside of the uncoupled absorbance spectrum. Remarkably, we show that the responsivity of a polariton OPD can be pushed to near infrared wavelengths with external quantum efficiencies exceeding those of a control device.

[22]  arXiv:1802.06230 (cross-list from nlin.PS) [pdf, ps, other]
Title: Dark-bright soliton pairs: bifurcations and collisions
Subjects: Pattern Formation and Solitons (nlin.PS); Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)

The statics, stability and dynamical properties of dark-bright soliton pairs are investigated motivated by applications in a homogeneous system of two-component repulsively interacting Bose-Einstein condensate. One of the intra-species interaction coefficients is used as the relevant parameter controlling the deviation from the integrable Manakov limit. Two different families of stationary states are identified consisting of dark-bright solitons that are either anti-symmetric (out-of-phase) or asymmetric (mass imbalanced) with respect to their bright soliton. Both of the above dark-bright configurations coexist at the integrable limit of equal intra- and inter-species repulsions and are degenerate in that limit. However, they are found to bifurcate from it in a transcritical bifurcation. The latter interchanges the stability properties of the bound dark-bright pairs rendering the anti-symmetric states unstable and the asymmetric ones stable past the associated critical point (and vice versa before it). Finally, on the dynamical side, it is found that large kinetic energies and thus rapid soliton collisions are essentially unaffected by the intra-species variation, while cases involving near equilibrium states or breathing dynamics are significantly modified under such a variation.

[23]  arXiv:1802.06236 (cross-list from cond-mat.mes-hall) [pdf, other]
Title: Quantum phase transitions and the degree of nonidentity in the system with two different species of vector bosons
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

We address the system with two species of vector bosons in an optical lattice. In addition to the the standard parameters characterizing such a system, we are dealing here with the "degree of atomic nonidentity", manifesting itself in the difference of tunneling amplitudes and on-site Coulomb interactions. We obtain a cascade of quantum phase transitions occurring with the increase in the degree of atomic nonidentity. In particular, we show that the phase diagram for strongly distinct atoms is qualitatively different from that for (nearly) identical atoms considered earlier. The resulting phase diagrams evolve from the images similar to the "J. Mir\'o-like paintings" to "K. Malewicz-like" ones.

[24]  arXiv:1802.06348 (cross-list from physics.soc-ph) [pdf, other]
Title: Quantitative Predictions in Quantum Decision Theory
Comments: Latex file, 17 pages, 1 figure
Journal-ref: IEEE Trans. Syst. Man Cybern. Syst. 48 (2018) 366-381
Subjects: Physics and Society (physics.soc-ph); Quantum Physics (quant-ph)

Quantum Decision Theory, advanced earlier by the authors, and illustrated for lotteries with gains, is generalized to the games containing lotteries with gains as well as losses. The mathematical structure of the approach is based on the theory of quantum measurements, which makes this approach relevant both for the description of decision making of humans and the creation of artificial quantum intelligence. General rules are formulated allowing for the explicit calculation of quantum probabilities representing the fraction of decision makers preferring the considered prospects. This provides a method to quantitatively predict decision-maker choices, including the cases of games with high uncertainty for which the classical expected utility theory fails. The approach is applied to experimental results obtained on a set of lottery gambles with gains and losses. Our predictions, involving no fitting parameters, are in very good agreement with experimental data. The use of quantum decision making in game theory is described. A principal scheme of creating quantum artificial intelligence is suggested.

[25]  arXiv:1802.06417 (cross-list from physics.optics) [pdf, other]
Title: Superthermal photon bunching in terms of simple probability distributions
Subjects: Optics (physics.optics); Quantum Physics (quant-ph)

We analyze the second-order photon autocorrelation function $g^{(2)}$ with respect to the photon probability distribution and discuss the generic features of a distribution that result in superthermal photon bunching ($g^{(2)}>2$). Superthermal photon bunching has been reported for a number of optical microcavity systems that exhibit processes like superradiance or mode competition. We show that a superthermal photon number distribution cannot be constructed from the principle of maximum entropy, if only the intensity and the second-order autocorrelation are given. However, for bimodal systems an unbiased superthermal distribution can be constructed from second-order correlations and the intensities alone. Our findings suggest modeling superthermal single-mode distributions by a mixture of a thermal and a lasing like state and thus reveal a generic mechanism in the photon probability distribution responsible for creating superthermal photon bunching. We relate our general considerations to a physical system, a (single-emitter) bimodal laser, and show that its statistics can be approximated and understood within our proposed model. Furthermore the excellent agreement of the statistics of the bimodal laser and our model reveal that the bimodal laser is an ideal source of bunched photons, in the sense that it can generate statistics that contain no other features but the superthermal bunching.

[26]  arXiv:1802.06438 (cross-list from cond-mat.mes-hall) [pdf, ps, other]
Title: Theory of nonlinear microwave absorption by interacting two-level systems
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

The microwave absorption and noise caused by quantum two-level systems (TLS) dramatically suppress the coherence in Josephson junction qubits that are promising candidates for a quantum information applications. Microwave absorption by TLSs is not clearly understood yet because of the complexity of their interactions leading to the spectral diffusion. Here, the theory of the non-linear absorption in the presence of spectral diffusion is developed using the generalized master equation formalism. The theory predicts that the linear absorption regime holds while a TLS Rabi frequency is smaller than their phase decoherence rate. At higher external fields, a novel non-linear absorption regime is found with the loss tangent inversely proportional to the intensity of the field. The theory can be generalized to acoustic absorption and lower dimensions realized in superconducting qubits.

[27]  arXiv:1802.06490 (cross-list from cond-mat.stat-mech) [pdf, ps, other]
Title: On the exact solvability of the anisotropic central spin model: An operator approach
Authors: Ning Wu
Comments: 5 pages, 0 figure, to appear in Physica A: Statistical Mechanics and its Applications
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)

Using an operator approach based on a commutator scheme that has been previously applied to Richardson's reduced BCS model and the inhomogeneous Dicke model, we obtain general exact solvability requirements for an anisotropic central spin model with $XXZ$-type hyperfine coupling between the central spin and the spin bath, without any prior knowledge of integrability of the model. We outline the basic steps of the usage of the operator approach, and pedagogically summarize them into two \emph{Lemmas} and two \emph{Constraints}. Through a step-by-step construction of the eigen-problem, we show that the condition $g'^2_j-g_j^2=c$ naturally arises for the model to be exactly solvable, where $c$ is a constant independent of the bath-spin index $j$, and $\{g_j\}$ and $\{g'_j\}$ are the longitudinal and transverse hyperfine interactions, respectively. The obtained conditions and the resulting Bethe ansatz equations are consistent with that in previous literature.

[28]  arXiv:1802.06510 (cross-list from cond-mat.quant-gas) [pdf, other]
Title: Thermodynamics and structural transition of binary atomic Bose-Fermi mixtures in box or harmonic potentials: A path-integral study
Subjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)

Experimental realizations of a variety of atomic binary Bose-Fermi mixtures have brought opportunities for studying composite quantum systems with different spin-statistics. The binary atomic mixtures can exhibit a structural transition from a mixture into phase separation as the boson-fermion interaction increases. By using a path-integral formalism to evaluate the grand partition function and thermodynamic grand potential, we obtain the effective potential of binary Bose-Fermi mixtures. Thermodynamic quantities in a broad range of temperatures and interactions are also derived. The structural transition can be identified as a loop of the effective potential curve, and the volume fraction of phase separation can be determined by the lever rule. For $^6$Li-$^7$Li and $^6$Li-$^{41}$K mixtures, we present the phase diagrams of the mixtures in a box potential at zero and finite temperatures. Due to the flexible densities of atomic gases, the construction of phase separation is more complicated when compared to conventional liquid or solid mixtures where the individual densities are fixed. For harmonically trapped mixtures, we use the local density approximation to map out the finite-temperature density profiles and present typical trap structures, including the mixture, partially separated phases, and fully separated phases.

[29]  arXiv:1802.06523 (cross-list from cond-mat.mes-hall) [pdf, ps, other]
Title: Chern-Simons layers on dielectrics and metals
Comments: 15 pages, 8 figures, Latex2e
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)

A diffraction problem for a flat Chern-Simons layer on the surface of a dielectric semispace is solved. The crossing from the repulsive to the attractive Casimir force is analyzed for two Au and two Si semispaces covered by Chern-Simons layers and separated by a vacuum slit.

[30]  arXiv:1802.06540 (cross-list from physics.atom-ph) [pdf, other]
Title: Generalized perspective on chiral measurements without magnetic interactions
Subjects: Atomic Physics (physics.atom-ph); Atomic and Molecular Clusters (physics.atm-clus); Chemical Physics (physics.chem-ph); Optics (physics.optics); Quantum Physics (quant-ph)

We present a unified description of several methods of chiral discrimination based on electric-dipole interactions. It includes photoelectron circular dichroism (PECD), microwave rotational enantio-sensitive spectroscopy, photoexcitation circular dichroism (PXCD) and photoelectron photoexcitation circular dichroism (PXECD). We show that, inspite of the fact that the physics underlying the appearance of a chiral response is very different in all these methods, the chiral observable in all cases has a unique form. It is a polar vector given by the product of (i) the molecular pseudoscalar and (ii) the field pseudovector specified by configurations of the electromagnetic fields interacting with an isotropic ensemble of chiral molecules. The molecular pseudoscalar is a rotationally invariant property, which is composed from different molecule-specific vectors and in the simplest case is a triple product of such vectors. The key property that enables the chiral response is the non coplanarity of the vectors forming such triple product. The key property that enables chiral detection without using chiral electromagnetic fields is the vectorial nature of the enantio-sensitive observable. Our compact and general expression for this observable shows what ultimately determines the efficiency of the chiral signal and if, or when, it can reach 100%. We also discuss the differences between the two phenomena, which rely on the bound states, PXCD and microwave enantio-sensitive spectroscopy, and the two phenomena using the continuum states, PECD and PXECD. Finally, we extend these methods to arbitrary polarizations of electromagnetic fields used to induce and probe the chiral response.

[31]  arXiv:1802.06554 (cross-list from cond-mat.stat-mech) [pdf, other]
Title: Speed Limit for Classical Stochastic Processes
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)

Speed limit for classical stochastic Markov processes with discrete states is studied. We find that a trade-off inequality exists between the speed of the state transformation and the entropy production. The dynamical activity determines the time scale and plays a crucial role in the inequality. For systems with stationary current, a similar trade-off inequality with the Hatano-Sasa entropy production gives a much better bound on the speed of the state transformation. Our inequalities contain only physically well-defined quantities, and thus the physical picture of these inequalities is clear.

[32]  arXiv:1802.06599 (cross-list from physics.atom-ph) [pdf, other]
Title: Study of Rydberg blockade in thermal vapor
Subjects: Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)

We present the experimental demonstration of Rydberg blockade in thermal atomic vapor where the atoms are not necessarily be frozen. We show that not all the interacting atoms but only the atoms with same velocity collectively participate in the blockade process. Using this observation, we formulated a suitable model based on super atom picture to study blockade interaction in thermal vapor. We performed an experiment to measure Rydberg population in rubidium thermal vapor using optical heterodyne detection technique and density dependent suppression of Rydberg population is observed in suitable experimental parameter regime. Further analysis of the experimental data using the model verifies the scaling law for van der Waals interaction strength $(C_{6})$ with principal quantum number of the Rydberg state with $11\%$ error. Our result suggests multi-photon excitation in thermal vapor with suitable laser configuration to probe Rydberg blockade interaction based optical nonlinearity and many body effects.

[33]  arXiv:1802.06693 (cross-list from cond-mat.quant-gas) [pdf, ps, other]
Title: Quench Dynamics of Finite Bosonic Ensembles in Optical Lattices with Spatially Modulated Interactions
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)

The nonequilibrium quantum dynamics of few boson ensembles which experience a spatially modulated interaction strength and are confined in finite optical lattices is investigated. Performing quenches either on the wavevector or the phase of the interaction profile an enhanced imbalance of the interatomic repulsion between distinct spatial regions of the lattice is induced. Following both quench protocols triggers various tunneling channels and a rich excitation dynamics consisting of a breathing and a cradle mode. All modes are shown to be amplified for increasing inhomogeneity amplitude of the interaction strength. Especially the phase quench induces a directional transport enabling us to discern energetically, otherwise, degenerate tunneling pathways. Moreover, a periodic population transfer between distinct momenta for quenches of increasing wavevector is observed, while a directed occupation of higher momenta can be achieved following a phase quench. Finally, during the evolution regions of partial coherence are revealed between the predominantly occupied wells.

[34]  arXiv:1802.06704 (cross-list from cond-mat.quant-gas) [pdf, other]
Title: Quantum simulation of lattice gauge theories using Wilson fermions
Subjects: Quantum Gases (cond-mat.quant-gas); High Energy Physics - Lattice (hep-lat); High Energy Physics - Phenomenology (hep-ph); Quantum Physics (quant-ph)

Quantum simulators have the exciting prospect of giving access to real-time dynamics of lattice gauge theories, in particular in regimes that are difficult to compute on classical computers. Future progress towards scalable quantum simulation of lattice gauge theories, however, hinges crucially on the efficient use of experimental resources. As we argue in this work, due to the fundamental non-uniqueness of discretizing the relativistic Dirac Hamiltonian, the lattice representation of gauge theories allows for an optimization that up to now has been left unexplored. We exemplify our discussion with lattice quantum electrodynamics in two-dimensional space-time, where we show that the formulation through Wilson fermions provides several advantages over the previously considered staggered fermions. Notably, it enables a strongly simplified optical lattice setup and it reduces the number of degrees of freedom required to simulate dynamical gauge fields. Exploiting the optimal representation, we propose an experiment based on a mixture of ultracold atoms trapped in a tilted optical lattice. Using numerical benchmark simulations, we demonstrate that a state-of-the-art quantum simulator may access the Schwinger mechanism and map out its non-perturbative onset.

[35]  arXiv:1802.06714 (cross-list from cond-mat.mtrl-sci) [pdf, other]
Title: Identification and tunable optical coherent control of transition-metal spins in silicon carbide
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

Color centers in wide-bandgap semiconductors are attractive systems for quantum technologies since they can combine long-coherent electronic spin and bright optical properties. Several suitable centers have been identified, most famously the nitrogen-vacancy defect in diamond. However, integration in communication technology is hindered by the fact that their optical transitions lie outside telecom wavelength bands. Several transition-metal impurities in silicon carbide do emit at and near telecom wavelengths, but knowledge about their spin and optical properties is incomplete. We present all-optical identification and coherent control of molybdenum-impurity spins in silicon carbide with transitions at near-infrared wavelengths. Our results identify spin $S=1/2$ for both the electronic ground and excited state, with highly anisotropic spin properties that we apply for implementing optical control of ground-state spin coherence. Our results show optical lifetimes below 100 ns and inhomogenous spin dephasing times near 1 $\mu$s, establishing relevance quantum spin-photon interfacing.

[36]  arXiv:1802.06740 (cross-list from hep-th) [pdf, other]
Title: Complexity Growth with Lifshitz Scaling and Hyperscaling Violation
Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)

Using complexity=action proposal we study the growth rate of holographic complexity for Lifshitz and hyperscaling violating geometries. We will consider both one and two sided black branes in an Einstein-Maxwell-Dilaton gravitational theory. We find that in either case Lloyd's bound is violated and the rate of growth of complexity saturates to a value which is greater than twice the mass of the corresponding black brane. This value reduces to the mass of the black brane in the isotropic case. We show that in two sided black brane the saturation happens from above while for one sided black brane it happens from below.

### Replacements for Tue, 20 Feb 18

[37]  arXiv:1305.3083 (replaced) [pdf, ps, other]
Title: Anomalous second order coherence and $g^{(2)}$ complementarity
Authors: Ron Folman
Comments: Invited talk for SPIE 2013
Subjects: Quantum Physics (quant-ph)
[38]  arXiv:1510.04038 (replaced) [pdf, ps, other]
Title: Quantum Discord of Cosmic Inflation: Can we Show that CMB Anisotropies are of Quantum-Mechanical Origin?
Comments: discussion below equation (86) corrected
Journal-ref: Phys. Rev. D 93, 023505 (2016)
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
[39]  arXiv:1702.05660 (replaced) [pdf, other]
Title: At the limits of criticality-based quantum metrology: apparent super-Heisenberg scaling revisited
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)
[40]  arXiv:1705.05789 (replaced) [pdf, ps, other]
Title: Constrains of Charge-to-Mass Ratios on Noncommutative Phase Space
Authors: Kai Ma
Comments: 6pages; v2: text are improved and several references are added. v3: match to the published version
Journal-ref: Adv.High Energy Phys. 2017 (2017) 1945156
Subjects: High Energy Physics - Phenomenology (hep-ph); Quantum Physics (quant-ph)
[41]  arXiv:1705.09136 (replaced) [pdf, other]
Journal-ref: Science 359, 662-666 (2018)
Subjects: Quantum Physics (quant-ph); Atomic Physics (physics.atom-ph); Optics (physics.optics)
[42]  arXiv:1708.00924 (replaced) [pdf, ps, other]
Title: Discrete Lorentz symmetry and discrete time translational symmetry
Authors: Pei Wang
Comments: 17 pages, 4 figures, the published version
Journal-ref: New J. Phys. 20, 023042 (2018)
Subjects: Statistical Mechanics (cond-mat.stat-mech); High Energy Physics - Lattice (hep-lat); Quantum Physics (quant-ph)
[43]  arXiv:1709.07378 (replaced) [pdf, other]
Title: Nonlinear Quantum Rabi Model in Trapped Ions
Journal-ref: Phys. Rev. A 97, 023624 (2018)
Subjects: Quantum Physics (quant-ph)
[44]  arXiv:1711.01284 (replaced) [pdf, other]
Title: Quantum Quenches and Relaxation Dynamics in the Thermodynamic Limit
Comments: 5 pages, 3 figures, as published
Journal-ref: Phys. Rev. Lett. 120, 070603 (2018)
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
[45]  arXiv:1711.01740 (replaced) [pdf, ps, other]
Title: Semiclassical theory of strong localization for quantum thermalization
Comments: 13 pages, 8 figures, improved version with extra figures
Journal-ref: Phys. Rev. E 97, 022127 (2018)
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Nuclear Theory (nucl-th)
[46]  arXiv:1711.07738 (replaced) [pdf, ps, other]
Title: Antiferromagnetic order without recourse to staggered fields
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
[47]  arXiv:1711.07739 (replaced) [pdf, other]
Title: Information-reality complementarity: The role of measurements and quantum reference frames
Comments: 11 pages, 1 figure, typos removed, closer to the published version, selected as Editors' Suggestion
Journal-ref: Phys. Rev. A 97, 022107 (2018)
Subjects: Quantum Physics (quant-ph)
[48]  arXiv:1711.08914 (replaced) [pdf, other]
Title: Fermionic reaction coordinates and their application to an autonomous Maxwell demon in the strong coupling regime
Comments: 18 pages incl. references, appendix and 10 figures; slightly changed version with 5 new Refs
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Statistical Mechanics (cond-mat.stat-mech)
[49]  arXiv:1712.01912 (replaced) [pdf, ps, other]
Title: Quantum dynamics of the intramolecular vibrational energy redistribution in OCS: From localization to quasi-thermalization
Subjects: Quantum Physics (quant-ph); Chemical Physics (physics.chem-ph)
[50]  arXiv:1801.01195 (replaced) [pdf, other]
Title: Joint Spectral Characterization of Photon-Pair Sources
Subjects: Quantum Physics (quant-ph)
[51]  arXiv:1801.02533 (replaced) [pdf, ps, other]
Title: Noncommutative effects on the fluid dynamics and modifications of the Freidmann equation
Authors: Kai Ma
Comments: 19 pages; v2: title is changed, texts are improved, references are added
Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)
[52]  arXiv:1801.03793 (replaced) [pdf, other]
Title: Ultralong Dephasing Times in Solid-State Spin Ensembles via Quantum Control
Subjects: Quantum Physics (quant-ph)
[53]  arXiv:1802.01524 (replaced) [pdf, other]
Title: Diffraction of a CW atom laser in the Raman-Nath regime
Comments: 5 pages and 3 figures
Subjects: Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
[54]  arXiv:1802.04196 (replaced) [pdf, ps, other]
Title: Universal quantum computing and three-manifolds
Comments: 15 pages, 4 figures, 6 tables
Subjects: Quantum Physics (quant-ph); Group Theory (math.GR); Geometric Topology (math.GT)
[55]  arXiv:1802.05687 (replaced) [pdf, other]
Title: Effect of Hilbert space truncation on Anderson localization