Agents, subsystems, and the conservation of information. (arXiv:1804.01943v2 [quant-ph] UPDATED) Quantum Physics - Mon, 2018-04-30 11:45

Dividing the world into subsystems is an important component of the scientific method. The choice of subsystems, however, is not defined a priori. Typically, it is dictated by experimental capabilities, which may be different for different agents. Here we propose a way to define subsystems in general physical theories, including theories beyond quantum and classical mechanics. Our construction associates every agent A with a subsystem SA, equipped with its set of states and its set of transformations. In quantum theory, this construction accommodates the notion of subsystems as factors of a tensor product Hilbert space, as well as the notion of subsystems associated to a subalgebra of operators. Classical systems can be interpreted as subsystems of quantum systems in different ways, by applying our construction to agents who have access to different sets of operations, including multiphase covariant channels and certain sets of free operations arising in the resource theory of quantum coherence. After illustrating the basic definitions, we restrict our attention to closed systems, that is, systems where all physical transformations act invertibly and where all states can be generated from a fixed initial state. For closed systems, we propose a dynamical definition of pure states, and show that all the states of all subsystems admit a canonical purification. This result extends the purification principle to a broader setting, in which coherent superpositions can be interpreted as purifications of incoherent mixtures.

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Resource theory of non-Gaussian operations. (arXiv:1803.07580v2 [quant-ph] UPDATED) Quantum Physics - Mon, 2018-04-30 11:45

Non-Gaussian states and operations are crucial for various continuous-variable quantum information processing tasks. To quantitatively understand non-Gaussianity beyond states, we establish a resource theory for non-Gaussian operations. In our framework, we consider Gaussian operations as free operations, and non-Gaussian operations as resources. We define entanglement-assisted non-Gaussianity generating power and show that it is a monotone that is non-increasing under the set of free super-operations, i.e., concatenation and tensoring with Gaussian channels. For conditional unitary maps, this monotone can be analytically calculated. As examples, we show that the non-Gaussianity of ideal photon-number subtraction and photon-number addition equal the non-Gaussianity of the single-photon Fock state. Based on our non-Gaussianity monotone, we divide non-Gaussian operations into two classes: (1) the finite non-Gaussianity class, e.g., photon-number subtraction, photon-number addition and all Gaussian-dilatable non-Gaussian channels; and (2) the diverging non-Gaussianity class, e.g., the binary phase-shift channel and the Kerr nonlinearity. This classification also implies that not all non-Gaussian channels are exactly Gaussian-dilatable. Our resource theory enables a quantitative characterization and a first classification of non-Gaussian operations, paving the way towards the full understanding of non-Gaussianity.

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Berry phase in controlled light propagation and storage

Author(s): Andrzej Raczyński, Jarosław Zaremba, and Sylwia Zielińska-Raczyńska

It is shown that during light storage in an atomic medium in the Λ configuration, with not only the amplitude of the control field but also its phase changing adiabatically, a photon gains a Berry (geometric) phase. In the case of the tripod configuration with two probe fields the Berry phase is rep...

[Phys. Rev. A 97, 043861] Published Fri Apr 27, 2018

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Scalable approach to generation of large symmetric Dicke states

Author(s): Sachin Kasture

Symmetric Dicke states represent a class of genuinely entangled multipartite states with superior resistance to loss and entanglement characteristics, even for low fidelity. A scalable and resource-intensive method is proposed using hybrid spatiotemporal encoding using only linear optics for generat...

[Phys. Rev. A 97, 043862] Published Fri Apr 27, 2018

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Focus: Why Your Pupils Wobble

APS Physics - Fri, 2018-04-27 12:00

Author(s): Katherine Wright

A model that describes eye behavior during and after a sudden gaze shift could help improve the interpretation of eye motion measurements for cognitive tests and eye-tracking studies.

[Physics 11, 41] Published Fri Apr 27, 2018

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Structural physical approximation for the realization of the optimal singlet fraction with two measurements

PRA: Quantum information - Fri, 2018-04-27 12:00

Author(s): Satyabrata Adhikari

Structural physical approximation (SPA) has been exploited to approximate nonphysical operation such as partial transpose. It has already been studied in the context of detection of entanglement and found that if the minimum eigenvalue of SPA to partial transpose is less than 29 then the two-qubit s...

[Phys. Rev. A 97, 042344] Published Fri Apr 27, 2018

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Experimental realization of noise-induced adiabaticity in nuclear magnetic resonance

PRA: Quantum information - Fri, 2018-04-27 12:00

Author(s): Bi-Xue Wang, Tao Xin, Xiang-Yu Kong, Shi-Jie Wei, Dong Ruan, and Gui-Lu Long

The adiabatic evolution is the dynamics of an instantaneous eigenstate of a slowly varing Hamiltonian. Recently, an interesting phenomenon shows up that white noises can enhance and even induce adiabaticity, which is in contrast to previous perception that environmental noises always modify and even...

[Phys. Rev. A 97, 042345] Published Fri Apr 27, 2018

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Robustness of Topological Order in the Toric Code with Open Boundaries. (arXiv:1804.09718v1 [cond-mat.str-el]) Quantum Physics - Fri, 2018-04-27 10:32

We analyze the robustness of topological order in the toric code in an open boundary setting in the presence of perturbations. The boundary conditions are introduced on a cylinder, and are classified into condensing and non-condensing classes depending on the behavior of the excitations at the boundary under perturbation. For the non-condensing class, we see that the topological order is more robust when compared to the case of periodic boundary conditions while in the condensing case topological order is lost as soon as the perturbation is turned on. In most cases, the robustness can be understood by the quantum phase diagram of a equivalent Ising model.

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Measurement of the entanglement spectrum of a symmetry-protected topological state using the IBM quantum computer. (arXiv:1804.09725v1 [cond-mat.mes-hall]) Quantum Physics - Fri, 2018-04-27 10:32

Entanglement properties are routinely used to characterize phases of quantum matter in theoretical computations. For example the spectrum of the reduced density matrix, or so-called "entanglement spectrum", has become a widely used diagnostic for universal topological properties of quantum phases. However, while being convenient to calculate theoretically, it is notoriously hard to measure in experiments. Here we use the IBM quantum computer to make the first ever measurement of the entanglement spectrum of a symmetry-protected topological state. We are able to distinguish its entanglement spectrum from those we measure for trivial and long-range ordered states.

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The zeroth law of thermodynamics is redundant. (arXiv:1804.09726v1 [math-ph]) Quantum Physics - Fri, 2018-04-27 10:32

We reconsider the basic building blocks of classical phenomenological thermodynamics. While doing so we show that the zeroth law is a redundant postulate for the theory by deriving it from the first and the second laws. This is in stark contrast to the prevalent conception that the three laws, the zeroth, first and second, are all necessary and independent axioms.

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Emergent statistical mechanics of entanglement in random unitary circuits. (arXiv:1804.09737v1 [cond-mat.stat-mech]) Quantum Physics - Fri, 2018-04-27 10:32

We map the dynamics of entanglement in random unitary circuits, with finite on-site Hilbert space dimension $q$, to an effective classical statistical mechanics. We demonstrate explicitly the emergence of a `minimal membrane' governing entanglement growth, which in 1+1D is a directed random walk in spacetime (or a variant thereof). Using the replica trick to handle the logarithm in the definition of the $n$th R\'enyi entropy $S_n$, we map the calculation of the entanglement after a quench to a problem of interacting random walks. For the second R\'enyi entropy, $S_2$, we are able to take the replica limit explicitly. This gives a mapping between entanglement growth and a directed polymer in a random medium at finite temperature (confirming KPZ scaling for entanglement growth in generic noisy systems). We find that the entanglement growth rate (`speed') $v_n$ depends on the R\'enyi index $n$, and we calculate $v_2$ and $v_3$ in an expansion in the inverse local Hilbert space dimension, $1/q$. These rates are determined by the free energy of a random walk, and of a bound state of two random walks, respectively, and include contributions of `energetic' and `entropic' origin. We give a combinatorial interpretation of the Page-like subleading corrections to the entanglement at late times and discuss the dynamics of the entanglement close to and after saturation. We briefly discuss the application of these insights to time-independent Hamiltonian dynamics.

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Agency and the physics of numbers. (arXiv:1804.09786v1 [physics.hist-ph]) Quantum Physics - Fri, 2018-04-27 10:32

Analogous to G\"odel's incompleteness theorems is a theorem in physics to the effect that the set of explanations of given evidence is uncountably infinite. An implication of this theorem is that contact between theory and experiment depends on activity beyond computation and measurement -- physical activity of some agent making a guess. Standing on the need for guesswork, we develop a representation of a symbol-handling agent that both computes and, on occasion, receives a guess from interaction with an oracle. We show: (1) how physics depends on such an agent to bridge a logical gap between theory and experiment; (2) how to represent the capacity of agents to communicate numerals and other symbols, and (3) how that communication is a foundation on which to develop both theory and implementation of spacetime and related competing schemes for the management of motion.

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High-resolution antenna near-field imaging and sub-THz measurements with a small atomic vapor-cell sensing element. (arXiv:1804.09789v1 [physics.ins-det]) Quantum Physics - Fri, 2018-04-27 10:32

Atomic sensing and measurement of millimeter-wave (mmW) and THz electric fields using quantum-optical EIT spectroscopy of Rydberg states in atomic vapors has garnered significant interest in recent years towards the development of atomic electric-field standards and sensor technologies. Here we describe recent work employing small atomic vapor cell sensing elements for near-field imaging of the radiation pattern of a K$_u$-band horn antenna at 13.49 GHz. We image fields at a spatial resolution of $\lambda/10$ and measure over a 72 to 240 V/m field range using off-resonance AC-Stark shifts of a Rydberg resonance. The same atomic sensing element is used to measure sub-THz electric fields at 255 GHz, an increase in mmW-frequency by more than one order of magnitude. The sub-THz field is measured over a continuous $\pm$100 MHz frequency band using a near-resonant mmW atomic transition.

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Non-equilibrium absorbing state phase transitions in discrete-time quantum dynamics. (arXiv:1804.09794v1 [quant-ph]) Quantum Physics - Fri, 2018-04-27 10:32

We introduce a discrete-time quantum dynamics on a two-dimensional lattice that describes the evolution of a $1+1$-dimensional spin system. The underlying quantum map is constructed such that the reduced state at each time step is separable. We show that for long times this state becomes stationary and displays a continuous phase transition in the density of excited spins. This phenomenon can be understood through a connection to the so-called Domany-Kinzel automaton, which implements a classical non-equilibrium process that features a transition to an absorbing state. Near the transition density-density correlations become long-ranged, but interestingly the same is the case for quantum correlations despite the separability of the stationary state. We quantify quantum correlations through the local quantum uncertainty and show that in some cases they may be determined experimentally solely by measuring expectation values of classical observables. This work is inspired by recent experimental progress in the realization of Rydberg lattice quantum simulators, which - in a rather natural way - permit the realization of conditional quantum gates underlying the discrete-time dynamics discussed here.

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Enriching a Linear/Non-linear Lambda Calculus: A Programming Language for String Diagrams. (arXiv:1804.09822v1 [cs.LO]) Quantum Physics - Fri, 2018-04-27 10:32

Linear/non-linear (LNL) models, as described by Benton, soundly model a LNL term calculus and LNL logic closely related to intuitionistic linear logic. Every such model induces a canonical enrichment that we show soundly models a LNL lambda calculus for string diagrams, introduced by Rios and Selinger (with primary application in quantum computing). Our abstract treatment of this language leads to simpler concrete models compared to those presented so far. We also extend the language with general recursion and prove soundness. Finally, we present an adequacy result for the diagram-free fragment of the language which corresponds to a modified version of Benton and Wadler's adjoint calculus with recursion.

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Optical Bistability in a Low Photon-Density Regime. (arXiv:1804.09853v1 [cond-mat.stat-mech]) Quantum Physics - Fri, 2018-04-27 10:32

We give a microscopic description of the optical bistability, where the transmission coefficient has two different values as a function of input light intensity, and the system exhibits a discontinuous jump with a hysteresis loop. We developed an efficient numerical algorithm to treat the quantum master equation for hybridized systems of many photons and a large number of two-level atoms. By using this method, we characterize the bistability from the viewpoint of eigenmodes and eigenvalues of the time evolution operator of the quantum master equation. We investigate the optical bistability within the low photon-density regime, where the hybridization of photon and atom degrees of freedom occurs and the resonance spectrum has a double peak structure. We compared it with the standard optical bistability between the low photon-density regime and the high photon-density regime, where the photons can be treated as a classical electromagnetic field and the resonance spectrum has a single peak structure. We discuss the steady-state properties of the optical bistability: dependencies of the photon number density on the intensity and the double peak structure of the photon number distribution inside the bistable region. As for the dynamical properties, we find that the relaxation timescale shows an exponential growth with the system size, and reveal how the hysteresis loop of the optical bistability depends on the size of the system and the sweeping rate of the driving amplitude. Finally, by investigating the effects of detuning frequency of the input field, we clarify the characteristic properties of the present optical bistability within the low photon-density regime, which are qualitatively different from the standard optical bistable phenomena.

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Detecting Non-Commutativity of Unknown Hamiltonians via Extreme Gain of Correlations. (arXiv:1804.09925v1 [quant-ph]) Quantum Physics - Fri, 2018-04-27 10:32

Non-commutativity is one of the most elementary non-classical features of quantum observables. Here we present a method to detect non-commutativity of Hamiltonians, which is applicable even if these Hamiltonians are completely unknown. We consider two probe objects individually interacting with the third object (mediator) but not with each other. If the probe-mediator Hamiltonians commute, we derive upper bounds on correlations between the probes that depend on the dimension of the mediator. We then demonstrate that these bounds can be violated with correlation dynamics generated by non-commuting Hamiltonians. An intuitive explanation is provided in terms of multiple exchanges of a virtual particle which lead to the excessive accumulation of correlations. A plethora of correlation quantifiers are helpful in our method, e.g. quantum entanglement, discord, mutual information, and even classical correlation, to detect non-classicality in the form of non-commutativity.

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Negative-Temperature State Relaxation and Reservoir-Assisted Quantum Entanglement in Double Spin Domain Systems. (arXiv:1804.09926v1 [quant-ph]) Quantum Physics - Fri, 2018-04-27 10:32

Spin collective phenomena including superradiance are even today being intensively investigated with experimental tests performed based on state-of-the-art quantum technologies. Such attempts are not only for the simple experimental verification of predictions from the last century but also as a motivation to explore new applications of spin collective phenomena and the coherent control of the coupling between spin ensembles and reservoirs. In this paper, we investigate the open quantum dynamics of two spin ensembles (double spin domains) coupled to a common bosonic reservoir. We analyze in detail the dynamics of our collective state and its structure by focusing on both the symmetry and asymmetry of this coupled spin system. We find that when the spin size of one of the double domains is larger than that of the other domain, at the steady state this system exhibits two novel collective behaviors: the negative-temperature state relaxation in the smaller spin domain and the reservoir-assisted quantum entanglement between the two domains. These results are the consequence of the asymmetry of this system and the decoherence driven by the common reservoir.

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Thouless time for mass-deformed SYK. (arXiv:1804.09934v1 [hep-th]) Quantum Physics - Fri, 2018-04-27 10:32

We study the onset of RMT dynamics in the mass-deformed SYK model (i.e. an SYK model deformed by a quadratic random interaction) in terms of the strength of the quadratic deformation. We use as chaos probes both the connected unfolded Spectral Form Factor (SFF) as well as the Gaussian-filtered SFF, which has been recently introduced in the literature. We show that they detect the chaotic/integrable transition of the mass-deformed SYK model at different values of the mass deformation: the Gaussian-filtered SFF sees the transition for large values of the mass deformation; the connected unfolded SFF sees the transition at small values. The latter is in qualitative agreement with the transition as seen by the OTOCs. We argue that the chaotic/integrable deformation affect the energy levels inhomogeneously: for small values of the mass deformation only the low-lying states are modified while for large values of the mass deformation also the states in the bulk of the spectrum move to the integrable behavior.

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Quantum Dynamic Programming Algorithm for DAGs. Applications for AND-OR DAG Evaluation and DAG's Diameter Search. (arXiv:1804.09950v1 [cs.DS]) Quantum Physics - Fri, 2018-04-27 10:32

In this paper, we present Quantum Dynamic Programming approach for problems on directed acycling graphs (DAGs). The algorithm has time complexity $O(\sqrt{\hat{n}m}\log \hat{n})$ comparing to a deterministic one that has time complexity $O(n+m)$. Here $n$ is a number of vertexes, $\hat{n}$ is a number of vertexes with at least one outgoing edge; and $m$ is a number of edges. We show that we can solve problems that have OR, AND, NAND, MAX and MIN functions as the main transition step. The approach is useful for a couple of problems. One of them is computing Boolean formula that represented by DAG with AND and OR boolean operations in vertexes. Another one is DAG's diameter search.

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