Physics

Direct computation of the quantum partition function by path-integral nested sampling. (arXiv:1804.05987v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

In the present work we introduce a computational approach to the absolute rovibrational quantum partition function using the path-integral formalism of quantum mechanics in combination with the nested sampling technique. The numerical applicability of path-integral nested sampling is demonstrated for small molecules of spectroscopic interest. The computational cost of the method is determined by the evaluation time of a point on the potential-energy surface (PES). For efficient PES implementations, the path-integral nested-sampling method can be a viable alternative to the direct Boltzmann summation technique of variationally computed rovibrational energies, especially for medium-sized molecules and at elevated temperatures.

Categories: Journals, Physics

square affinity of coherence and least square measurement. (arXiv:1804.06090v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

We establish an operational measure for quantum coherence with affinity, a notion similar to fidelity. That is, we introduce the square affinity coherence and give the analytic formula for this coherence measure. Moreover, we provide an operational interpretation for this coherence measure, by proving that the corresponding coherence is equal to the error probability to discrimination a set of pure states with least square measurement. Besides, we study its convex roof and show that it is a different coherence measure, not like geometric coherence. Based on the roles they play in quantum state discrimination, we make a comparison between geometric coherence and these quantifiers. Following the same idea, we introduce a family of coherence measures with the generalization of affinity, these are also the special case of $\alpha$-$z$-relative R$\acute{\mathrm{e}}$nyi entropy.

Categories: Journals, Physics

Exact solution of generalized Dirac oscillator in electric field. (arXiv:1804.06091v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

We study generalized (1+1)-dimensional Dirac oscillator in nonuniform electric field. It is shown that in the case of specially chosen electric field the eigenvalue equation can be casted in the form of supersymmetric quantum mechanics. It gives a possibility to find exact solution for the energy spectrum of the generalized Dirac oscillator in nonuniform electric field. Explicit examples of exact solutions are presented. We show that sufficiently large electric field destroys the bounded eigenstates.

Categories: Journals, Physics

A simple protocol for fault tolerant verification of quantum computation. (arXiv:1804.06105v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

With experimental quantum computing technologies now in their infancy, the search for efficient means of testing the correctness of these quantum computations is becoming more pressing. An approach to the verification of quantum computation within the framework of interactive proofs has been fruitful for addressing this problem. Specifically, an untrusted agent (prover) alleging to perform quantum computations can have his claims verified by another agent (verifier) who only has access to classical computation and a small quantum device for preparing or measuring single qubits. However, when this quantum device is prone to errors, verification becomes challenging and often existing protocols address this by adding extra assumptions, such as requiring the noise in the device to be uncorrelated with the noise on the prover's devices. In this paper, we present a simple protocol for verifying quantum computations, in the presence of noisy devices, with no extra assumptions. This protocol is based on post hoc techniques for verification, which allow for the prover to know the desired quantum computation and its input. We also perform a simulation of the protocol, for a one-qubit computation, and find the error thresholds when using the qubit repetition code as well as the Steane code.

Categories: Journals, Physics

Perfect strategies for non-signalling games. (arXiv:1804.06151v1 [math.OA])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

We unify and consolidate various results about non-signall-ing games, a subclass of non-local two-player one-round games, by introducing and studying several new families of games and establishing general theorems about them, which extend a number of known facts in a variety of special cases. Among these families are {\it reflexive games,} which are characterised as the hardest non-signalling games that can be won using a given set of strategies. We introduce {\it imitation games,} in which the players display linked behaviour, and which contains as subclasses the classes of variable assignment games, binary constraint system games, synchronous games, many games based on graphs, and {\it unique} games. We associate a C*-algebra $C^*(\mathcal{G})$ to any imitation game $\mathcal{G}$, and show that the existence of perfect quantum commuting (resp.\ quantum, local) strategies of $\mathcal{G}$ can be characterised in terms of properties of this C*-algebra, extending known results about synchronous games. We single out a subclass of imitation games, which we call {\it mirror games,} and provide a characterisation of their quantum commuting strategies that has an algebraic flavour, showing in addition that their approximately quantum perfect strategies arise from amenable traces on the encoding C*-algebra. We describe the main classes of non-signalling correlations in terms of states on operator system tensor products.

Categories: Journals, Physics

Classicalization Clearly: Quantum Transition into States of Maximal Memory Storage Capacity. (arXiv:1804.06154v1 [hep-th])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

Classicalization is a phenomenon of redistribution of energy - initially stored in few hard quanta - into the high occupation numbers of the soft modes, described by a final state that is approximately classical. Using an effective Hamiltonian, we first show why the transition amplitudes that increase occupation numbers are exponentially suppressed and how a very special family of classicalizing theories compensates this suppression. This is thanks to a large micro-state entropy generated by the emergent gapless modes around the final classical state. The dressing of the process by the super-soft quanta of these modes compensates the exponential suppression of the transition probability. Hence, an unsuppressed classicalization takes place exclusively into the states of exponentially enhanced memory storage capacity. Next, we describe this phenomenon in the language of a quantum neural network, in which the neurons are represented as interconnected quantum modes with gravity-like negative-energy synaptic connections. We show that upon an injection of energy in form of a hard quantum stimulus, the network reaches the classicalized state of exponentially enhanced memory capacity with order one probability. We construct a simple model in which the transition results into classical states that carry an area-law micro-state entropy. In this language, a non-Wilsonian UV-completion of the Standard Model via classicalization implies that above cutoff energy the theory operates as a brain network that softens the high energy quanta by bringing itself into the state of a maximal memory capacity. A similar interpretation applies to black hole formation in particle collision.

Categories: Journals, Physics

Increased Dimensionality of Raman Cooling in a Slightly Nonorthogonal Optical Lattice. (arXiv:1804.06155v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

We experimentally study the effect of a slight nonorthogonality in a two-dimensional optical lattice onto resolved-sideband Raman cooling. We find that when the trap frequencies of the two lattice directions are equal, the trap frequencies of the combined potential exhibit an avoided crossing and the corresponding eigenmodes are rotated by 45 gerees relative to the lattice beams. Hence, tuning the trap frequencies makes it possible to rotate the eigenmodes such that both eigenmodes have a large projection onto any desired direction in the lattice plane, in particular, onto the direction along which Raman cooling works. Using this, we achieve two-dimensional Raman ground-state cooling in a geometry where this would be impossible, if the eigenmodes were not rotated. Our experiment is performed with a single atom inside an optical resonator but this is inessential and the scheme is expected to work equally well in other situations.

Categories: Journals, Physics

Schwarzian correction to quantum correlation in SYK model. (arXiv:1804.06164v1 [hep-th])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

We study a class of SYK-type models in large N limit from the gravity dual side in terms of Schwarzian action analytically. The quantum correction to two point correlation function due to the Schwarzian action produces transfer of degree of freedom from the quasiparticle peak to Hubbard band in density of states (DOS), a signature strong correlation. In Schwinger-Keldysh (SK) formalism, we calculate higher point thermal out-of-time order correlation (OTOC) functions, which indicate quantum chaos by having Lyapunov exponent. Higher order local spin-spin correlations are also calculated, which can be related to the dynamical local susceptibility of quantum liquids such as spin glasses, disordered metals.

Categories: Journals, Physics

Generalized Weyl-Heisenberg algebra, qudit systems and entanglement measure of symmetric states via spin coherent states. (arXiv:1804.06184v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

A relation is established in the present paper between Dicke states in a d-dimensional space and vectors in the representation space of a generalized Weyl-Heisenberg algebra of finite dimension d. This provides a natural way to deal with the separable and entangled states of a system of N = d-1 symmetric qubit states. Using the decomposition property of Dicke states, it is shown that the separable states coincide with the Perelomov coherent states associated with the generalized Weyl-Heisenberg algebra considered in this paper. In the so-called Majorana scheme, the qudit (d-level) states are represented by N points on the Bloch sphere; roughly speaking, it can be said that a qudit (in a d-dimensional space) is describable by a N-qubit vector (in a N-dimensional space). In such a scheme, the permanent of the matrix describing the overlap between the N qubits makes it possible to measure the entanglement between the N qubits forming the qudit. This is confirmed by a Fubini-Study metric analysis. A new parameter, proportional to the permanent and called perma-concurrence, is introduced for characterizing the entanglement of a symmetric qudit arising from N qubits. For d=3 (i.e., N = 2), this parameter constitutes an alternative to the concurrence for two qubits. Other examples are given for d=4 and 5. A connection between Majorana stars and zeros of a Bargmmann function for qudits closes this article.

Categories: Journals, Physics

Geometric and algebraic origins of additive uncertainty relations. (arXiv:1804.06191v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

Constructive techniques to establish state-independent uncertainty relations for the sum of variances of arbitrary two observables are presented. We investigate the range of simultaneously attainable pairs of variances, which can be applied to a wide variety of problems including detection of quantum entanglement. Resulting uncertainty relations are state-independent, semianalytical, bound-error and can be made arbitrarily tight. The advocated approach allows one to improve earlier numerical works and to derive semianalitical tight bounds for the uncertainty relation for the sum of variances constrained to finding roots of a polynomial of a single real variable. In several cases it is possible to solve the problem completely by establishing exact analytical bounds.

Categories: Journals, Physics

Multiscale Cyclic Dynamics in Light Harvesting Complex in presence of vibrations and noise. (arXiv:1804.06260v1 [physics.chem-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

Starting from the many-body Schr\"odinger equation, we derive a new type of Lindblad Master equations describing a cyclic exciton/electron dynamics in the light harvesting complex and the reaction center. These equations resemble the Master equations for the electric current in mesoscopic systems, and they go beyond the single-exciton description by accounting for the multi-exciton states accumulated in the antenna, as well as the charge-separation, fluorescence and photo-absorption. Although these effects take place on very different timescales, their inclusion is necessary for a consistent description of the exciton dynamics. Our approach reproduces both coherent and incoherent dynamics of exciton motion along the antenna in the presence of vibrational modes and noise. We applied our results to evaluate energy (exciton) and fluorescent currents as a function of sunlight intensity.

Categories: Journals, Physics

Effective control of chemical potentials by Rabi coupling with rf-fields in ultracold mixtures. (arXiv:1804.06266v1 [cond-mat.quant-gas])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

We show that a linear term coupling the atoms of an ultracold binary mixture provides a simple method to induce an effective and tunable population imbalance between them. This term is easily realized by a Rabi coupling between different hyperfine levels of the same atomic species. The resulting effective imbalance holds for one-particle states dressed by the Rabi coupling and obtained diagonalizing the mixing matrix of the Rabi term. This way of controlling the chemical potentials applies for both bosonic and fermionic atoms and it allows also for spatially and temporally dependent imbalances. As a first application, we show that, in the case of two attractive fermionic hyperfine levels with equal chemical potentials and coupled by the Rabi pulse, the same superfluid properties of an imbalanced binary mixture are recovered. We finally discuss the properties of m-species mixtures in the presence of SU(m)-invariant interactions.

Categories: Journals, Physics

Self-similarity analysis of the non-linear Schr\"odinger equation in the Madelung form. (arXiv:1804.06274v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

In the present study a particular case of Gross-Pitaevskii or non-linear Schr\"odinger equation is rewritten to a form similar to a hydrodynamic Euler equation using the Madelung transformation. The obtained system of differential equations is highly nonlinear. Regarding solutions, a larger coefficient of the nonlinear term yields stronger deviation of the solution from the linear case.

Categories: Journals, Physics

Cavity correlated electron-nuclear dynamics from first principles. (arXiv:1804.06295v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

The rapidly developing and converging fields of polaritonic chemistry and quantum optics necessitate a unified approach to predict strongly-correlated light-matter interactions with atomic-scale resolution. Combining concepts from both fields presents an opportunity to create a predictive theoretical and computational approach to describe cavity correlated electron-nuclear dynamics from first principles. Towards this overarching goal, we introduce a general time-dependent density-functional theory to study correlated electron, nuclear and photon interactions on the same quantized footing. In our work we demonstrate the arising one-to-one correspondence in quantum-electrodynamical density-functional theory, introduce Kohn-Sham systems, and discuss possible routes for approximations to the emerging exchange-correlation potentials. We complement our theoretical formulation with the first ab initio calculation of a correlated electron-nuclear-photon system. From the time-dependent dipole moment of a CO$_2$ molecule in an optical cavity, we construct the infrared spectra and time-dependent quantum-electrodynamical observables such as the electric displacement field, Rabi splitting between the upper and lower polaritonic branches and cavity-modulated molecular motion. This cavity-modulated molecular motion has the potential to alter and open new chemical reaction pathways as well as create new hybrid states of light and matter. Our work opens an important new avenue in introducing ab initio methods to the nascent field of collective strong vibrational light-matter interactions.

Categories: Journals, Physics

Photon-graviton scattering: a new way to detect anisotropic gravitational waves?. (arXiv:1804.06298v1 [gr-qc])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

Gravitons are the quantum counterparts of gravitational waves in low-energy theories of gravity. Using Feynman rules one can compute scattering amplitudes describing the interaction between gravitons and other fields. Here, we consider the interaction between gravitons and photons. Using the quantum Boltzmann equation formalism, we derive fully general equations describing the radiation transfer of photon polarization, due to the forward scattering with gravitons. We show that the Q and U photon linear polarization modes couple with the V photon circular polarization mode, if gravitons have anisotropies in their power-spectrum statistics. As an example, we apply our results to the case of primordial gravitons, considering models of inflation where an anisotropic primordial graviton distribution is produced. Finally, we evaluate the effect on Cosmic Microwave Background (CMB) polarization, showing that in general the expected effects on the observable CMB frequencies are very small. However, our result is promising, since it could provide a novel tool for detecting anisotropic backgrounds of gravitational waves, as well as for getting further insight on the physics of gravitational waves.

Categories: Journals, Physics

Pair-correlated variational approach for interacting ultra-cold atoms in arbitrary one-dimensional confinement. (arXiv:1804.06342v1 [cond-mat.quant-gas])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

Standard analytical construction of the many-body wave function of interacting particles, beyond mean-field theory, is based on the Jastrow approach. Many-body interacting ground state is build up from the ground state of the non-interacting system and the product of solutions of the corresponding interacting two-body problem. However, this is possible only if the center of mass motion in decoupled from the mutual interactions. In our work we present an alternative approach to the standard \textit{pair-correlation} wave-function construction which is based on the general constraints given by contact nature of the atom-atom interactions. Within the proposed ansatz we study the many-body properties of trapped bosons as well as fermionic mixtures and compare its predictions with the exact diagonalization approach in a wide range of particle numbers, interaction strengths, and different trapping potentials.

Categories: Journals, Physics

Hyperbolic quantum color codes. (arXiv:1804.06382v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

Current work presents a new approach to quantum color codes on compact surfaces with genus $g \geq 2$ using the identification of these surfaces with hyperbolic polygons and hyperbolic tessellations. We show that this method may give rise to color codes with a very good parameters and we present tables with several examples of these codes whose parameters had not been shown before. We also present a family of codes with minimum distance $d=4$ and the encoding rate asymptotically going to 1 while $n \rightarrow \infty$.

Categories: Journals, Physics

Relativistic entanglement of two particles driven by continuous product momenta. (arXiv:1409.1316v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

In this paper we explore the entanglement of two relativistic spin-$1/2$ particles with continuous momenta. The spin state is described by the Bell state and the momenta are given by Gaussian distributions of product form. Transformations of the spins are systematically investigated in different boost scenarios by calculating the orbits and concurrence of the spin degree of freedom. By visualizing the behavior of the spin state we get further insight into how and why the entanglement changes in different boost situations.

Categories: Journals, Physics

Anonymous broadcasting of classical information with a continuous-variable topological quantum code. (arXiv:1503.00717v4 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

Broadcasting information anonymously becomes more difficult as surveillance technology improves, but remarkably, quantum protocols exist that enable provably traceless broadcasting. The difficulty is making scalable entangled resource states that are robust to errors. We propose an anonymous broadcasting protocol that uses a continuous-variable surface-code state that can be produced using current technology. High squeezing enables large transmission bandwidth and strong anonymity, and the topological nature of the state enables local error mitigation.

Categories: Journals, Physics

Quantum Approximate Markov Chains are Thermal. (arXiv:1609.06636v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Wed, 2018-04-18 05:45

We prove that any one-dimensional (1D) quantum state with small quantum conditional mutual information in all certain tripartite splits of the system, which we call a quantum approximate Markov chain, can be well-approximated by a Gibbs state of a short-range quantum Hamiltonian. Conversely, we also derive an upper bound on the (quantum) conditional mutual information of Gibbs states of 1D short-range quantum Hamiltonians. We show that the conditional mutual information between two regions A and C conditioned on the middle region B decays exponentially with the square root of the length of B.

These two results constitute a variant of the Hammersley-Clifford theorem (which characterizes Markov networks, i.e. probability distributions which have vanishing conditional mutual information, as Gibbs states of classical short-range Hamiltonians) for 1D quantum systems. The result can be seen as a strengthening - for 1D systems - of the mutual information area law for thermal states. It directly implies an efficient preparation of any 1D Gibbs state at finite temperature by a constant-depth quantum circuit.

Categories: Journals, Physics
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