Entropy rate of message sources driven by quantum walks

The amount of information generated by a discrete time stochastic processes in a single step can be quantified by the entropy rate. We investigate the differences between two discrete time walk models, the discrete time quantum walk and the classical random walk in terms of entropy rate. We develop analytical methods to calculate and approximate it. This allows us to draw conclusions about the differences between classical stochastic and quantum processes in terms of the classical information theory.Comment: 12 pages, 4 figure

Optimization of periodic single-photon sources

We introduce a theoretical framework which is suitable for the description of all spatial and time-multiplexed periodic single-photon sources realized or proposed thus far. Our model takes into account all possibly relevant loss mechanisms. This statistical analysis of the known schemes shows that multiplexing systems can be optimized in order to produce maximal single-photon probability for various sets of loss parameters by the appropriate choice of the number of multiplexed units of spatial multiplexers or multiplexed time intervals and the input mean photon pair number, and reveals the physical reasons of the existence of the optimum. We propose a novel time-multiplexed scheme to be realized in bulk optics, which, according to the present analysis, would have promising performance when experimentally realized. It could provide a single-photon probability of 85\% with a choice of experimental parameters which are feasible according to the experiments known from the literature.Comment: 13 pages, 18 figure

Orders of chaoticity of unitaries

We introduce the concept of K-th order chaoticity of unitaries, and analyze it for the case of two-level quantum systems. This property is relevant in a certain quantum random number generation scheme. We show that no unitaries exist with an arbitrary order of chaoticity

Adiabatikus kontroll a kvantumoptikában és a kvantuminformatikában = Adiabatic control in quantum optics and in quantum informatics

Kiterjesztettük a STIRAP eljárást impulzusmomentum-állapotokból álló sokszintes degenerált kvantumrendszerekre, továbbá eljárást dolgoztunk ki állapotuk rekonstruálására. Alapvető kvantumkapu műveleteket implementáltunk a STIRAP módszer felhasználásával félvezető nanostruktúrákban. Disszipatív rendszerekben alkalmazható robusztus eljárást mutattunk be tetszőleges kevert állapot előállítására, mely kvantumbitek preparálására is használható. A kvantumtrajektória megközelítést felhasználva hatékony módszereket dolgoztunk ki az időfejlesztő operátor valamint kétidős korrelációs függvények meghatározására. Módszert javasoltunk fény nemlineáris frekvenciakonverziójára fáziskoherens közegekben. Bebizonyítottuk, hogy a fáziskoherens közegben modellezett hullámkatasztrófákra jellemző logaritmikus fázisszingularitásnál a fáziskitevő valós része egész vagy félegész szám. Megmutattuk, hogy Bose-Einstein kondenzátumban kvázi-egydimenziós áramlásnál az analóg Hawking-sugárzás hőmérséklete csak a csapdázó potenciáltól függhet. Összefonódott állapot létrehozására alkalmas algoritmust fejlesztettünk ki a STIRAP módszer alkalmazásával. Eljárást dolgoztunk ki összefonódott kvantumállapotok preparálására spin-láncokban. Numerikus optimalizációt végeztünk a végesen korrelált állapotok körében a végtelen, transzlációinvariáns spin-láncban a legközelebbi szomszéd összefonódottság maximumának meghatározására. | We have extended the STIRAP (Stimulated Raman Adiabatic Passage) method to degenerate systems consisting of angular momentum states. Moreover, we have developed a reconstruction scheme to retrieve their quantum state. We have applied our quantum state control schemes to nanostructures for implementing basic quantum logic gates. We have developed a robust method to prepare pure and mixed states in dissipative systems. We have developed efficient methods using the quantum trajectory approach for numerically determining two-time correlation functions and the time-evolution operator in open quantum systems. We have developed a method for efficient nonlinear frequency conversion in phase coherent media. We have proved that by logarithmic phase singularities, modeled in phase coherent media, at wave catastrophes the wave amplitude grows with a half-integer power. In a quasi one dimensional flow of Bose-Einstein condensed atoms the Hawking temperature is entirely determined by the curvature of the trapping potential. We have developed a method based on the STIRAP scheme for creating entangled states in compound systems. We have proposed a scheme to create entangled states in spin-chains. We have performed a numerical optimization to determine the maximum nearest neighbor entanglement in a translationally invariant infinite qubit chain based on finitely correlated states

Direct versus measurement assisted bipartite entanglement in multi-qubit systems and their dynamical generation in spin systems

We consider multi-qubit systems and relate quantitatively the problems of generating cluster states with high value of concurrence of assistance, and that of generating states with maximal bipartite entanglement. We prove an upper bound for the concurrence of assistance. We consider dynamics of spin-1/2 systems that model qubits, with different couplings and possible presence of magnetic field to investigate the appearance of the discussed entanglement properties. We find that states with maximal bipartite entanglement can be generated by an XY Hamiltonian, and their generation can be controlled by the initial state of one of the spins. The same Hamiltonian is capable of creating states with high concurrence of assistance with suitably chosen initial state. We show that the production of graph states using the Ising Hamiltonian is controllable via a single-qubit rotation of one spin-1/2 subsystem in the initial multi-qubit state. We shown that the property of Ising dynamics to convert a product state basis into a special maximally entangled basis is temporally enhanced by the application of a suitable magnetic field. Similar basis transformations are found to be feasible in the case of isotropic XY couplings with magnetic field.Comment: (14 pages, 7 figures, RevTeX4

Quantum homogenization and state randomization in semi-quantal spin systems

We investigate dynamics of semi-quantal spin systems in which quantum bits are attached to classically and possibly stochastically moving classical particles. The interaction between the quantum bits takes place when the respective classical particles get close to each other in space. We find that with Heisenberg XX couplings quantum homogenization takes place after a time long enough, regardless of the details of the underlying classical dynamics. This is accompanied by the development of a stationary bipartite entanglement. If the information on the details of the motion of a stochastic classical system is disregarded, the stationary state of the whole quantum subsystem is found to be a complete mixture in the studied cases, though the transients depend on the properties of the classical motion.Comment: 10 pages, 10 figures (included

Application of a Hybrid Algorithm Based on Quantum Annealing to Solve a Metropolitan Scale Railway Dispatching Problem

We address the applicability of quantum-classical hybrid solvers for practical railway dispatching/conflict management problems, with a demonstration on real-life metropolitan-scale network traffic. The railway network includes both single-and double segments and covers all the requirements posed by the operator of the network. We build a linear integer model for the problem and solve it with D-Wave's quantum-classical hybrid solver as well as with CPLEX for comparison. The computational results demonstrate the readiness for application and benefits of quantum-classical hybrid solvers in the a realistic railway scenario: they yield acceptable solutions on time; a critical requirement in a dispatching situation. Though they are heuristic they offer a valid alternative and outperform classical solvers in some cases

Quantum readiness for scheduling of Automatic Guided Vehicles (AGVs) as job-shop problem

A case study based on a real-life production environment for the scheduling of automated guided vehicles (AGVs) is presented. A linear programming model is formulated for scheduling AGVs with given paths and task assignments. Using the new model, a moderate size instance of 15 AGVs (all using the same main lane connecting most of the crucial parts of the factory) can be solved approximately with a CPLEX solver in seconds. The model is also solved with a state-of-the art hybrid quantum-classical solver of the noisy intermediate size quantum (NISQ) devices' era (D-Wave BQM and CQM). It is found that it performs similarly to CPLEX, thereby demonstrating the ``quantum readiness'' of the model. The hybrid solver reports non-zero quantum processing times, hence, its quantum part contributes to the solution efficiency