55 research outputs found
Progettazione di nuovi materiali per l'abbattimento di inquinanti : Come proteggere i nostri monumenti
No full textSi illustra una parte dei risultati del progetto LISA 2013 di cui M. Ceotto è il PI. Co-PI L. Lo Presti e D. Tamascelli
Excitation Dynamics in Chain-Mapped Environments
No full textThe chain mapping of structured environments is a most powerful tool for the simulation of open quantum system dynamics. Once the environmental bosonic or fermionic degrees of freedom are unitarily rearranged into a one dimensional structure, the full power of Density Matrix Renormalization Group (DMRG) can be exploited. Beside resulting in efficient and numerically exact simulations of open quantum systems dynamics, chain mapping provides an unique perspective on the environment: the interaction between the system and the environment creates perturbations that travel along the one dimensional environment at a finite speed, thus providing a natural notion of light-, or causal-, cone. In this work we investigate the transport of excitations in a chain-mapped bosonic environment. In particular, we explore the relation between the environmental spectral density shape, parameters and temperature, and the dynamics of excitations along the corresponding linear chains of quantum harmonic oscillators. Our analysis unveils fundamental features of the environment evolution, such as localization, percolation and the onset of stationary currents
Speed and entropy of an interacting continuous time quantum walk
No full textWe present some dynamic and entropic considerations about the evolution of a continuous time quantum walk implementing the clock of an autonomous
machine. On a simple model, we study in quite explicit terms the Lindblad
evolution of the clocked subsystem, relating the evolution of its entropy to the spreading of the wave packet of the clock. We explore possible ways of
reducing the generation of entropy in the clocked subsystem, as it amounts to a
deficit in the probability of finding the target state of the computation. We are thus led to examine the benefits of abandoning some classical prejudice about how a clocking mechanism should operate
Noise-assisted quantum transport and computation
No full textThe transmission of an excitation along a spin chain can be hindered by the presence of small fixed imperfections that create trapping regions where the excitation may get caught (Anderson localization). A certain degree of noise, ensuing from the interaction with a thermal bath, allows us to overcome localization (noise-assisted transport). In this paper, we investigate the relation between the noise-assisted transport and (quantum) computation. In particular, we prove that noise does assist classical computation on a quantum computing device, but hinders the possibility of creating entanglement
Entropy generation in a model of reversible computation
No full textWe present a model in which, due to the quantum nature of the signals controlling the implementation time of successive unitary
computational steps, physical irreversibility appears in the execution
of a logically reversible computation
Quantum annealing and the Schrödinger-Langevin-Kostin equation
No full textWe show, in the context of quantum combinatorial optimization, or quantum annealing, how the nonlinear Schrödinger-Langevin-Kostin equation can dynamically drive the system toward its ground state. We illustrate, moreover, how a frictional force of Kostin type can prevent the appearance of genuinely quantum problems such as Bloch oscillations and Anderson localization which would hinder an exhaustive search
Dissipative dynamics of a spin system with three-body interaction
No full textIn this paper, we explicitly solve the Lindblad equation for a system of three spins with a three-body interaction, coupled to the environment by bath operators that inject or absorb spin carriers. We exemplify the properties of this solution in the context of a simple instance of Feynman's quantum computer in which a two-qubit program line is executed, applying the primitive to a one-qubit register
Dynamical kickback and noncommuting impurities in a spin chain
No full textIn an interacting continuous time quantum walk, while the walker (the cursor) ismoving
on a graph, computational primitives (unitary operators associated to the edges) are applied to ancillary qubits (the register). The model with one walker was originally proposed by R. Feynman, who thus anticipated many features of the Continuous Time Quantum Walk (CTWQ) computing paradigm. In this note we examine the behaviour of an interacting CTQW with two walkers and examine the interaction of the walkers with noncommuting primitives. We endow such a walk with a notion of trajectory, in the sense of sample path of an associated Markov process, in order to use such notions as sojourn time and first passage time as heuristic tools for gaining intuition about its behaviour
Grover’s algorithm on a Feynman computer
No full textWe present an implementation of Grover’s algorithm in the framework of Feynman’s cursor model of a quantum computer. The cursor degrees of
freedom act as a quantum clocking mechanism, and allow Grover’s algorithm to be performed using a single, time-independent Hamiltonian. We examine
issues of locality and resource usage in implementing such a Hamiltonian.
In the familiar language of Heisenberg spin–spin coupling, the clocking mechanism appears as an excitation of a basically linear chain of spins, with occasional controlled jumps that allow for motion on a planar graph: in this sense our model implements the idea of ‘timing’ a quantum algorithm using a continuous-time random walk. In this context we examine some consequences
of the entanglement between the states of the input/output register and the states of the quantum clock
Quantum timing and synchronization problems
No full textFeynman’s model of a quantum computer provides an example of a continuous-time quantum walk. Its clocking mechanism is an excitation of a basically linear chain of spins with occasional controlled jumps which allow for motion on a planar graph. The spreading of the wave packet poses limitations on the probability of ever completing the s elementary steps of a computation: an additional amount of storage space δ is needed in order to achieve an assigned completion probability. In this note we study the
END instruction, viewed as a measurement of the position of the clocking excitation: a
π-pulse indefinitely freezes the contents of the input/output register, with a probability
depending only on the ratio δ/s
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