70 research outputs found
Quantum walks on two-dimensional grids with multiple marked locations
The running time of a quantum walk search algorithm depends on both the
structure of the search space (graph) and the configuration of marked
locations. While the first dependence have been studied in a number of papers,
the second dependence remains mostly unstudied.
We study search by quantum walks on two-dimensional grid using the algorithm
of Ambainis, Kempe and Rivosh [AKR05]. The original paper analyses one and two
marked location cases only. We move beyond two marked locations and study the
behaviour of the algorithm for an arbitrary configuration of marked locations.
In this paper we prove two results showing the importance of how the marked
locations are arranged. First, we present two placements of marked
locations for which the number of steps of the algorithm differs by
factor. Second, we present two configurations of and
marked locations having the same number of steps and probability to
find a marked location
Quantum walks can find a marked element on any graph
We solve an open problem by constructing quantum walks that not only detect
but also find marked vertices in a graph. In the case when the marked set
consists of a single vertex, the number of steps of the quantum walk is
quadratically smaller than the classical hitting time of any
reversible random walk on the graph. In the case of multiple marked
elements, the number of steps is given in terms of a related quantity
which we call extended hitting time.
Our approach is new, simpler and more general than previous ones. We
introduce a notion of interpolation between the random walk and the
absorbing walk , whose marked states are absorbing. Then our quantum walk
is simply the quantum analogue of this interpolation. Contrary to previous
approaches, our results remain valid when the random walk is not
state-transitive. We also provide algorithms in the cases when only
approximations or bounds on parameters (the probability of picking a
marked vertex from the stationary distribution) and are
known.Comment: 50 page
Hitting time for quantum walks on the hypercube
Hitting times for discrete quantum walks on graphs give an average time
before the walk reaches an ending condition. To be analogous to the hitting
time for a classical walk, the quantum hitting time must involve repeated
measurements as well as unitary evolution. We derive an expression for hitting
time using superoperators, and numerically evaluate it for the discrete walk on
the hypercube. The values found are compared to other analogues of hitting time
suggested in earlier work. The dependence of hitting times on the type of
unitary ``coin'' is examined, and we give an example of an initial state and
coin which gives an infinite hitting time for a quantum walk. Such infinite
hitting times require destructive interference, and are not observed
classically. Finally, we look at distortions of the hypercube, and observe that
a loss of symmetry in the hypercube increases the hitting time. Symmetry seems
to play an important role in both dramatic speed-ups and slow-downs of quantum
walks.Comment: 8 pages in RevTeX format, four figures in EPS forma
Non-Markovian dynamics of a qubit coupled to an Ising spin bath
We study the analytically solvable Ising model of a single qubit system
coupled to a spin bath. The purpose of this study is to analyze and elucidate
the performance of Markovian and non-Markovian master equations describing the
dynamics of the system qubit, in comparison to the exact solution. We find that
the time-convolutionless master equation performs particularly well up to
fourth order in the system-bath coupling constant, in comparison to the
Nakajima-Zwanzig master equation. Markovian approaches fare poorly due to the
infinite bath correlation time in this model. A recently proposed
post-Markovian master equation performs comparably to the time-convolutionless
master equation for a properly chosen memory kernel, and outperforms all the
approximation methods considered here at long times. Our findings shed light on
the applicability of master equations to the description of reduced system
dynamics in the presence of spin-baths.Comment: 17 pages, 16 figure
Hitting time for the continuous quantum walk
We define the hitting (or absorbing) time for the case of continuous quantum
walks by measuring the walk at random times, according to a Poisson process
with measurement rate . From this definition we derive an explicit
formula for the hitting time, and explore its dependence on the measurement
rate. As the measurement rate goes to either 0 or infinity the hitting time
diverges; the first divergence reflects the weakness of the measurement, while
the second limit results from the Quantum Zeno effect. Continuous-time quantum
walks, like discrete-time quantum walks but unlike classical random walks, can
have infinite hitting times. We present several conditions for existence of
infinite hitting times, and discuss the connection between infinite hitting
times and graph symmetry.Comment: 12 pages, 1figur
Ergodicity breaking in a model showing many-body localization
We study the breaking of ergodicity measured in terms of return probability
in the evolution of a quantum state of a spin chain. In the non ergodic phase a
quantum state evolves in a much smaller fraction of the Hilbert space than
would be allowed by the conservation of extensive observables. By the anomalous
scaling of the participation ratios with system size we are led to consider the
distribution of the wave function coefficients, a standard observable in modern
studies of Anderson localization. We finally present a criterion for the
identification of the ergodicity breaking (many-body localization) transition
based on these distributions which is quite robust and well suited for
numerical investigations of a broad class of problems.Comment: 5 pages, 5 figures, final versio
Mutation of the Traj18 gene segment using TALENs to generate Natural Killer T cell deficient mice.
Invariant Natural Killer T (iNKT) cells are a unique subset of T lymphocytes that have been implicated in both promoting and suppressing a multitude of immune responses. In mice, iNKT cells express T cell antigen receptors (TCRs) comprising a unique TCRα rearrangement between the Trav11 and Traj18 gene segments. When paired with certain Trbv TCRβ chains, these TCRs recognize lipid antigens presented by the major histocompatibility complex (MHC) class I-like molecule, CD1d. Until recently, the sole model of iNKT deficiency targeted the Jα18, which is absolutely required to form the TCR with the appropriate antigenic specificity. However, these mice were demonstrated to have a large reduction in TCR repertoire diversity, which could confound results arising from studies using these mice. Here, we have created a new NKT-deficient mouse strain using transcription activator-like effector nuclease (TALEN) technology to only disrupt the expression of Jα18, leaving the remaining Jα repertoire unperturbed. We confirm that these mice lack iNKT cells and do not respond to lipid antigen stimulation while the development of conventional T cells, regulatory T cells, and type Ib NKT cells is normal. This new mouse strain will serve as a new model of iNKT cell deficiency to facilitate our understanding of iNKT biology
Quantum walks with infinite hitting times
Hitting times are the average time it takes a walk to reach a given final
vertex from a given starting vertex. The hitting time for a classical random
walk on a connected graph will always be finite. We show that, by contrast,
quantum walks can have infinite hitting times for some initial states. We seek
criteria to determine if a given walk on a graph will have infinite hitting
times, and find a sufficient condition, which for discrete time quantum walks
is that the degeneracy of the evolution operator be greater than the degree of
the graph. The set of initial states which give an infinite hitting time form a
subspace. The phenomenon of infinite hitting times is in general a consequence
of the symmetry of the graph and its automorphism group. Using the irreducible
representations of the automorphism group, we derive conditions such that
quantum walks defined on this graph must have infinite hitting times for some
initial states. In the case of the discrete walk, if this condition is
satisfied the walk will have infinite hitting times for any choice of a coin
operator, and we give a class of graphs with infinite hitting times for any
choice of coin. Hitting times are not very well-defined for continuous time
quantum walks, but we show that the idea of infinite hitting-time walks
naturally extends to the continuous time case as well.Comment: 28 pages, 3 figures in EPS forma
Quantum walks on quotient graphs
A discrete-time quantum walk on a graph is the repeated application of a
unitary evolution operator to a Hilbert space corresponding to the graph. If
this unitary evolution operator has an associated group of symmetries, then for
certain initial states the walk will be confined to a subspace of the original
Hilbert space. Symmetries of the original graph, given by its automorphism
group, can be inherited by the evolution operator. We show that a quantum walk
confined to the subspace corresponding to this symmetry group can be seen as a
different quantum walk on a smaller quotient graph. We give an explicit
construction of the quotient graph for any subgroup of the automorphism group
and illustrate it with examples. The automorphisms of the quotient graph which
are inherited from the original graph are the original automorphism group
modulo the subgroup used to construct it. We then analyze the behavior of
hitting times on quotient graphs. Hitting time is the average time it takes a
walk to reach a given final vertex from a given initial vertex. It has been
shown in earlier work [Phys. Rev. A {\bf 74}, 042334 (2006)] that the hitting
time can be infinite. We give a condition which determines whether the quotient
graph has infinite hitting times given that they exist in the original graph.
We apply this condition for the examples discussed and determine which quotient
graphs have infinite hitting times. All known examples of quantum walks with
fast hitting times correspond to systems with quotient graphs much smaller than
the original graph; we conjecture that the existence of a small quotient graph
with finite hitting times is necessary for a walk to exhibit a quantum
speed-up.Comment: 18 pages, 7 figures in EPS forma
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