14,675 research outputs found
Quantum Walks on the Line with Phase Parameters
In this paper, a study on discrete-time coined quantum walks on the line is
presented. Clear mathematical foundations are still lacking for this quantum
walk model. As a step towards this objective, the following question is being
addressed: {\it Given a graph, what is the probability that a quantum walk
arrives at a given vertex after some number of steps?} This is a very natural
question, and for random walks it can be answered by several different
combinatorial arguments. For quantum walks this is a highly non-trivial task.
Furthermore, this was only achieved before for one specific coin operator
(Hadamard operator) for walks on the line. Even considering only walks on
lines, generalizing these computations to a general SU(2) coin operator is a
complex task. The main contribution is a closed-form formula for the amplitudes
of the state of the walk (which includes the question above) for a general
symmetric SU(2) operator for walks on the line. To this end, a coin operator
with parameters that alters the phase of the state of the walk is defined.
Then, closed-form solutions are computed by means of Fourier analysis and
asymptotic approximation methods. We also present some basic properties of the
walk which can be deducted using weak convergence theorems for quantum walks.
In particular, the support of the induced probability distribution of the walk
is calculated. Then, it is shown how changing the parameters in the coin
operator affects the resulting probability distribution.Comment: In v2 a small typo was fixed. The exponent in the definition of N_j
in Theorem 3 was changed from -1/2 to 1. 20 pages, 3 figures. Presented at
10th Asian Conference on Quantum Information Science (AQIS'10). Tokyo, Japan.
August 27-31, 201
The Witten index for one-dimensional split-step quantum walks under the non-Fredholm condition
Split-step quantum walks possess chiral symmetry. From a supercharge, that is
the imaginary part of the time evolution operator, the Witten index can be
naturally defined and it gives the lower bound of the dimension of eigenspaces
of or . The first three authors studied the Witten index as a Fredholm
index under the condition that the supercharge satisfies the Fredholm
condition. In this paper, we establish the Witten index formula under the
non-Fredholm condition. To derive it, we employ the spectral shift function
induced by the fourth-order difference operator with a rank one perturbation on
the half-line. Under two-phase limit conditions and trace conditions, the
Witten index only depends on parameters of two-side limits. Especially,
half-integer indices appear.Comment: 26 pages, 1 figure
Recurrence of biased quantum walks on a line
The Polya number of a classical random walk on a regular lattice is known to
depend solely on the dimension of the lattice. For one and two dimensions it
equals one, meaning unit probability to return to the origin. This result is
extremely sensitive to the directional symmetry, any deviation from the equal
probability to travel in each direction results in a change of the character of
the walk from recurrent to transient. Applying our definition of the Polya
number to quantum walks on a line we show that the recurrence character of
quantum walks is more stable against bias. We determine the range of parameters
for which biased quantum walks remain recurrent. We find that there exist
genuine biased quantum walks which are recurrent.Comment: Journal reference added, minor corrections in the tex
Quantum walks on a circle with optomechanical systems
We propose an implementation of a quantum walk on a circle on an
optomechanical system by encoding the walker on the phase space of a radiation
field and the coin on a two-level state of a mechanical resonator. The dynamics
of the system is obtained by applying Suzuki-Trotter decomposition. We
numerically show that the system displays typical behaviors of quantum walks,
namely, the probability distribution evolves ballistically and the standard
deviation of the phase distribution is linearly proportional to the number of
steps. We also analyze the effects of decoherence by using the phase damping
channel on the coin space, showing the possibility to implement the quantum
walk with present day technology.Comment: 6 figures, 16 pages in Quantum Information Processing, July 201
Coined quantum walks on percolation graphs
Quantum walks, both discrete (coined) and continuous time, form the basis of
several quantum algorithms and have been used to model processes such as
transport in spin chains and quantum chemistry. The enhanced spreading and
mixing properties of quantum walks compared with their classical counterparts
have been well-studied on regular structures and also shown to be sensitive to
defects and imperfections in the lattice. As a simple example of a disordered
system, we consider percolation lattices, in which edges or sites are randomly
missing, interrupting the progress of the quantum walk. We use numerical
simulation to study the properties of coined quantum walks on these percolation
lattices in one and two dimensions. In one dimension (the line) we introduce a
simple notion of quantum tunneling and determine how this affects the
properties of the quantum walk as it spreads. On two-dimensional percolation
lattices, we show how the spreading rate varies from linear in the number of
steps down to zero, as the percolation probability decreases to the critical
point. This provides an example of fractional scaling in quantum walk dynamics.Comment: 25 pages, 14 figures; v2 expanded and improved presentation after
referee comments, added extra figur
Localization, delocalization, and topological phase transitions in the one-dimensional split-step quantum walk
Quantum walks are promising for information processing tasks because on
regular graphs they spread quadratically faster than random walks. Static
disorder, however, can turn the tables: unlike random walks, quantum walks can
suffer Anderson localization, whereby the spread of the walker stays within a
finite region even in the infinite time limit. It is therefore important to
understand when we can expect a quantum walk to be Anderson localized and when
we can expect it to spread to infinity even in the presence of disorder. In
this work we analyze the response of a generic one-dimensional quantum walk --
the split-step walk -- to different forms of static disorder. We find that
introducing static, symmetry-preserving disorder in the parameters of the walk
leads to Anderson localization. In the completely disordered limit, however, a
delocalization sets in, and the walk spreads subdiffusively. Using an efficient
numerical algorithm, we calculate the bulk topological invariants of the
disordered walk, and interpret the disorder-induced Anderson localization and
delocalization transitions using these invariants.Comment: version 2, submitted to Phys. Rev.
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