6,781 research outputs found
Decoherence can be useful in quantum walks
We present a study of the effects of decoherence in the operation of a
discrete quantum walk on a line, cycle and hypercube. We find high sensitivity
to decoherence, increasing with the number of steps in the walk, as the
particle is becoming more delocalised with each step. However, the effect of a
small amount of decoherence is to enhance the properties of the quantum walk
that are desirable for the development of quantum algorithms. Specifically, we
observe a highly uniform distribution on the line, a very fast mixing time on
the cycle, and more reliable hitting times across the hypercube.Comment: (Imperial College London) 6 (+epsilon) pages, 6 embedded eps figures,
RevTex4. v2 minor changes to correct typos and refs, submitted version. v3
expanded into article format, extra figure, updated refs, Note on "glued
trees" adde
Limits of quantum speedup in photosynthetic light harvesting
It has been suggested that excitation transport in photosynthetic light
harvesting complexes features speedups analogous to those found in quantum
algorithms. Here we compare the dynamics in these light harvesting systems to
the dynamics of quantum walks, in order to elucidate the limits of such quantum
speedups. For the Fenna-Matthews-Olson (FMO) complex of green sulfur bacteria,
we show that while there is indeed speedup at short times, this is short lived
(70 fs) despite longer lived (ps) quantum coherence. Remarkably, this time
scale is independent of the details of the decoherence model. More generally,
we show that the distinguishing features of light-harvesting complexes not only
limit the extent of quantum speedup but also reduce rates of diffusive
transport. These results suggest that quantum coherent effects in biological
systems are optimized for efficiency or robustness rather than the more elusive
goal of quantum speedup.Comment: 9 pages, 6 figures. To appear in New Journal Physics, special issue
on "Quantum Effects and Noise in Biomolecules." Updated to accepted versio
Anomalous momentum diffusion in a dissipative many-body system
Decoherence is ubiquitous in quantum physics, from the conceptual foundations
to quantum information processing or quantum technologies, where it is a threat
that must be countered. While decoherence has been extensively studied for
simple, well-isolated systems such as single atoms or ions, much less is known
for many-body systems where inter-particle correlations and interactions can
drastically alter the dissipative dynamics. Here we report an experimental
study of how spontaneous emission destroys the spatial coherence of a gas of
strongly interacting bosons in an optical lattice. Instead of the standard
momentum diffusion expected for independent atoms, we observe an anomalous
sub-diffusive expansion, associated with a universal slowing down of the decoherence dynamics. This algebraic decay reflects the
emergence of slowly-relaxing many-body states, akin to sub-radiant states of
many excited emitters. These results, supported by theoretical predictions,
provide an important benchmark in the understanding of open many-body systems.Comment: Supplementary material available as ancillary fil
Quantum walks on two kinds of two-dimensional models
In this paper, we numerically study quantum walks on two kinds of
two-dimensional graphs: cylindrical strip and Mobius strip. The two kinds of
graphs are typical two-dimensional topological graph. We study the crossing
property of quantum walks on these two models. Also, we study its dependence on
the initial state, size of the model. At the same time, we compare the quantum
walk and classical walk on these two models to discuss the difference of
quantum walk and classical walk
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