13,539 research outputs found
Quantum simulation of topologically protected states using directionally unbiased linear-optical multiports
It is shown that quantum walks on one-dimensional arrays of special
linear-optical units allow the simulation of discrete-time Hamiltonian systems
with distinct topological phases. In particular, a slightly modified version of
the Su-Schrieffer-Heeger (SSH) system can be simulated, which exhibits states
of nonzero winding number and has topologically protected boundary states. In
the large-system limit this approach uses quadratically fewer resources to
carry out quantum simulations than previous linear-optical approaches and can
be readily generalized to higher-dimensional systems. The basic optical units
that implement this simulation consist of combinations of optical multiports
that allow photons to reverse direction
Photonic simulation of entanglement growth and engineering after a spin chain quench
The time evolution of quantum many-body systems is one of the most important processes for benchmarking quantum simulators. The most curious feature of such dynamics is the growth of quantum entanglement to an amount proportional to the system size (volume law) even when interactions are local. This phenomenon has great ramifications for fundamental aspects, while its optimisation clearly has an impact on technology (e.g., for on-chip quantum networking). Here we use an integrated photonic chip with a circuit-based approach to simulate the dynamics of a spin chain and maximise the entanglement generation. The resulting entanglement is certified by constructing a second chip, which measures the entanglement between multiple distant pairs of simulated spins, as well as the block entanglement entropy. This is the first photonic simulation and optimisation of the extensive growth of entanglement in a spin chain, and opens up the use of photonic circuits for optimising quantum devices
Twisted Photons: New Quantum Perspectives in High Dimensions
Quantum information science and quantum information technology have seen a
virtual explosion world-wide. It is all based on the observation that
fundamental quantum phenomena on the individual particle or system-level lead
to completely novel ways of encoding, processing and transmitting information.
Quantum mechanics, a child of the first third of the 20th century, has found
numerous realizations and technical applications, much more than was thought at
the beginning. Decades later, it became possible to do experiments with
individual quantum particles and quantum systems. This was due to technological
progress, and for light in particular, the development of the laser. Hitherto,
nearly all experiments and also nearly all realizations in the fields have been
performed with qubits, which are two-level quantum systems. We suggest that
this limitation is again mainly a technological one, because it is very
difficult to create, manipulate and measure more complex quantum systems. Here,
we provide a specific overview of some recent developments with
higher-dimensional quantum systems. We mainly focus on Orbital Angular Momentum
(OAM) states of photons and possible applications in quantum information
protocols. Such states form discrete higher-dimensional quantum systems, also
called qudits. Specifically, we will first address the question what kind of
new fundamental properties exist and the quantum information applications which
are opened up by such novel systems. Then we give an overview of recent
developments in the field by discussing several notable experiments over the
past 2-3 years. Finally, we conclude with several important open questions
which will be interesting for investigations in the future.Comment: 15 pages, 7 figure
Quantum walks: a comprehensive review
Quantum walks, the quantum mechanical counterpart of classical random walks,
is an advanced tool for building quantum algorithms that has been recently
shown to constitute a universal model of quantum computation. Quantum walks is
now a solid field of research of quantum computation full of exciting open
problems for physicists, computer scientists, mathematicians and engineers.
In this paper we review theoretical advances on the foundations of both
discrete- and continuous-time quantum walks, together with the role that
randomness plays in quantum walks, the connections between the mathematical
models of coined discrete quantum walks and continuous quantum walks, the
quantumness of quantum walks, a summary of papers published on discrete quantum
walks and entanglement as well as a succinct review of experimental proposals
and realizations of discrete-time quantum walks. Furthermore, we have reviewed
several algorithms based on both discrete- and continuous-time quantum walks as
well as a most important result: the computational universality of both
continuous- and discrete- time quantum walks.Comment: Paper accepted for publication in Quantum Information Processing
Journa
Universally Optimal Noisy Quantum Walks on Complex Networks
Transport properties play a crucial role in several fields of science, as
biology, chemistry, sociology, information science, and physics. The behavior
of many dynamical processes running over complex networks is known to be
closely related to the geometry of the underlying topology, but this connection
becomes even harder to understand when quantum effects come into play. Here, we
exploit the Kossakoski-Lindblad formalism of quantum stochastic walks to
investigate the capability to quickly and robustly transmit energy (or
information) between two distant points in very large complex structures,
remarkably assisted by external noise and quantum features as coherence. An
optimal mixing of classical and quantum transport is, very surprisingly, quite
universal for a large class of complex networks. This widespread behaviour
turns out to be also extremely robust with respect to geometry changes. These
results might pave the way for designing optimal bio-inspired geometries of
efficient transport nanostructures that can be used for solar energy and also
quantum information and communication technologies.Comment: 17 pages, 12 figure
Quantum Google in a Complex Network
We investigate the behavior of the recently proposed quantum Google
algorithm, or quantum PageRank, in large complex networks. Applying the quantum
algorithm to a part of the real World Wide Web, we find that the algorithm is
able to univocally reveal the underlying scale-free topology of the network and
to clearly identify and order the most relevant nodes (hubs) of the graph
according to their importance in the network structure. Moreover, our results
show that the quantum PageRank algorithm generically leads to changes in the
hierarchy of nodes. In addition, as compared to its classical counterpart, the
quantum algorithm is capable to clearly highlight the structure of secondary
hubs of the network, and to partially resolve the degeneracy in importance of
the low lying part of the list of rankings, which represents a typical
shortcoming of the classical PageRank algorithm. Complementary to this study,
our analysis shows that the algorithm is able to clearly distinguish scale-free
networks from other widespread and important classes of complex networks, such
as Erd\H{o}s-R\'enyi networks and hierarchical graphs. We show that the ranking
capabilities of the quantum PageRank algorithm are related to an increased
stability with respect to a variation of the damping parameter that
appears in the Google algorithm, and to a more clearly pronounced power-law
behavior in the distribution of importance among the nodes, as compared to the
classical algorithm. Finally, we study to which extent the increased
sensitivity of the quantum algorithm persists under coordinated attacks of the
most important nodes in scale-free and Erd\H{o}s-R\'enyi random graphs
Interdisciplinary and physics challenges of Network Theory
Network theory has unveiled the underlying structure of complex systems such
as the Internet or the biological networks in the cell. It has identified
universal properties of complex networks, and the interplay between their
structure and dynamics. After almost twenty years of the field, new challenges
lie ahead. These challenges concern the multilayer structure of most of the
networks, the formulation of a network geometry and topology, and the
development of a quantum theory of networks. Making progress on these aspects
of network theory can open new venues to address interdisciplinary and physics
challenges including progress on brain dynamics, new insights into quantum
technologies, and quantum gravity.Comment: (7 pages, 4 figures
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