80,312 research outputs found
Relative phase change during quantum operation
Quantum operations represented by completely positive maps encompass many of
the physical processes and have been very powerful in describing quantum
computation and information processing tasks. We introduce the notion of
relative phase change for a quantum system undergoing quantum operation. We
find that the relative phase shift of a system not only depends on the state of
the system, but also depends on the initial state of the ancilla with which it
might have interacted in the past. The relative phase change during a sequence
of quantum operations is shown to be non-additive in nature. This property can
attribute a `memory' to a quantum channel. Also the notion of relative phase
shift helps us to define what we call `in-phase quantum channels'. We will
present the relative phase shift for a qubit undergoing depolarizing channel
and complete randomization and discuss their implications.Comment: Latex file, article style, 15 pages, no figures. Invited talk
presented at First Feynman Festival on Quantum Computation at University of
Maryland, College Park from August 23-28th, 2002 under the title ``Quantum
phase during quantum operation'
Quantum Thermodynamics
Quantum thermodynamics is an emerging research field aiming to extend
standard thermodynamics and non-equilibrium statistical physics to ensembles of
sizes well below the thermodynamic limit, in non-equilibrium situations, and
with the full inclusion of quantum effects. Fuelled by experimental advances
and the potential of future nanoscale applications this research effort is
pursued by scientists with different backgrounds, including statistical
physics, many-body theory, mesoscopic physics and quantum information theory,
who bring various tools and methods to the field. A multitude of theoretical
questions are being addressed ranging from issues of thermalisation of quantum
systems and various definitions of "work", to the efficiency and power of
quantum engines. This overview provides a perspective on a selection of these
current trends accessible to postgraduate students and researchers alike.Comment: 48 pages, improved and expanded several sections. Comments welcom
Coulomb entangler and entanglement testing network for waveguide qubits
We present a small network for the testing of the entanglement of two
ballistic electron waveguide qubits. The network produces different output
conditional on the presence or absence of entanglement. The structure of the
network allows for the determination of successful entanglement operations
through the measurement of the output of a single qubit. We also present a
simple model of a dynamic coulomb-like interaction and use it to describe some
characteristics of a proposed scheme for the entanglement of qubits in
ballistic electron waveguides.Comment: 12 pages of text plus 7 figures: total 19 page
Quantum phase estimation algorithms with delays: effects of dynamical phases
The unavoidable finite time intervals between the sequential operations
needed for performing practical quantum computing can degrade the performance
of quantum computers. During these delays, unwanted relative dynamical phases
are produced due to the free evolution of the superposition wave-function of
the qubits. In general, these coherent "errors" modify the desired quantum
interferences and thus spoil the correct results, compared to the ideal
standard quantum computing that does not consider the effects of delays between
successive unitary operations. Here, we show that, in the framework of the
quantum phase estimation algorithm, these coherent phase "errors", produced by
the time delays between sequential operations, can be avoided by setting up the
delay times to satisfy certain matching conditions.Comment: 10 pages, no figur
A 2D Quantum Walk Simulation of Two-Particle Dynamics
Multi-dimensional quantum walks can exhibit highly non-trivial topological
structure, providing a powerful tool for simulating quantum information and
transport systems. We present a flexible implementation of a 2D optical quantum
walk on a lattice, demonstrating a scalable quantum walk on a non-trivial graph
structure. We realized a coherent quantum walk over 12 steps and 169 positions
using an optical fiber network. With our broad spectrum of quantum coins we
were able to simulate the creation of entanglement in bipartite systems with
conditioned interactions. Introducing dynamic control allowed for the
investigation of effects such as strong non-linearities or two-particle
scattering. Our results illustrate the potential of quantum walks as a route
for simulating and understanding complex quantum systems
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