30,262 research outputs found
Quantum cryptography as a retrodiction problem
We propose a quantum key distribution protocol based on a quantum
retrodiction protocol, known as the Mean King problem. The protocol uses a two
way quantum channel. We show security against coherent attacks in a
transmission error free scenario, even if Eve is allowed to attack both
transmissions. This establishes a connection between retrodiction and key
distribution.Comment: 5 pages, 1 figur
Propagation and spectral properties of quantum walks in electric fields
We study one-dimensional quantum walks in a homogeneous electric field. The
field is given by a phase which depends linearly on position and is applied
after each step. The long time propagation properties of this system, such as
revivals, ballistic expansion and Anderson localization, depend very
sensitively on the value of the electric field , e.g., on whether
is rational or irrational. We relate these properties to the
continued fraction expansion of the field. When the field is given only with
finite accuracy, the beginning of the expansion allows analogous conclusions
about the behavior on finite time scales.Comment: 7 pages, 4 figure
Quantum Walks with Non-Orthogonal Position States
Quantum walks have by now been realized in a large variety of different
physical settings. In some of these, particularly with trapped ions, the walk
is implemented in phase space, where the corresponding position states are not
orthogonal. We develop a general description of such a quantum walk and show
how to map it into a standard one with orthogonal states, thereby making
available all the tools developed for the latter. This enables a variety of
experiments, which can be implemented with smaller step sizes and more steps.
Tuning the non-orthogonality allows for an easy preparation of extended states
such as momentum eigenstates, which travel at a well-defined speed with low
dispersion. We introduce a method to adjust their velocity by momentum shifts,
which allows to investigate intriguing effects such as the analog of Bloch
oscillations.Comment: 5 pages, 4 figure
Origin and reduction of wakefields in photonic crystal accelerator cavities
Photonic crystal (PhC) defect cavities that support an accelerating mode tend
to trap unwanted higher-order modes (HOMs) corresponding to zero-group-velocity
PhC lattice modes at the top of the bandgap. The effect is explained quite
generally from photonic band and perturbation theoretical arguments. Transverse
wakefields resulting from this effect are observed in a hybrid dielectric PhC
accelerating cavity based on a triangular lattice of sapphire rods. These
wakefields are, on average, an order of magnitude higher than those in the
waveguide-damped Compact Linear Collider (CLIC) copper cavities. The avoidance
of translational symmetry (and, thus, the bandgap concept) can dramatically
improve HOM damping in PhC-based structures.Comment: 11 pages, 18 figures, 2 table
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