65 research outputs found
Comment on "Quantum dense key distribution"
In this Comment we question the security of recently proposed by Degiovanni
et al. [Phys. Rev. A 69 (2004) 032310] scheme of quantum dense key
distribution
Discrete Time Quantum Walk Approach to State Transfer
We show that a quantum state transfer, previously studied as a continuous
time process in networks of interacting spins, can be achieved within the model
of discrete time quantum walks with position dependent coin. We argue that due
to additional degrees of freedom, discrete time quantum walks allow to observe
effects which cannot be observed in the corresponding continuous time case.
First, we study a discrete time version of the engineered coupling protocol due
to Christandl et. al. [Phys. Rev. Lett. 92, 187902 (2004)] and then discuss the
general idea of conversion between continuous time quantum walks and discrete
time quantum walks.Comment: 9 pages, 6 figures, comments welcom
Quantum walk as a generalized measuring device
We show that a one-dimensional discrete time quantum walk can be used to
implement a generalized measurement in terms of positive operator value measure
(POVM) on a single qubit. More precisely, we show that for a single qubit any
set of rank 1 and rank 2 POVM elements can be generated by a properly
engineered quantum walk. In such a scenario the measurement of particle at
position x=i corresponds to a measurement of a POVM element E_i on a qubit. We
explicitly construct quantum walks implementing unambiguous state
discrimination and SIC-POVM.Comment: 6 pages, 1 figur
Probabilistic coding of quantum states
We discuss properties of probabilistic coding of two qubits to one qutrit and
generalize the scheme to higher dimensions. We show that the protocol
preservers entanglement between qubits to be encoded and environment and can be
also applied to mixed states. We present the protocol which enables encoding of
n qudits to one qudit of dimension smaller than the Hilbert space of the
original system and then probabilistically but error-free decode any subset of
k qudits. We give a formula for the probability of successful decoding
New data on the late Badenian-Sarmatian depositsof the Nowy Sącz Basin (Magura Nappe, Polish Outer Carpathians) and their palaeogeographical implications
In the Nowy Sącz Basin, newly exposed Middle Miocene deposits have been studied and sam pled in the Kamienica Nawojowska, Poprad
and Dunajec rivers. The calcareous nannoplankton of the fresh water to marine deposits was examined. Pale grey and brown clayey shales
with plant remains and thin seams of lignite represent the freshwater deposits of the Biegonice Formation. These deposits pass upwards
into ca. 50 m thick packet of brackish and marine deposits, represented mainly by dark marly shales with bivalves and gastropods of the
Iwkowa and Niskowa formations. These deposits contain relatively rich late Badenian to Sarmatian calcareous nannoplakton (NN6/7Zone)
Towards symmetric scheme for superdense coding between multiparties
Recently Liu, Long, Tong and Li [Phys. Rev. A 65, 022304 (2002)] have
proposed a scheme for superdense coding between multiparties. This scheme seems
to be highly asymmetric in the sense that only one sender effectively exploits
entanglement. We show that this scheme can be modified in order to allow more
senders to benefit of the entanglement enhanced information transmission.Comment: 6 page
Quantum walks on cycles
We consider asymptotic behaviour of a Hadamard walk on a cycle. For a walk
which starts with a state in which all the probability is concentrated on one
node, we find the explicit formula for the limiting distribution and discuss
its asymptotic behaviour when the length of the cycle tends to infinity. We
also demonstrate that for a carefully chosen initial state, the limiting
distribution of a quantum walk on cycle can lie further away from the uniform
distribution than its initial state
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