137 research outputs found
Quantum cryptography via parametric downconversion
The use of quantum bits (qubits) in cryptography holds the promise of secure
cryptographic quantum key distribution schemes. It is based usually on
single-photon polarization states. Unfortunately, the implemented ``qubits'' in
the usual weak pulse experiments are not true two-level systems, and quantum
key distribution based on these imperfect qubits is totally insecure in the
presence of high (realistic) loss rate. In this work, we investigate another
potential implementation: qubits generated using a process of parametric
downconversion. We find that, to first (two-photon) and second (four-photon)
order in the parametric downconversion small parameter, this implementation of
quantum key distribution is equivalent to the theoretical version.
Once realistic measurements are taken into account, quantum key distribution
based on parametric downconversion suffers also from sensitivity to extremely
high (nonrealistic) losses. By choosing the small parameter of the process
according to the loss rates, both implementations of quantum key distribution
can in principle become secure against the attack studied in this paper.
However, adjusting the small parameter to the required levels seems to be
impractical in the weak pulse process. On the other hand, this can easily be
done in the parametric downconversion process, making it a much more promising
implementation.Comment: 6 pages, Latex (a special style file is attached). Presented in
QCM'98 conference. Similar results regarding the insecurity of weak-pulse
schemes were also presented by Norbert Lutkenhaus in the same conferenc
Broadband channel capacities
We study the communication capacities of bosonic broadband channels in the
presence of different sources of noise. In particular we analyze lossy channels
in presence of white noise and thermal bath. In this context, we provide a
numerical solution for the entanglement assisted capacity and upper and lower
bounds for the classical and quantum capacities.Comment: 11 pages, 7 figures, 3 table
Common origin of no-cloning and no-deleting principles - Conservation of information
We discuss the role of the notion of information in the description of
physical reality. We consider theories for which dynamics is linear with
respect to stochastic mixing. We point out that the no-cloning and no-deleting
principles emerge in any such theory, if law of conservation of information is
valid, and two copies contain more information than one copy. We then describe
the quantum case from this point of view.Comment: This paper is dedicated to Asher Peres on the occasion of his
seventieth birthda
Quantum Gambling Using Three Nonorthogonal States
We provide a quantum gambling protocol using three (symmetric) nonorthogonal
states. The bias of the proposed protocol is less than that of previous ones,
making it more practical. We show that the proposed scheme is secure against
non-entanglement attacks. The security of the proposed scheme against
entanglement attacks is shown heuristically.Comment: no essential correction, 4 pages, RevTe
Quantum cryptography using balanced homodyne detection
We report an experimental quantum key distribution that utilizes balanced
homodyne detection, instead of photon counting, to detect weak pulses of
coherent light. Although our scheme inherently has a finite error rate, it
allows high-efficiency detection and quantum state measurement of the
transmitted light using only conventional devices at room temperature. When the
average photon number was 0.1, an error rate of 0.08 and "effective" quantum
efficiency of 0.76 were obtained.Comment: Errors in the sentence citing ref.[20] are correcte
All Inequalities for the Relative Entropy
The relative entropy of two n-party quantum states is an important quantity
exhibiting, for example, the extent to which the two states are different. The
relative entropy of the states formed by reducing two n-party to a smaller
number of parties is always less than or equal to the relative entropy of
the two original n-party states. This is the monotonicity of relative entropy.
Using techniques from convex geometry, we prove that monotonicity under
restrictions is the only general inequality satisfied by relative entropies. In
doing so we make a connection to secret sharing schemes with general access
structures.
A suprising outcome is that the structure of allowed relative entropy values
of subsets of multiparty states is much simpler than the structure of allowed
entropy values. And the structure of allowed relative entropy values (unlike
that of entropies) is the same for classical probability distributions and
quantum states.Comment: 15 pages, 3 embedded eps figure
HYPERION: An open-source parallelized three-dimensional dust continuum radiative transfer code
HYPERION is a new three-dimensional dust continuum Monte-Carlo radiative
transfer code that is designed to be as generic as possible, allowing radiative
transfer to be computed through a variety of three-dimensional grids. The main
part of the code is problem-independent, and only requires an arbitrary
three-dimensional density structure, dust properties, the position and
properties of the illuminating sources, and parameters controlling the running
and output of the code. HYPERION is parallelized, and is shown to scale well to
thousands of processes. Two common benchmark models for protoplanetary disks
were computed, and the results are found to be in excellent agreement with
those from other codes. Finally, to demonstrate the capabilities of the code,
dust temperatures, SEDs, and synthetic multi-wavelength images were computed
for a dynamical simulation of a low-mass star formation region. HYPERION is
being actively developed to include new features, and is publicly available
(http://www.hyperion-rt.org).Comment: Accepted for publication in Astronomy & Astrophysics. HYPERION is
being prepared for release at the start of 2012, but you can already sign up
to the mailing list at http://www.hyperion-rt.org to be informed once it is
available for downloa
General impossible operations in quantum information
We prove a general limitation in quantum information that unifies the
impossibility principles such as no-cloning and no-anticloning. Further, we
show that for an unknown qubit one cannot design a universal Hadamard gate for
creating equal superposition of the original and its complement state.
Surprisingly, we find that Hadamard transformations exist for an unknown qubit
chosen either from the polar or equatorial great circles. Also, we show that
for an unknown qubit one cannot design a universal unitary gate for creating
unequal superpositions of the original and its complement state. We discuss why
it is impossible to design a controlled-NOT gate for two unknown qubits and
discuss the implications of these limitations.Comment: 15 pages, no figures, Discussion about personal quantum computer
remove
Optimal discrimination of mixed quantum states involving inconclusive results
We propose a generalized discrimination scheme for mixed quantum states. In
the present scenario we allow for certain fixed fraction of inconclusive
results and we maximize the success rate of the quantum-state discrimination.
This protocol interpolates between the Ivanovic-Dieks-Peres scheme and the
Helstrom one. We formulate the extremal equations for the optimal positive
operator valued measure describing the discrimination device and establish a
criterion for its optimality. We also devise a numerical method for efficient
solving of these extremal equations.Comment: 5 pages, 1 figur
Pilot-aided estimation and equalisation of a Radio-over-Fibre system in Wideband Code Division Multiple Access
In this study, the impact of a Radio-over-Fibre (RoF) subsystem on the capacity performance of wideband code division multiple access is evaluated. This study investigates the use of pilot-aided channel estimation to compensate for the optical subsystem non-linearities for different channel conditions, estimation intervals and coding schemes. The results show that pilot-aided channel estimation is an effective method for compensating the composite impairments of the optical subsystem and the radio frequency (RF) channel. It is found that there is always a suitable pilot power level which maximises the system capacity performance regardless of coding scheme and channel condition. Also, the peak capacity is only slightly affected by a decrease in the estimation interval
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