216 research outputs found
Probabilistic quantum multimeters
We propose quantum devices that can realize probabilistically different
projective measurements on a qubit. The desired measurement basis is selected
by the quantum state of a program register. First we analyze the
phase-covariant multimeters for a large class of program states, then the
universal multimeters for a special choice of program. In both cases we start
with deterministic but erroneous devices and then proceed to devices that never
make a mistake but from time to time they give an inconclusive result. These
multimeters are optimized (for a given type of a program) with respect to the
minimum probability of inconclusive result. This concept is further generalized
to the multimeters that minimize the error rate for a given probability of an
inconclusive result (or vice versa). Finally, we propose a generalization for
qudits.Comment: 12 pages, 3 figure
Several experimental realizations of symmetric phase-covariant quantum cloner of single-photon qubits
We compare several optical implementations of phase-covariant cloning
machines. The experiments are based on copying of the polarization state of a
single photon in bulk optics by special unbalanced beam splitter or by balanced
beam splitter accompanied by a state filtering. Also the all-fiber based setup
is discussed, where the information is encoded into spatial modes, i.e., the
photon can propagate through two optical fibers. Each of the four
implementations possesses some advantages and disadvantages that are discussed.Comment: 8 pages, 11 figure
Security Proof for Quantum Key Distribution Using Qudit Systems
We provide security bounds against coherent attacks for two families of
quantum key distribution protocols that use -dimensional quantum systems. In
the asymptotic regime, both the secret key rate for fixed noise and the
robustness to noise increase with . The finite-key corrections are found to
be almost insensitive to .Comment: 5 pages, 1 figure, version 3 corrects equations (9) and (11), and
slightly modifies the figure to reflect the change to equation (11
Passive sources for the Bennett-Brassard 1984 quantum key distribution protocol with practical signals
Most experimental realizations of quantum key distribution are based on the
Bennett-Brassard 1984 (so-called BB84) protocol. In a typical optical
implementation of this scheme, the sender uses an active source to produce the
required BB84 signal states. While active state preparation of BB84 signals is
a simple and elegant solution in principle, in practice passive state
preparation might be desirable in some scenarios, for instance, in those
experimental setups operating at high transmission rates. Passive schemes might
also be more robust against side-channel attacks than active sources. Typical
passive devices involve parametric down-conversion. In this paper, we show that
both coherent light and practical single photon sources are also suitable for
passive generation of BB84 signal states. Our method does not require any
external-driven element, but only linear optical components and photodetectors.
In the case of coherent light, the resulting key rate is similar to the one
delivered by an active source. When the sender uses practical single photon
sources, however, the distance covered by a passive transmitter might be longer
than the one of an active configuration.Comment: 14 pages, 11 figure
Experimental asymmetric phase-covariant quantum cloning of polarization qubits
We report on two optical realizations of the asymmetric
phase-covariant cloning machines for polarization states of single photons. The
experimental setups combine two-photon interference and tunable polarization
filtering that enables us to control the asymmetry of the cloners. The first
scheme involves a special unbalanced bulk beam splitter exhibiting different
splitting ratios for vertical and horizontal polarizations, respectively. The
second implemented scheme consists of a balanced fiber coupler where photon
bunching occurs, followed by a free-space part with polarization filters. With
this later approach we were able to demonstrate very high cloning fidelities
which are above the universal cloning limit.Comment: 7 pages, 8 figure
Unambiguous state discrimination in quantum cryptography with weak coherent states
The use of linearly independent signal states in realistic implementations of
quantum key distribution (QKD) enables an eavesdropper to perform unambiguous
state discrimination. We explore quantitatively the limits for secure QKD
imposed by this fact taking into account that the receiver can monitor to some
extend the photon number statistics of the signals even with todays standard
detection schemes. We compare our attack to the beamsplitting attack and show
that security against beamsplitting attack does not necessarily imply security
against the attack considered here.Comment: 10 pages, 6 figures, updated version with added discussion of
beamsplitting attac
Passive-scheme analysis for solving untrusted source problem in quantum key distribution
As a practical method, the passive scheme is useful to monitor the photon
statistics of an untrusted source in a "Plug & Play" quantum key distribution
(QKD) system. In a passive scheme, three kinds of monitor mode can be adopted:
average photon number (APN) monitor, photon number analyzer (PNA) and photon
number distribution (PND) monitor. In this paper, the security analysis is
rigorously given for APN monitor, while for PNA, the analysis including
statistical fluctuation and random noise, is addressed with a confidence level.
The results show that the PNA can achieve better performance than the APN
monitor and can asymptotically approach the theoretical limit of the PND
monitor. Also, the passive scheme with the PNA works efficiently when the
signal-to-noise ratio () is not too low and so is highly applicable to
solve the untrusted source problem in the QKD system.Comment: 8 pages, 6 figures, published versio
On single-photon quantum key distribution in the presence of loss
We investigate two-way and one-way single-photon quantum key distribution
(QKD) protocols in the presence of loss introduced by the quantum channel. Our
analysis is based on a simple precondition for secure QKD in each case. In
particular, the legitimate users need to prove that there exists no separable
state (in the case of two-way QKD), or that there exists no quantum state
having a symmetric extension (one-way QKD), that is compatible with the
available measurements results. We show that both criteria can be formulated as
a convex optimisation problem known as a semidefinite program, which can be
efficiently solved. Moreover, we prove that the solution to the dual
optimisation corresponds to the evaluation of an optimal witness operator that
belongs to the minimal verification set of them for the given two-way (or
one-way) QKD protocol. A positive expectation value of this optimal witness
operator states that no secret key can be distilled from the available
measurements results. We apply such analysis to several well-known
single-photon QKD protocols under losses.Comment: 14 pages, 6 figure
Glass transition of an epoxy resin induced by temperature, pressure and chemical conversion: a configurational entropy rationale
A comparative study is reported on the dynamics of a glass-forming epoxy
resin when the glass transition is approached through different paths: cooling,
compression, and polymerization. In particular, the influence of temperature,
pressure and chemical conversion on the dynamics has been investigated by
dielectric spectroscopy. Deep similarities are found in dynamic properties. A
unified reading of our experimental results for the structural relaxation time
is given in the framework of the Adam-Gibbs theory. The quantitative agreement
with the experimental data is remarkable, joined with physical values of the
fitting parameters. In particular, the fitting function of the isothermal
tau(P) data gives a well reasonable prediction for the molar thermal expansion
of the neat system, and the fitting function of the isobaric-isothermal tau(C)
data under step- polymerization conforms to the prediction of diverging tau at
complete conversion of the system.Comment: 16 pages, 8 figures, from the talk given at the 4th International
Discussion Meeting on Relaxations in Complex Systems (IDMRCS), Hersonissos,
Helaklion, Crete (Greece), 17-23 June 200
Upper bounds for the secure key rate of decoy state quantum key distribution
The use of decoy states in quantum key distribution (QKD) has provided a
method for substantially increasing the secret key rate and distance that can
be covered by QKD protocols with practical signals. The security analysis of
these schemes, however, leaves open the possibility that the development of
better proof techniques, or better classical post-processing methods, might
further improve their performance in realistic scenarios. In this paper, we
derive upper bounds on the secure key rate for decoy state QKD. These bounds
are based basically only on the classical correlations established by the
legitimate users during the quantum communication phase of the protocol. The
only assumption about the possible post-processing methods is that double click
events are randomly assigned to single click events. Further we consider only
secure key rates based on the uncalibrated device scenario which assigns
imperfections such as detection inefficiency to the eavesdropper. Our analysis
relies on two preconditions for secure two-way and one-way QKD: The legitimate
users need to prove that there exists no separable state (in the case of
two-way QKD), or that there exists no quantum state having a symmetric
extension (one-way QKD), that is compatible with the available measurements
results. Both criteria have been previously applied to evaluate single-photon
implementations of QKD. Here we use them to investigate a realistic source of
weak coherent pulses. The resulting upper bounds can be formulated as a convex
optimization problem known as a semidefinite program which can be efficiently
solved. For the standard four-state QKD protocol, they are quite close to known
lower bounds, thus showing that there are clear limits to the further
improvement of classical post-processing techniques in decoy state QKD.Comment: 10 pages, 3 figure
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