17,035 research outputs found
Quantum Cryptography Approaching the Classical Limit
We consider the security of continuous-variable quantum cryptography as we
approach the classical-limit, i.e., when the unknown preparation noise at the
sender's station becomes significantly noisy or thermal (even by as much as
10,000 times the variance of the vacuum mode). We show that, provided the
channel transmission losses do not exceed 50%, the security of quantum
cryptography is not dependent on the channel transmission, and is therefore,
incredibly robust against significant amounts of excess preparation noise. We
extend these results to consider for the first time quantum cryptography at
wavelengths considerably longer than optical and find that regions of security
still exist all the way down to the microwave.Comment: Letter (4 pages) followed by appendix (4 pages). Updated from
published version with some minor correction
Continuous-Variable Quantum Key Distribution using Thermal States
We consider the security of continuous-variable quantum key distribution
using thermal (or noisy) Gaussian resource states. Specifically, we analyze
this against collective Gaussian attacks using direct and reverse
reconciliation where both protocols use either homodyne or heterodyne
detection. We show that in the case of direct reconciliation with heterodyne
detection, an improved robustness to channel noise is achieved when large
amounts of preparation noise is added, as compared to the case when no
preparation noise is added. We also consider the theoretical limit of infinite
preparation noise and show a secure key can still be achieved in this limit
provided the channel noise is less than the preparation noise. Finally, we
consider the security of quantum key distribution at various electromagnetic
wavelengths and derive an upper bound related to an entanglement-breaking
eavesdropping attack and discuss the feasibility of microwave quantum key
distribution.Comment: 12 pages, 11 figures. Updated from published version with some minor
correction
A quantum version of free energy - irreversible work relations
We give a quantum version of the Jarzynski relation between the distribution
of work done over a certain time-interval on a system and the difference of
equilibrium free energies. The main new ingredient is the identification of
work depending on the quantum history of the system and the proper definition
of various quantum ensembles over which the averages should be made. We also
discuss a number of different regimes that have been considered by other
authors and which are unified in the present set-up. In all cases, and quantum
or classical, it is a general relation between heat and time-reversal that
makes the Jarzynski relation so universally valid
The merger rate of massive galaxies
We calculate the projected two point correlation function for samples of
luminous and massive galaxies in the COMBO-17 photometric redshift survey,
focusing particularly on the amplitude of the correlation function at small
projected radii and exploring the constraints such measurements can place on
the galaxy merger rate. For nearly volume-limited samples with 0.4<z<0.8, we
find that 4+/-1% of luminous M_B<-20 galaxies are in close physical pairs (with
real space separation of <30 proper kpc). The corresponding fraction for
massive galaxies with M_*>2.5e10 M_sun is 5+/-1%. Incorporating close pair
fractions from the literature, the 2dFGRS and the SDSS, we find a fairly rapid
evolution of the merger fraction of massive galaxies between z=0.8 and the
present day. Assuming that the major merger timescale is of order the dynamical
timescale for close massive galaxy pairs, we tentatively infer that ~50% (70%)
of all galaxies with present-day masses M_*>5e10 M_sun (remnants of mergers
between galaxies with M_*>2.5e10 M_sun) have undergone a major merger since
z=0.8(1): major mergers between massive galaxies are a significant driver of
galaxy evolution over the last eight billion years.Comment: ApJ, in press. 8 pages, 3 figures. Expanded discussion section with
explicit discussion of merger fraction vs. close pair fraction. Change of
typical close pair timescale results in increased inferred merger rat
Dynamical fluctuations for semi-Markov processes
We develop an Onsager-Machlup-type theory for nonequilibrium semi-Markov
processes. Our main result is an exact large time asymptotics for the joint
probability of the occupation times and the currents in the system,
establishing some generic large deviation structures. We discuss in detail how
the nonequilibrium driving and the non-exponential waiting time distribution
influence the occupation-current statistics. The violation of the Markov
condition is reflected in the emergence of a new type of nonlocality in the
fluctuations. Explicit solutions are obtained for some examples of driven
random walks on the ring.Comment: Minor changes, accepted for publication in Journal of Physics
Experimental quantum communication complexity
We prove that the fidelity of two exemplary communication complexity
protocols, allowing for an N-1 bit communication, can be exponentially improved
by N-1 (unentangled) qubit communication. Taking into account, for a fair
comparison, all inefficiencies of state-of-the-art set-up, the experimental
implementation outperforms the best classical protocol, making it the candidate
for multi-party quantum communication applications.Comment: 4 pages, 2 eps figures, RevTEX4; submitted June 23, 200
Encoding Context-Sensitivity in Reo into Non-Context-Sensitive Semantic Models (Technical Report)
Reo is a coordination language which can be used to model the interactions among a set of components or services in a compositional manner using connectors. The language concepts of Reo include synchronization, mutual exclusion, data manipulation, memory and context-dependency. Context-dependency facilitates the precise specification of a connector's possible actions in situations where it would otherwise exhibit nondeterministic behavior. All existing formalizations of context-dependency in Reo are based on extended semantic models that provide constructs for modeling the presence and absence of I/O requests at the ports of a connector.
In this paper, we show that context-dependency in Reo can be encoded in basic semantic models, namely connector coloring with two colors and constraint automata, by introducing additional fictitious ports for Reo's primitives. Both of these models were considered as not expressive enough to handle context-dependency up to now. We demonstrate the usefulness of our approach by incorporating context-dependency into the constraint automata based Vereofy model checker
Is a soft nuclear equation of state extracted from heavy-ion data incompatible with pulsar data?
We discuss the recent constraints on the nuclear equation of state from
pulsar mass measurements and from subthreshold production of kaons in heavy-ion
collisions. While recent pulsar data points towards a hard equation of state,
the analysis of the heavy-ion data allows only for soft equations of state. We
resolve the apparent contradiction by considering the different density regimes
probed. We argue that future measurements of global properties of low-mass
pulsars can serve as an excellent cross-check to heavy-ion data.Comment: 8 pages, 1 figure, contribution to the proceedings of the
international conference on 'Nuclear Physics in Astrophysics III', Dresden,
Germany, March 26-31, 2007, minor corrections to match published version, JPG
in pres
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