155 research outputs found
Heralded qubit amplifiers for practical device-independent quantum key distribution
Device-independent quantum key distribution does not need a precise quantum
mechanical model of employed devices to guarantee security. Despite of its
beauty, it is still a very challenging experimental task. We compare a recent
proposal by Gisin et al. [Phys. Rev. Lett. 105, 070501 (2010)] to close the
detection loophole problem with that of a simpler quantum relay based on
entanglement swapping with linear optics. Our full-mode analysis for both
schemes confirms that, in contrast to recent beliefs, the second scheme can
indeed provide a positive key rate which is even considerably higher than that
of the first alternative. The resulting key rates and required detection
efficiencies of approx. 95% for both schemes, however, strongly depend on the
underlying security proof.Comment: 5 pages, 3 figure
Security of distributed-phase-reference quantum key distribution
Distributed-phase-reference quantum key distribution stands out for its easy
implementation with present day technology. Since many years, a full security
proof of these schemes in a realistic setting has been elusive. For the first
time, we solve this long standing problem and present a generic method to prove
the security of such protocols against general attacks. To illustrate our
result we provide lower bounds on the key generation rate of a variant of the
coherent-one-way quantum key distribution protocol. In contrast to standard
predictions, it appears to scale quadratically with the system transmittance.Comment: 4 pages + appendix, 4 figure
Unconditional Security of Single-Photon Differential Phase Shift Quantum Key Distribution
In this Letter, we prove the unconditional security of single-photon
differential phase shift quantum key distribution (DPS-QKD) protocol, based on
the conversion to an equivalent entanglement-based protocol. We estimate the
upper bound of the phase error rate from the bit error rate, and show that
DPS-QKD can generate unconditionally secure key when the bit error rate is not
greater than 4.12%. This proof is the first step to the unconditional security
proof of coherent state DPS-QKD.Comment: 5 pages, 2 figures; shorten the length, improve clarity, and correct
typos; accepted for publication in Physical Review Letter
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
Effect of detector dead-times on the security evaluation of differential-phase-shift quantum key distribution against sequential attacks
We investigate limitations imposed by detector dead-times on the performance
of sequential attacks against a differential-phase-shift (DPS) quantum key
distribution (QKD) protocol with weak coherent pulses. In particular, we
analyze sequential attacks based on unambiguous state discrimination of the
signal states emitted by the source and we obtain ultimate upper bounds on the
maximal distance achievable by a DPS QKD scheme both in the so-called trusted
and untrusted device scenarios, respectively.Comment: 21 pages, 14 figure
On the geometric distance between quantum states with positive partial transposition and private states
We prove an analytic positive lower bound for the geometric distance between
entangled positive partial transpose (PPT) states of a broad class and any
private state that delivers one secure key bit. Our proof holds for any Hilbert
space of finite dimension. Although our result is proven for a specific class
of PPT states, we show that our bound nonetheless holds for all known entangled
PPT states with non-zero distillable key rates whether or not they are in our
special class.Comment: 16 page
Practical quantum key distribution: On the security evaluation with inefficient single-photon detectors
Quantum Key Distribution with the BB84 protocol has been shown to be
unconditionally secure even using weak coherent pulses instead of single-photon
signals. The distances that can be covered by these methods are limited due to
the loss in the quantum channel (e.g. loss in the optical fiber) and in the
single-photon counters of the receivers. One can argue that the loss in the
detectors cannot be changed by an eavesdropper in order to increase the covered
distance. Here we show that the security analysis of this scenario is not as
easy as is commonly assumed, since already two-photon processes allow
eavesdropping strategies that outperform the known photon-number splitting
attack. For this reason there is, so far, no satisfactory security analysis
available in the framework of individual attacks.Comment: 11 pages, 6 figures; Abstract and introduction extended, Appendix
added, references update
Intercept-resend attacks in the Bennett-Brassard 1984 quantum key distribution protocol with weak coherent pulses
Unconditional security proofs of the Bennett-Brassard protocol of quantum key
distribution have been obtained recently. These proofs cover also practical
implementations that utilize weak coherent pulses in the four signal
polarizations. Proven secure rates leave open the possibility that new proofs
or new public discussion protocols obtain larger rates over increased distance.
In this paper we investigate limits to error rate and signal losses that can be
tolerated by future protocols and proofs.Comment: 11 pages, 3 figures. Version accepted for publication in Phys. Rev.
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
Dynamic scaling regimes of collective decision making
We investigate a social system of agents faced with a binary choice. We
assume there is a correct, or beneficial, outcome of this choice. Furthermore,
we assume agents are influenced by others in making their decision, and that
the agents can obtain information that may guide them towards making a correct
decision. The dynamic model we propose is of nonequilibrium type, converging to
a final decision. We run it on random graphs and scale-free networks. On random
graphs, we find two distinct regions in terms of the "finalizing time" -- the
time until all agents have finalized their decisions. On scale-free networks on
the other hand, there does not seem to be any such distinct scaling regions
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