1,209 research outputs found
Theoretical investigation of electron-hole complexes in anisotropic two-dimensional materials
Trions and biexcitons in anisotropic two-dimensional materials are
investigated within an effective mass theory. Explicit results are obtained for
phosphorene and arsenene, materials that share features such as a direct
quasi-particle gap and anisotropic conduction and valence bands. Trions are
predicted to have remarkably high binding energies and an elongated
electron-hole structure with a preference for alignment along the armchair
direction, where the effective masses are lower. We find that biexciton binding
energies are also notably large, especially for monolayer phosphorene, where
they are found to be twice as large as those for typical monolayer transition
metal dichalcogenides.Comment: 3 figures, 5 pages + Supplementary Material, accepted for publication
in Phys. Rev.
Deuteron Momentum Distribution in KD2HPO4
The momentum distribution in KD2PO4(DKDP) has been measured using neutron
Compton scattering above and below the weakly first order
paraelectric-ferroelectric phase transition(T=229K). There is very litte
difference between the two distributions, and no sign of the coherence over two
locations for the proton observed in the paraelectric phase, as in KH2PO4(KDP).
We conclude that the tunnel splitting must be much less than 20mev. The width
of the distribution indicates that the effective potential for DKDP is
significantly softer than that for KDP. As electronic structure calculations
indicate that the stiffness of the potential increases with the size of the
coherent region locally undergoing soft mode fluctuations, we conclude that
there is a mass dependent quantum coherence length in both systems.Comment: 6 pages 5 figure
Effects of detector efficiency mismatch on security of quantum cryptosystems
We suggest a type of attack on quantum cryptosystems that exploits variations
in detector efficiency as a function of a control parameter accessible to an
eavesdropper. With gated single-photon detectors, this control parameter can be
the timing of the incoming pulse. When the eavesdropper sends short pulses
using the appropriate timing so that the two gated detectors in Bob's setup
have different efficiencies, the security of quantum key distribution can be
compromised. Specifically, we show for the Bennett-Brassard 1984 (BB84)
protocol that if the efficiency mismatch between 0 and 1 detectors for some
value of the control parameter gets large enough (roughly 15:1 or larger), Eve
can construct a successful faked-states attack causing a quantum bit error rate
lower than 11%. We also derive a general security bound as a function of the
detector sensitivity mismatch for the BB84 protocol. Experimental data for two
different detectors are presented, and protection measures against this attack
are discussed.Comment: v3: identical to the journal version. However, after publication we
have discovered that Eq. 11 is incorrect: the available bit rate after
privacy amplification is reduced even in the case (QBER)=0 [see Quant. Inf.
Comp. 7, 73 (2007)
Security Trade-offs in Ancilla-Free Quantum Bit Commitment in the Presence of Superselection Rules
Security trade-offs have been established for one-way bit commitment in
quant-ph/0106019. We study this trade-off in two superselection settings. We
show that for an `abelian' superselection rule (exemplified by particle
conservation) the standard trade-off between sealing and binding properties
still holds. For the non-abelian case (exemplified by angular momentum
conservation) the security trade-off can be more subtle, which we illustrate by
showing that if the bit-commitment is forced to be ancilla-free an
asymptotically secure quantum bit commitment is possible.Comment: 7 pages Latex; v2 has 8 pages and additional references and
clarifications, this paper is to appear in the New Journal of Physic
Optimal ratio between phase basis and bit basis in QKD
In the original BB84 protocol, the bit basis and the phase basis are used
with equal probability. Lo et al (J. of Cryptology, 18, 133-165 (2005))
proposed to modify the ratio between the two bases by increasing the final key
generation rate. However, the optimum ratio has not been derived. In this
letter, in order to examine this problem, the ratio between the two bases is
optimized for exponential constraints given Eve's information
distinguishability and the final error probability
Experimental study on Gaussian-modulated coherent states quantum key distribution over standard telecom fiber
In this paper, we present a fully fiber-based one-way Quantum Key
Distribution (QKD) system implementing the Gaussian-Modulated Coherent States
(GMCS) protocol. The system employs a double Mach-Zehnder Interferometer (MZI)
configuration in which the weak quantum signal and the strong Local Oscillator
(LO) go through the same fiber between Alice and Bob, and are separated into
two paths inside Bob's terminal. To suppress the LO leakage into the signal
path, which is an important contribution to the excess noise, we implemented a
novel scheme combining polarization and frequency multiplexing, achieving an
extinction ratio of 70dB. To further minimize the system excess noise due to
phase drift of the double MZI, we propose that, instead of employing phase
feedback control, one simply let Alice remap her data by performing a rotation
operation. We further present noise analysis both theoretically and
experimentally. Our calculation shows that the combined polarization and
frequency multiplexing scheme can achieve better stability in practice than the
time-multiplexing scheme, because it allows one to use matched fiber lengths
for the signal and the LO paths on both sides of the double MZI, greatly
reducing the phase instability caused by unmatched fiber lengths. Our
experimental noise analysis quantifies the three main contributions to the
excess noise, which will be instructive to future studies of the GMCS QKD
systems. Finally, we demonstrate, under the "realistic model" in which Eve
cannot control the system within Bob's terminal, a secure key rate of
0.3bit/pulse over a 5km fiber link. This key rate is about two orders of
magnitude higher than that of a practical BB84 QKD system.Comment: 21 pages, 9 figure
Experimental quantum tossing of a single coin
The cryptographic protocol of coin tossing consists of two parties, Alice and
Bob, that do not trust each other, but want to generate a random bit. If the
parties use a classical communication channel and have unlimited computational
resources, one of them can always cheat perfectly. Here we analyze in detail
how the performance of a quantum coin tossing experiment should be compared to
classical protocols, taking into account the inevitable experimental
imperfections. We then report an all-optical fiber experiment in which a single
coin is tossed whose randomness is higher than achievable by any classical
protocol and present some easily realisable cheating strategies by Alice and
Bob.Comment: 13 page
Security of practical private randomness generation
Measurements on entangled quantum systems necessarily yield outcomes that are
intrinsically unpredictable if they violate a Bell inequality. This property
can be used to generate certified randomness in a device-independent way, i.e.,
without making detailed assumptions about the internal working of the quantum
devices used to generate the random numbers. Furthermore these numbers are also
private, i.e., they appear random not only to the user, but also to any
adversary that might possess a perfect description of the devices. Since this
process requires a small initial random seed, one usually speaks of
device-independent randomness expansion.
The purpose of this paper is twofold. First, we point out that in most real,
practical situations, where the concept of device-independence is used as a
protection against unintentional flaws or failures of the quantum apparatuses,
it is sufficient to show that the generated string is random with respect to an
adversary that holds only classical-side information, i.e., proving randomness
against quantum-side information is not necessary. Furthermore, the initial
random seed does not need to be private with respect to the adversary, provided
that it is generated in a way that is independent from the measured systems.
The devices, though, will generate cryptographically-secure randomness that
cannot be predicted by the adversary and thus one can, given access to free
public randomness, talk about private randomness generation.
The theoretical tools to quantify the generated randomness according to these
criteria were already introduced in [S. Pironio et al, Nature 464, 1021
(2010)], but the final results were improperly formulated. The second aim of
this paper is to correct this inaccurate formulation and therefore lay out a
precise theoretical framework for practical device-independent randomness
expansion.Comment: 18 pages. v3: important changes: the present version focuses on
security against classical side-information and a discussion about the
significance of these results has been added. v4: minor changes. v5: small
typos correcte
Finite-key security against coherent attacks in quantum key distribution
The work by Christandl, K\"onig and Renner [Phys. Rev. Lett. 102, 020504
(2009)] provides in particular the possibility of studying unconditional
security in the finite-key regime for all discrete-variable protocols. We spell
out this bound from their general formalism. Then we apply it to the study of a
recently proposed protocol [Laing et al., Phys. Rev. A 82, 012304 (2010)]. This
protocol is meaningful when the alignment of Alice's and Bob's reference frames
is not monitored and may vary with time. In this scenario, the notion of
asymptotic key rate has hardly any operational meaning, because if one waits
too long time, the average correlations are smeared out and no security can be
inferred. Therefore, finite-key analysis is necessary to find the maximal
achievable secret key rate and the corresponding optimal number of signals.Comment: 9 pages, 4 figure
Does Social Presence or the Potential for Interaction reduce Social Gaze in Online Social Scenarios? Introducing the "Live Lab" paradigm.
Research has shown that people’s gaze is biased away from faces in the real-world but
towards them when they are viewed onscreen. Non-equivalent stimulus conditions may have
represented a confound in this research however, as participants viewed onscreen stimuli as
pre-recordings where interaction was not possible, compared to real-world stimuli which
were viewed in real-time where interaction was possible. We assessed the independent
contributions of online social presence and ability for interaction on social gaze by
developing the “live lab” paradigm. Participants in three groups (N = 132) viewed a
confederate either as a) a live webcam stream where interaction was not possible (one-way),
b) a live webcam stream where an interaction was possible (two-way) or c) as a prerecording.
Potential for interaction, rather than online social presence, was the primary
influence on gaze behaviour: Participants in the pre-recorded and one-way conditions looked
more to the face than those in the two-way condition, particularly when the confederate made
“eye contact”. Fixation durations to the face were shorter when the scene was viewed live,
particularly during a bid for eye contact
Our findings support the dual function of gaze, but suggest that online social presence alone
is not sufficient to activate social norms of civil inattention. Implications for the
reinterpretation of previous research are discussed
- …