1,770 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.
Unconditional security at a low cost
By simulating four quantum key distribution (QKD) experiments and analyzing
one decoy-state QKD experiment, we compare two data post-processing schemes
based on security against individual attack by L\"{u}tkenhaus, and
unconditional security analysis by Gottesman-Lo-L\"{u}tkenhaus-Preskill. Our
results show that these two schemes yield close performances. Since the Holy
Grail of QKD is its unconditional security, we conclude that one is better off
considering unconditional security, rather than restricting to individual
attacks.Comment: Accepted by International Conference on Quantum Foundation and
Technology: Frontier and Future 2006 (ICQFT'06
Generalized self-testing and the security of the 6-state protocol
Self-tested quantum information processing provides a means for doing useful
information processing with untrusted quantum apparatus. Previous work was
limited to performing computations and protocols in real Hilbert spaces, which
is not a serious obstacle if one is only interested in final measurement
statistics being correct (for example, getting the correct factors of a large
number after running Shor's factoring algorithm). This limitation was shown by
McKague et al. to be fundamental, since there is no way to experimentally
distinguish any quantum experiment from a special simulation using states and
operators with only real coefficients.
In this paper, we show that one can still do a meaningful self-test of
quantum apparatus with complex amplitudes. In particular, we define a family of
simulations of quantum experiments, based on complex conjugation, with two
interesting properties. First, we are able to define a self-test which may be
passed only by states and operators that are equivalent to simulations within
the family. This extends work of Mayers and Yao and Magniez et al. in
self-testing of quantum apparatus, and includes a complex measurement. Second,
any of the simulations in the family may be used to implement a secure 6-state
QKD protocol, which was previously not known to be implementable in a
self-tested framework.Comment: To appear in proceedings of TQC 201
Coin Tossing is Strictly Weaker Than Bit Commitment
We define cryptographic assumptions applicable to two mistrustful parties who
each control two or more separate secure sites between which special relativity
guarantees a time lapse in communication. We show that, under these
assumptions, unconditionally secure coin tossing can be carried out by
exchanges of classical information. We show also, following Mayers, Lo and
Chau, that unconditionally secure bit commitment cannot be carried out by
finitely many exchanges of classical or quantum information. Finally we show
that, under standard cryptographic assumptions, coin tossing is strictly weaker
than bit commitment. That is, no secure classical or quantum bit commitment
protocol can be built from a finite number of invocations of a secure coin
tossing black box together with finitely many additional information exchanges.Comment: Final version; to appear in Phys. Rev. Let
Three-intensity decoy state method for device independent quantum key distribution with basis dependent errors
We study the measurement device independent quantum key distribution (MDIQKD)
in practice with limited resource, when there are only 3 different states in
implementing the decoy-state method and when there are basis dependent coding
errors. We present general formulas for the decoy-state method for two-pulse
sources with 3 different states, which can be applied to the recently proposed
MDIQKD with imperfect single-photon source such as the coherent states or the
heralded states from the parametric down conversion. We point out that the
existing result for secure QKD with source coding errors does not always hold.
We find that very accurate source coding is not necessary. In particular, we
loosen the precision of existing result by several magnitude orders for secure
QKD.Comment: Published version with Eq.(17) corrected. We emphasize that our major
result (Eq.16) for the decoy-state part can be applied to generate a key rate
very close to the ideal case of using infinite different coherent states, as
was numerically demonstrated in Ref.[21]. Published in PRA, 2013, Ja
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)
Efficient Heralding of Photonic Qubits with Apllications to Device Independent Quantum Key Distribution
We present an efficient way of heralding photonic qubit signals using linear
optics devices. First we show that one can obtain asymptotically perfect
heralding and unit success probability with growing resources. Second, we show
that even using finite resources, we can improve qualitatively and
quantitatively over earlier heralding results. In the latte r scenario, we can
obtain perfect heralded photonic qubits while maintaining a finite success
probability. We demonstrate the advantage of our heralding scheme by predicting
key rates for device independent quantum key distribution, taking imperfections
of sources and detectors into account
Unconditionally Secure Key Distribution Based on Two Nonorthogonal States
We prove the unconditional security of the Bennett 1992 protocol, by using a
reduction to an entanglement distillation protocol initiated by a local
filtering process. The bit errors and the phase errors are correlated after the
filtering, and we can bound the amount of phase errors from the observed bit
errors by an estimation method involving nonorthogonal measurements. The angle
between the two states shows a trade-off between accuracy of the estimation and
robustness to noises.Comment: 5 pages, 1 figur
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
Recommended from our members
Schism: When Research and Practice Fail to Meet
The authors explore the hypothesis generation, intervention formation and operationalization of PL 107-110, as a cautionary example of the social cost of failure to effectively integrate empirical research to professional practice. They suggest a reexamination of traditional dichotomous University identification as Research versus Practitioner and present a case for a stronger emphasis upon the integration of research and practice through applied experiential activity during the period of University education
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