23,690 research outputs found
On the Application of Gluon to Heavy Quarkonium Fragmentation Functions
We analyze the uncertainties induced by different definitions of the momentum
fraction in the application of gluon to heavy quarkonium fragmentation
function. We numerically calculate the initial fragmentation
functions by using the non-covariant definitions of with finite gluon
momentum and find that these fragmentation functions have strong dependence on
the gluon momentum . As , these fragmentation
functions approach to the fragmentation function in the light-cone definition.
Our numerical results show that large uncertainties remains while the
non-covariant definitions of are employed in the application of the
fragmentation functions. We present for the first time the polarized gluon to
fragmentation functions, which are fitted by the scheme exploited in
this work.Comment: 11 pages, 7 figures;added reference for sec.
Monotonicity and logarithmic convexity relating to the volume of the unit ball
Let stand for the volume of the unit ball in for
. In the present paper, we prove that the sequence
is logarithmically convex and that the sequence
is strictly
decreasing for . In addition, some monotonic and concave properties of
several functions relating to are extended and generalized.Comment: 12 page
Calculation of the current noise spectrum in mesoscopic transport: an efficient quantum master equation approach
Based on our recent work on quantum transport [Li et al., Phys. Rev. B 71,
205304 (2005)], where the calculation of transport current by means of quantum
master equation was presented, in this paper we show how an efficient
calculation can be performed for the transport noise spectrum. Compared to the
longstanding classical rate equation or the recently proposed quantum
trajectory method, the approach presented in this paper combines their
respective advantages, i.e., it enables us to tackle both the many-body Coulomb
interactionand quantum coherence on equal footing and under a wide range of
setup circumstances. The practical performance and advantages are illustrated
by a number of examples, where besides the known results and new insights
obtained in a transparent manner, we find that this alternative approach is
much simpler than other well-known full quantum mechanical methods such as the
Landauer-B\"uttiker scattering matrix theory and the nonequilibrium Green's
function technique.Comment: 13 pages, 3 figures, submitted to PR
A General Information Theoretical Proof for the Second Law of Thermodynamics
We show that the conservation and the non-additivity of the information,
together with the additivity of the entropy make the entropy increase in an
isolated system. The collapse of the entangled quantum state offers an example
of the information non-additivity. Nevertheless, the later is also true in
other fields, in which the interaction information is important. Examples are
classical statistical mechanics, social statistics and financial processes. The
second law of thermodynamics is thus proven in its most general form. It is
exactly true, not only in quantum and classical physics but also in other
processes, in which the information is conservative and non-additive.Comment: 4 page
Comment on "Resilience of gated avalanche photodiodes against bright illumination attacks in quantum cryptography"
This is a comment on the publication by Yuan et al. [Appl. Phys. Lett. 98,
231104 (2011); arXiv:1106.2675v1 [quant-ph]].Comment: 2 page
Robust and Efficient Sifting-Less Quantum Key Distribution Protocols
We show that replacing the usual sifting step of the standard
quantum-key-distribution protocol BB84 by a one-way reverse reconciliation
procedure increases its robustness against photon-number-splitting (PNS)
attacks to the level of the SARG04 protocol while keeping the raw key-rate of
BB84. This protocol, which uses the same state and detection than BB84, is the
m=4 member of a protocol-family using m polarization states which we introduce
here. We show that the robustness of these protocols against PNS attacks
increases exponentially with m, and that the effective keyrate of optimized
weak coherent pulses decreases with the transmission T like T^{1+1/(m-2)}
Phase-Remapping Attack in Practical Quantum Key Distribution Systems
Quantum key distribution (QKD) can be used to generate secret keys between
two distant parties. Even though QKD has been proven unconditionally secure
against eavesdroppers with unlimited computation power, practical
implementations of QKD may contain loopholes that may lead to the generated
secret keys being compromised. In this paper, we propose a phase-remapping
attack targeting two practical bidirectional QKD systems (the "plug & play"
system and the Sagnac system). We showed that if the users of the systems are
unaware of our attack, the final key shared between them can be compromised in
some situations. Specifically, we showed that, in the case of the
Bennett-Brassard 1984 (BB84) protocol with ideal single-photon sources, when
the quantum bit error rate (QBER) is between 14.6% and 20%, our attack renders
the final key insecure, whereas the same range of QBER values has been proved
secure if the two users are unaware of our attack; also, we demonstrated three
situations with realistic devices where positive key rates are obtained without
the consideration of Trojan horse attacks but in fact no key can be distilled.
We remark that our attack is feasible with only current technology. Therefore,
it is very important to be aware of our attack in order to ensure absolute
security. In finding our attack, we minimize the QBER over individual
measurements described by a general POVM, which has some similarity with the
standard quantum state discrimination problem.Comment: 13 pages, 8 figure
Side-channel-free quantum key distribution
Quantum key distribution (QKD) offers the promise of absolutely secure
communications. However, proofs of absolute security often assume perfect
implementation from theory to experiment. Thus, existing systems may be prone
to insidious side-channel attacks that rely on flaws in experimental
implementation. Here we replace all real channels with virtual channels in a
QKD protocol, making the relevant detectors and settings inside private spaces
inaccessible while simultaneously acting as a Hilbert space filter to eliminate
side-channel attacks. By using a quantum memory we find that we are able to
bound the secret-key rate below by the entanglement-distillation rate computed
over the distributed states.Comment: Considering general quantum systems, we extended QKD to the presence
of an untrusted relay, whose measurement creates secret correlations in
remote stations (achievable rate lower-bounded by the coherent information).
This key ingredient, i.e., the use of a measurement-based untrusted relay,
has been called 'measurement-device independence' in another arXiv submission
(arXiv:1109.1473
Uniqueness of nontrivially complete monotonicity for a class of functions involving polygamma functions
For , let
on . In the
present paper, we prove using two methods that, among all for
, only is nontrivially completely monotonic on
. Accurately, the functions and are
completely monotonic on , but the functions for
are not monotonic and does not keep the same sign on
.Comment: 9 page
Experimental demonstration of phase-remapping attack in a practical quantum key distribution system
Unconditional security proofs of various quantum key distribution (QKD)
protocols are built on idealized assumptions. One key assumption is: the sender
(Alice) can prepare the required quantum states without errors. However, such
an assumption may be violated in a practical QKD system. In this paper, we
experimentally demonstrate a technically feasible "intercept-and-resend" attack
that exploits such a security loophole in a commercial "plug & play" QKD
system. The resulting quantum bit error rate is 19.7%, which is below the
proven secure bound of 20.0% for the BB84 protocol. The attack we utilize is
the phase-remapping attack (C.-H. F. Fung, et al., Phys. Rev. A, 75, 32314,
2007) proposed by our group.Comment: 16 pages, 6 figure
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