3,207 research outputs found
A de Finetti representation theorem for infinite dimensional quantum systems and applications to quantum cryptography
According to the quantum de Finetti theorem, if the state of an N-partite
system is invariant under permutations of the subsystems then it can be
approximated by a state where almost all subsystems are identical copies of
each other, provided N is sufficiently large compared to the dimension of the
subsystems. The de Finetti theorem has various applications in physics and
information theory, where it is for instance used to prove the security of
quantum cryptographic schemes. Here, we extend de Finetti's theorem, showing
that the approximation also holds for infinite dimensional systems, as long as
the state satisfies certain experimentally verifiable conditions. This is
relevant for applications such as quantum key distribution (QKD), where it is
often hard - or even impossible - to bound the dimension of the information
carriers (which may be corrupted by an adversary). In particular, our result
can be applied to prove the security of QKD based on weak coherent states or
Gaussian states against general attacks.Comment: 11 pages, LaTe
(WP 2010-11) The Benefits of Environmental Improvement: Estimates From Space-time Analysis
This paper develops estimates of environmental improvement based on a two-stage hedonic price analysis of the single family housing market in the Puget Sound region of Washington State. The analysis — which focuses specifically on several EPA-designated environmental hazards and involves 226,918 transactions for 177,303 unique properties that took place between January 2001 and September 2009 — involves four steps: (i) ten hedonic price functions are estimated year-by-year, one for each year of the 2000s; (ii) the hedonic estimates are used to compute the marginal implicit price of distance from air release, superfund, and toxic release sites; (iii) the marginal implicit prices, which vary through time, are used to estimate a series of implicit demand functions describing the relationship between the price of distance and the quantity consumed; and, finally (iv) the demand estimates are compared to those obtained in other research and then used evaluate the potential scale of benefits associated with some basic environmental improvement scenarios. Overall, the analysis provides further evidence that it is possible to develop a structural model of implicit demand within a single housing market and suggests that the benefits of environmental improvement are substantial
An information-theoretic security proof for QKD protocols
We present a new technique for proving the security of quantum key
distribution (QKD) protocols. It is based on direct information-theoretic
arguments and thus also applies if no equivalent entanglement purification
scheme can be found. Using this technique, we investigate a general class of
QKD protocols with one-way classical post-processing. We show that, in order to
analyze the full security of these protocols, it suffices to consider
collective attacks. Indeed, we give new lower and upper bounds on the
secret-key rate which only involve entropies of two-qubit density operators and
which are thus easy to compute. As an illustration of our results, we analyze
the BB84, the six-state, and the B92 protocol with one-way error correction and
privacy amplification. Surprisingly, the performance of these protocols is
increased if one of the parties adds noise to the measurement data before the
error correction. In particular, this additional noise makes the protocols more
robust against noise in the quantum channel.Comment: 18 pages, 3 figure
Multipartite Bound Information exists and can be activated
We prove the conjectured existence of Bound Information, a classical analog
of bound entanglement, in the multipartite scenario. We give examples of
tripartite probability distributions from which it is impossible to extract any
kind of secret key, even in the asymptotic regime, although they cannot be
created by local operations and public communication. Moreover, we show that
bound information can be activated: three honest parties can distill a common
secret key from different distributions having bound information. Our results
demonstrate that quantum information theory can provide useful insight for
solving open problems in classical information theory.Comment: four page
The Benefits of Environmental Improvement: Estimates From Space-time Analysis
This paper develops estimates of environmental improvement based on a two-stage hedonic price analysis of the single family housing market in the Puget Sound region of Washington State. The analysis — which focuses specifically on several EPA-designated environmental hazards and involves 226,918 transactions for 177,303 unique properties that took place between January 2001 and September 2009 — involves four steps: (i) ten hedonic price functions are estimated year-by-year, one for each year of the 2000s; (ii) the hedonic estimates are used to compute the marginal implicit price of distance from air release, superfund, and toxic release sites; (iii) the marginal implicit prices, which vary through time, are used to estimate a series of implicit demand functions describing the relationship between the price of distance and the quantity consumed; and, finally (iv) the demand estimates are compared to those obtained in other research and then used evaluate the potential scale of benefits associated with some basic environmental improvement scenarios. Overall, the analysis provides further evidence that it is possible to develop a structural model of implicit demand within a single housing market and suggests that the benefits of environmental improvement are substantial.Hedonic housing model, benefits, environmental improvement
The Uncertainty Principle in the Presence of Quantum Memory
The uncertainty principle, originally formulated by Heisenberg, dramatically
illustrates the difference between classical and quantum mechanics. The
principle bounds the uncertainties about the outcomes of two incompatible
measurements, such as position and momentum, on a particle. It implies that one
cannot predict the outcomes for both possible choices of measurement to
arbitrary precision, even if information about the preparation of the particle
is available in a classical memory. However, if the particle is prepared
entangled with a quantum memory, a device which is likely to soon be available,
it is possible to predict the outcomes for both measurement choices precisely.
In this work we strengthen the uncertainty principle to incorporate this case,
providing a lower bound on the uncertainties which depends on the amount of
entanglement between the particle and the quantum memory. We detail the
application of our result to witnessing entanglement and to quantum key
distribution.Comment: 5 pages plus 12 of supplementary information. Updated to match the
journal versio
Valaciclovir for Chronic Hepatitis BVirus Infection after Lung Transplantation
Abstract. : We report on a chronic asymptomatic hepatitis B surface antigen (HBsAg) carrier who developed an increase in aminotransferase and HBsAg levels 1 year after lung transplantation. During treatment for cutaneous herpes simplex virus (HSV) infection with oral valaciclovir there was a marked decrease in replicating hepatitis B virus (HBV)-DNA and aminotransferase levels, which was sustained for 9 months by continuing low-dose valaciclovir. A second rise in aminotransferase levels again responded to a valaciclovir dose increase and the HBV-DNA levels declined further. Although we cannot exclude a spontaneous variation of the serologic parameters, our observation suggests that valaciclovir may represent a valuable therapeutic option in the treatment of chronic hepatitis B after lung transplantatio
Efficient Quantum Polar Coding
Polar coding, introduced 2008 by Arikan, is the first (very) efficiently
encodable and decodable coding scheme whose information transmission rate
provably achieves the Shannon bound for classical discrete memoryless channels
in the asymptotic limit of large block sizes. Here we study the use of polar
codes for the transmission of quantum information. Focusing on the case of
qubit Pauli channels and qubit erasure channels, we use classical polar codes
to construct a coding scheme which, using some pre-shared entanglement,
asymptotically achieves a net transmission rate equal to the coherent
information using efficient encoding and decoding operations and code
construction. Furthermore, for channels with sufficiently low noise level, we
demonstrate that the rate of preshared entanglement required is zero.Comment: v1: 15 pages, 4 figures. v2: 5+3 pages, 3 figures; argumentation
simplified and improve
Itinerant in-plane magnetic fluctuations and many-body correlations in NaCoO
Based on the {\it ab-initio} band structure for NaCoO we derive the
single-electron energies and the effective tight-binding description for the
bands using projection procedure. Due to the presence of the
next-nearest-neighbor hoppings a local minimum in the electronic dispersion
close to the point of the first Brillouin zone forms. Correspondingly,
in addition to a large Fermi surface an electron pocket close to the
point emerges at high doping concentrations. The latter yields the new
scattering channel resulting in a peak structure of the itinerant magnetic
susceptibility at small momenta. This indicates dominant itinerant in-plane
ferromagnetic fluctuations above certain critical concentration , in
agreement with neutron scattering data. Below the magnetic susceptibility
shows a tendency towards the antiferromagnetic fluctuations. We further analyze
the many-body effects on the electronic and magnetic excitations using various
approximations applicable for different ratio.Comment: 10 page
Lower and upper bounds on the secret key rate for QKD protocols using one--way classical communication
We investigate a general class of quantum key distribution (QKD) protocols
using one-way classical communication. We show that full security can be proven
by considering only collective attacks. We derive computable lower and upper
bounds on the secret key rate of those QKD protocol involving only entropies of
two--qubit density operators. As an illustration of our results, we determine
new bounds for the BB84, the six-state, and the B92 protocol. We show that in
all these cases the first classical processing that the legitimate partners
should apply consists in adding noise. This is precisely why any entanglement
based proof would generally fail here.Comment: minor changes, results for BB84 and B92 adde
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