36 research outputs found
Optimal State Merging Without Decoupling
We construct an optimal state merging protocol by adapting a
recently-discovered optimal entanglement distillation protcol [Renes and
Boileau, Phys. Rev. A . 73, 032335 (2008)]. The proof of optimality relies only
on directly establishing sufficient "amplitude" and "phase" correlations
between Alice and Bob and not on usual techniques of decoupling Alice from the
environment. This strengthens the intuition from quantum error-correction that
these two correlations are all that really matter in two-party quantum
information processing.Comment: 10 pages, accepted contribution to TQC 200
Conjectured Strong Complementary Information Tradeoff
We conjecture a new entropic uncertainty principle governing the entropy of
complementary observations made on a system given side information in the form
of quantum states, generalizing the entropic uncertainty relation of Maassen
and Uffink [Phys. Rev. Lett. 60, 1103 (1988)]. We prove a special case for
certain conjugate observables by adapting a similar result found by Christandl
and Winter pertaining to quantum channels [IEEE Trans. Inf. Theory 51, 3159
(2005)], and discuss possible applications of this result to the decoupling of
quantum systems and for security analysis in quantum cryptography.Comment: 4 page
The Physical Underpinning of Security Proofs for Quantum Key Distribution
The dawn of quantum technology unveils a plethora of new possibilities and challenges in the world of information technology, one of which is the quest for secure information transmission. A breakthrough in classical algorithm or the development of a quantum computer could threaten the security of messages encoded using public key cryptosystems based on one-way function such as RSA. Quantum key distribution (QKD) offers an unconditionally secure alternative to such schemes, even in the advent of a quantum computer, as it does not rely on mathematical or technological assumptions, but rather on the universality of the laws of quantum mechanics.
Physical concepts associated with quantum mechanics, like the uncertainty principle or entanglement, paved the way to the first successful security proof for QKD. Ever since, further development in security proofs for QKD has been remarkable. But the connection between entanglement distillation and the uncertainty principle has remained hidden under a pile of mathematical burden. Our main goal is to dig the physics out of the new advances in security proofs for QKD. By introducing an alternative definition of private state, which elaborates the ideas of Mayers and Koashi, we explain how the security of all QKD protocols follows from an entropic uncertainty principle. We show explicitly how privacy amplification protocol can be reduced to a private state distillation protocol constructed from our observations about the uncertainty principle. We also derive a generic security proof for one-way permutation-invariant QKD protocols. Considering collective attack, we achieve the same secret key generation rate as the Devetak-Winter's bound. Generalizing an observation from Kraus, Branciard and Renner, we have provided an improved version of the secret key generation rates by considering a different symmetrization. In certain situations, we argue that Azuma's inequality can simplify the security proof considerably, and we explain the implication, on the security level, of reducing a QKD protocol to an entanglement or a more general private state distillation protocol.
In a different direction, we introduce a QKD protocol with multiple-photon encoding that can be implemented without a shared reference frame. We prove the unconditional security of this protocol, and discuss some features of the efficiency of multiple-photon QKD schemes in general
Using error correction to determine the noise model
Quantum error correcting codes have been shown to have the ability of making
quantum information resilient against noise. Here we show that we can use
quantum error correcting codes as diagnostics to characterise noise. The
experiment is based on a three-bit quantum error correcting code carried out on
a three-qubit nuclear magnetic resonance (NMR) quantum information processor.
Utilizing both engineered and natural noise, the degree of correlations present
in the noise affecting a two-qubit subsystem was determined. We measured a
correlation factor of c=0.5+/-0.2 using the error correction protocol, and
c=0.3+/-0.2 using a standard NMR technique based on coherence pathway
selection. Although the error correction method demands precise control, the
results demonstrate that the required precision is achievable in the
liquid-state NMR setting.Comment: 10 pages, 3 figures. Added discussion section, improved figure
Physical Underpinnings of Privacy
One of the remarkable features of quantum mechanics is the ability to ensure
secrecy. Private states embody this effect, as they are precisely those
multipartite quantum states from which two parties can produce a shared secret
that cannot in any circumstance be correlated to an external system. Naturally,
these play an important role in quantum key distribution (QKD) and quantum
information theory. However, a general distillation method has heretofore been
missing. Inspired by Koashi's complementary control scenario (arXiv:0704.3661v1
[quant-ph]), we give a new definition of private states in terms of one party's
potential knowledge of two complementary measurements made on the other and use
this to construct a general method of private state distillation using quantum
error-correcting codes. The procedure achieves the same key rate as recent,
more information-theoretic approaches while demonstrating the physical
principles underlying privacy of the key. Additionally, the same approach can
be used to establish the hashing inequality for entanglement distillation, as
well as the direct quantum coding theorem.Comment: 12.5 page
Growth rings show limited evidence for ungulates' potential to suppress shrubs across the Arctic
Global warming has pronounced effects on tundra vegetation, and rising mean temperatures increase plant growth potential across the Arctic biome. Herbivores may counteract the warming impacts by reducing plant growth, but the strength of this effect may depend on prevailing regional climatic conditions. To study how ungulates interact with temperature to influence growth of tundra shrubs across the Arctic tundra biome, we assembled dendroecological data from 20 sites, comprising 1153 individual shrubs and 223 63 annual growth rings. Evidence for ungulates suppressing shrub radial growth was only observed at intermediate summer temperatures (6.5 degrees C-9 degrees C), and even at these temperatures the effect was not strong. Multiple factors, including forage preferences and landscape use by the ungulates, and favourable climatic conditions enabling effective compensatory growth of shrubs, may weaken the effects of ungulates on shrubs, possibly explaining the weakness of observed ungulate effects. Earlier local studies have shown that ungulates may counteract the impacts of warming on tundra shrub growth, but we demonstrate that ungulates' potential to suppress shrub radial growth is not always evident, and may be limited to certain climatic conditions
Growth rings show limited evidence for ungulates’ potential to suppress shrubs across the Arctic
Global warming has pronounced effects on tundra vegetation, and rising mean temperatures
increase plant growth potential across the Arctic biome. Herbivores may counteract the warming
impacts by reducing plant growth, but the strength of this effect may depend on prevailing regional
climatic conditions. To study how ungulates interact with temperature to influence growth of
tundra shrubs across the Arctic tundra biome, we assembled dendroecological data from 20 sites,
comprising 1153 individual shrubs and 223 63 annual growth rings. Evidence for ungulates
suppressing shrub radial growth was only observed at intermediate summer temperatures
(6.5 ◦C–9 ◦C), and even at these temperatures the effect was not strong. Multiple factors, including
forage preferences and landscape use by the ungulates, and favourable climatic conditions enabling
effective compensatory growth of shrubs, may weaken the effects of ungulates on shrubs, possibly
explaining the weakness of observed ungulate effects. Earlier local studies have shown that
ungulates may counteract the impacts of warming on tundra shrub growth, but we demonstrate
that ungulates’ potential to suppress shrub radial growth is not always evident, and may be limited
to certain climatic conditions.Research Council of Norwayhttp://iopscience.iop.org/1748-9326dm2022Mammal Research InstituteZoology and Entomolog
Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo
Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level
Ultralight vector dark matter search using data from the KAGRA O3GK run
Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM