359 research outputs found
A comparison of X-ray stress measurement methods \\based on the fundamental equation
Stress measurement methods using X-ray diffraction (XRD methods) are based on
so-called fundamental equations. The fundamental equation is described in the
coordinate system that best suites the measurement situation, and, thus, making
a comparison between different XRD methods is not straightforward. However, by
using the diffraction vector representation, the fundamental equations of
different methods become identical. Furthermore, the differences between the
various XRD methods are in the choice of diffraction vectors and the way of
calculating the stress from the measured data. The stress calculation methods
can also be unified using the general least-squares method, which is a common
least-squares method of multivariate analysis. Thus, the only difference
between these methods turns out to be in the choice of the set of diffraction
vectors. In light of these ideas, we compare three commonly used XRD methods:
the sin^2 psi method, the XRD^2 method, and the cos alpha method using the
estimation of the measurement errors
Separability of N-particle Fermionic States for Arbitrary Partitions
We present a criterion of separability for arbitrary s partitions of
N-particle fermionic pure states. We show that, despite the superficial
non-factorizability due to the antisymmetry required by the
indistinguishability of the particles, the states which meet our criterion have
factorizable correlations for a class of observables which are specified
consistently with the states. The separable states and the associated class of
observables share an orthogonal structure, whose non-uniqueness is found to be
intrinsic to the multi-partite separability and leads to the non-transitivity
in the factorizability in general. Our result generalizes the previous result
obtained by Ghirardi et. al. [J. Stat. Phys. 108 (2002) 49] for the s = 2 and s
= N case.Comment: 21 pages, 2 figure
Security of differential quadrature phase shift quantum key distribution
One of the simplest methods for implementing quantum key distribution over
fiber-optic communication is the Bennett-Brassard 1984 protocol with phase
encoding (PE-BB84 protocol), in which the sender uses phase modulation over
double pulses from a laser and the receiver uses a passive delayed
interferometer. Using essentially the same setup and by regarding a train of
many pulses as a single block, one can carry out the so-called differential
quadrature phase shift (DQPS) protocol, which is a variant of differential
phase shift (DPS) protocols. Here we prove the security of the DQPS protocol
based on an adaptation of proof techniques for the BB84 protocol, which
inherits the advantages arising from the simplicity of the protocol, such as
accommodating the use of threshold detectors and simple off-line calibration
methods for the light source. We show that the secure key rate of the DQPS
protocol in the proof is eight thirds as high as the rate of the PE-BB84
protocol.Comment: Citation of the Fig.1 in the text correcte
Refined security proof of the round-robin differential phase shift quantum key distribution and its improved performance in the finite-sized case
Among many quantum key distribution (QKD) protocols, the round-robin
differential phase shift (RRDPS) protocol is unique in that it can upper-bound
the amount of the information leakage without monitoring the signal
disturbance. To expedite implementation of the protocol, however, the number of
pulses forming a single block should be kept small, which significantly
decreases the key rates in the original security proof. In the present paper,
we refine the security proof of the RRDPS protocol in the finite-sized regime
and achieve a tighter estimation for the information leakage without changing
the original experimental setups. As a consequence, we obtain better key rates
in both asymptotic and finite-sized cases while keeping the preferable features
of the protocol, such as omission of phase randomization.Comment: 25 pages, 5 figure
Complex Probability Measure and Aharonov's Weak Value
We present a complex probability measure relevant for double (pairs of)
states in quantum mechanics, as an extension of the standard probability
measure for single states that underlies Born's statistical rule. When the
double states are treated as the initial and final states of a quantum process,
we find that Aharonov's weak value, which has acquired a renewed interest as a
novel observable quantity inherent in the process, arises as an expectation
value associated with the probability measure. Despite being complex, our
measure admits the physical interpretation as mixed processes, i.e., an
ensemble of processes superposed with classical probabilities.Comment: 7 pages, 1 figure
Quantum key distribution protocols with slow basis choice
Many quantum key distribution (QKD) protocols require random choice of
measurement basis for each pulse or each train of pulses. In some QKD
protocols, such as the Round-Robin Differential Phase Shift (RRDPS) QKD
protocol, this requirement is a bit challenging as randomly choosing hundreds
of settings for every, say, 100 pulses may be too fast with current
technologies. In this paper, we solve this issue by proving the security of QKD
protocols with slow basis choice without compromising the secret key rate. We
also show that the random choice of the bases for the state preparation can be
made slow if the signals do not leak any information on the basis. Examples of
QKD protocols that our technique can apply include the RRDPS protocol and
BB84-type protocols, and our technique relaxes demands for the implementation
of QKD systems.Comment: 7 pages, 4 figure
Universal Separability and Entanglement in Identical Particle Systems
Entanglement is known to be a relative notion, defined with respect to the
choice of physical observables to be measured (i.e., the measurement setup
used). This implies that, in general, the same state can be both separable and
entangled for different measurement setups, but this does not exclude the
existence of states which are separable (or entangled) for all possible setups.
We show that for systems of bosonic particles there indeed exist such
universally separable states: they are i.i.d. pure states. In contrast, there
is no such state for fermionic systems with a few exceptional cases. We also
find that none of the fermionic and bosonic systems admits universally
entangled states.Comment: 11 pages, 1 figur
Rigorous calibration method for photon-number statistics
Characterization of photon statistics of a light source is one of the most
basic tools in quantum optics. Although the outcome from existing methods is
believed to be a good approximation when the measured light is sufficiently
weak, there is no rigorous quantitative bounds on the degree of the
approximation. As a result, they fail to fulfill the demand arising from
emerging applications of quantum information such as quantum cryptography.
Here, we propose a calibration method to produce rigorous bounds for a
photon-number probability distribution by using a conventional
Hanbury-Brown-Twiss setup with threshold photon detectors. We present a general
framework to treat any number of detectors and non-uniformity of their
efficiencies. The bounds are conveniently given as closed-form expressions of
the observed coincidence rates and the detector efficiencies. We also show
optimality of the bounds for light with a small mean photon number. As an
application, we show that our calibration method can be used for the light
source in a decoy-state quantum key distribution protocol. It replaces the a
priori assumption on the distribution that has been commonly used, and achieves
almost the same secure key rate when four detectors are used for the
calibration.Comment: 8 pages, 3 figure
Experimental quantum key distribution without monitoring signal disturbance
Since the invention of Bennett-Brassard 1984 (BB84) protocol, many quantum
key distribution (QKD) protocols have been proposed and some protocols are
operated even in field environments. One of the striking features of QKD is
that QKD protocols are provably secure unlike cryptography based on
computational complexity assumptions. It has been believed that, to guarantee
the security of QKD, Alice and Bob have to monitor the statistics of the
measurement outcomes which are used to determine the amount of the privacy
amplification to generate a key. Recently a new type of QKD protocol, called
round robin differential phase shift (RRDPS) protocol, was proposed, and
remarkably this protocol can generate a key without monitoring any statistics
of the measurement outcomes. Here we report an experimental realization of the
RRDPS protocol. We used a setup in which Bob randomly chooses one from four
interferometers with different pulse delays so that he could implement phase
difference measurements for all possible combinations with five-pulse time-bin
states. Using the setup, we successfully distributed keys over 30 km of fiber,
making this the first QKD experiment that does not rely on signal disturbance
monitoring.Comment: 8 pages, 4 figure
Quantum key distribution with simply characterized light sources
To guarantee the security of quantum key distribution (QKD), several
assumptions on light sources must be satisfied. For example, each random bit
information is precisely encoded on an optical pulse and the photon-number
probability distribution of the pulse is exactly known. Unfortunately, however,
it is hard to check if all the assumptions are really met in practice, and it
is preferable that we have minimal number of device assumptions. In this paper,
we adopt the differential-phase-shift (DPS) QKD protocol and drastically
mitigate the requirements on light sources. Specifically, we only assume the
independence among emitted pulses, the independence of the vacuum emission
probability from a chosen bit, and upper bounds on the tail distribution
function of the total photon number in a single block of pulses for single, two
and three photons. Remarkably, no other detailed characterizations, such as the
amount of phase modulation, are required. Our security proof significantly
relaxes demands for light sources, which paves a route to guarantee
implementation security with simple verification of the devices.Comment: 10 pages, 2 figure
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