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 ...

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 ar...

Prog. Theor. Exp. Phys. 2013 053A02 May 2013 [Refereed]

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 a...

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 diff...

Journal of the Physical Society of Japan 81 064005 May 2012 [Refereed]

We show that the various intermediate states appearing in the process of
one-way computation at a given step of measurement are all equivalent modulo
local unitary transformations. This implies, in particular, that all those
intermediate states sh...

We present a general criterion for entanglement of N indistinguishable
particles decomposed into arbitrary s subsystems based on the unambiguous
measurability of correlation. Our argument provides a unified viewpoint on the
entanglement of indisti...

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 sta...

We present a family of entanglement measures R_m which act as indicators for
separability of n-qubit quantum states into m subsystems for arbitrary 2 \leq m
\leq n. The measure R_m vanishes if the state is separable into m subsystems,
and for m = ...

Entanglement of multipartite systems is studied based on exchange symmetry
under the permutation group S_N. With the observation that symmetric property
under the exchange of two constituent states and their separability are
intimately linked, we ...

Akihiro Mizutani, Toshihiko Sasaki, Go Kato, Yuki Takeuchi, Kiyoshi Tamaki

Apr 2017

We show the information-theoretic security proof of the
differential-phase-shift (DPS) quantum key distribution (QKD) protocol based on
the complementarity approach [arXiv:0704.3661 (2007)]. Our security proof
provides a slightly better key genera...

The round-robin differential phase shift (RRDPS) quantum key distribution
(QKD) protocol is a unique quantum key distribution protocol whose security has
not been understood through an information-disturbance trade-off relation, and
a sufficient a...

An essential step in quantum key distribution is the estimation of parameters
related to the leaked amount of information, which is usually done by sampling
of the communication data. When the data size is finite, the final key rate
depends on how...

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 bi...