Research Articles:
Teleportation of composite
systems for communication and information processing
(pp0001-0015)
S.G.R.
Louis, A.D. Greentree, W.J. Munro, and K. Nemoto
We describe two protocols for efficient data transmission using a single
passive bus. Different types of interactions are obtained enabling the
deterministic transfer and teleportation of composite quantum systems
for arbitrary subsystem dimension and for arbitrary numbers of
subsystems. The subsystems may become entangled in the transmission in
which case the protocols can serve generalized teleportation based
information processing as well as storage and transmission functions. We
explore the cases of two qubits and two qutrits in detail, obtaining a
maximally entangling mapping of the composite systems and discuss the
use of a continuous variable bus.
Computing stabilized norms for
quantum operations
(pp0016-0035)
N.
Johnston, D.W. Kribs, and V.I. Paulsen
The diamond and completely bounded norms for linear maps play an
increasingly important role in quantum information science, providing
fundamental stabilized distance measures for differences of quantum
operations. We give a brief introduction to the theory of completely
bounded maps. Based on this theory, we formulate an algorithm to compute
the norm of an arbitrary linear map. We present an implementation of the
algorithm via MATLAB, discuss its efficiency, and consider the
case of differences of unitary maps.
Generation of entanglement between
spin of an electron and polarization of a photon
(pp0036-0061)
N.
Chandra and R. Ghosh
This paper shows an electron and a photon, emitted in two consecutive
steps from an atom following the absorption of a single photon, may be
entangled in the presence of the spin-orbit interaction only. This
entanglement strongly depends upon the polarization of the absorbed and
of the radiated photons, kinematics of two emitted particles, and
dynamics of photoionization; however, the photoemission dynamics plays
no role in this entanglement. This hybrid entanglement can be used in
teleporting a quantum state encoded in a flying/stationary material
particle onto a light pulse, or vice versa. Such an electron-photon
entanglement, in addition, will make it possible to learn about the
polarization of a single photon or spin-polarization of a free electron
without making any measurements on the corresponding particle itself.
Upper bounds on the performance of
differential-phase-shift quantum key distribution
(pp0062-0080)
H.
Gomez-Sousa and M. Curty
In this paper, we investigate limitations imposed by sequential attacks
on the performance of a differential-phase-shift (DPS) quantum key
distribution (QKD) protocol with weak coherent pulses. Specifically, we
analyze a sequential attack based on optimal unambiguous discrimination
of the relative phases between consecutive signal states emitted by the
source. We show that this attack can provide tighter upper bounds for
the security of a DPS QKD scheme than those derived from sequential
attacks where the eavesdropper aims to identify the state of each signal
emitted by the source unambiguously.
On perfect completeness for QMA
(pp0081-0089)
S.
Aaronson
Whether the class QMA (Quantum Merlin Arthur)\ is equal to
QMA_1, or QMA with one-sided error, has been an open problem
for years. This note helps to explain why the problem is difficult,\ by
using ideas from real analysis to give a "quantum" relative to which
QMA \neq QMA_1. As a byproduct, we find that there are facts
about quantum complexity classes that are classically relativizing but
not quantumly relativizing, among them such "trivial" containments as
BQP \subseteq ZQEXP.
Violation of equalities in bipartite
qutrits systems
(pp0090-0102)
H.
Movahhedian
We have recently shown that for the special case of a bipartite system
with binary inputs and outputs there exist equalities in local theories
which are violated by quantum theory. The amount of white noise
tolerated by these equalities are twice that of inequalities. In this
paper we will first introduce an inequality in bipartite qutrits systems
which, if non-maximally entangled state is used instead of maximally
entangled state, is violated more strongly by quantum theory. Hence
reproducing the results obtained in the literature. We will then prove
that our equalities in this case are violated by quantum theory too, and
they tolerate much more white noise than inequalities.
Sub- and super-fidelity as bounds for
quantum fidelity
(pp0103-0130)
J.A.
Miszczak, Z. Puchala, P. Horodecki, A. Uhlmann, and K. Zyczkowski
We derive several bounds on fidelity between quantum states. In
particular we show that fidelity is bounded from above by a simple to
compute quantity we call super--fidelity. It is analogous to another
quantity called sub--fidelity. For any two states of a two--dimensional
quantum system (N=2) all three quantities coincide. We
demonstrate that sub-- and super--fidelity are concave functions. We
also show that super--fidelity is super--multiplicative while
sub--fidelity is sub--multiplicative and design feasible schemes to
measure these quantities in an experiment.Super--fidelity can be used to
define a distance between quantum states. With respect to this metric
the set of quantum states forms a part of a N^2-1 dimensional
hypersphere.
Security proof of quantum key
distribution with detection efficiency mismatch
(pp0131-0165)
C.-H.
F. Fung, K. Tamaki, B. Qi, H.-K. Lo, and X. Ma
In theory, quantum key distribution
(QKD) offers unconditional security based on the laws of physics.
However, as demonstrated in recent quantum hacking theory and
experimental papers, detection efficiency loophole can be fatal to the
security of practical QKD systems. Here, we describe the physical origin
of detection efficiency mismatch in various domains including spatial,
spectral, and time domains and in various experimental set-ups. More
importantly, we prove the unconditional security of QKD even with
detection efficiency mismatch. We explicitly show how the key generation
rate is characterized by the maximal detection efficiency ratio between
the two detectors. Furthermore, we prove that by randomly switching the
bit assignments of the detectors, the effect of detection efficiency
mismatch can be completely eliminated.
Generalized concurrences do not
provide necessary and sufficient conditions for entanglement detection
(pp0166-0180)
L.
Cattaneo and D. D'Alessandro
We study generalized concurrences as a tool to detect the
entanglement of bipartite quantum systems. By considering the case of
2x4 states of rank 2, we prove that generalized concurrences do not, in
general, give a necessary and sufficient condition of separability. We
identify a set of entangled states which are undetected by this method.
