QIC Abstracts

 Vol.9 No.1&2 January 1, 2009

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.

back to QIC online Front page