1,354 research outputs found
Cloning a Qutrit
We investigate several classes of state-dependent quantum cloners for
three-level systems. These cloners optimally duplicate some of the four
maximally-conjugate bases with an equal fidelity, thereby extending the
phase-covariant qubit cloner to qutrits. Three distinct classes of qutrit
cloners can be distinguished, depending on two, three, or four
maximally-conjugate bases are cloned as well (the latter case simply
corresponds to the universal qutrit cloner). These results apply to symmetric
as well as asymmetric cloners, so that the balance between the fidelity of the
two clones can also be analyzed.Comment: 14 pages LaTex. To appear in the Journal of Modern Optics for the
special issue on "Quantum Information: Theory, Experiment and Perspectives".
Proceedings of the ESF Conference, Gdansk, July 10-18, 200
From Bell's Theorem to Secure Quantum Key Distribution
Any Quantum Key Distribution (QKD) protocol consists first of sequences of
measurements that produce some correlation between classical data. We show that
these correlation data must violate some Bell inequality in order to contain
distillable secrecy, if not they could be produced by quantum measurements
performed on a separable state of larger dimension. We introduce a new QKD
protocol and prove its security against any individual attack by an adversary
only limited by the no-signaling condition.Comment: 5 pages, 2 figures, REVTEX
General properties of Nonsignaling Theories
This article identifies a series of properties common to all theories that do
not allow for superluminal signaling and predict the violation of Bell
inequalities. Intrinsic randomness, uncertainty due to the incompatibility of
two observables, monogamy of correlations, impossibility of perfect cloning,
privacy of correlations, bounds in the shareability of some states; all these
phenomena are solely a consequence of the no-signaling principle and
nonlocality. In particular, it is shown that for any distribution, the
properties of (i) nonlocal, (ii) no arbitrarily shareable and (iii) positive
secrecy content are equivalent.Comment: 10 page
Quantum correlations and secret bits
It is shown that (i) all entangled states can be mapped by single-copy
measurements into probability distributions containing secret correlations, and
(ii) if a probability distribution obtained from a quantum state contains
secret correlations, then this state has to be entangled. These results prove
the existence of a two-way connection between secret and quantum correlations
in the process of preparation. They also imply that either it is possible to
map any bound entangled state into a distillable probability distribution or
bipartite bound information exists.Comment: 4 pages, published versio
Characterizing the nonlocal correlations of particles that never interacted
Quantum systems that have never interacted can become nonlocally correlated
through a process called entanglement swapping. To characterize nonlocality in
this context, we introduce local models where quantum systems that are
initially uncorrelated are described by uncorrelated local variables. While a
pair of maximally entangled qubits prepared in the usual way (i.e., emitted
from a common source) requires a visibility close to 70% to violate a Bell
inequality, we show that an entangled pair generated through entanglement
swapping will already violate a Bell inequality for visibilities as low as 50%
under our assumption.Comment: 5 pages, 2 figure
Long-distance Bell-type tests using energy-time entangled photons
Long-distance Bell-type experiments are presented. The different experimental
challenges and their solutions in order to maintain the strong quantum
correlations between energy-time entangled photons over more than 10 km are
reported and the results analyzed from the point of view of tests of
fundamental physics as well as from the more applied side of quantum
communication, specially quantum key distribution. Tests using more than one
analyzer on each side are also presented.Comment: 22 pages including 7 figures and 5 table
Security of Quantum Key Distribution with Entangled Qutrits
The study of quantum cryptography and quantum non-locality have
traditionnally been based on two-level quantum systems (qubits). In this paper
we consider a generalisation of Ekert's cryptographic protocol [Ekert] where
qubits are replaced by qutrits. The security of this protocol is related to
non-locality, in analogy with Ekert's protocol. In order to study its
robustness against the optimal individual attacks, we derive the information
gained by a potential eavesdropper applying a cloning-based attack.Comment: 9 pages original version: july 2002, replaced in january 2003
(reason: minor changes
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