261 research outputs found
Entanglement, which-way measurements, and a quantum erasure
We present a didactical approach to the which-way experiment and the
counterintuitive effect of the quantum erasure for one-particle quantum
interferences. The fundamental concept of entanglement plays a central role and
highlights the complementarity between quantum interference and knowledge of
which path is followed by the particle.Comment: 5 pages, 4 figures; with some clarifications and added reference
Experimental quantum key distribution based on a Bell test
We report on a complete free-space field implementation of a modified Ekert91
protocol for quantum key distribution using entangled photon pairs. For each
photon pair we perform a random choice between key generation and a Bell
inequality. The amount of violation is used to determine the possible knowledge
of an eavesdropper to ensure security of the distributed final key.Comment: 5 pages ReVTeX, 3 figures; version v2 with updated references and
minor corrections, author spelling fixe
Collisional quantum thermometry
We introduce a general framework for thermometry based on collisional models,
where ancillas probe the temperature of the environment through an intermediary
system. This allows for the generation of correlated ancillas even if they are
initially independent. Using tools from parameter estimation theory, we show
through a minimal qubit model that individual ancillas can already outperform
the thermal Cramer-Rao bound. In addition, due to the steady-state nature of
our model, when measured collectively the ancillas always exhibit superlinear
scalings of the Fisher information. This means that even collective
measurements on pairs of ancillas will already lead to an advantage. As we find
in our qubit model, such a feature may be particularly valuable for weak
system-ancilla interactions. Our approach sets forth the notion of metrology in
a sequential interactions setting, and may inspire further advances in quantum
thermometry
Quantum cloning with an optical fiber amplifier
It has been shown theoretically that a light amplifier working on the
physical principle of stimulated emission should achieve optimal quantum
cloning of the polarization state of light. We demonstrate close-to-optimal
universal quantum cloning of polarization in a standard fiber amplifier for
telecom wavelengths. For cloning 1 --> 2 we find a fidelity of 0.82, the
optimal value being 5/6 = 0.83.Comment: 4 pages, 3 figure
Thermalizing Quantum Machines: Dissipation and Entanglement
We study the relaxation of a quantum system towards the thermal equilibrium
using tools developed within the context of quantum information theory. We
consider a model in which the system is a qubit, and reaches equilibrium after
several successive two-qubit interactions (thermalizing machines) with qubits
of a reservoir. We characterize completely the family of thermalizing machines.
The model shows a tight link between dissipation, fluctuations, and the maximal
entanglement that can be generated by the machines. The interplay of quantum
and classical information processes that give rise to practical irreversibility
is discussed.Comment: 4 pages, 1 figur
Quantum correlations in Newtonian space and time: arbitrarily fast communication or nonlocality
We investigate possible explanations of quantum correlations that satisfy the
principle of continuity, which states that everything propagates gradually and
continuously through space and time. In particular, following [J.D. Bancal et
al, Nature Physics 2012], we show that any combination of local common causes
and direct causes satisfying this principle, i.e. propagating at any finite
speed, leads to signalling. This is true even if the common and direct causes
are allowed to propagate at a supraluminal-but-finite speed defined in a
Newtonian-like privileged universal reference frame. Consequently, either there
is supraluminal communication or the conclusion that Nature is nonlocal (i.e.
discontinuous) is unavoidable.Comment: It is an honor to dedicate this article to Yakir Aharonov, the master
of quantum paradoxes. Version 2 contains some more references and a clarified
conclusio
Violation of Bell inequality for thermal states of interaction qubits via a multi-qubit Heisenberg model
We study the violations of Bell inequality for thermal states of qubits in a
multi-qubit Heisenberg model as a function of temperature and external magnetic
fields. Unlike the behaviors of the entanglement the violation can not be
obtained by increasing the temperature or the magnetic field. The threshold
temperatures of the violation are found be less than that of the entanglement.
We also consider a realistic cavity-QED model which is a special case of the
mutli-qubit Heisenberg model.Comment: 5 pages, 5 figures, few changed, accepted by New J. Phy
One-sided Device-Independent Quantum Key Distribution: Security, feasibility, and the connection with steering
We analyze the security and feasibility of a protocol for Quantum Key
Distribution (QKD), in a context where only one of the two parties trusts his
measurement apparatus. This scenario lies naturally between standard QKD, where
both parties trust their measurement apparatuses, and Device-Independent QKD
(DI-QKD), where neither does, and can be a natural assumption in some practical
situations. We show that the requirements for obtaining secure keys are much
easier to meet than for DI-QKD, which opens promising experimental
opportunities. We clarify the link between the security of this one-sided
DI-QKD scenario and the demonstration of quantum steering, in analogy to the
link between DI-QKD and the violation of Bell inequalities.Comment: v2 replaces the 3-page abstract posted as v1: our results are now
given with all necessary details. v3: published versio
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