Publications

Quantum Communication: real-world applications and academic research

The field of quantum communication is mature enough to be clearly divided into an applied side, around Quantum Key Distribution, and a fundamental research program. The applied side is driven by real-world applications with concerns like reliability and costs. The second aspect is still mainly curiosity driven and aims, among other things, at extending the distance of fiber-based quantum communication to continental (or even inter-continental) distances. In this tutorial I will give an intuitive perspective on some recent advances in the state of the art.

Entanglement of two quantum bits is equivalent to secure key distribution

We study the problem of secret key distillation from bipartite states in the scenario where Alice and Bob can only perform measurements at the single-copy level and classically process the obtained outcomes. Even with these limitations, secret bits can be asymptotically distilled by the honest parties from any two-qubit entangled state, under any individual attack. Our results point out a complete equivalence between two-qubit entanglement and secure key distribution: a key can be established through a one-qubit channel if and only if it allows to distribute entanglement. These results can be generalized to higher dimension for all those states that are one-copy distillable.

Advisory Board

No abstract is provided for this article.

Measurement-Device-Independent Entanglement Witnesses for All Entangled Quantum States

The problem of demonstrating entanglement is central to quantum information processing applications. Resorting to standard entanglement witnesses requires one to perfectly trust the implementation of the measurements to be performed on the entangled state, which may be an unjustified assumption. Inspired by the recent work of F. Buscemi [Phys. Rev. Lett. 108, 200401 (2012)], we introduce the concept of measurement-device-independent entanglement witnesses (MDI-EWs), which allow one to demonstrate entanglement of all entangled quantum states with untrusted measurement apparatuses. We show how to systematically obtain such MDI-EWs from standard entanglement witnesses. Our construction leads to MDI-EWs that are loss tolerant and can be implemented with current technology.

Quantum cryptography and long distance Bell experiments: How to control decoherence

Physics without determinism: Alternative interpretations of classical physics

Classical physics is generally regarded as deterministic, as opposed to quantum mechanics that is considered the first theory to have introduced genuine indeterminism into physics. We challenge this view by arguing that the alleged determinism of classical physics relies on the tacit, metaphysical assumption that there exists an actual value of every physical quantity, with its infinite predetermined digits (which we name principle of infinite precision). Building on recent information-theoretic arguments showing that the principle of infinite precision (which translates into the attribution of a physical meaning to mathematical real numbers) leads to unphysical consequences, we consider possible alternative indeterministic interpretations of classical physics. We also link those to well-known interpretations of quantum mechanics. In particular, we propose a model of classical indeterminism based on finite information quantities (FIQs). Moreover, we discuss the perspectives that an indeterministic physics could open (such as strong emergence), as well as some potential problematic issues. Finally, we make evident that any indeterministic interpretation of physics would have to deal with the problem of explaining how the indeterminate values become determinate, a problem known in the context of quantum mechanics as (part of) the ``quantum measurement problem.'' We discuss some similarities between the classical and the quantum measurement problems, and propose ideas for possible solutions (e.g., ``collapse models'' and ``top-down causation'').

High-sensitivity-coherent optical frequency-domain reflectometry for characterization of fiber-optic network components

The authors have performed high-sensitivity-coherent optical frequency-domain reflectometry (OFDR) measurements of pig-tailed optical devices with centimeter resolution. The oscillations of the Rayleigh backscattering level produced by coherent fading noise was eliminated, strongly improving the capability of OFDR measurements of low level reflections and losses. Sensitivities down to -105 dB were achieved with minute measurement times.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quantum Cloning for Absolute Radiometry

In the quantum regime information can be copied with only a finite fidelity. This fidelity gradually increases to 1 as the system becomes classical. In this Letter we show how this fact can be used to directly measure the amount of radiated power. We demonstrate how these principles can be used to build a practical primary standard.

Semi-device-independent characterization of multipartite entanglement of states and measurements

The semi-device-independent framework allows one to draw conclusions about properties of an unknown quantum system under weak assumptions. Here we present a semi-device-independent scheme for the characterization of multipartite entanglement based around a game played by several isolated parties whose devices are uncharacterized beyond an assumption about the dimension of their Hilbert spaces. Our scheme can simultaneously certify that an $n$-partite high-dimensional quantum state features genuine multipartite entanglement, and that a joint measurement on $n$ subsystems is entangled. Moreover, it provides a lower bound on the number of entangled measurement operators. These tests are strongly robust to noise, and even optimal for certain classes of states and measurements, as we demonstrate with illustrative examples. Notably, our scheme allows the certification of many entangled states admitting a local model, which therefore cannot violate any Bell inequality.

Macroscopic quantum measurements: In what direction does a magnet point?

We introduce the concept of macroscopic quantum measurement, that is, a quantum formalism describing the measurements we perform continuously in our everyday life; for example, when looking at a magnet. We idealize the problem by considering parallel spins whose direction has to be estimated. We present a physical measurement model weak enough to almost not disturb the quantum state, but strong enough to provide almost the maximal amount of information in a single shot. Interestingly an intermediate coupling strength between spin system and measurement apparatus achieves better results than a strong coupling.

Overall polarization dispersion after propagation through different fibres

Polarization dispersion is one of the effect which can give rise to data rate limitation. An actual optical link can be installed using different appended fibres, manufactured by different producers, processing techniques or batches. Knowing the polarization dispersion of each individual fibre, it could be difficult to forecast the overall dispersion of such an heterogeneous link. Fortunately recent models make the prediction of the global polarization dispersion possible. An experimental verification of this model using two different methods is reported, so that the predicted dispersion of two appended non-similar fibres could be compared with actual measurements.

Quantum cryptography and long distance Bell experiments: How to control decoherence

No abstract is provided for this article.

Experimental investigations of the statistical properties of polarization mode dispersion in single mode fibers

Polarization mode dispersion of installed optical cables and of short concatenations of hi-bi fibers have been measured with polarimetric and interferometric instruments. The results confirm the theoretical models, in particular the predicted relations between the two measurement methods. The importance of a statistical treatment of polarization mode dispersion is underscored by the observed instability of the principal states and the remarkable long-term stability of their statistics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quantum approach to coupling classical and quantum dynamics

We present a consistent framework of coupled classical and quantum dynamics. Our result allows us to overcome severe limitations of previous phenomenological approaches, such as evolutions that do not preserve the positivity of quantum states or that allow one to activate quantum nonlocality for superluminal signaling. A ``hybrid'' quantum-classical density is introduced, and its evolution equation derived. The implications and applications of our result are numerous: it incorporates the back-reaction of quantum on classical variables, and it resolves fundamental problems encountered in standard mean-field theories, and remarkably, also in the quantum measurement process; i.e., the most controversial example of quantum-classical interaction is consistently described within our approach, leading to a theory of dynamical collapse.

Quantum Memories for long distance Quantum Communication

Quantum communications is the art of transferring a quantum state from one location to a distant one. On the application side, quantum communication is already relatively advanced with Quantum Random Number Generators and Quantum Key Distribution (QKD) systems having found niche markets. However, on the academic research side quantum communication has still a long way to go until a functional quantum repeater can extend the distances to continental scales. Quantum repeaters are based on quantum teleportation, the most fascinating application of entanglement. Additionally, quantum repeaters require quantum memories with memory times close to a second; this represents one grand challenge for quantum communication.

Faint laser quantum key distribution: eavesdropping exploiting multiphoton pulses

No abstract is provided for this article.

Refracted-near-field analyses for refractive-index profiles of integrated LiNbO3 waveguides

The development of any new technology requires new characterization tools both for research and development laboratories and for production control. It is thus important to develop characterization methods for LiNbO3 waveguides. In this contribution we concentrate on a basic physical property of a waveguide, its refractive-index profile,1 for which we present a direct and nondestructive measurement technique. This technique generalizes the refracted-near-field (RNF) method, which is a well-known optical-fiber characterization tool.2 However, specific adaptations are necessary because of noncylindrical geometry, because of small dimensions, especially for the most interesting cases of single-mode guides, and because of high refractive indices, around 2.25, for which the index-matching liquids are corrosive and toxic and should thus be avoided. We have circumvented these difficulties by collecting the unguided light that emerges from one end of the sample when the beam is focused on the other end, as illustrated in Fig. 1. With this configuration we move somewhat away from the traditional RNF configuration; in particular, several sections of the incident beam will be detected.3

Non-linear quantum state transformation of

A non-linear quantum state transformation is presented. The transformation, which operates on pairs of spin- 1 2 , can be used to distinguish optimally between two non-orthogonal states. Similar transformations applied locally on each component of an entangled pair of spin- 1 2 can be used to transform a mixed non-local state into a quasi-pure maximally entangled singlet state. In both cases the transformation makes use of the basic building block of the quantum computer, namely the quantum-XOR gate.