Publications

A framework for the study of symmetric full-correlation Bell-like inequalities

Full-correlation Bell-like inequalities represent an important subclass of Bell-like inequalities that have found applications in both a better understanding of fundamental physics and in quantum information science. Loosely speaking, these are inequalities where only measurement statistics involving all parties play a role. In this paper, we provide a framework for the study of a large family of such inequalities that are symmetrical with respect to arbitrary permutations of the parties. As an illustration of the power of our framework, we derive (i) a new family of Svetlichny inequalities for arbitrary numbers of parties, settings and outcomes, (ii) a new family of two-outcome device-independent entanglement witnesses for genuine n-partite entanglement and (iii) a new family of two-outcome Tsirelson inequalities for arbitrary numbers of parties and settings. We also discuss briefly the application of these new inequalities in the characterization of quantum correlations.

How much free will is needed to demonstrate nonlocality

Bell's inequality holds for all non-product states

We prove that any non-product state of two-particle systems violates a Bell inequality.

Quantum physics needs complex numbers

Complex numbers, i.e., numbers with a and an imaginary part, are essential for mathematical analysis, while their role in other subjects, such as electromagnetism or special relativity, is far less fundamental. Quantum is the only physical theory where these numbers seem to play an indispensible role, as the theory is explicitly formulated in terms of operators acting on complex Hilbert spaces. The occurrence of complex numbers within the quantum formalism has nonetheless puzzled countless physicists, including the fathers of the theory, for whom a version of quantum physics, where states and observables are represented by operators, seemed much more natural. In fact, previous works showed that such real quantum physics can reproduce the outcomes of any multipartite experiment, as long as the parts share arbitrary quantum states. Thus, are complex numbers really needed for a quantum description of nature? Here, we show this to be case by proving that and complex quantum make different predictions in network scenarios comprising independent quantum state sources. This allows us to devise a Bell-type quantum experiment whose input-output correlations cannot be approximated by any quantum model. The successful realization of such an experiment would disprove quantum physics, in the same way as standard Bell experiments disproved local physics.

Parametric interaction between two single photons

We report the experimental demonstration of an interaction between two independent single photons. The interacting photons are generated via parametric downconversion in distinct sources. An efficient waveguide is employed to realise the first single photon demonstration of sum frequency generation.

“Plug and Play” Quantum Cryptography

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text B. Huttner, A. Muller, G. Ribordy, W. Tittel, H. Zbinden, and N. Gisin, "“Plug and Play” Quantum Cryptography," Optics & Photonics News 8(12), 38-39 (1997) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Heralded Single-Phonon Preparation, Storage, and Readout in Cavity Optomechanics

We show how to use the radiation pressure optomechanical coupling between a mechanical oscillator and an optical cavity field to generate in a heralded way a single quantum of mechanical motion (a Fock state). Starting with the oscillator close to its ground state, a laser pumping the upper motional sideband produces correlated photon-phonon pairs via optomechanical parametric down-conversion. Subsequent detection of a single scattered Stokes photon projects the macroscopic oscillator into a single-phonon Fock state. The nonclassical nature of this mechanical state can be demonstrated by applying a readout laser on the lower sideband to map the phononic state to a photonic mode and performing an autocorrelation measurement. Our approach proves the relevance of cavity optomechanics as an enabling quantum technology.

SECOQC White Paper on Quantum Key Distribution and Cryptography

The SECOQC White Paper on Quantum Key Distribution and Cryptography is the outcome on a thorough consultation and discussion among the participants of the European project SECOQC (www.secoqc.net). This paper is a review article that attempts to position Quantum Key Distribution (QKD) in terms of cryptographic applications. A detailed comparison of QKD with the solutions currently in use to solve the key distribution problem, based on classical cryptography, is provided. We also detail how the work on QKD networks lead within SECOQC will allow the deployment of long-distance secure communication infrastructures based on quantum cryptography. The purpose of the White Paper is finally to promote closer collaboration between classical and quantum cryptographers. We believe that very fruitful research, involving both communities, could emerge in the future years and try to sketch what may be the next challenges in this direction.

Triangle Nonlocality : genuine quantum nonlocality and quantum Finner inequality

Definitions of multipartite nonlocality

In a multipartite setting, it is possible to distinguish quantum states that are genuinely $n$-way entangled from those that are separable with respect to some bipartition. Similarly, the nonlocal correlations that can arise from measurements on entangled states can be classified into those that are genuinely $n$-way nonlocal, and those that are local with respect to some bipartition. Svetlichny introduced an inequality intended as a test for genuine tripartite nonlocality. This work introduces two alternative definitions of $n$-way nonlocality, which we argue are better motivated both from the point of view of the study of nature, and from the point of view of quantum information theory. We show that these definitions are strictly weaker than Svetlichny's, and introduce a series of suitable Bell-type inequalities for the detection of three-way nonlocality. Numerical evidence suggests that all three-way entangled pure quantum states can produce three-way nonlocal correlations.

Experimental heralded amplification of time-bin qubits

We present an experimental realisation of heralded amplification of time-bin qubits for quantum communication, based on a source of separable photons at telecom wavelengths, with Hong-Ou-Mandel visibility > 90% without any filtering and high coupling efficiency.

Separation of quantum correlated particles by a heat bath

We consider the action of a heat bath on one particle of a pair of spin-$\frac{1}{2}$ particles in a singlet state. The related relaxation effects are described by the Pauli master equation. We show that the density matrix of the two-particle system tends toward a product. The interpretation of this result is, however, complicated by the nonuniqueness of the decomposition of a given density matrix into pure states. To circumvent this difficulty we study separately the terms describing the fluctuations and the friction. The first one does not modify the correlations, although it screens them but the second one separates the systems. The action of a fluctuating magnetic field on one particle of a correlated pair is also investigated.

Optical fiber characterization by simultaneous measurement of the transmitted and refracted near field

An experimental setup is presented which permits, in a routine way for R&D purposes, simultaneous measurement of the transmitted near field at 1300 nm and 1550 nm, and the refracted near field at 820 nm. A new method for the calibration of the refractive index is proposed. The obtained accuracy for the refractive index is dn+or-0.0002. The reproducibility of measurements of geometrical parameters like the mode field diameter, the core and cladding diameters, and concentricity error, is +or-0.1 mu m. Measurements of the mode field as a function of polarization state for four different hi-bi fiber are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Distributed-gain and optimum length of erbium-doped fibre amplifiers

The distributed-gain along erbium doped fibres was measured with coherent optical frequency-domain reflectometry. The optimum length for different amplifier specifications were measured and compared to the cutback technique.

Simulating Maximal Quantum Entanglement without Communication

It is known that all causal correlations between two parties which output each 1 bit, a and b, when receiving each 1 bit, x and y, can be expressed as convex combinations of local correlations (i.e., correlations that can be simulated with local random variables) and nonlocal correlations of the form a+b=xy mod 2. We show that a single instance of the latter elementary nonlocal correlation suffices to simulate exactly all possible projective measurements that can be performed on a maximally entangled state of two qubits, with no communication needed at all. This elementary nonlocal correlation thus defines some unit of nonlocality, which we call a nl bit.

Optical storage for 0.53 s in a solid-state atomic frequency comb memory using dynamical decoupling

Quantum memories with long storage times are key elements in long-distance quantum networks. The atomic frequency comb (AFC) memory in particular has shown great promise to fulfill this role, having demonstrated multimode capacity and spin-photon quantum correlations. However, the memory storage times have so-far been limited to about one millisecond, realized in a Eu${}^{3+}$ doped Y${}_2$SiO${}_5$ crystal at zero applied magnetic field. Motivated by studies showing increased spin coherence times under applied magnetic field, we developed a AFC spin-wave memory utilizing a weak 15 mT magnetic field in a specific direction that allows efficient optical and spin manipulation for AFC memory operations. With this field configuration the AFC spin-wave storage time increased to 40 ms using a simple spin-echo sequence. Furthermore, by applying dynamical decoupling techniques the spin-wave coherence time reaches 530 ms, a 300-fold increase with respect to previous AFC spin-wave storage experiments. This result paves the way towards long duration storage of quantum information in solid-state ensemble memories.

Which features of quantum physics are not fundamentally quantum but are due to indeterminism?

What is fundamentally quantum? We argue that most of the features, problems, and paradoxes -- such as the measurement problem, the Wigner's friend paradox and its proposed solutions, single particle nonlocality, and no-cloning -- allegedly attributed to quantum physics have a clear classical analogue if one is to interpret classical physics as fundamentally indeterministic. What really characterizes quantum physics boils down only to phenomena that involve $\hbar$, i.e., incompatible observables.

Quantum Trajectories for Brownian Motion

We present the stochastic Schr\"odinger equation for the dynamics of a quantum particle coupled to a high temperature environment and apply it to the dynamics of a driven, damped, nonlinear quantum oscillator. Apart from an initial slip on the environmental memory time scale, in the mean, our result recovers the solution of the known non-Lindblad quantum Brownian motion master equation. A remarkable feature of our powerful stochastic approach is its localization property: individual quantum trajectories remain localized wave packets for all times, even for the classically chaotic system considered here, the localization being stronger as $\ensuremath{\Elzxh}\ensuremath{\rightarrow}0$.