Publications

Measurement of nonlinear polarization rotation in a highly birefringent optical fibre using a Faraday mirror

No abstract is provided for this article.

A solid-state light–matter interface at the single-photon level

No abstract is provided for this article.

Device-Independent Quantum Key Distribution with Local Bell Test

Existing protocols for device-independent quantum key distribution solve the problem of inadvertently flawed devices, but still rely on two-party Bell tests across almost lossless quantum channels that are hard to realize over long distances. Now theorists propose a new device-independent protocol that allows Bell testing to be carried out by only one party, potentially enabling long-distance key distributions.

Two-mode squeezed state as Schrodinger-cat-like state

Viewpoint: 0 + 0 > 0

No abstract is provided for this article.

Bilocal versus nonbilocal correlations in entanglement-swapping experiments

Entanglement swapping is a process by which two initially independent quantum systems can become entangled and generate nonlocal correlations. To characterize such correlations, we compare them to those predicted by bilocal models, where systems that are initially independent are described by uncorrelated states. We extend in this paper the analysis of bilocal correlations initiated in [Phys. Rev. Lett. 104, 170401 (2010)]. In particular, we derive new Bell-type inequalities based on the bilocality assumption in different scenarios, we study their possible quantum violations, and analyze their resistance to experimental imperfections. The bilocality assumption, being stronger than Bell's standard local causality assumption, lowers the requirements for the demonstration of quantumness in entanglement swapping experiments.

Towards a Quantum Memory for Photons in Erbium Doped Materials

Violation of Bell's Inequality using interferometers physically separated by 35 meters

An experimental demonstration of quantum correlations is presented. Energy and time entangled photons at wavelengths of 704 and 1310 nm are produced by parametric downconversion in KNbO3 and are sent through optical fibers into a bulk-optical (704 nm) and an all-fiber Michelson-interferometer (1310 nm), respectively. The two interferometers are located 35 meters aside from one another. Using Faraday-mirrors in the fiber-interferometer, all birefringence effects in the fibers are automatically compensated. We obtained two-photon fringe visibilities of up to 95 % from which one can project a violation of Bell's inequality by 8 standard deviations. The good performance and the auto-aligning feature of Faraday-mirror interferometers show their potential for a future test of Bell's inequalities in order to examine quantum-correlations over long distances.

Avoided crossing is insufficient to witness large-scale quantum coherence in flux qubits

Do experiments based on superconducting loops segmented with Josephson junctions (e.g., flux qubits) show macroscopic quantum behavior in the sense of Schr\odinger's cat example? Various arguments based on microscopic and phenomenological models were recently adduced in this debate. We approach this problem by adapting --to flux qubits-- the framework of large-scale quantum coherence, which was already successfully applied to spin ensembles and photonic systems. We show that contemporary experiments might show quantum coherence more than one hundred times larger than experiments in the classical regime. However, we argue that the often used demonstration of an avoided crossing in the energy spectrum is not sufficient to conclude about the presence of large-scale quantum coherence. Alternative, rigorous witnesses are proposed.

Quantum cryptography over 23 km in installed under-lake telecom fibre

A quantum cryptographic channel using a 23 km long installed standard telecom optical cable is reported. The key is encoded in the polarization of very weak laser pulses of average photon number 0.12. The measured error rate is lower than 3.4%. The ability of the system to establish a quantum key at standard telecom wavelength (1300 nm) using installed cables is thus assessed.

Demonstrating the power of quantum computers, certification of highly entangled measurements and scalable quantum nonlocality

Increasingly sophisticated quantum computers motivate the exploration of their abilities in certifying genuine quantum phenomena. Here, we demonstrate the power of state-of-the-art IBM quantum computers in correlation experiments inspired by quantum networks. Our experiments feature up to 12 qubits and require the implementation of paradigmatic Bell-State Measurements for scalable entanglement-swapping. First, we demonstrate quantum correlations that defy classical models in up to nine-qubit systems while only assuming that the quantum computer operates on qubits. Harvesting these quantum advantages, we are able to certify 82 basis elements as entangled in a 512-outcome measurement. Then, we relax the qubit assumption and consider quantum nonlocality in a scenario with multiple independent entangled states arranged in a star configuration. We report quantum violations of source-independent Bell inequalities for up to ten qubits. Our results demonstrate the ability of quantum computers to outperform classical limitations and certify scalable entangled measurements.

On the simulation of two-qubit states with non-local resources

No abstract is provided for this article.

Photon-counting optical-fiber sensor for the detection of ammonia in neurochemical applications

We report our progress in developing a miniaturized version of an optical-fiber sensor for the measurement of ammonia. The principle of the measurement is based on the combination of photon counting and fluorescence from a pH indicator. This sensor has been developed in order to monitor the release of ammonia by photoreceptors of the honeybee drone retina under stimulation by light or by a chemical. It has a dynamic of 25 dB and a detection limit of 30 nM. Sensors for other chemicals could be constructed on the same principle, by simply changing the indicator used.

Reply to comment of “Hidden quantum nonlocality revealed by local filters” by K. Berndl and S. Teufel

The logic of Bell's theorem is summarized.

Why Bohmian Mechanics? One- and Two-Time Position Measurements, Bell Inequalities, Philosophy, and Physics

In Bohmian mechanics particles follow continuous trajectories, hence 2-time position correlations are well defined. Nevertheless, Bohmian mechanics predicts the violation of Bell inequalities. Motivated by this fact we investigate position measurements in Bohmian mechanics by coupling the particles to macroscopic pointers. This explains the violation of Bell inequalities despite 2-time position correlations. We relate this fact to so-called surrealistic trajectories that, in our model, correspond to slowly moving pointers. Next, we emphasize that the nice feature of Bohmian mechanics, which doesn't distinguish microscopic and macroscopic systems, implies that the quantum weirdness of quantum physics also shows up at the macro-scale. Finally, we discuss the fact that Bohmian mechanics is attractive to philosophers, but not so much to physicists and argue that the Bohmian community is responsible for the latter.

Quantum cryptography

We live in a quantum world – something that physicists have considered with amazement for more than seventy years. But we now realize that quantum physics is more than a radical departure from classical physics, It also offers many new possibilities for information processing.

Photon-Number-Splitting versus Cloning Attacks in Practical Quantum Cryptography

In practical quantum cryptography, the source sometimes produces multi-photon pulses, thus enabling the eavesdropper Eve to perform the powerful photon-number-splitting (PNS) attack. Recently, it was shown by Curty and Lutkenhaus [Phys. Rev. A 69, 042321 (2004)] that the PNS attack is not always the optimal attack when two photons are present: if errors are present in the correlations Alice-Bob and if Eve cannot modify Bob's detection efficiency, Eve gains a larger amount of information using another attack based on a 2->3 cloning machine. In this work, we extend this analysis to all distances Alice-Bob. We identify a new incoherent 2->3 cloning attack which performs better than those described before. Using it, we confirm that, in the presence of errors, Eve's better strategy uses 2->3 cloning attacks instead of the PNS. However, this improvement is very small for the implementations of the Bennett-Brassard 1984 (BB84) protocol. Thus, the existence of these new attacks is conceptually interesting but basically does not change the value of the security parameters of BB84. The main results are valid both for Poissonian and sub-Poissonian sources.

Photon-number-splitting versus cloning attacks in practical implementations of the Bennett-Brassard 1984 protocol for quantum cryptography

In practical quantum cryptography, the source sometimes produces multiphoton pulses, thus enabling the eavesdropper Eve to perform the powerful photon-number-splitting (PNS) attack. Recently, it was shown by Curty and L\"utkenhaus [Phys. Rev. A 69, 042321 (2004)] that the PNS attack is not always the optimal attack when two photons are present: if errors are present in the correlations Alice-Bob and if Eve cannot modify Bob's detection efficiency, Eve gains a larger amount of information using another attack based on a $2\ensuremath{\rightarrow}3$ cloning machine. In this work, we extend this analysis to all distances Alice-Bob. We identify a new incoherent $2\ensuremath{\rightarrow}3$ cloning attack which performs better than those described before. Using it, we confirm that, in the presence of errors, Eve's better strategy uses $2\ensuremath{\rightarrow}3$ cloning attacks instead of the PNS. However, this improvement is very small for the implementations of the Bennett-Brassard 1984 (BB84) protocol. Thus, the existence of these new attacks is conceptually interesting but basically does not change the value of the security parameters of BB84. The main results are valid both for Poissonian and sub-Poissonian sources.