Publications

Performance of stochastic clocks in the Alternate Ticks Game

In standard quantum theory, time is not an observable. It enters as a parameter in the Schrödinger equation, but there is no measurement operator associated to it. Nevertheless, one may take an operational viewpoint and regard time as the information one can read from clocks. The Alternate Ticks Game, introduced in arXiv:1506.01373, is a completely operational means to quantify the accuracy of time scales generated by clocks. The idea is to count the number of ticks that two copies of a clock can produce until they run out of synchronisation. Here we investigate the performance of stochastic clocks in this game. These are clocks which are classical in the sense that they do not exploit quantum coherence. Our results support earlier conjectures that their accuracy grows linearly in the size of the clockwork, measured in terms of the dimension of the associated Hilbert space. In particular, we derive explicit bounds on the accuracy of a natural class of stochastic clocks, the stochastic ladder clocks.

Advisory Board

No abstract is provided for this article.

Advances in Quantum Cryptography and Relays

Advances in quantum cryptography extend from commercial demonstrations and quantum random number generators to improved detectors, new protocols and architectures, and even to quantum teleportation and quantum memories. We discuss our recent results in this field.

Quantum communications with optical fibers

Towards efficient quantum memory with very long storage time

No abstract is provided for this article.

Local and Nonlocal Correlations

No abstract is provided for this article.

Simulating Quantum Systems Using Real Hilbert Spaces

We develop a means of simulating the evolution and measurement of a multipartite quantum state under discrete or continuous evolution using another quantum system with states and operators lying in a real Hilbert space. This extends previous results which were unable to simulate local evolution and measurements with local operators and was limited to discrete evolution. We also detail applications to Bell inequalities and self-testing of the quantum apparatus.

Non-local two-photon correlations 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.

Trojan-horse attacks on quantum-key-distribution systems

General Trojan horse attacks on quantum key distribution systems are analyzed. We illustrate the power of such attacks with today's technology and conclude that all system must implement active counter-measures. In particular all systems must include an auxiliary detector that monitors any incoming light. We show that such counter-measures can be efficient, provided enough additional privacy amplification is applied to the data. We present a practical way to reduce the maximal information gain that an adversary can gain using Trojan horse attacks. This does reduce the security analysis of the 2-way {\it Plug-&-Play} system to those of the standard 1-way systems.

Interferometry with Faraday mirrors for quantum cryptography

Quantum Cryptography over 23km of installed Telecom fiber using a novel interferometer with Faraday mirrors is presented. The interferometer needs no alignment nor polarization control and features 99.8% fringe visibility. A secret key of 20kbit length with a error rate of 1.35% for 0.1 photon per pulse was produced.

Interferometry for polarization mode dispersion measurements in single fibres

We present two interferometric techniques for the measurements of the polarization mode dispersion of standard (i.e. circular) single-mode fibers. The first method uses an interferometric loop and works in the frequency domain. The second uses a polarization maintaining Michelson interferometer and works in the time domain. Their output are related by a Fourier transformation. Both techniques provide information about the entire statistical distribution of the polarization mode delays. Also, both techniques can be applied to the characterization of installed lines. The setups are presented, the principle of the measurements are discussed and examples of results are shown.

Optical tests of quantum nonlocality: from EPR‐Bell tests towards experiments with moving observers

Past, present and future experimental tests of quantum nonlocality are discussed. Consequences of assuming that the state‐vector collapse is a real physical phenomenon in space‐time are developed. These lead to experiments feasible with today's technology.

Partial list of bipartite Bell inequalities with four binary settings

We give a partial list of 26 tight Bell inequalities for the case where Alice and Bob choose among four two-outcome measurements. All tight Bell inequalities with less settings are reviewed as well. For each inequality we compute numerically the maximal quantum violation, the resistance to noise and the minimal detection efficiency required for closing the detection loophole. Surprisingly, most of these inequalities are outperformed by the CHSH inequality.

Stochastic wave equations versus parallel world components

In response to a recent Letter of Zeh, we present the case for adding stochastic terms to the Schrödinger equation, rather than introducing different observers in different world-components. The similarities and the contrasts between these alternative approaches are presented, with emphasis on recent results obtained with the first approach.

Quantum Measurements and Non-locality

No abstract is provided for this article.

Photonic entangled states at telecom wavelengths

Summary form only given. Time-bins provide a practical mean to play with arbitrary dimension Hilbert spaces using photons in standard telecom fibers. Examples encompass: the simple case of quantum cryptography using qubits, entangled qubit pairs in any freely chosen state, entangled qutrit pairs, two entangled systems each of dimension chosen freely between 1 and more than 10, and the more complex scenario involving more than two photons, like quantum teleportation and entanglement swapping. After an introduction to time-bins as a basis, each of the above examples will be introduced, explained and illustrated with experimental results.

SiPM for fast Photon-Counting and Multiphoton Detection

We demonstrate fast counting and multiphoton detection abilities of a Silicon Photo Multiplier (SiPM). In fast counting mode we are able to detect two consecutive photons separated by only 2.3 ns corresponding to 430 MHz. The counting efficiency for small optical intensities at lambda= 532 nm was found to be around 16% with a dark count rate of 52 kHz at T= -5 masculine C. Using the SiPM in multiphoton detection mode, we find a good signal discrimination for different numbers of simultaneously detected photons.

Book review