Publications

Characterizing the Nonlocal Correlations Created via Entanglement Swapping

Quantum systems that have never interacted can become nonlocally correlated through a process called entanglement swapping. To characterize nonlocality in this context, we introduce local models where quantum systems that are initially uncorrelated are described by uncorrelated local variables. This additional assumption leads to stronger tests of nonlocality. We show, in particular, that an entangled pair generated through entanglement swapping will already violate a Bell-type inequality for visibilities as low as 50% under our assumption.

OFDR measurements of insertion and return losses of integrated optic devices

The insertion loss of an integrated optic device is measured by means of the OFDR teclmique. Measurements of Rayleigh BackScattered intensity in the input and output fibers of the device allow direct detection of light power guided in the fiber before and after the component. The insertion and return losses of the component can thus be directly evaluated. Experimental results are in good agreement with measurements performed by the cut-back method.

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.

Atomic frequency comb memory with spin-wave storage in¹⁵³Eu^{3 +}:Y₂SiO₅

153Eu3 +:Y2SiO5 is a very attractive candidate for a long-lived, multimode quantum memory due to the long spin coherence time (∼15 ms), the relatively large hyperfine splitting (100 MHz) and the narrow optical homogeneous linewidth (∼100 Hz). Here we show an atomic frequency comb memory with spin-wave storage in a promising material 153Eu3 +:Y2SiO5, reaching storage times slightly beyond 10 μs. We analyse the efficiency of the storage process and discuss ways of improving it. We also measure the inhomogeneous spin linewidth of 153Eu3 +:Y2SiO5, which we find to be 69 ± 3 kHz. These results represent a further step towards realizing a long-lived, multimode solid-state quantum memory.

Experimental Tests of Nonlocality with Entangled Photons

We report on a variety of Bell tests performed with a high-quality photonic entanglement source. These tests begin to quantify nonlocal resources available in quantum mechanics, as well as place bounds on beyond-quantum theories.

Performance of InGaAs/InP avalanche photodiodes as gated-mode photon counters

We investigate the performance of separate absorption multiplication InGaAs/InP avalanche photodiodes as single-photon detectors for 1.3- and 1.55-mum wavelengths. First we study afterpulses and choose experimental conditions to limit this effect. Then we compare the InGaAs/InP detector with a germanium avalanche photodiode; the former shows a lower dark-count rate. The effect of operating temperature is studied for both wavelengths. At 173 K and with a dark-count probability per gate of 10(-4), detection efficiencies of 16% for 1.3 mum and 7% for 1.55 mum are obtained. Finally, a timing resolution of less than200 ps is demonstrated.

Repeaters for quantum communication

Results about quantum information teleportation and coherent transfer between photons at different wavelengths are reported. Recent progress and ongoing projects are also reviewed.

Comment on "no-signaling condition and quantum dynamics".

Quantum nonlocality: How does Nature perform the trick?(1)

Since our early childhood we know in our bones that in order to interact with an object we have either to go to it or to throw something at it. Yet, contrary to all our daily experience, Nature is nonlocal: there are spatially separated systems that exhibit nonlocal correlations. In recent years this led to new experiments, deeper understanding of the tension between quantum physics and relativity and to proposals for disruptive technologies. Consider two spatially separated quantum systems, one controlled by Alice, the other by Bob, in a pure state . Alice and Bob may perform some measurements x and y on their systems and collect the results a and b, respectively. This situation is described by a conditional probability distribution p (a,bj x,y). In general this correlation doesn’t factorize: p (a,bj x,y) 6 p (aj x) � p (bj y), i.e. the two systems are correlated. At first, this is no surprise, correlations are everywhere. For example, consider two cups of the same color, either both red or both green, one in Alice’s and one in Bob’s hands. If they looks at the color of their cups, Alice and Bob’s results are correlated. In this example the origin of the correlation is obvious, Alice and Bob had only partial information: they knew that both have the same color, but they ignored which color. This differs deeply from the quantum situation, as quantum theory claims that a pure state provides a complete description of the two systems. This led EPR[2] to believe that quantum theory is incomplete in the same sense as the description ”of the same color” provides only an incomplete description of the color state of the cups.

Quantum Communications with Optical Fibers

No abstract is provided for this article.

Experimental comparison between two different methods for measuring polarisation mode dispersion in singlemode fibres

Two polarisation mode dispersion measurement methods are experimentally compared. The first is based on theory of the principal state of polarisation and the second on the measurement of the autocorrelation function of the light at the output of the fibre. The results agree with each other, within experimental accuracy.

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.

Measurements of the nonlinear coefficient of standard, SMF, DSF, and DCF fibers using a self-aligned interferometer and a Faraday mirror

Using a method based on the detection of the Kerr phase shift by a self-aligned interferometer, we present measurements of the nonlinear coefficient n/sub 2//A/sub eff/ for standard single-mode fiber (SMF), dispersion-shifted fibers, and dispersion compensating fibers. The presence of a Faraday mirror in the interferometer makes the setup very robust, and different test fibers can be measured without any further readjustments. Interlaboratory comparisons show that the values found with our method are in good agreement with the other ones. Further, analysis of a SMF fiber with large chromatic dispersion shows a good reproducibility of the n/sub 2//A/sub eff/ measurements as a function of fiber length.

Two-mode squeezed states as Schrödinger cat-like states

In recent years, there has been an increased interest in the generation of superposition of coherent states with opposite phases, the so-called photonic Schrödinger-cat states.These experiments are very challenging and so far, cats involving small photon numbers only have been implemented.Here, we propose to consider two-mode squeezed states as examples of a Schrödinger-cat-like state.In particular, we are interested in several criteria aiming to identify quantum states that are macroscopic superpositions in a more general sense.We show how these criteria can be extended to continuous variable entangled states.We apply them to various squeezed states, argue that two-mode squeezed vacuum states belong to a class of general Schrödinger-cat states and compare the size of states obtained in several experiments.Our results not only promote two-mode squeezed states for exploring quantum effects at the macroscopic level but also provide direct measures to evaluate their usefulness for quantum metrology.

Towards Quantum Experiments with Human Eyes as Detectors Based on Cloning via Stimulated Emission

We show theoretically that a large Bell inequality violation can be obtained with human eyes as detectors, in a ``micro-macro'' experiment where one photon from an entangled pair is greatly amplified via stimulated emission. The violation is robust under photon loss. This leads to an apparent paradox, which we resolve by noting that the violation proves the existence of entanglement before the amplification. The same is true for the micro-macro experiments performed so far with conventional detectors. However, we also prove that there is genuine micro-macro entanglement even for high loss.

Time Really Passes, Science Can’t Deny That

No abstract is provided for this article.

POTDR, depolarization and detection of sections with large PMD

No abstract is provided for this article.

No-Go Theorem for Generic Simulation of Qubit Channels with Finite Classical Resources

The mathematical framework of quantum theory, though fundamentally distinct from classical physics, raises the question of whether quantum processes can be efficiently simulated using classical resources. For instance, a sender (Alice) possessing the classical description of a qubit state can simulate the action of a qubit channel through finite classical communication with a receiver (Bob), enabling Bob to reproduce measurement statistics for any observable on the state. Here, we contend that a more general simulation requires reproducing statistics of joint measurements, potentially involving entangled effects, on Alice's system and an additional system held by Bob, even when Bob's system state is unknown or entangled with a larger system. We establish a no-go result, demonstrating that such a general simulation for the perfect qubit channel is impossible with finite classical communication. Furthermore, we show that entangled effects render classical simulation significantly more challenging compared to unentangled effects. On the other hand, for noisy qubit channels, such as those with depolarizing noise, we demonstrate that general simulation is achievable with finite communication. Notably, the required communication increases as the noise decreases, revealing an intricate relationship between the noise in the channel and the resources necessary for its classical simulation.