Publications

Quantum Memories. A Review based on the European Integrated Project "Qubit Applications (QAP)"

We perform a review of various approaches to the implementation of quantum memories, with an emphasis on activities within the quantum memory sub-project of the EU Integrated Project Qubit Applications. We begin with a brief overview over different applications for quantum memories and different types of quantum memories. We discuss the most important criteria for assessing quantum memory performance and the most important physical requirements. Then we review the different approaches represented in Qubit Applications in some detail. They include solid-state atomic ensembles, NV centers, quantum dots, single atoms, atomic gases and optical phonons in diamond. We compare the different approaches using the discussed criteria.

Loss of EPR correlations leads to signaling

We consider alternative models to quantum mechanics, that have been proposed in the recent years in order to explain the EPR correlations between two particles. Moving to the case of three particles, we show that these models violate the no-signaling condition. This shows that superluminal hidden communication (or influences) alone cannot explain the correlations between entangled quantum particles.

Mapping multiple photonic qubits into and out of one solid-state atomic ensemble

No abstract is provided for this article.

Refractive index profile and geometry measurements in multicore fibres

No abstract is provided for this article.

Retinal and post-retinal contributions to the quantum efficiency of the human eye

The retina is one of the best known quantum detectors with rods able to respond to a single photon. However, estimates on the number of photons eliciting conscious perception, based on signal detection theory, are systematically above these values. One possibility is that post-retinal processing significantly contributes to the decrease in the quantum efficiency determined by signal detection. We carried out experiments in humans using controlled sources of light while recording EEG and reaction times. Half of the participants behaved as noisy detectors reporting perception in trials where no light was sent. DN subjects were significantly faster to take decisions. Reaction times significantly increased with the decrease in the number of photons. This trend was reflected in the latency and onset of the EEG responses over frontal and parietal contacts where the first significant differences in latency comparable to differences in reaction time appeared. Delays in latency of neural responses across intensities were observed later over visual areas suggesting that they are due to the time required to reach the decision threshold in decision areas rather than to longer integration times at sensory areas. Our results suggest that post-retinal processing significantly contribute to increase detection noise and thresholds, decreasing the efficiency of the retina brain detector system.

Correlations of entangled quantum states cannot be classically simulated

Simulation tasks are insightful tools to compare information-theoretic resources. Considering a generalization of usual Bell scenarios where external quantum inputs are provided to the parties, we show that any entangled quantum state exhibits correlations that cannot be simulated using only shared randomness and classical communication, even when the amount and rounds of classical communication involved are unrestricted. We indeed construct explicit Bell-like inequalities that are necessarily satisfied by such classical resources but nevertheless violated by correlations obtainable from entangled quantum states, when measured a single copy at a time.

Polarization mode dispersion measurements on long buried cables

This paper describes the polarization mode dispersion (PMD) measurements of long standard fibers, dispersion-shifted (DS-) and dispersion-flattened (DF-) fibers uncabled on bobbins and in cables installed in the test links Berlin IV (18 km long) and Berlin VI (22.6 km). The PMD is measured with the aid of the interferometric technique, using a Michelson-interferometer. This method is not restricted to long fibers with low birefringence, it can be used to measuring the PMD and group beatlength of short high-birefringent fibers with a high accuracy. It will be shown that the cabling procedure increases the PMD values considerably, up to one order of magnitude.

Quantum Cryptography Protocols Robust against Photon Number Splitting Attacks for Weak Laser Pulse Implementations

We introduce a new class of quantum key distribution protocols, tailored to be robust against photon number splitting (PNS) attacks. We study one of these protocols, which differs from the original protocol by Bennett and Brassard (BB84) only in the classical sifting procedure. This protocol is provably better than BB84 against PNS attacks at zero error.

Towards Quantum Memory

Exploring the Limits of Quantum Nonlocality with Entangled Photons

Nonlocality is arguably among the most counterintuitive phenomena predicted by quantum theory. In recent years, the development of an abstract theory of nonlocality has brought a much deeper understanding of the subject, revealing a rich and complex phenomenon. In the current work, we present a systematic experimental exploration of the limits of quantum nonlocality. Using a versatile and high-fidelity source of pairs of polarization-entangled photons, we explore the boundary of quantum correlations, demonstrate the counterintuitive effect of more nonlocality with less entanglement, present the most nonlocal correlations ever reported, and achieve quantum correlations requiring the use of complex qubits. All of our results are in remarkable agreement with quantum predictions, and thus represent a thorough test of quantum theory. Pursuing such an approach is nevertheless highly desirable, as any deviation may provide evidence of new physics beyond the quantum model.Received 16 July 2015DOI:https://doi.org/10.1103/PhysRevX.5.041052This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical Society

Direct Measurement of Superluminal Group Velocity and Signal Velocity in an Optical Fiber

We present an easy way of observing superluminal group velocities using a birefringent optical fiber and other standard devices. In the theoretical analysis, we show that the optical properties of the setup can be described using the notion of "weak value." The experiment shows that the group velocity can indeed exceed c in the fiber; and we report the first direct observation of the so-called "signal velocity," the speed at which information propagates and that cannot exceed c.

Is realism compatible with true randomness ?

It is argued that realism and true randomness are fully compatible. Realistic true random events are acts of pure creation that obey strict laws, but do not necessarily satisfy Kolmogorov's axioms of probabilities. Realistic true randomness is some sort of nondeterministic force, or propensity of physical systems to manifest such and such properties under such and such conditions. Realistic random events reflect preexisting properties, as required by realism, simply the reflection is not deterministic; still, the preexisting properties determine the propensities of the different possible events. It is argued that deterministic extensions of quantum physics are necessarily incompatible with special relativity. Hence, from today's violations of Bell's inequalities one can conclude that all future physics theories will display true randomness as does quantum physics. It is argued that accepting true randomness and realism leads to new questions with interesting answers, allowing one 1) to study nonlocality in configurations with many independent sources and 2) to bound how much free will is needed for a proper violation of Bell's inequality.

Electromagnetic field confined and tailored with a few air holes in a photonic-crystal fiber

No abstract is provided for this article.

The Multiqubit Elegant Joint Measurement

The Elegant Joint Measurement (EJM) is a highly symmetric, partially entangled two-qubit measurement whose local marginals form a regular tetrahedron on the Bloch sphere and which has a low entanglement cost for local implementation. It plays a central role in quantum networks exhibiting nonclassical correlations and serves as a paradigmatic example of an entangled measurement with local structure. Despite its significance, generalizing the EJM beyond two qubits has remained unresolved. Here, we extend the EJM to the multipartite setting by identifying all tetrahedrally symmetric, efficiently localizable multiqubit bases. For two qubits, these criteria uniquely select the EJM. For three or more, they yield a discrete set of equivalence classes, reflecting the richer structure of multiparticle entanglement.

Fidelity of an Optical Memory Based on Stimulated Photon Echoes

We investigated the preservation of information encoded into the relative phase and amplitudes of optical pulses during storage and retrieval in an optical memory based on stimulated photon echo. By interfering photon echoes produced in a single-mode $\mathrm{Ti}\mathrm{\text{:}}\mathrm{Er}\mathrm{\text{:}}{\mathrm{LiNbO}}_{3}$ waveguide, we found that decoherence in the medium translates only as loss and not as degradation of information. We measured a visibility for interfering echoes close to 100%. These results may have important implications for future long-distance quantum communication protocols.

New Additions to the Schrödinger Cat Family

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Interferometric polarization mode dispersion measurements with femtosecond sensitivity

The standard interferometric measurement technique for polarization mode dispersion (PMD) measures the cross correlation of the electromagnetic field at the output of the fiber or optical device under test when a broadband light source is used. The sensitivity (smallest measurable PMD delay) of this technique is limited by the coherence time of the light source. In this contribution, we critically analyze extensions of the interferometric technique to lower values and present a new extension which provides a sensitivity of a few femtoseconds.

Long range quantum communications