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N°33 I quarterly I April 2014 Live from the Labs | 11 Packing More Quantum Information BY jason brown w Quantum communication relies on the laws of quantum physics to facilitate ultra-secure communications, a vital capability in a modern economy. Photons are among the particles used to carry information as quantum bits (qubits). Until recently, quantum communication consisted in encoding information in the polarization state of photons, which can be either vertical or horizontal. But researchers have recently managed to encode information into the orbital angular momentum (OAM) of photons, which in theory has an infinite number of states. This allows information to be transmitted at a much higher density, since each OAM state can be used as a separate channel in a communications system, thus theoretically offering an unlimited number of channels. Now, Julien Laurat and colleagues at the LKB 1 have shown that the information packed in the OAM of photons could be stored in a quantum memory, and retrieved later on demand.2 To store OAM qubits, Laurat and his team used a container of very cold gas of cesium atoms to record the quantum state of single photons. Practically, two light beams were sent into the cold cesium gas: an auxiliary beam, and a beam carrying qubits. When the auxiliary beam was switched off, the information stored in qubits was transferred to the cesium atoms. By sending an auxiliary beam once more into the gas, the cesium atoms re-emitted the stored data, thus reconstituting the initial photonic qubits. By using different light beams carrying specific quantum information, the researchers could demonstrate that the initial information had been preserved through the whole process. This type of memory would be an integral part of future quantum repeaters, which would make long-distance quantum communications possible. This study shows that the OAM of photons can be used in a quantum communication network. The researchers will now focus on increasing the number of bits carried by each photon to expand the information density and eventually, store numerous qubits in a single memory. 01. Laboratoire Kastler Brossel (Université Pierre et Marie Curie / CNRS / ENS / Collège de France). 02. A. Nicolas et al., “A quantum memory for orbital angular momentum photonic qubits,” Nature Photonics, 2014. 8: 234–8. Physics Contact information: LKB, Paris. Julien Laurat > Julien.laurat@upmc.fr The team is now investigating which physiological parameters best match these critical rhythms of clock gene expression. “Measuring genetic expression rhythms in humans is still quite complex. So we are looking for other, more accessible variables—like body temperature or cortisol secretion—that might reflect these genetic parameters in humans,” concludes Lévi. 01. F. Trinter et al., “Resonant Auger decay driving intermolecular Coulombic decay in molecular dimers,” Nature, 2014. 505: 664-6. doi:10.1038/ nature12927. 02. Goethe University of Frankfurt, Institute of Nuclear Physics (Germany). 03. Laboratoire de chimie physique matière et rayonnement (CNRS / UPMC). 04. Supported by the French National Research Agency and the European Research Area Networks in systems biology (ERASysBIO). 05. Rythmes biologiques et cancer (Inserm / Université Paris-Sud). 06. XM. Li et al., “A circadian clock transcription model for the personalization of cancer chronotherapy,” Cancer Res., 2013. 73: 7176-88. doi: 10.1158/0008-5472. CAN-13-1528. Contact information: LC PMR, Paris. Marc Simon > marc.simon@upmc.fr Goethe University, Frankfurt am Main. Florian Trinter > trinter@atom.uni-frankfurt.de Inserm, Paris. Francis Lévi > francis.levi@inserm.fr q Diagram representing the light beams sent through the cold cesium gas. Paris © Laboratoire Kastler Brossel © illustration : s. Kie hl for cnrs international maga zine


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