Embryonic Quantum Network at a telecom wavelength – e-QUANET

The breakthrough of quantum information turned the fundamental and conceptual property of entanglement into a resource for quantum communication protocols and quantum computational tasks. For instance, entanglement is now at the heart of successful point-to-point quantum cryptography links, but future heads towards more complex networking applications. In this context, telecom photons at 1550 nm represent the natural qubit carriers for distribution through optical fibers, whereas atoms and ions, whose interaction wavelengths are below 900 nm, are more appropriate for storage and computing tasks. Although light-matter interaction should allow switching from one type of carrier to the other, major difficulties remain because of significant mismatches in the operation wavelength and bandwidth. The realization of a quantum network made of interconnected nodes, where storage and computational tasks are possible, essentially depends on the coupling between these different types of qubits. This is currently considered as one of the main challenges quantum communication groups are facing. The e-QUANET consortium is gathering the Laboratoire de Physique de la Matière Condensée (LPMC) in Nice, the Laboratoire de Traitement et Communication de l'Information (LTCI) in Paris, and the Laboratoire Aimé Cotton (LAC) in Orsay. The main goal of our project is to unite complementary skills in order to build a pioneering embryonic quantum network operating at a telecom wavelength, in which generation, distribution, and storage of quantum information are possible. Our experimental scheme is based on interfacing a source of polarization entangled photons emitted at a telecom wavelength, to allow optical fiber transmission, with a narrow bandwidth quantum memory operating around 800 nm. The relevance of our scheme lies in the demonstration that entanglement created by the EPR-source is preserved after having experienced both the quantum interface and the memory storage and read out when connected altogether. Such an experiment has never been achieved so far and its success will rely on merging both fundamental and technological advances. After a 6-month period (WP1) of specifications optimization, the e-QUANET project will be based on four complementary and interdependent work-packages: WP2: The LTCI will study the realization of a polarization-entangled photon-pair source based on a bulk PPLN crystal. This source should feature outstanding performances: a very high brightness, i.e. a high coincidence rate (a few kHz) of highly entangled photon-pairs within a very narrow bandwidth (about 40 MHz), and a wavelength tunability around 1560 nm. WP3: In order to connect the photon-pair source and the quantum memory, the LPMC will design a wavelength conversion interface based on integrated optics onto a PPLN crystal. It will be relying on a ancillary classical pump to up-convert the qubit photons from the telecom wavelength to that of the memory of 790 nm. The figures of merit are a quantum fidelity and a conversion probability near unity. WP4: Based on its important experience, the LAC will investigate a storage protocol based on atomic frequency comb (AFC) applied to a thulium-doped crystal for building a solid-state quantum memory stage. WP5: The three components will be merged into a single setup for the final quantum networking experiment. The characterization of the 'embryonic quantum network' will consist in coincidence measurements using heralded photons and then in a Bell test using entangled photons. In both cases, a telecom photon is simply sent to Alice, and the other one to Bob after storage in the memory, and release. During the three years of this ambitious project, our consortium will merge worldwide recognized complementary skills in non-linear and quantum optics, rare-earth ion doped crystals and quantum communication. This is a key advantage for such a multidisciplinary project to succeed in a very competitive national and international context.