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Ensemble based advanced quantum light matter interfaces

Final Report Summary - QULIMA (Ensemble based advanced quantum light matter interfaces)

The main scientific objective of the QuLIMA project was to demonstrate ensemble based novel quantum light matter interfaces with enhanced storage capabilities and unprecedented performances. We studied two different physical systems with complementary properties: solid-state quantum memories implemented with rare-earth doped solids and cold atomic gases.

The first part of the project was devoted to solid-state quantum memories based on Praseodymium doped crystals, which are currently one of the best systems for classical light storage. We demonstrated new capabilities, including the quantum storage of photonic polarization qubits in a solid-state device and the storage of quantum light in this material, thanks to the development of a tailored ultra-narrowband quantum light source. An important experimental effort was devoted to the development of spin-wave storage with on demand readout. We demonstrated the first solid-state spin-wave quantum memory for time-bin qubits and the most efficient single-photon level solid-state spin wave optical memory to date. These efforts culminated with the demonstration of quantum correlations between a photon at telecommunication wavelengths and a multimode solid-state spin-wave quantum memory, as well as with demonstration of a solid-state source of non-classical photon pairs with embedded quantum memory. These results pave the way towards the demonstration of long-lived heralded entanglement between remote solid-state quantum memories. Beyond these results, we have proposed and demonstrated a novel platform for integrated quantum memories based on laser written waveguides.

In the second part of the project, we investigated quantum memories based on cold rubidium atoms, following the protocol of Duan, Lukin, Cirac and Zoller (DLCZ). Progress beyond the state of the art with this system went along three lines. First, we used the memory as a source of single photon with widely tunable waveshape. Second, we showed that these photons can be converted to telecom wavelengths by using an integrated quantum frequency conversion device, while preserving quantum properties. Third, we have combined a DLCZ building block with spin rephasing techniques, in order to enable time multiplexing. We have shown controlled dephasing and rephasing of a single spin wave, and have shown entanglement between a time-bin photonic qubit and a spin-wave. With the goal of progressing towards a quantum memory with embedded processing capabilities, we have investigated a light matter interface between photons and collective Rydberg excitations in cold atomic rubidium ensembles. We demonstrated the storage and retrieval of an externally generated correlated single photon onto a highly excited Rydberg level using electromagnetically induced transparency, an enabling step towards the demonstration of strong interactions between two single photon wavepackets. We also demonstrated storage enhanced non-linearity in the cold Rydberg ensemble, with input states containing on average tens of photons.

Finally, to progress towards heterogeneous quantum networks, we connected quantum memory systems of different kind. Central to this endeavour is the ability to convert coherently the frequency and waveform of single photons in order to connect quantum systems emitting and absorbing light at different frequencies. Using a cascaded a quantum frequency conversion interface and a photon at telecom wavelengths, we demonstrated the first photonic state transfer between disparate quantum memory systems, a cold atomic gas and a doped crystal.