The objectives of the project were the realization of a large optical depth cold atomic ensemble and the demonstration of different multiplexed quantum memory experiments based on this system.
High-OD cold atomic ensemble.
The first experimental key step was the realization of a high-OD cold atomic ensemble. The setup is based on an elongated 2D magneto-optical trap of cesium atoms. The obtained cigar-shaped ensemble has a length of 2.5 cm, and the optical depth is increased by linearly ramping up the magnetic field gradient to radially compress the ensemble. Finally, a large optical depth of 300 has been obtained. Cancellation of residual magnetic field at the few millGauss level has also been achieved.
Highly-efficient quantum memory.
Based on this high-OD cold atomic ensemble, we have demonstrated the quantum storage of polarization qubits with a record efficiency of 70%, while maintaining a fidelity with the initial quantum bit beyond 99%, thereby demonstrating for the first time a reversible qubit mapping where more information is retrieved than lost. The qubits are encoded with weak coherent states at the single-photon level and the memory is based on electromagnetically-induced transparency in the elongated ensemble that has been spatially multiplexed for dual-rail storage. This implementation preserves high optical depth on both rails, without compromise between multiplexing and storage efficiency. The results have been published in Nature Communications 9 (1), 363 (2018).
Multiplexer system for memory operation.
An optical multiplexer /demultiplexer system was setup to generate a large number of orthogonal LP modes which share the same path at the output in free space. This system has been fully aligned and calibrated. It will be used to investigate the scalability of quantum network protocols taking benefit from spatial multiplexing.
Dissemination.
The different results obtained during the fellowship have been presented in various international conferences in the field of atomic physics, quantum physics and quantum information, such as:
• The 4th Quantum Engineering, from Fundamental Aspects to Application (Nice, France)
• The Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2017, Paris, France)
• The 4th International Conference on optical orbital angular momentum (ICOAM17, Anacapri, Italy)
• The 50th Anniversary European Group on Atomic Systems (EGAS 2018, Kraków, Poland).
• The 26th International Conference on Atomic Physics (ICAP 2018, Barcelona, Spain).
