A reversible optical memory for twisted photons

Summary form only given. The orbital angular momentum (OAM) of light has raised a great deal of applications [1], ranging from the trapping of particles, the driving of optical micromachines to applications in astrophysics studies. In quantum optics, twisted single-photons have also been identified as promising carriers for the implementation of various quantum information protocols. They indeed offer the possibility to encode and process information in high-dimensional Hilbert spaces and could enable to reach higher efficiencies and enhanced information capacity. For further applications in quantum networks involving light-matter interfaces, a critical capability is the ability to reversibly map OAM into and out of atomic memories. While the storage of bright beams of light in Laguerre-Gaussian modes has been reported in the past few years [2, 3], we report here the first demonstration of storage in the single-photon regime [4]. The experimental realization proceeds in the following way. First, LG modes are generated using a spatial light modulator. These modes, which are weak coherent states at the single-photon level, are then mapped into an ensemble of cold cesium atoms in a magneto-optical trap using the dynamic electromagnetically-induced transparency (EIT) protocol. The signal quantum properties, including its OAM, are expected to be transferred to the atomic medium and can be stored for a programmable time, limited by the lifetime of the atomic coherence. After this reversible storage, unlike for bright beams, the detection of twisted single-photons cannot be performed by imaging on a CCD camera. Here we have chosen to work with a combination of a computergenerated hologram, which adds or subtracts one unity of OAM, and a single-mode fiber to implement a mode discriminator. In our experiment, the output mode is actually split on a 50/50 beam splitter and one discriminator is installed in each of the two output paths so that we can quantify both the LG+1 and- LG-1 components of the retrieved light. We stored and retrieved LG+1 and LG-1 modes. As can be seen in fig. 1, detection events in the detection path with l opposite to the one of the incoming mode are negligible, within the distinction ratio. For both modes, the overall storage and retrieval efficiency is (16 ± 2%). We thus demonstrated that such pulses carrying OAM can be stored in the single-photon regime into and out the ensemble with preserved handedness of the helical phase structure. Mapping of a TEM10 mode, i.e. an equal-weight superposition of LG+1 and LG-1 modes was also performed. Within our experimental precision, the achieved storage and retrieval efficiency is the same here each LG components than for the individual storage presented previously. Further works should include reconstruction of a full density matrix, including coherences not measured here, and the calculation of process fidelity. We implemented a sophisticated detection technique based on a Mach-Zehnder interferometer. Preliminary results will be reported. Extensions to higher alphabets for the realization of high-dimensional quantum information networks is also work for future.