Spectral-hole memory for light at the single-photon level

Posted 12 July, 2016


The activity of the Quantum Photonics with Solids and Atoms group at ICFO is devoted to the study of the interaction between light and matter at the level of single quanta. One of the systems exploited are cryogenically cooled rare earth doped crystals, because they provide a unique system in which a large number of ions, with excellent optical and spin coherence properties, are trapped in an ordered transparent matrix. Using these crystals, the group is exploring various protocols in order to generate, store, and on-demand retrieve quantum states of light. In particular the group recently demonstrated, in the framework of an international collaboration involving researchers from Laborariore Aimé Cotton in Paris and Laboratorio de Fotónica y Optoelectrónica in Bariloche, a new storage scheme, based on the preparation of a narrow spectral hole in a Pr3+: Y2SiO5 crystal. An input pulse is slowed down by the spectral hole and, subsequently, a strong and short Raman pulse is used to transfer the optical excitation to an empty spin-state, effectively stopping the light. After a controllable delay, a second Raman pulse is sent to retrieve the stored light.


In order to exploit the good coherence properties of the optically active ions, the sample has to be cooled down to temperatures below 4K. The possibility to see the samples from different angle is important in the first-stage alignment of this experiment because the laser beam used to prepare the memory via optical pumping and to operate the Raman transfer to the spin-storage state has to be spatially separated by the input pulses, in order to suppress the noise in the detection of the retrieved pulses. The two beams only overlap at the input facet of the memory crystal. To further suppress the noise from the Raman pulses, we implement a spectral filter with a second Pr3+: Y2SiO5 crystal, which also has to be hosted in the same Cryostation. When working at the single photon level, the measurements can extend over a long time. Also, it is crucial that the input pulses reach the memory crystal in a region homogeneously prepared via optical pumping. In this respect the mechanical stability of the Cryostation was fundamental to always count on optimized and stable beam overlap. To ensure the system is not affected by the small vibrations left, the experiment was also synchronized to the Cryostation cycle.


The group has demonstrated a new light storage protocol based on stopped light in a spectral hole in a Pr3+: Y2SiO5 crystal achieving a storage and retrieval efficiency of up to 39%. Thanks to a low unconditional noise floor, the stored and retrieved single-photon-level pulses were detected with high signal-to-noise ratio. This demonstrates that the memory can work in the quantum regime, with the highest efficiency so far obtained for spin-wave solid state optical memories.


These results are promising for the realization of robust, highly efficient and long-lived spin-wave solid state quantum memories. In this experiment, single mode weak coherent pulses were stored and retrieved. When extended to the storage of true single photons, the protocol could be exploited to demonstrate entanglement between remote crystals.


Spectral-hole memory for light at the single-photon level http://dx.doi.org/10.1103/PhysRevA.93.040302
Article and photos courtesy of Kutlu Kutluer, Prof. Hugues de Riedmatten, and Dr. Margherita Mazzera of the Institute of Photonic Sciences

This work was performed using a Montana Instruments Cryostation. This article should not be considered an endorsement of any product.