ICFO researchers report simultaneously two studies that demonstrate the creation of quantum correlations between single photons and single spin waves in a crystal.Photonic quantum memories are essential resources in quantum information science, in particular for the development of quantum repeaters. Solid state devices based on rare-earth doped crystals present appealing properties for quantum storage of light, with long coherence times and prospects for integration. In this type of memory, photonic quantum information is stored as collective atomic excitations.
Up to now, most of the research in rare-earth based quantum memories focused on photon storage in collective optical excitations, leading to short and pre-determined storage time. Now, three experiments, two from ICFO and one from the University of Geneva, show that single photons can be stored as collective spin-excitations, or spin-waves, in a crystal, potentially enabling much longer storage times and on-demand read-out of the stored quantum information.
In a first paper, ICFO researchers Alessandro Seri, Dr Andreas Lenhard, and Dr Margherita Mazzera and former ICFOnians Dr Daniel Rieländer, Dr Mustafa Gündoğan and Dr Patrick Ledingham, from the group of Prof Hugues de Riedmatten, demonstrate a temporally multimode spin-wave quantum memory for single photons, based on praseodymium-doped crystals. Using an external quantum light source, they could generate a photon pair where one photon is at telecommunication wavelength, while the other is resonant with the quantum memory. This allowed the researchers to demonstrate quantum correlations between a photon a telecom wavelength, thus readily compatible with telecommunication networks, and a spin wave in a crystal, an important resource for quantum repeaters. This study appeared in PRX.
Using a different approach, ICFO researchers Kutlu Kutluer, Dr Margherita Mazzera and Prof Hugues de Riedmatten demonstrated in a similar crystal a photon pair source with an embedded multimode quantum memory. By exciting the crystal with a laser pulse, they could create a non-classically correlated state between a photon and a spin-wave, using a modified version of a technique originally developed for atomic gases by Duan, Lukin, Cirac and Zoller. This spin wave can then be converted into a single photon after a potentially long programmable time. This technique has the great advantage of combining a quantum light source and a quantum memory in the same solid-state device. The study appeared in PRL as Editor´s suggestion, together with a similar work using a different rare-earth ion by the group of Mikael Afzelius and Nicolas Gisin from the University of Geneva.
The results of these three experiments provide an important resource for quantum repeaters and pave the way for the implementation of quantum information networks with distant solid-state quantum nodes. The papers have been highlighted as a Viewpoint in Physics, written by J. Nunn.