ICFO boosts performances of fiber-integrated quantum memories

11 Jul 2022

Team demonstrates entanglement between quantum memory and telecoms-wavelength photons.

Researchers from ICFO (Spain), IFN-CNR (Italy), and Heriot-Watt University (UK), have demonstrated entanglement between a fiber-integrated quantum memory and a telecommunications-wavelength photon.

The work is described in Science Advances.

ICFO says that quantum memories are one of the building blocks of the future quantum internet: “Without them, it would be rather impossible to transmit quantum information over long distances and expand into a real quantum network,” stated the institute.

Such memories have the function of receiving the quantum information encoded in a photon in the form of qubits, storing it and then retrieving it. Quantum memories can be realized in different material systems, for example ensembles of cold atoms or doped crystals.

In order to be useful, the memories must fulfil several requirements, such as the efficiency, duration, and multiplexing of their storage capability, to ensure the quality of the quantum communications that they will support. One other requirement that has become a matter of considerable research is designing quantum memories that can be directly integrated in a fiber-optic network.

Such a fully integrated approach brings several physical and engineering challenges, including finding a solution that preserves good coherence properties, providing an efficient and stable system to transfer photons from optical fibers to the quantum memory, as well as the miniaturisation of the control system and its interface with incoming light.

With these objectives clear, ICFO researchers Jelena Rakonjac, Dario Lago-Rivera, Alessandro Seri and Samuele Grandi, led by ICREA Prof. at ICFO Hugues de Riedmatten, in collaboration with Giacomo Corrielli and Roberto Osellame from IFN-CNR and Margherita Mazzera from Heriot-Watt, have been able to demonstrate entanglement between a fiber-integrated quantum memory and a telecommunications-wavelength photon.

Praseodymium-doped

In their experiment, the team used a crystal doped with praseodymium as their quantum memory. A waveguide was then laser-written inside the memory. This is a micrometer-scale canal within the crystal which confines and guides the photon in a tight space.

Two identical optical fibers were attached to both sides of the crystal to provide a direct interface between photons carrying quantum information and the memory. This experimental setup enabled an all-fiber connection between the quantum memory and a source of photons.

With this novel setup, they were able to store photons from 2 µs up to 28 µs and preserve the entanglement of the photon pairs after storage. The result obtained is of significant improvement since the entanglement storage time shown by the team is 1000 longer (three orders of magnitude) than any other previous fiber-integrated device used until now, and approaching the performances observed in bulk quantum memories.

Researcher Jelena Rakonjac commented, “This experiment has given us great hopes in the sense that we envision that many waveguides can be fabricated in one crystal, which would allow for many photons to be stored simultaneously in a small region and maximize the capability features of the quantum memory.”

Fellow researcher Hugues de Riedmatten added, “We are thrilled with this result which opens many possibilities for fibre integrated memories. What is clear is that this particular material and way of creating waveguides allows us to achieve performances close to bulk memories. In the future, extending the storage to spin states will allow on-demand retrieval of the stored photons and lead to the long storage times that we have been aiming for. This fibre-integrated quantum memory definitely shows great promise for future use in quantum networks”.