A performance boost in fiber-integrated quantum memories

Researchers from ICFO, IFN-CNR and Heriot-Watt University report in Researchers are making progress the demonstration of the entanglement between a quantum memory integrated in the fiber and a photon of telecommunication wavelength.

Quantum memories are one of the building blocks of the quantum internet of the future. Without them, it would be quite impossible to transmit quantum information over long distances and evolve into a true quantum network. The mission of these memories is to receive quantum information encoded in a photon in the form of qubits, to store it and then retrieve it. Quantum memories can be realized in different material systems, for example sets of cold atoms or doped crystals.

To be useful memories, they must meet several requirements, such as efficiency, duration, and multiplexing of their storage capacity, to ensure the quality of quantum communication that they will support. Another requirement that is the subject of much research is the design of quantum memories that can be integrated directly into the fiber optic network.

In recent years and with the boom in quantum technologies, many works are aimed at improving the scalability of existing quantum memories (making them smaller and / or simpler units) to facilitate their integration and rollout in a real work network. Such a fully integrated approach comes with several physical and technical obstacles, including finding a solution that maintains good coherence properties, provides an efficient and stable system for transferring photons from optical fibers to quantum memory, and miniaturizes the quantum memory control system and its incoming light interface. All this must be achieved by achieving the same level of performance achieved in mass “standard” versions of the device. So far, this has proven to be difficult, and current results in fiber-integrated quantum memories are far from what can be achieved in mass memories.

With these clear goals, in a recent work published in Researchers are making progressICFO researchers Jelena Rakonjac, Dario Lago-Rivera, Alessandro Seri and Samuele Grandi, led by ICFO professor ICREA Hugues de Riedmatten, in collaboration with Giacomo Corrielli and Roberto Osellame from IFN-CNR and Margherita Mazzera from Heriot-Watt University, were able to demonstrate the entanglement. between a fiber-integrated quantum memory and a telecommunication wavelength photon.

A special quantum memory

In their experiment, the team used a crystal doped with praseodymium as a quantum memory. A waveguide was then laser etched inside the memory. It is a micrometric channel inside the crystal that confines and directs the photon in a confined space. Two identical optical fibers were then fixed on each side of the crystal to provide a direct interface between the photons carrying quantum information and the memory. This experimental setup enabled a fiber connection between the quantum memory and a photon source.

To prove that this embedded quantum memory can store entanglement, the team used an entangled photon pair source, where one photon is memory compatible while the other is at the telecommunication wavelength. With this new configuration, they were able to store photons from 2 tils to 28 µs and preserve the entanglement of photon pairs after storage. The result obtained is a significant improvement, as the entanglement storage time shown by the team is 1000 times longer (three orders of magnitude) than any other fiber embedded device that has been used so far, and comes close to performance observed in quantum mass memories. This was possible due to the fully integrated nature of the device, which allowed the use of a more sophisticated control system than previous results. Finally, when the entanglement was divided between a visible photon stored in the quantum memory and a photon at telecommunication wavelengths, the team also proved that the system is fully compatible with telecommunication infrastructures and suitable for communication.

Demonstrating this kind of embedded quantum memory opens up many new possibilities, including multiplexing, scalability, and deeper integration. As Jelena Rakonjac points out, “This experiment gave us high hopes in the sense that we imagine that many waveguides can be made in a crystal, which would allow many photons to be stored simultaneously in a small area and to maximize the capacity characteristics of quantum memory. Since the device is already fiber optic, it can also be more easily connected to other fiber based components.

Hugues de Riedmatten concludes that “we are pleased with this result, which opens up many possibilities for memories integrated with fiber. What is clear is that precisely this material and this way of creating waveguides gives us in the future, extending storage to spin modes will enable on-demand retrieval of stored photons and lead to the long storage times we aim for. This fiber-embedded quantum memory is certainly very promising for future use in quantum network.

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