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Quantum Memory Entangled with a Photon at Telecommunication Wavelength

Scientists from ICFO, IFN-CNR, and Heriot-Watt University demonstrated the entanglement between a fiber-integrated quantum memory and a telecommunications-wavelength photon.

Picture of the quantum memory attached to the optical fiber. Image Credit: ICFO/S. Grandi.

The study was published in the Science Advances journal.

One of the elements that will make up the future quantum internet is quantum memory. Without it, it would be difficult to send quantum data over great distances to grow into a true quantum network.

These memory tasks include receiving and storing qubits—quantum bits—that are the result of quantum information being encoded in a photon. In a variety of material systems, such as ensembles of cold atoms or doped crystals, quantum memories can be produced.

They must meet several criteria, including those for efficiency, longevity, and multiplexing of their storage capability, to guarantee the caliber of the quantum communication they will serve. Designing quantum memory that can be directly incorporated into the fiber-optic network is another necessity that has become a focus of intensive research.

There has been a lot of effort recently, especially with the rise of quantum technologies, to make existing quantum memories more scalable (i.e., smaller and/or simpler devices), which will make it easier to deploy and integrate them into real-world networks.

Coming up with a solution that maintains good coherence features, providing a useful and stable system to transmit photons from optical fibers to the quantum memory, as well as the miniaturization of the quantum memory’s control system and its interface with incoming light, are just a few of the physical and engineering challenges that come with a completely integrated method.

All of this should be done while maintaining the device’s performance levels found in “regular” bulk versions. This has been difficult thus far, and current fiber-integrated quantum memory realizations fall far short of what is possible with bulk memories.

With these goals in mind, ICFO researchers Jelena Rakonjac, Dario Lago-Rivera, Alessandro Seri, and Samuele Grandi, under the direction of ICREA Professor at ICFO Hugues de Riedmatten, in conjunction with Giacomo Corrielli and Roberto Osellame from IFN-CNR and Margherita Mazzera from Heriot-Watt University, were able to show entanglement between a fiber-integrated quantum memory and a telecommunications-wavelength photon.

A Special Quantum Memory

The team’s quantum memory in the experiment was a crystal doped with praseodymium. The memory was afterward laser-written with a waveguide. The photon is contained and guided in a small space by this canal, which is a micrometer-scale opening within the crystal.

The crystal was subsequently fitted with two identical optical fibers, which served as a direct link between photons conveying quantum information and memory. This experimental configuration made it possible to connect the quantum memory and a photon source entirely through fiber.

The scientists employed a supply of entangled photon pairs with one photon suitable for the memory and the other at telecom wavelength to demonstrate that this embedded quantum memory can retain entanglement. With this innovative technique, they were able to store photons from 2 µs up to 28 µs while still maintaining the entanglement of the photon pairs.

Since the entanglement storage period demonstrated by the team is 1000 times longer (three orders of magnitude) than any other fiber-integrated device previously employed and is beginning to approach the performances seen in bulk quantum memories, the result gained is a significant enhancement.

This was made possible by the device’s completely integrated design, which permitted the deployment of a more sophisticated control system than was possible with earlier realizations.

The team also demonstrated that the device is completely compatible with telecommunications infrastructure and appropriate for long-distance quantum communication since the entanglement was exchanged between a visible photon kept in the quantum memory and one at telecom wavelengths.

Many new opportunities are made possible by the demonstration of this kind of embedded quantum memory, especially in terms of multiplexing, scaling, and further integration.

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. Since the device is already fiber-coupled, it can also be more readily interfaced with other fiber-based components.

Jelena Rakonjac, Researcher, ICFO

Hugues de Riedmatten concludes by testifying, “We are thrilled with this result which opens many possibilities for fiber integrated memories. What is clear is that this particular material and way of creating waveguides allows us to achieve performances close to bulk memories.”

Hugues de Riedmatten adds, “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 fiber-integrated quantum memory shows great promise for future use in quantum networks.”

Journal Reference

Rakonjac, J. V., et al. (2022) Storage and analysis of light-matter entanglement in a fiber-integrated system. Science Advances.

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