A boost in performance in fiber-integrated quantum memories


Science Advances reports on the demonstration of entanglement between a fiber-integrated quantum memory and a photon of a communications wavelength by researchers from ICFO, IFN-CNR, and Heriot-Watt University.

One of the components of the upcoming quantum internet are quantum memory. Without them, extending into a true quantum network and transmitting quantum information over great distances would be fairly difficult. These memories have the task of accepting the qubits, or quantum bits, that are encoded in a photon, storing them, and then retrieving them. Different material systems, such as ensembles of cold atoms or doped crystals, can be used to construct quantum memories.

They must meet a number of criteria, including those for efficiency, longevity, and multiplexing of their storage capabilities, in order to guarantee the caliber of the quantum communication they will enable. Designing quantum memory that can be directly incorporated in the fiber-optic network is another necessity that has become a focus of intensive study.

There has been a lot of effort done recently, especially with the rise of quantum technologies, to make current 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. Finding a solution that maintains good coherence properties, providing an effective and stable system to transfer 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 such a fully integrated approach. While accomplishing all of this, the device should maintain the same degree of performance as its "normal" bulk equivalents. This has been difficult so far, and current fiber-integrated quantum memory realizations fall well 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 demonstrate entanglement between

Unique quantum memory

The team's quantum memory in the experiment was a crystal doped with praseodymium. The memory was afterwards laser-written with a waveguide. The photon is contained and guided in a small region 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 the 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 compatible with the memory and the other at telecom wavelength to demonstrate that this integrated quantum memory can store entanglement. With this innovative technique, scientists were able to store photons for up to 28 seconds while still maintaining the entanglement of the photon pairs. Since the entanglement storage period demonstrated by the researchers 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 memory, the result gained is a significant advance. This was made feasible by the device's completely integrated design, which permitted the deployment of a more complex control system than was possible with earlier realizations. The researchers also demonstrated that the device is completely compatible with telecommunications infrastructure and appropriate for long-distance quantum communication since the entanglement was shared between a visible photon stored in the quantum memory and one at telecom wavelengths.

Many new opportunities are made possible by the demonstration of this kind of integrated quantum memory, especially in terms of multiplexing, scaling, and further integration. According to Jelena Rakonjac, "This experiment has raised our aspirations in the sense that we believe many waveguides might be created in a single crystal, maximizing the capabilities of the quantum memory by allowing multiple photons to be concurrently stored in a tiny area. The gadget can also more easily connect with other fiber-based components because it is already fiber linked."

Hugues de Riedmatten declares in his conclusion that "We are overjoyed with this outcome, which offers a wealth of opportunities for fiber-integrated memories. It is evident that we can attain performances that are comparable to bulk memory using this specific material and method of making waveguides. The lengthy storage times that we have been striving for will be achieved in the future by expanding the storage to spin states, which will enable on-demand recovery of the stored photons. For use in upcoming quantum networks, this fiber-integrated quantum memory most certainly offers a lot of promise."