A boost in performances in fibre-integrated quantum memories
Researchers from ICFO, partners of the DAALI project, IFN-CNR and Heriot-Watt University report in Science Advances the demonstration of entanglement between a fibre-integrated quantum memory and a telecommunications-wavelength photon.
Key breakthrough in the integration of quantum memories into fiber-optic networks
Quantum memories are crucial components of the future quantum internet. We cannot transmit quantum information over long distances or establish a comprehensive quantum network without them. These memories receive quantum information encoded in photons as qubits, store this information, and then retrieve it. Different material systems, such as cold atom ensembles or doped crystals, can serve as quantum memories.
In order to be useful memories, they need to fulfil several requirements, such as the efficiency, duration, and multiplexing of their storage capability, to ensure the quality of the quantum communication 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 the fibre-optic network.
Challenges and Progress in Fibre-Integrated Quantum Memories
In recent years, with the boom of quantum technologies, researchers have extensively worked on improving the scalability of existing quantum memories. They aim to make them smaller and simpler, facilitating their integration and deployment in real-world networks. However, this fully integrated approach presents several physical and engineering hurdles. Researchers must find a solution that preserves good coherence properties while providing an efficient and stable system to transfer photons from optical fibres to the quantum memory. Additionally, they must miniaturize the quantum memory’s control system and its interface with incoming light. Achieving all of this while matching the performance of “standard” bulk versions of the device has proven challenging. Current realizations of fibre-integrated quantum memories fall short of the capabilities of bulk memories.
With these objectives clear, in a recent work published in Science Advances, ICFO researchers Jelena Rakonjac, Dario Lago-Rivera, Alessandro Seri and Samuele Grandi, led by ICREA Prof. at ICFO Hugues de Riedmatten, partners of the DAALI project, in collaboration with Giacomo Corrielli and Roberto Osellame from IFN-CNR and Margherita Mazzera from Heriot-Watt University, have been able to demonstrate entanglement between a fibre-integrated quantum memory and a telecommunications-wavelength photon.
A special Quantum Memory
The team’s experiment utilized a crystal doped with praseodymium as their quantum memory. They then laser-wrote a waveguide, a micrometer-scale canal that confines and guides photons, within the memory. To create a direct interface between the memory and photons carrying quantum information, the team attached two identical optical fibers to either side of the crystal. This experimental setup allowed an all-fiber connection between the memory and a photon source.
To prove the integrated quantum memory’s capability to store entanglement, the team used a source of entangled photon pairs, with one photon compatible with the memory and the other at telecom wavelength. This novel setup allowed them to store photons from 2 µs up to 28 µs and maintain entanglement in the photon pairs after storage. The team’s results show a major improvement, with a 1000-fold increase in entanglement storage time over any previous fiber-integrated device. The results even approach the performance of bulk quantum memories. The fully integrated nature of the device, enabling the use of a more sophisticated control system, made this possible. The entanglement between a visible photon stored in the memory and one at telecom wavelengths demonstrates the system’s compatibility with telecommunications infrastructure, making it suitable for long-distance quantum communication.
Fiber-Integrated Quantum Memory: A Promising Future for Quantum Networks
Moreover, this demonstration of an integrated quantum memory opens many possibilities, particularly in multiplexing, scalability, and further integration. For instance, Jelena Rakonjac envisions fabricating numerous waveguides within a single crystal, allowing for simultaneous storage of many photons in a small region and maximizing the quantum memory’s capabilities. Furthermore, the device’s fiber-coupled nature also makes it easier to interface with other fiber-based components.
Similarly, Hugues de Riedmatten expresses excitement over the results, which open many possibilities for fiber integrated memories. In fact, the material and waveguide creation method allow for performance close to that of bulk memories. In addition, future extensions of storage to spin states will enable on-demand retrieval of the stored photons and achieve the desired long storage times. Consequently, this fiber-integrated quantum memory holds great promise for future quantum network applications.
