Map of the Quantum Communications Fiber Used in this Experiment. Credit: AWS

There have been a number of quantum networking testbeds created over the past few years (see our article listing them here), but Harvard University and the Amazon Web Services Center for Quantum Networking (AWS CQN) have created one with a new twist. A key technology that several quantum communications researchers are working to develop is called a Quantum Memory. This is a device that can receive a qubit and hold it for an extended period of time for use in the future. What Harvard and AWS CQN have create is a quantum network that demonstrates distributing entanglement between two photons in Quantum Memories placed at the end of a 35 kilometer fiberoptic cable loop (Node A to Node B).

A key to the quantum memory is to exploit the properties of an atomic defect in diamond called the Silicon Vacancy Center (SiV). This material places a silicon atom to displace a carbon atom within a diamond lattice as shown in the diagram below. In addition, the network requires the employment of frequency conversion devices which can convert back and forth the wavelength of the photons from the visible light range, where the quantum memory works best, to the telecom wavelengths (1,350 nm) that are best for the fiber optic cables.

Diagram of SiV Material. Carbon atoms are in silver, silicon atoms are in gold, and the white dots shows locations of displaced carbon atoms. Credit: AWS

A diagram of the experimental setup showing the SiV based quantum memories, frequency conversion devices, and other apparatus is shown below.

Dagram of the experimental setup. Credit: AWS

There is still a lot of work that needs to be done to utilize this technology for a usable quantum network. First, the SiV material is challenging to build and requires low temperature operation, but AWS CQN is working on ways of improving this. Second, the communication efficiencies and rates need to improve.

But, most important a working quantum repeater needs to be developed that uses a quantum memories is the key component. The biggest issue hindering faster deployment of quantum networks is the distance limitations due to losses in the optical fibers. Classical networks use photonic repeater devices space periodically along the line to capture and regenerate classical bits to compensate for the loss in the cable. But classical photonic repeaters won’t work with quantum networks because of the No Cloning theorem. Quantum repeaters are being researched by several groups that utilize quantum memories such as these SiV devices along with other devices that implement entanglement swapping. But quantum repeaters are still in a very early research phase and many years away from commercial deployment. As a result, quantum networks are only useful today for metropolitan networks that don’t require a repeater or networks that contain classical trusted nodes that would not be able to distribute entanglement.

For additional information about this experiment, you can access a blog article posted on the AWS website here and a technical paper posted on the Nature magazine website here. Additional material can be found in a blog article that provides an introduction to quantum networking here and another blog article here that describes AWS’ collaboration with Element Six to develop SiV based diamond material. Also, AWS has created a short video that talks about quantum repeaters and diamond materials for quantum use that can be viewed here.

May 17, 2024