Amazon and Harvard University have created a “quantum network” that transmitted an entangled photon from one quantum computer to another over 35 kilometers of fiber-optic cable.

Researchers from Harvard and Amazon’s AWS Center for Quantum Networking put a set of nodes around the Boston area to build a network capable of “efficiently catching, storing, and transferring information initially stored in light.” Like the internet we know, quantum networks send information carried by light — in this case, quantum-entangled photons. But they need “repeaters” to prevent those photons from scattering across long distances, as light is wont to do, and the repeaters have to be able to send the photon without breaking its entanglement and modifying the information.

Harvard and AWS say the experimental nodes use cavities in diamonds that “trap light and force it to interact with quantum memories.” These nodes can be mass-produced with existing nanofabrication technology. During their experiments, the team took a qubit encoded into a photon and bounced it off a quantum memory in a Harvard lab. From the post:

When the photon interacts with the quantum memory, it becomes entangled with the memory – meaning that measurements performed on either the photon or the memory would provide information on (and thus modify) the state of the other.

However, instead of measuring the photon (and thus extracting the information), the photon undergoes quantum frequency conversion from visible frequency (where the quantum memory operates) to telecom frequency (where losses in optical fiber are minimized). The (now telecom-frequency) photon then makes a round trip through an underground fiber network before returning to Harvard, where it is converted back to visible frequency.

This journey accomplished, the photon is bounced off a different quantum memory in a different lab, thus transferring the entanglement from the photon onto this second memory. Finally, the photon, having bounced off the second memory, is then routed to a detector which notes the presence of a photon, but does not reveal any of the underlying quantum information contained in the light.It entangled with the memory — meaning that measuring either the photon or the memory modifies the state of the other. The photon then converts from a visible frequency to a telecom frequency, which then bounces to a different lab, thus completing the journey. 

AWS says early experiments showed the quantum-entangled photon traveled more than 35 kilometers. The entangled photon was stored for over a second, which the company says is “sufficient for light to be able to travel more than 300,000 kilometers” and is more than enough time to go around the world 7.5 times.

Quantum networks use the same principles as quantum computing by using the quantum state of photons to carry information, the Department of Energy explains. Experiments with quantum networks have been around for a while, but no one has yet to make a fully commercial version. 

AWS says many more improvements are needed before its quantum network will be scalable and commercially viable. So far, it’s slow and only sends one quantum memory at a time.