One thing – dreaming of a quantum internet that could send hack-proof information around the world via photons superimposed in different quantum states. It is quite another thing to physically show that it is possible.
That's exactly what Harvard physicists have done, using Boston's existing telecommunications fiber to demonstrate the world's longest fiber distance between two quantum memory nodes. Think of it as a simple closed Internet between points A and B, transmitting a signal encoded not in classic bits like the existing Internet, but in completely safe individual particles of light.
Innovative work entitled “Entanglement of nanophotonic quantum memory nodes in a telecommunications network” and published in Nature was conducted under the direction of Mykhailo Lukin, Professor of Physics at Joshua University and Beth Friedman, in collaboration with Harvard professors Marco Lonchar and Hongkun. Park, who together with Amazon Web Services researchers are members of the Harvard Quantum Initiative.
A Harvard team laid the groundwork for the first quantum Internet by connecting two quantum memory nodes separated by a fiber-optic link spread over a roughly 22-mile loop through Cambridge, Somerville, Watertown and Boston. The two nodes were located on the same floor in the Harvard Integrated Science and Technology Laboratory.
Analogous to classical computer memory, quantum memory is an important component of the interconnected future of quantum computing, as it enables complex network operations, as well as information storage and retrieval. While other quantum networks have been built in the past, the Harvard team's is the longest fiber-optic network between devices that can store, process and move information.
Each node — it is a very small quantum computer made from a piece of diamond that has a defect in its atomic structure called a silicon vacancy center. Inside the diamond, carved structures smaller than a hundredth of a human hair enhance the interaction between the silicon vacancy center and light.
A silicon vacancy center contains two qubits, or bits of quantum information: one in the form of an electron spin used for communication, and the other in a long-lived nuclear spin used as a memory qubit to store entanglement (a quantum mechanical property that allows information to be perfectly correlated at any distance).
Both spins are fully controlled by microwave pulses. These diamond devices — just a few square millimeters — placed inside refrigerated dilution units that reach temperatures of -459°F.
Using silicon vacancy centers as quantum memory devices for individual photons has been a multi-year research program at Harvard. This technology solves the main problem of the theorized quantum internet: signal loss that cannot be amplified by traditional means.
A quantum network cannot use standard optical fiber signal repeaters because it is impossible to copy arbitrary quantum information, which makes the information safe, but also very difficult to transport over long distances.
Network nodes based on silicon vacancy centers can catch, store and scramble bits of quantum information while correcting for signal loss. After the nodes are cooled to absolute zero, the light passes through the first node and, by the nature of the atomic structure of the silicon vacancy center, becomes entangled with it.
"Since the light is already entangled with the first node, it can transfer this entanglement to the second node", — explained first author Kahn Knauth, a student in the Kenneth S. Griffin Graduate School of Arts and Sciences in Lukin's lab. "We call this photonic entanglement".
For the past few years, researchers have rented optical fiber from a company in Boston to conduct their experiments, installing their demonstration network on top of existing fiber to show that creating a quantum Internet with similar network lines will be possible.
«Showing that quantum network nodes can be entangled in a real environment of a very busy urban area is an important step towards a practical network between quantum computers’, — said Lukin.
A two-node quantum network – this is just the beginning. Researchers are hard at work expanding the performance of their network by adding nodes and experimenting with more network protocols.