Researchers Take Milestone in Quantum Communications Using Light

Few terms are more ubiquitous in the scientific arena today than “quantum”.

Technologies based on the notoriously delicate laws of quantum mechanics promise to make computers far more powerful than today’s fastest supercomputers, unassailable secure communications and unprecedented detection capabilities needed for new scientific discoveries.

But for these technologies to emerge, researchers must develop efficient quantum communications networks that connect quantum devices while preserving the delicate states of the particles used to transmit information.

A team of researchers at the Department of Energy’s Oak Ridge National Laboratory, along with colleagues at Purdue University, have taken an important step toward this goal by harnessing the frequency, or color, of light. Such capabilities could contribute to more practical, large-scale quantum networks that are exponentially more powerful and secure than the classical networks we have today.

Specifically, the team is harnessing the properties of light and the principles of quantum mechanics to transfer information, making the network itself a processor of photonic quantum information. This approach is promising for several reasons.

For starters, photons travel at the speed of light, allowing information to travel from point A to point B as quickly as possible. Photons generally do not interact with each other or with the surrounding environment, ensuring that information will not be scrambled or corrupted during transport. “Light is really the only viable option for quantum communications over long distances,” said project leader Joseph Lukens, ORNL researcher, Wigner Fellow and winner of the DOE Early Career Award who helped detail the results of the team Physical examination letters.

The team used light to produce frequency qubits, or single photons that reside simultaneously in two different frequencies, to demonstrate for the first time completely arbitrary communication operations in frequency coding. While frequency coding and entanglement appear in many systems and are naturally compatible with fiber optics, using these phenomena to perform data manipulation and processing operations has traditionally proven difficult. Such operations, however, are necessary for basic network functions in quantum communications and, by extension, the realization of a wide range of quantum technologies.

Using technology developed at ORNL known as the Quantum Frequency Processor, the researchers demonstrated widely applicable quantum gates, or the logical operations required to run quantum communication protocols. In these protocols, researchers must be able to manipulate photons in a user-defined manner, often in response to measurements made on particles elsewhere in the array. While traditional operations used in classical computers and communication technologies, such as AND and OR, operate on digital zeros and ones individually, quantum gates operate on simultaneous overlays of zeros and ones, keeping quantum information protected during their passage, a necessary phenomenon. to create a true quantum network.

By proving that their setup could turn any qubit state into a different qubit state, the team demonstrated practical information transfer. “If you can perform arbitrary operations, you can perform any of the fundamental quantum communication protocols such as frequency conversion-based routing,” Lukens said.

Their system is one of many different systems, but one of the most promising given the results. As an example, the team was able to demonstrate over 98% fidelity – a quantitative measure of accuracy – using their custom setup.

While the bin-of-frequency quantum network has historically been difficult to control, the team’s toolkit, Lukens said, makes it much more controllable. Not only that, it is a naturally produced system that translates well into existing optical fibers. In fact, the system was designed using conventional telecommunications components such as phase modulators. These factors make the technology cheaper and more attractive to industries looking to apply it. Additionally, this domino effect simultaneously advances classical and quantum communications, advancing the team’s methods and eventually bringing large-scale quantum networks closer to reality.

Their next experiment will be to implement their system on a photonic integrated circuit. “There are a lot of unplanned applications,” Lukens said. “Frequency encoding is naturally produced by many different systems, and it is very well suited for fiber optics, so the potential application space should be wide.”