Pushing Quantum Information Across the Channel

The information age is built on efficiently processing digital data. Today this data is processed classically as bits of zeros and ones. But there exists a more powerful kind of information processing. Quantum information, which obeys the laws of quantum mechanics, can not only carry those classic ones and zeros but it can impart them with unbreakable security and process them with computational power that may go beyond the reach of the most powerful supercomputers.

To better harness quantum information, physicists and engineers have built new devices, such as quantum computers. However, because of imperfections in manufacturing these devices, quantum information gets clouded with noise. As a result, when quantum light particles are sent across a fiber optic cable, they tend to get lost in transmission and the information deteriorates in quality.

Understanding and correcting noise introduced by these quantum devices, also called quantum channels, is a fundamental challenge for quantum information science and technology. Quantum channels are much quirkier than their classical counterparts. For example, it is relatively easy to know when a classical communication channel has stopped working: a phone call is interrupted by white noise, a video call gets stuck on a frozen screen.

In the quantum world, it’s a lot harder to decipher whether a channel can send information properly. It’s not as obvious when and how quantum information gets lost in transmission. This makes the study of quantum information both more complex and intriguing.

Vikesh Siddhu, who graduated with his Ph.D. in physics from Carnegie Mellon University, focused on the intriguing properties of quantum information as part of Emeritus Professor of Physics Bob Griffiths quantum theory group. During his doctoral program, Siddhu developed a new mathematical tool to find noisy quantum channels that can send noiseless information. The tool, referred to as a log-singularity-based mechanism, tracks changes in the rates at which quantum noise can be corrected.

His analysis, recently published in Nature Communications, helped to resolve a long-standing problem in understanding when the simplest of quantum channels can send quantum information. He also uncovered instances where noiseless quantum information can be sent even when the noise is expected to be so strong it would typically prevent any information being sent at all. Removing noise from quantum information is key to making quantum computing and communication a reality.

Siddhu's work also expanded on non-additivity in quantum communication, a mysterious behavior where two quantum channels send more information together than the sum of what each channel sends separately. He found non-additivity in extremely simple low-noise quantum channels, bringing us closer to realizing non-additivity in practice. If non-additive quantum communication were to become a reality, it would be akin to upgrading to a high-speed 5G network without building new physical infrastructure but by using the 4G network in a radically different way.

A deeper understanding of quantum information is necessary for building more efficient quantum computers and smarter quantum communication devices. Siddhu, a current postdoc at JILA University of Colorado/NIST, hopes to continue using the new tools developed in this work to get further insights into quantum information, including its non-additive aspects. Such insights could help find new ways to fine-tune the function of quantum devices that several labs and companies are trying to build. The results in this work also provide other researchers with new tools for tackling fundamental problems about when and how much quantum information can be sent across quantum channels.

This work was funded by the National Science Foundation.