New chip offers way to make use of quantum system ‘imperfections’

“Understanding how quantum systems behave under this messiness is crucial if we want our experiments to say something about nature as it really is, not just idealized setups,” says Govind Krishna, PhD student at KTH Royal Institute of Technology.

In a recent study at KTH, Kishna led the development of a chip that enables researchers to simulate the way many quantum systems behave when they lose energy or information to their surroundings. The results were published in Nature Communications.

Krishna says the research strengthens the role of quantum computers as simulators of quantum systems and quantum processes in nature.

“Our research presents a method that lets us safely test how ‘imperfection’ in quantum systems behave,” he says, “and even explore ideas where ‘imperfection’ itself becomes an asset we can use, instead of only a problem we try to get rid of.”

Krishna says the study uses light particles, or photons, on the chip as stand‑ins for the particles in whatever system is being modelled. Using this tightly‑controlled quantum system enables replaying and studying the photon behavior in other kinds of systems.

The device is an integrated photonic circuit: light travels through microscopic tracks, or waveguides, on a silicon chip, much like electricity moves through wires on a computer chip. In its experiments, the team added an extra “side track” that plays the role of a loss channel. Using electrical signals, they could tune how strongly the main tracks connected to this side track.

“In many quantum experiments, anything that does not fit the ideal textbook picture is simply treated as loss and ignored,” Krishna says. “The chip enables us to simulate those non‑ideal processes in a controlled way.”

Part of the quantum light is redirected into a separate output that plays the role of the environment or loss channel. This channel is measured in order to track the fate of photons.  

“The chip works a bit like a programmable railway junction for quantum light,” says Ali Elshaari, associate professor at KTH and senior author of the study. “By changing the control signals, we can decide whether the photons mostly stay on the main track, are mostly diverted to the loss channel, or end up in superpositions that depend on their quantum interference.”

Although the study was focused on basic physics and proof‑of‑principle demonstrations, the researchers say that understanding these effects is important for future quantum technologies. 

“Real quantum devices will always have leaks and noise,” says co‑author Jun Gao, associate professor at Huazhong University of Science and Technology in Wuhan, China. “Our chip gives us a controlled way to study how quantum information flows under those conditions, and when elements that used to be seen only as problems - like loss, might be turned into useful resources instead.