Researchers from Hy-Q solve central problem in quantum physics

12. November 2018

Professor Albert Schliesser is the team leader for a research team at the DNRF’s Center for Hybrid Quantum Networks (Hy-Q) that, with the help of ultra-precise sensors, has found the solution to a big problem in quantum physics. The study has been published in Nature.

From left: Professor and team leader Albert Schliesser with researchers Dr. David Mason, Massimiliano Rossi and Junxin Chen. Photo: The Niels Bohr Institute.

Professor Albert Schliesser, from the basic research center Hy-Q at the Neils Bohr Institute at the University of Copenhagen, together with his research team, including post-doc David Mason, Ph.D. fellow Massimiliano Rossi, and Junxon Chen, from Hy-Q’s research group for Quantum Optics, has solved a central problem in quantum physics. The researchers have found a solution to measure objects on a quantum scale with extreme precision and without disturbances.

When one measures an object on a quantum scale, the object is disturbed by the measurement itself. If the researcher, for example, uses a laser beam to determine the exact position of the quantum object in question, this determination is disturbed by “noise” in the shape of photons – light particles – from the laser beam, which makes the object move in tune with the photons when they hit the object.

Because the photons hit the object with a random frequency, an extra movement occurs besides the original one that the object made before the use of the laser beam. This makes the initial condition of the object’s movement extremely difficult to measure – a problem that researchers across the world have tried to solve for more than two decades. But now, Professor Schliesser and the rest of the research team from Hy-Q have made it possible to measure the object’s movement with the help of a special millimeter-sized ceramic membrane.

“A strong measurement is needed, even though it results in quantum backaction. All we have to do is to measure and undo the quantum backaction. And that is basically what we’ve succeeded in doing,” explained Professor Schliesser, the senior author of the study.

With the help of an experiment, including a so-called 3×3 mm membrane made of ceramic silicon nitride that is under high tension and vibrates like a drumhead when struck by movement, the researchers from Hy-Q can exclude external noise. This exclusion is caused by an extreme isolation in the membrane due to a special hole pattern developed by the Schliesser laboratory.

“Our experiments offer us a really unique opportunity: our data very clearly show quantum effects, such as quantum backaction, in the measurement of mechanical motion. So we can test in our labs whether clever modifications of the measurement apparatus can improve precision—using tricks that in the last few decades could only be theorized.”

The exclusion of outside disturbances through extreme isolation means that Professor Schliesser and the rest of the research team can focus on the quantum effects of the measurement with no disturbance from background noise. By using a stable laser, the researchers can both measure the vibration and the so-called “backaction” from the measurement all the way down to a quantum level, which enables the researchers to determine the movement in the quantum membrane solely in a quantum state.

“The remarkable thing is that we can then take this measurement record, run it through some electronics, and apply a counteracting force to the membrane, to undo the random effects of quantum backaction. It basically works like a set of noise-cancelling headphones, just in the quantum regime,” explains Ph.D. fellow Massimiliano Rossi from Hy-Q, one of the study’s lead authors.

Read the scientific article in Nature here.

More information about the study can be found in a press release from the University of Copenhagen here.