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Mechanical Oscillator ‘Interprets’ Optical Signals

EUGENE, Ore., Nov. 15, 2012 — If you want two parties to communicate and they don’t speak the same language, you have to find a good interpreter.

In the case of quantum computing, getting different quantum systems, or nodes, to understand each other is a significant challenge because each system communicates with photons of distinct wavelengths, or colors. But a “dark mode” that converts an optical field, or signal, from one color to another could be just the interpreter needed in efforts to build and connect quantum computers.

Color conversion will be crucial for networking quantum systems and for building a quantum Internet where photons carry information, said University of Oregon physics professor Hailin Wang, a member of the Oregon Center for Optics and leader of the research group that made the discovery.

“Optomechanical systems can be used to store light and change its color — operations that are important for a quantum network,” said Chunhua Dong, a postdoctoral research associate in Wang’s lab and co-author of a report on the experiment.

To change the color of a light pulse, the physicists coupled tiny radiation pressure forces generated by light circulating inside a glass microsphere to the mechanical breathing motion of the microsphere. Exciting a mechanical vibration through the optomechanical coupling generates a new light pulse at the desired color.

Physicists at the University of Oregon used a theorized “dark mode” to convert an optical field, or signal, from one color to another. From left, the graduate student co-authors, Chunhua Dong, Victor Fiore and Mark Kuzyk, who worked with their faculty adviser Hailin Wang, on the research. Courtesy of University of Oregon.

Current conversion methods are limited by thermal mechanical motion — only at temperatures approaching absolute zero can such an approach be useful for quantum applications. The scientists equated the challenge to a tuning fork.

“If you were able to look at the tuning fork very closely, you would see that it is always vibrating a little bit on its own, just from the thermal motion,” said graduate student Victor Fiore. “This causes a problem, because the noise from the thermal motion can swamp out the signal that we care about.”

To address this, the team demonstrated a dark mode approach proposed earlier this year by theoretical physicists from McGill University and the University of California, Merced. By achieving the dark mode, the color-conversion process becomes immune to thermal noise, even though the conversion is still mediated by the same mechanical oscillator. This method offers an alternative to cooling the mechanical oscillator to eliminate noise.

The dark mode is difficult to describe, so graduate student Mark C. Kuzyk suggests picturing three children sitting on swings, holding hands.

“The two outermost kids are the photons of different colors, and the middle child is the mechanical oscillator,” Kuzyk said. “When all three children are sitting still, there are no photons or vibrations in the system. If we push one of the swings, all three kids will start moving. In the dark-mode approach, we push and pull on the swings in a special way that generates a very particular pattern of swinging.

“As the child on the left-hand side moves forward, the child on the right-hand side moves backward, such that the middle child never moves. This is interesting because, even though the middle child never moves, she is a necessary part of the system. Without her, there would be no way to couple the two outermost swings.”

To use the approach in quantum Internet applications, Wang said, the team will have to demonstrate that the process can work at the single-photon level and can be implemented on a semiconductor chip.

The study was published in Science (doi: 10.1126/science.1228370). 

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