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Mechanical oscillator transfers optical signals

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Suppose you needed two PCs to exchange information. The process would be relatively easy because they speak the same language. Between a PC and a Mac, however, you would need a middleman, although that problem is still relatively simple. But what if you had two quantum systems? How would you mediate that communication?

A possible solution, proposed by a team led by Dr. Hailin Wang at the University of Oregon, is to use certain optomechanical effects to translate the data from one quantum node to another.

The data in quantum systems is stored by atoms as photons of certain wavelengths that correspond to different colors. If a “blue” atom needs to communicate with a “green” atom, a properly coupled system must be excited to convert the blue light into green light.

“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 their paper published in the December 2012 issue of Science (doi: 10.1126/science.1228370).

“Dark mode is a special normal mode – in our case, a superposition of the two optical modes,” Wang told Photonics Spectra. “This superposition mode is decoupled from the mechanical oscillator but can still mediate mechanical coupling.”

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, graduate student co-authors Chunhua Dong, Victor Fiore and Mark Kuzyk, who worked with their faculty adviser, Hailin Wang, on the research.

To create the dark-mode system, the physicists coupled the radiation pressure force generated by light circulating inside a glass microsphere to the mechanical breathing motion of the microsphere. By exciting the mechanical vibration through the coupled system, they can generate new light pulses of any color.

The dark-mode method is superior to current ones because it is not limited by thermal mechanical motion; i.e., the experiment does not have to be performed at absolute zero. The noise from the thermal motion interferes with the signal the researchers are looking for from the oscillator. In dark-mode experiments, the process is immune from thermal noise because of the optomechanical coupling.

To properly understand the dark mode, Mark C. Kuzyk, another graduate student in Wang’s lab, suggests considering children on a swing set. “The two outermost kids are the photons (light) of different colors, and the middle child is the mechanical oscillator. 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.”

As a translator, dark mode can be useful for linking quantum systems into a network to serve as secure communication channels or as a resource to scientific investigation. However, to use this dark-mode approach in developing a quantum Internet, it must be shown that the process can work on the single-photon level and be implemented on a semiconductor chip.

“Although there are also more conventional applications of the dark mode [such as optical switching and optical storage], they are not economically or technologically competitive at this stage,” Wang said.

Photonics Spectra
Feb 2013
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