Device Offers Quantum-Compatible Link Between Microwave, Optical Domains

A device that uses a small plate to absorb microwave energy and bounce it into laser light could provide a solution for sending quantum signals over long distances.

Scientists at JILA, a joint institute of the University of Colorado Boulder and the National Institute of Standards and Technology, have developed a mechanically mediated microwave-optical converter. The research targets an important step for practical quantum computing — the ability to convert microwave signals, produced by quantum chips, into light beams that can travel down fiber optic cables.

This chip, designed by researchers at JILA and measuring less than a half-inch across, converts microwave energy into laser light. Courtesy of Peter Burns and Dan Schmidt.

“Currently, there’s no way to convert a quantum signal from an electrical signal to an optical signal,” said researcher Peter Burns. “We’re anticipating a growth in quantum computing and are trying to create a link that will be usable for these networks.”

Operating at T <100 mK temperatures, as required for quantum electrical circuits, the converter demonstrated 47 percent conversion efficiency — a significantly better performance than other methods for converting microwaves into light, such as methods using crystals or magnets.

Even in the ultracold facilities where quantum chips are stored, trace amounts of heat can cause the device to shake, sending out excess photons that contaminate the signal. Researchers discovered that noise emitted from the two converter output ports was strongly correlated, because both outputs recorded thermal motion of the same mechanical mode. They used a classical feed-forward protocol to reduce added noise to 38 photons.

Robert Peterson, a former graduate student at JILA, works with a 'dilution refrigerator.' This equipment can cool scientific instruments, such as a new quantum 'trampoline,' down to a fraction of a degree above absolute zero. Courtesy of Peter Burns.

“What we do is measure that noise on the microwave side of the device, and that allows us to distinguish on the optical side between the signal and the noise,” Burns said.

Researchers believe that a quantum feed-forward protocol could allow quantum information to be transferred even when thermal phonons enter the mechanical element faster than the electro-optic conversion rate.

The team will need to bring down the noise even more for the device to become a practical tool. But the potential benefits are huge, said professor Konrad Lehnert.

“It’s clear that we are moving toward a future where we will have little prototype quantum computers. It will be a huge benefit if we can network them together,” Lehnert said.

The research was published in Nature Physics (doi:10.1038/s41567-018-0210-0).