Phillip F. Schewe 301-405-0989
COLLEGE PARK, Md. - Ever lose your mobile-phone reception or GPS signal? This occurs because of weak radio wave (phone) or microwave (GPS) signals. The widely shared problem of detecting weak signals also hampers astronomers looking back at the early universe and physicians seeking to locate and identify cancer tumors using MRI scans. All four of these areas--communications, navigation, astronomy, and medical imaging--depend on discriminating weak radio or microwave signals from a noisy environment. These and other applications, including quantum computing, could all benefit from a new approach to detecting and strengthening such signals developed by an international collaboration of scientists from Maryland and Denmark.
Created by a team from the Joint Quantum Institute (JQI) at the University of Maryland, the Niels Bohr Institute at the University of Copenhagen, and the Technical University of Denmark, this new radio band detector coverts radio waves to light, providing the first all-optical detection of microwaves and other radio band waves, while reducing background noise a thousand times better than existing methods. The building and testing of this new device was reported in the 6 March 2014 issue of the journal Nature.
The new device not only produces a much higher degree of noise reduction but it does this at room temperature. The development is based upon prior theoretical work by the same collaboration. "This device is the first room-temperature transducer of radio waves to optical waves at the quantum level and the first to entail a threefold electrical-mechanical-optical conversion. Previous efforts have bridged the electrical and mechanical or the mechanical and optical, but not all three realms," said team member and JQI Fellow Jacob Taylor.
A Radio Signal Converter
The detector is a mechanical-electrical device that converts one form of electromagnetic energy, radio waves, into another, light waves. It works like this: radio waves strike an antenna which constitutes one element in an electronic circuit. Another element in that circuit is a device with two electrodes called a capacitor. One of these electrodes consists of a flexible membrane. Light at visible frequencies reflects off the back side of this membrane. Depending on the radio signal arriving at the antenna, the silicon-nitride membrane (coated with a 50-nm-thick film of silver) mechanically alters its shape accordingly. This in turn modulates the visible light waves in a consistent way, thus converting a radio signal into an optical one.
Radio waves, microwaves, visible light and x-rays are among the many types of electromagnetic waves. They differ from each other in wavelength and energy.
"In the first place, this is a completely new way to measure electrical signals: making them excite a tiny membrane which we monitor with laser light," says team leader Albert Schliesser of the Niels Bohr Institute. "It may sound surprising, but this approach is so sensitive that it can out-perform conventional electronic amplifiers. That means, for example, that it could be a new way of getting clearer MRI images, or maps of the sky recorded by radio telescopes. We are currently trying to extend our work--which so far is really just a demonstrator of the concept--to attain a smaller detector which is more sensitive and capable of handling a wider band of radio signals."
This up-conversion from radio to optical has several advantages. First, it allows a radio signal to be converted into light and shot down an optical fiber rather than being sent down a copper wire, where it would suffer considerable energy loss. The radio-optical conversion will also help facilitate the development of devices that handle quantum information, such as a quantum computer. In a regular microphone, sound is converted into electrical signals sent down a wire. In the analogous quantum microphone, quantum information could be interconverted between radio and optical waves alternatively for transport or processing.
In this artist's impression of the device, radio waves (green) arrive and are sent to the membrane (center) via gold wires. Electric forces make the membrane move. This motion is detected with a laser beam (red) and translated into a light signal, resulting in a device with a very high sensitivity to radio waves. Credit: Niels Bohr Institute
A second, but no less important advantage of this device is its mitigation of noise. Radio waves were a boon for communications, starting with Marconi and the first "wireless" transmission of information in the early 20th century. As radio electronics grew in sophistication scientists and engineers became more and more concerned with Johnson noise, the ubiquitous radio noise present by virtue of the simple fact that Earth's surface is a warm place; our world positively glows in radio waves. Named for Bell Labs researcher John B. Johnson, this thermal noise competes with whatever radio signals are being processed in devices. One can amplify a weak signal, but the noise gets amplified along with the signal.
Even more unwanted noise is added in the amplifiers that bring the signal to a level at which it can be processed. For years special transistors have accomplished this task. One major drawback of this approach has been the need to chill the converters to very low temperatures to reach their best performance. One example of this kind of device is the one used on the orbiting Planck Telescope, which maps the microwave background. When the craft's coolant is depleted, the mission ends.
JQI's Taylor says that the whole up-conversion process can be done in reverse. Again for the purpose of quantum communication, there might be a need to convert microwave or radio signals into optical form and then back into radio after transmission from one quantum device to another.
The Joint Quantum Institute is based at the University of Maryland in College Park and is operated jointly by UMD and the National Institute of Standards and Technology in Gaithersburg, MD.