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September 5, 2018 12:20 am

Get Ready For Atomic Radio

An anonymous reader quotes a report from MIT Technology Review: David Anderson at Rydberg Technologies in Ann Arbor, Michigan, and a couple of colleagues, have reinvented the antenna from scratch. Their new device works in an entirely different way from conventional antennas, using a laser to measure the way radio signals interact with certain types of atoms. The secret sauce in the new device is Rydberg atoms. These are cesium atoms in which the outer electrons are so excited that they orbit the nucleus at great distance. At these distances, the electrons' potential energy levels are extremely closely spaced, and this gives them special properties. Indeed, any small electric field can nudge them from one level to another. Radio waves consist of alternating electric fields that readily interact with any Rydberg atoms they come across. This makes them potential sensors. But how to detect this interaction? A gas made of Rydberg atoms has another property that turns out to be useful -- it can be made transparent by a laser tuned to a specific frequency. This laser essentially saturates the gas's ability to absorb light, allowing another laser beam to pass through it. However, the critical frequency at which this happens depends crucially on the properties of the Rydberg atoms in the gas. When these atoms interact with radio waves, the critical frequency changes in response. That's the basis of the radio detection. Anderson and co create a gas of cesium atoms excited into Rydberg states. They then use a laser tuned to a specific frequency to make the gas transparent. Finally, they shine a second laser through the gas and measure how much light is absorbed, to see how the transparency varies with ambient radio waves. The signal from a simple light-sensitive photodiode then reveals the way the radio signals are frequency modulated or amplitude modulated. The atomic radio can detect a huge range of signals -- over four octaves from the C band to the Q band, or wavelengths from 2.5 to 15 centimeters. It also should be less insensitive to electromagnetic interference due to its lack of conventional radio circuitry. "The atomic radio wave receiver operates by direct real-time optical detection of the atomic response to AM and FM baseband signals, precluding the need for traditional de-modulation and signal-conditioning electronics," say Anderson and co.

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