A Room Temperature Maser to Amplify Weak Signals

The maser effect was achieved by placing a diamond in a sapphire ring and irradiating with green light from a laser. The diamond appears red due to fluorescence after excitation. (Source: J. Breeze)

The spacecraft Voyager 2 has travelled a barely ima­ginable 17 billion kilo­meters from Earth and continues to send signals to the ground station. This is made possible by the maser tech­nology, which works on similar prin­ciples to the laser and is able to amplify weak micro­wave signals almost noise-free. For regular communi­cation on Earth, this techno­logy is rarely used because it works only at low tempera­tures. However, a research team at the London Center for Nano­technology and the Univer­sity of Saarland has now developed a maser that can be used at room tempera­ture.

The physics behind the maser is similar to that of the laser using a microwave ampli­fication. Both generate coherent electro­magnetic radiation at a single wavelength. “Until now, masers were mainly used for communi­cation in space, for example to maintain radio contact with the Voyager space­craft. Because masers can amplify very weak signals almost noise-free this makes them interes­ting for future communi­cation techno­logies on Earth”, says Christopher Kay, professor of Physical Chemistry and Didactics of Chemistry at the Univer­sity of the Saarland.

A disadvan­tage of the existing maser tech­nology is that it needs very low tempera­tures, which could be reached only by the employ­ment of liquid helium. Together with fellow researchers at the London Center for Nano­technology, Christopher Kay has now developed a maser that can be operated at ambient tempera­tures. The key to the techno­logy is the use of a sapphire micro­wave reso­nator, held in a magnetic field. The resonator confines and concen­trates the micro­wave radia­tion and thereby it may be amplified it in a phase-stable manner.

The radiation itself is generated by optically exciting nitrogen vacancy centers in a diamond. In contrast to pure diamonds, which contain only carbon atoms and are therefore colorless, in the diamond used here a small number of carbon atoms are replaced by a nitrogen atom. The site next to the nitrogen atom, which usually contains one carbon atom, is empty. “This defect gives the diamond a purple color. It has a multi­tude of remarkable quantum properties and is therefore of interest for the develop­ment of new techno­logies, especially for nano appli­cations”, explains Kay. For example, a maser can be used for more precise measure­ments in space explo­ration or nano­metrology. “Wherever signals with low inten­sity are to be received, for example, over over long distances and need to be amplified without adding noise, the maser opens up new possi­bilities,” says Kay.

“It had already been suggested in the scien­tific community that diamonds with NV centers could be used as the basis for a maser, but the key to our success was to place a diamond in a sapphire reso­nator”, explains Jonathan Breeze of Imperial College, London, adding, “An exciting aspect of this tech­nology is in that the output frequency can be adjusted simply by changing the applied magnetic field. The current device operates at a frequency of 9 gigahertz.

For comparison, mobile phones work in the two-gigahertz range. With commer­cially available magnetic techno­logies, fre­quencies up to 200 giga­hertz could be achieved with our room tempera­ture maser.” As masers use optical photons to generate micro­wave photons, the researchers expect their work to open new avenues in the fields of diamond quantum tech­nology, magnetic resonance imaging and communi­cations. (Source: U Saarland)

Reference: J. D. Breeze et al.: Continuous-wave room-temperature diamond maser, Nature 555, 493 (2018); DOI: 10.1038/nature25970

Link: Group C. Kay, Dept. of Chemistry, University of Saarland, Saarbrücken, Germany • Dept. of Materials, Imperial College London, London, UK

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