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APPLIED PHYSICS
Applications for masers - the microwave equivalent of lasers - have been hindered by their extreme operating conditions and the inability to produce continuous emissions. A diamond maser overcomes these limitations. See Letter p.493
In 1954, scientists reported the first maser1 - a device similar to a laser, but operating at microwave frequencies. Although lasers were not demonstrated until six years later2, masers have not been as widely used as their optical counterparts. The bottleneck has been the need to operate masers under conditions of either high vacuum or extremely low temperature (a few kelvin). On page 493, Breeze et al.3 present, for the first time, a maser that works continuously under ambient conditions. Such a device could lead to advances in microwave metrology and communications, and in quantum many-body physics.
The key component of a maser (or a laser) is a material known as a gain medium. In ordinary materials, electrons usually exist in their lowest energy states and can absorb radiation by jumping to higher states. However, in a gain medium, the electron population is inverted: there are more electrons in higher states than in the lowest ones.
A photon passing through a gain medium can stimulate an electron in a higher state to jump to a lower state and emit an identical photon. If these photons are bounced back and forth between mirrors, or confined in a closed metal structure called a cavity, they can be copied many times before escaping from the system, generating a macroscopic quantum state of many identical photons. This process explains the name maser (laser): microwave (light) amplification by stimulated emission of radiation.
The different orientations of the tiny magnet (the spin) associated with an electron are known as electron spin states, and have energy separations of the right magnitude for microwave emission. However, these states are susceptible to the collisions, rotations and vibrations of atoms through an effect called spin-orbit coupling. Researchers have minimized atomic collisions in masers using highvacuum conditions and dilute gain media, such as ammonia molecules (as in the first reported maser1), hydrogen atoms, free electrons and rubidium gases4. Masers have also been built using solid-state gain media, such as ruby, and...