While it has become a word, laser used to be an acronym: “light amplification by stimulated emission of radiation”. But there is an even older technology called a maser, which is the same acronym but with light switched out for microwaves. If you’ve never heard of masers, you might be tempted to dismiss them as early proto-lasers that are obsolete. But you’d be wrong! Masers keep showing up in places you’d never expect: radio telescopes, atomic clocks, deep-space tracking, and even some bleeding-edge quantum experiments. And depending on how a few materials and microwave engineering problems shake out, masers might be headed for a second golden age.

Simplistically, the maser is — in one sense — a “lower frequency laser.” Just like a laser, stimulated emission is what makes it work. You prepare a bunch of atoms or molecules in an excited energy state (a population inversion), and then a passing photon of the right frequency triggers them to drop to a lower state while emitting a second photon that matches the first with the same frequency, phase, and direction. Do that in a resonant cavity and you’ve got gain, coherence, and a remarkably clean signal.

The Same but Different

However, there are many engineering challenges to building a maser. For one thing, cavities are bigger than required for lasers. Sources of noise and the mitigations are different, too.

The maser grew out of radar research in the early 1950s. Charles Townes and others at Columbia University used ammonia in a cavity to produce a 24 GHz maser, completing it in 1953. For his work, he would share the 1964 Nobel Prize for physics with two Soviet physicists, Nikolay Basov and Alexander Prokhorov, who had also built a maser.

Eclipsed but Useful

By 1960, the laser appeared, and the maser was nearly forgotten. After all, a visible-light laser is something anyone can immediately appreciate, and it has many spectacular applications.

At the time, the naming of maser vs laser was somewhat controversial. Townes wanted to recast the “M” in maser to mean “molecular,” and pushed to call lasers “optical masers.” But competitors wanted unique names for each type of emission, so lasers for light, grasers for gamma rays, xasers for X-rays, and so on. In the end, only maser and laser stuck.

Masers have uses beyond fancy physics experiments. Trying to detect signals that are just above the noise floor? Try a cryogenic maser amplifier. That’s one way the NASA Deep Space Network pulls in signals. (PDF) You cool a ruby, or other material, to just a bit of 4 °K and use the output of the resulting maser to pull out signals without adding much noise. This works well for radio astronomy, too.

Need an accurate time base? Over the long term, a cesium clock is the way to go. But over a short period, a hydrogen maser clock will offer less noise and drift. This is also important to radio astronomy for building systems to use very long baseline interferometry. The NASA network also uses masers as a frequency standard.

All Natural

While we didn’t have our own masers until 1953, nature forms them in space. Water, hydroxyl, and silicon monoxide molecules in space can form natural masers. Scientists can use these astrophysical masers to map regions of space and measure velocities using Doppler shifts.

Harold Weaver found these in 1965 and, as you might expect, they operate without cavities, but still emit microwaves and are an important source of data for scientists studying space.

Future

While traditional masers are difficult to build, modern material science may be setting the stage for a maser comeback. For example, using nitrogen-vacancy centers in diamonds rather than rubies can lead to masers that don’t require cryogenic cooling. A room-temperature maser could open up applications in much the same way that laser diodes made things possible that would not have been practical with high-voltage tubes and special gases.

Masers can produce signals that may be useful in quantum computing, too. So while you might think of the maser as a historical oddity, it is still around and still has an important job to do.

In a world where lasers are so cheap that they are a dollar-store cat toy, we’d love to see a cheap “maser on a chip” that works at room temperature might even put the maser in reach of us hackers. We hope we get there.