https://gizmodo.com/physicists-are-reinventing-the-laser-1846085004

A laser, in essence, is a megaphone for light. The word itself, originally an acronym, reflects this function: “light amplification by stimulated emission of radiation.” Send in a photon of the right frequency, and the laser makes copies of it, multiplying the original signal.

These photon clones exit the laser in sync with each other, traveling “in phase,” as the experts call it. You can think of it this way: Each photon is a wave, with its crest and trough lined up with its neighbor, marching together in lock-step out of the laser. This contrasts with most other light sources, such as your reading lamp or even the Sun, which both emit photons that disperse randomly.

The longer photons stay in sync, the more monochromatic the light. The color of a light source corresponds to the wavelength of its photons, with green light spanning roughly the 500 to 550 nanometer range, for example. For multiple photons to stay in sync a long time, their wavelengths must line up very precisely—meaning the photons need to be as close to one color as possible.

This synchrony of laser photons, known as temporal coherence, is one of the device’s most useful properties. Many technologies make use of laser light’s ridiculously fast and steady rhythm, its wave pattern repeating at hundreds of trillions of times a second for visible lasers. For example, this property underpins the world’s most precise timekeeping devices, known as optical lattice clocks.

But photons gradually lose sync after they leave the laser; how long they stick together is known as the laser’s coherence time. In 1958, physicists Arthur Schawlow and Charles Townes estimated the coherence time of a perfect laser. (This is a common physicist design strategy: Consider the most ideal version of something before building a far more lacking real-world device.) They found an equation thought to represent an ultimate coherence time limit for lasers, set by the laws of physics. Physicists refer to this as the Schawlow-Townes limit.

The two new papers find that the Schawlow-Townes limit is not the ultimate limit. “In principle, it should be possible to build lasers which are significantly more coherent,” said physicist David Pekker of the University of Pittsburgh, who led the other group. Their paper, currently under peer review, is posted as a pre-print on arXiv.

Both groups argue that the Schawlow-Townes limit rests on assumptions about the laser that are no longer true. Schawlow and Townes basically thought of the laser as a hollow box, in which photons multiply and leave at a rate proportional to the amount of light inside the box. Put another way, the photons flow out of Schawlow and Townes’s laser like water drains from a hole in a barrel. Water flows faster when the barrel is fuller, and vice versa.

But Wiseman and Pekker both found that if you place a valve on the laser to control the rate of the photon flow, you can actually make a laser coherent for much longer than the Schawlow-Townes limit. Wiseman’s paper takes this a step further. Allowing for these photon-controlling valves, his team re-estimates the coherence time limit for the perfect laser. “We show that ours is the ultimate quantum limit,” said Wiseman, meaning the true physical limit dictated by quantum mechanics.