In the 1950s, when physicists were racing to invent the first laser, they found that the rules of quantum mechanics restricted how pure the colour of their light could be. Since then, physicists and engineers have always built lasers with those restrictions in mind. But new theoretical research from two independent groups of physicists indicates that nature is more lax than previously thought. The findings could lead to improved, more monochromatic lasers for applications such as quantum computing, which the researchers illustrate in two proposed laser designs.
The work “overthrows 60 years of understanding about what limits lasers,” said physicist Howard Wiseman of Griffith University in Australia, whose group published their work in Nature Physics last October.
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, travelling “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 neighbour, 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 colour of a light source corresponds to the wavelength of its photons, with green light spanning roughly the 500 to 550 nanometre 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 colour 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.
Schawlow and Townes’s estimate, while not the fundamental restriction on lasers physicists originally thought, was reasonable for its time, said Wiseman. No one had any means for precisely controlling the flow of light out of a laser in the way that Wiseman and Pekker propose. But today’s lasers are a different story. Physicists can now control light with a multitude of devices developed for the budding quantum computing industry.
Pekker has teamed up with physicist Michael Hatridge, also of the University of Pittsburgh, to bring the new laser design to life. Hatridge’s expertise involves building circuits out of superconducting wire for storing and controlling microwave-frequency photons. They plan to build a microwave-emitting laser — known as a maser — for programming qubits inside a quantum computer made of superconducting circuits. Though building this new maser will take years of work and troubleshooting, Hatridge said they have all the tools and knowledge to make it possible. “That’s why we’re excited about it, because it’s just another engineering project,” Hatridge said.
Wiseman is looking for collaborators to build his design, also a maser. “I would really, really like this to happen, but I recognise it’s a long-term goal,” he said.
The designs are “completely feasible,” said physicist Steven Touzard of the National University of Singapore, who was not involved in either of the new papers. However, Pekker and Wiseman’s work may not directly lead to useful commercial lasers, according to Touzard. He pointed out that builders of lasers do not commonly use the Schawlow-Townes limit to direct their designs. So overturning the limit could be more of a theoretical advancement than an engineering one, he said.
Curiously, the two new designs also contradict another conventional wisdom about lasers. The devices do not produce light via so-called stimulated emission, which makes up the “s” and “e” in the acronym laser. Stimulated emission is a type of interaction between light and matter, in which a photon impinges upon an atom and “stimulates” the atom to emit an identical photon. If we imagine a laser as a box of light, as before, a laser that amplifies light using stimulated emission multiplies the signal proportionally to the amount of light already in the box. Another type of laser invented in 2012, known as a superradiant laser, also does not use stimulated emission to amplify light, according to Touzard.
The idea of a laser has outgrown its name. It is no longer exclusively “light amplification by stimulated emission of radiation.”
Of course, many such examples exist in the English language. The change in meaning is known as semantic shift and is common “wherever new technology is involved,” according to linguist Micha Elsner of the Ohio State University. “Ships still sail across the ocean, even when no actual sails are involved,” Elsner said in an email. “You can still dial someone’s number even though your phone doesn’t have a dial.”
“Even though a word’s etymology — its origin — certainly gives it a starting point, it does not determine its destiny forever going forward,” linguist Brian Joseph of the Ohio State University said in an email.
As Cold War goals transitioned into 21st century ones, lasers have evolved, too. They’ve been around long enough to integrate into nearly all aspects of modern life: They can correct human vision, read our grocery barcodes, etch computer chips, transmit video files from the Moon, help steer self-driving cars, and set the mood at psychedelic ragers. And now, the laser could be reinvented again. A 60-year-old device remains a symbol of a sci-fi future.