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Reinventing the laser: New theories challenge 60 years of common understanding

March 1, 2021

According to a study published in the journal Nature Physics, the work of two independent teams of physicists has challenged 60 years of prevailing consensus on lasers.


Since the first laser was invented in the 1950s, physicists have been building lasers based on quantum mechanical limits on the purity of their colors. Laser, short for "Light Amplification by Stimulated Emission of Radiation," works by generating a copy of the original signal when photons of the same frequency are shot in to excite atoms.


In the new theoretical study, two teams of physicists propose a solution to circumvent this longstanding limitation.


Lasers already have practical applications in everyday life, such as correcting vision, reading bar codes in grocery stores, etching computer chips, transmitting video files from the moon, and helping operate self-driving cars. Recent discoveries could add monochromatic lasers to this list and eventually use them for applications such as quantum computing.


The photons in the laser propagate in sync with each other, ejecting the laser at the same phase -- an alignment called "in phase" by scientists. Simply put, each photon is like a wave, with its peaks and troughs aligned with neighboring waves.


To achieve a monochromatic laser, photons need a longer time to synchronize, which means their wavelengths must be aligned precisely. The wavelength determines the color of the light source. For example, the wavelength of green light is between 500 and 550 nanometers.

The above-mentioned synchronization of laser photons is called temporal coherence, and this super-fast and stable frequency ensures that laser devices can be used for precision instruments.


The problem with traditional lasers, however, is that photons gradually fall out of sync as they leave the laser, and the time they stay in sync is known as the coherence time of the laser.


According to the laws of physics, scientists Arthur Schawlow and Charles Townes estimated the coherence time of a highly performing laser in 1958. This became known as the Schawlow-Townes limit, and it became the benchmark for developing lasers for decades.

"In principle, we should be able to make much more coherent lasers." David Pekker, a lead researcher saied.


A team of researchers led by physicist David Peck of the University of Pittsburgh is challenging this long-standing theory. They argue that the "Sholow-Townes limit" is not the ultimate limit. Their basic hypothesis is to be able to develop lasers that are constrained by the "Sholow-Townes limit" but are more coherent.


Instead of thinking of the laser as a hollow box with light in it, where photons replicate and leave at a rate proportional to the amount of light in the box, the latest research proposes a valve on the laser to control the speed at which photons flow. These physicists believe this will allow the laser to be coherent for much longer than previously thought.


Although the research team believes that Sholow and Townes' estimates of laser coherence were reasonable at the time, quantum technology has now enabled physicists to further refine the metric.

Some critics of the new work, however, say the design may not be suitable for commercial applications. Although it seems reasonable in theory, it does not suitable for practical commercial application. Take an example as the current laser manufacturers, most of them do not use the "Sholow-Townes limit" to guide their designs.


Nonetheless, the Peck team is confident that it will bring its new laser design into our lives. Their goal is to build a maser, for quantum programming in a quantum computer made of superconducting circuits. Keep in mind, however, that such an ambitious endeavor may require years of long-term research and many big problems to be solved.


This latest research may redefine what laser means, according to a peer review. Like the superradiant LASER, which was invented in 2012, the design contradicts the traditional definition of a LASER. They do not produce light through what is known as stimulated emission, so the "s" and "e" in the acronym "LASER" are no longer appropriate.