Pushing the laser limit

Australian quantum researchers have shown it’s possible to vastly improve the coherence of lasers, overcoming a bound that has been accepted as a fundamental limit for 60 years.

The results are published in the journal Nature Physics.

Lasers are defined as highly directional, monochromatic, coherent light. This means that light is emitted as a narrow beam in a specific direction, and every photon has the same wavelength and phase.

The coherence of a laser beam can more specifically be thought of as the number of photons that can be emitted in this manner, which is a property crucial in determining the performance of a laser in precision tasks like quantum computing.

A quantum limit of laser coherence was determined in a seminal 1958 paper by US physicists Arthur Schawlow, a Nobel Prize winner, and Charles Townes.

“They showed theoretically that the coherence of the beam cannot be greater than the square of the number of photons stored in the laser,” explains co-author Howard Wiseman, from Griffith University’s Centre for Quantum Dynamics.

“But they made assumptions about how energy is added to the laser and how it is released to form the beam.”

While these assumptions make sense and still apply to most of today’s lasers, they aren’t necessarily required when it comes to quantum mechanics.

“In the 21st century, our ability to engineer and control quantum systems has changed our conception of what is practical,” Wiseman and colleagues write in their paper. “At the same time, our understanding of quantum processes has been deepened.”

Now, the joint team from Griffith University and Macquarie University has used numerical simulations to demonstrate that a new limit is possible.

“We have shown that the true limit imposed by quantum mechanics is that the coherence cannot be greater than the fourth power of the number of photons stored in the laser,” says Macquarie’s Dominic Berry. 

“When the stored number of photons is large, as is typically the case, our new upper bound is much bigger than the old one.”

The team also found a quantum mechanical model for a laser that could theoretically achieve this upper bound for coherence.

But what does this mean in practice?

According to lead author Travis Baker, from Griffith, we probably won’t be seeing super-lasers anytime soon.

“But we do prove that it would be possible to construct our truly quantum-limited laser using superconducting technology,” he says. “This is the same technology used in the current best quantum computers, and our proposed device may have applications in that field.”

The results may also open up new lines of research into more energy-efficient lasers.