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Optics and photonics

Optics and photonics

Laser light goes for a quantum walk in a microchip

06 Dec 2023 Isabelle Dumé
Photograph of the laser used in the work. The laser is held between the thumb and forefinger of a person's blue-nitrile-gloved hand, against a blurred laboratory background
Photograph of the laser used in the work. The new optical comb device, which works thanks to a process known as a quantum walk, could be used to make miniaturized optical sensors. (Courtesy: ETH Zürich/D-PHYS/Kilian J Kessler)

Researchers at ETH Zürich in Switzerland have transformed a microchip laser that emits a single frequency (or colour) of light into one that emits light over a broad range of frequencies. The new optical comb device, which works thanks to a process known as a quantum walk, could be used to make miniaturized optical sensors for environmental and medical monitoring and to increase data transmission rates in telecommunications.

Led by physicist Jérôme Faist, the ETH researchers began with a quantum cascade laser integrated into a microchip. This device consists of a micro-ring structure made up of layers of arsenide, gallium, indium and aluminium. The ring confines and guides light and when connected to a direct source of electrical current, the electrons in it are stimulated to quickly jump across the different layers, emitting a cascade of photons. As the photons circulate in the ring, they multiply, producing coherent laser light with a single frequency.

Faist and colleagues found that if they excite this system with an additional alternating current oscillating at a certain resonance frequency, the light emitted goes from being a single colour to multiple colours in a space of just a few nanoseconds. Notably, before it stabilizes its final form, the spectrum of the emitted light resembles the motion of a so-called quantum walk.

A laser’s quantum walk

First proposed by the physicist and Nobel laureate Richard Feynman, the quantum walk is very different from the classical random walk commonly used to model the behaviour of physical systems ranging from fluctuating stock markets to the Brownian motion of pollen grains on the surface of a liquid. The classical random walk works like a lost hiker who chooses their next steps according to the toss of a coin. If the coin lands on heads, for example, the hiker might take a step to the left, whereas tails might call for a step to the right. After many coin tosses, the hiker’s position will be random, but likely close to their starting point.

In a quantum walk, in contrast, a quantum particle effectively moves in both directions at the same time after every toss, adopting a coherent superposition of right and left. This means there are always several possible paths the particle can take to arrive at its final position.

An optical comb-like spectrum

In the new device, this quantum walk has a remarkable outcome. “The different colours (or frequencies) add energy to the light emitted and create an optical comb-like spectrum,” Faist explains. “The optical frequencies are equidistant from each other, and their number is selected by the frequency and amplitude of the electrical oscillating signal sent to the laser.”

As for applications, the researchers say miniaturized optical sensors for environmental and medical monitoring are a possibility. In the longer term, Faist adds that such devices could increase the data transmission rate for optical communications, since each colour of light the laser emits – up to 100 colours in total – could serve as an independent communication channel.

The researchers report their findings in Science.

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