Light provides an unrivaled means of interacting with our cosmos. It has the ability to travel across cosmic distances and hit with our atmosphere, resulting in a shower of particles that tells a tale about past celestial events. Controlling light allows us to convey data from one side of the world to the other here on Earth.
Given its wide range of uses, it’s no surprise that light is crucial in allowing quantum information applications in the twenty-first century. Scientists, for example, utilize laser light to precisely regulate atoms, transforming them into ultra-sensitive time, acceleration, and even gravity sensors.
Currently, state-of-the-art quantum technology is constrained by size; state-of-the-art systems, let alone a chip, would not fit on a dining room table. Scientists and engineers must miniaturize quantum devices for practical usage, which necessitates rethinking some components for light-harvesting.
The lasers that we use to control atoms need isolators that block undesirable reflections. But so far the isolators that work well in large-scale experiments have proved tough to miniaturize.
Bahl
Gaurav Bahl, an IQUIST member, and his research team have created a simple, compact photonic circuit that uses sound waves to control light. The new research, which was published in the journal Nature Photonics on October 21st, provides a strong method for isolating or controlling the directionality of light.
According to the team’s tests, its technique to isolation now exceeds all prior on-chip alternatives and is ideal for use with atom-based sensors.
“Atoms are the perfect references anywhere in nature and provide a basis for many quantum applications,” said Bahl, a professor in Mechanical Science and Engineering (MechSe) at the University of Illinois at Urbana-Champaign. “The lasers that we use to control atoms need isolators that block undesirable reflections. But so far the isolators that work well in large-scale experiments have proved tough to miniaturize.”
Even under ideal conditions, light is difficult to regulate since it will reflect, absorb, and refract when it comes into contact with a surface. A mirror reflects light, a shard of glass bends light while allowing it to pass, and dark rocks absorb light and convert it to heat.
Essentially, the light will disperse in all directions from everything in its path. Because of this ungainly behavior, even a smidgeon of light can help you see in the dark.
Controlling light in large quantum devices is often a difficult process involving a plethora of mirrors, lenses, fibers, and other components. Many of these components must be approached differently in order to be miniaturized.
Scientists and engineers have made great progress in creating various light-controlling features on microchips in recent years. They can make waveguides, which are light-transporting conduits, and even modify the hue of the light using particular materials. It’s difficult to make light, which is made up of tiny blips called photons, go in one direction while suppressing unwanted backward reflections.
“An isolator is a device that allows light to pass uninterrupted one way and blocks it completely in the opposite direction,” said the study’s first author Benjamin Sohn, a former graduate student and postdoctoral researcher in Mechse who is now at NIST, Boulder.
“This unidirectionality cannot be achieved using just any common dielectric materials or glasses, and so we need to be a little more innovative. We also want the isolator to operate at wavelengths of light tuned to atomic sensors, which can be hard even at large scales.”
Magnets are the greatest instrument for attaining unidirectionality in most experiments. Nearly every laser, for example, contains a magneto-optic isolator that allows light to leave the laser but prevents it from flowing backward, which would interfere with its operation. While lasers can be downsized, traditional isolators are difficult to shrink for two reasons.
Magnetic fields would have a deleterious effect on neighboring atoms in tiny devices, for starters. Second, even if there was a way around it, the materials used inside the isolator don’t operate as well on a chip’s lower length scales.
Bahl’s group created a novel non-magnetic isolator that is simple in design, employs common optical materials, and is easily adjustable for varied light wavelengths.
“We wanted to design a device that naturally avoids loss, and the best way to do that is to have light propagate through nothing. The simplest bit of ‘nothing’ that can still guide photons along a controlled path is a waveguide, which is a very basic component in photonic circuits,” said Bahl.
The waveguide would direct laser light via a sequence of components to a tiny chamber holding atoms in a complete atom-based system. With this in mind, the researchers designed their semiconductor to work with light at 780 nanometers, which is the wavelength required to set up most rubidium-based sensors.
Because the light must be blocked in the other direction for isolation, this is merely the first part of the design. The scientists previously demonstrated that they could disrupt the symmetric flow of light by launching sound waves into a photonic circuit. In the present study, the researchers transformed this concept into a working chip piece.
A waveguide plus an adjacent ring resonator, which resembles an oblong racetrack, make up the entire photonic isolator. In normal circumstances, incoming light would simply travel through the waveguide and into the resonator, stopping all light passage.
The resonator, however, only collected light that was traveling backward through the waveguide when the scientists applied sound waves to the ring. Light went through the waveguide unhindered in the forward direction as if the resonator didn’t exist.
According to the researchers’ findings, virtually every photon passes through the waveguide in the forward direction, with barely one-in-ten-thousand photons passing through reverse. This implies the approach essentially eliminated losses, or unwanted light absorption, which had been a long-standing issue with earlier on-chip isolators.
The results reveal that the new devices have record-breaking on-chip isolation performance and perform as well as bigger magnet-based devices. Furthermore, the method is adaptable, since it may be employed for a variety of wavelengths without modifying the starting material.
“The simplicity in fabrication is key with our approach, you could print photonic isolators that work well for whatever wavelength you need, all on the same chip at the same time. This is just not possible with other approaches today,” said co-author Ogulcan Orsel, a graduate student in Electrical Engineering at the U of I.
This might make the novel design beneficial in other applications, such as quantum computing, where stray, uncontrolled magnetic fields and unwanted light can wreak havoc on overall device performance.
The Defense Advanced Research Projects Agency (DARPA), the Air Force Office of Scientific Research (AFOSR), the National Science Foundation (NSF), and the Office of Naval Research (ONR) all contributed to this research.
