Peter Rakich revives a 19th-century device to push quantum technology forward
This story originally appeared in Yale Engineering magazine.
So how does a 19th-century technology built around two mirrors address the very 21st-century issue of advancing quantum technology?
This device, known as the Fabry-Perot resonator, was invented by the French physicists Charles Fabry and Alfred Perot, and is indispensable for optical clocks, quantum technologies, and other applications. It was first used to measure extremely small distances, which proved useful in establishing the standard length of a meter. Among other scientific contributions, it helped bridge the gap between physics and astronomy; measure the energy emitted by the moon, stars, and the sun, and identify the ozone layer.
Today, it’s considered a key to ushering in the next generation of quantum communications, computation, and timekeeping technologies. It’s a structure that consists of two parallel mirrors precisely placed to reflect light back and forth. Its extreme stability comes from operating in a vacuum. The trick, though, is in finding a way to incorporate these resonators into photonic integrated circuits. These are microchips that use light, or photons, to provide significant advantages over conventional electronics in terms of efficiency, speed, and smaller devices.
The main challenge with incorporating Fabry-Perot resonators is that their strong mirror-like reflections can destabilize or even damage an on-chip laser.
A team of researchers in the lab of Peter Rakich, though, developed an ingenious solution. They introduced what they call “reflection transformation circuits” that essentially transform the light produced by a tiny, millimeter-scale Fabry-Perot resonator and tune it to the frequency of the on-chip laser. In doing so, they demonstrated for the first time an ultra-stable “self-injection” locked laser using a Fabry-Perot resonator. The study’s lead author, Haotian Cheng, a former grad student in Rakich’s lab and now an optics engineer at Apple, explains that “self-injection” means that a laser can effectively synchronize the laser oscillator with the resonance of this Fabry-Perot cavity, inheriting the remarkable low-noise properties of the cavity.
Through their innovation, they’ve created a device that produces an unprecedented suppression of fluctuations — or “noise” — in the laser’s frequencies. That’s important, because a laser with lower noise “gives us the ability to detect smaller signals or to sense motion, vibrations, etc. with much more precision,” says Rakich, the Donna L. Dubinsky Professor of Applied Physics. These features are crucial in that they have potential benefits for communications, sensing, laser radar, and imaging technologies. The laser’s low noise means that users can better identify the position of a vehicle, detect a microwave communications signal, or sense motion in a radar system.
Going forward, the researchers plan to use their current ultra-stable laser system as a building block for fiber-optic sensing applications. With a low-noise laser, such a system can sense any vibration in the fiber and detect earthquakes, submarines and other things. The technology also underpins Resonance Micro Technologies Inc., a company Rakich co-founded with Gregory G. Luther, which received a grant last year from the Roberts Innovation Fund — Yale Engineering’s accelerator for faculty innovations — to support commercialization efforts.
Cooling Sound
In another study, Rakich and his lab used the same device to achieve another breakthrough. In this case, they developed lasers to cool phonons — or particles of sound — within massive objects to their quantum ground state, the lowest possible energy in quantum mechanics.
“We have developed a very different type of bulk acoustic resonator that permits access to very high-frequency phonons using light,” Rakich says. They enhanced the interaction between light and the massive phonons using an optical Fabry-Perot resonator. The device’s highly reflective mirrors enhance the light field in the crystalline resonator. With this system, they were able to use lasers to cool the phonons within these objects to their lowest state of energy. In doing so, they were able to stabilize the phonons and enhance their quantum properties.
Rakich notes that, in the quantum realm, “massive” is a relative term. In this case, it means 10 micrograms of material in the acoustic wave motion, or an object that’s a bit smaller than a grain of sand. At the atomic scale, however, this corresponds to an enormous number of atoms (100 quadrillion) moving in quantum-coherent fashion.
This is a major advance — prior methods of using light to control motion at the quantum level have been limited to objects about a million times smaller. Scaling up to the Rakich lab’s system is critical because it allows for longer coherence times — that is, the duration of time that quantum information maintains its quantum properties before decaying.
Increasing coherence times is crucial to advancing quantum science and will be necessary to create practical quantum computers. The increase in size results in longer coherence because a smaller proportion of the atoms resides at the surface, where things can get tricky (even by quantum’s very tricky standards). Rakich recalls a quote from the late theoretical physicist Wolfgang Pauli: “God made the bulk; the surface was invented by the devil.”
“It’s notoriously difficult to control various interactions that occur at surfaces,” Rakich says. That’s why the Rakich lab’s approach works so well — by using light to access sound waves within the bulk of a crystal, they greatly reduce surface interactions. This effectively protects the system from unwanted quantum decoherence.
Hagai Diamandi, a former postdoctoral associate in Rakich’s lab who headed up the project, noted that the system provides excellent material properties, without many of the drawbacks of conventional methods.
“Having a system that can precisely control phonons while maintaining their unique properties opens up exciting possibilities for advancing the field of quantum research,” says Diamandi.
Guiding the path of light
For another project, Rakich and his team employed a more recently invented device, the optic isolator. With it, the researchers figured out a better way to control the path of light on small chips, a breakthrough that could lead to better and cheaper sensors, smartphones, and other devices.
Photonic integrated circuits (PICs) use light, or photons, to provide significant advantages over conventional electronics in terms of efficiency, speed, and allowing for smaller devices. The ability to generate, manipulate, and detect light using PICs of increasing complexity has opened the door to revolutionary advancements in classical and quantum computation, optical communications, as well as imaging and sensing technologies. Incorporating a component known as an optic isolator into PICs has the potential to make them even more powerful.
“It’s an essential component to protect lasers,” said Haotian Cheng, who also led this project. “It will let light pass in one direction but block it in the other direction.”
The challenge, though, is that optic isolators rely on magnetic materials, which are incompatible with the complementary metal-oxide-semiconductor (CMOS) materials used to make PICs.
“So it’s very hard to make that material out of your silicon chip,” Cheng said. “We need to find alternatives.”
To that end, the researchers made their own alternative by creating a non-magnetic optic isolator that allows for a wide range of light frequencies. Other researchers have demonstrated the use of optic isolators in PICs, but they come with limitations. They can only operate within a few gigahertz of optical bandwidth, for instance, which greatly limits functionality. The Yale researchers demonstrated that their approach allows the system to scale up the optical bandwidth to two terahertz (one terahertz equals 1,000 gigahertz) which means it can operate with multiple colors of light sources.
The system includes two acousto-optic beam splitters, each of which can split light beams and change their directions, creating a system that allows for easy control of the paths of light.
“If we can move everything on chip,” Cheng said, it could significantly reduce the cost of optical communications devices and other technologies. It will also make them more efficient and versatile.
“With this building block, we can achieve all the functions in silicon photonic chips,” he said. “That will make the industry change from a micro-assembled optical component to a fully on-chip platform, which will dramatically drop the cost.”
This story is a duplicate of the Yale Engineering news story of March 24, 2026, photos are credited to Tanner Pendleton. Please see below for a link to the original article and other related links.