The researchers trapped the atoms, forcing them to serve as photonic transistors

Purdue University researchers have trapped alkali atoms (cesium) in a photonic integrated circuit, which behaves like a transistor for photons (the smallest unit of light energy) similar to electronic transistors. These trapped atoms demonstrate the potential to build a quantum network based on nanophotonic circuits integrated with cold atoms.

The team, led by Chen-Lung Hung, associate professor of physics and astronomy at the Purdue University College of Science, published his discovery in Physical Review X.

“We have developed a technique to use lasers to cool and tightly trap atoms in a nanophotonic integrated circuit, where light propagates in a small photonic ‘wire’, or more precisely, a waveguide that is more than 200 times finer than a human hair.” explains Hung, who is also a member of the Purdue Quantum Science and Engineering Institute.

“These atoms are ‘frozen’ at negative 459.67 degrees Fahrenheit, or only 0.00002 degrees above absolute zero temperature, and are essentially stationary. At this cold temperature, the atoms can be captured by a ‘tractor beam’ intended for guidance of photonic wavelengths and are placed at a distance much shorter than the wavelength of light, around 300 nanometers or about the size of a virus.

“Using state-of-the-art nanofabrication tools at the Birck Nanotechnology Center, we model the photonic waveguide into a circular shape with a diameter of about 30 microns (three times smaller than a human hair) to form a so-called microring resonator. The light circulated in the microring resonator and interacted with the trapped atoms,” adds Hung.

A key aspect of the function the team demonstrates in this research is that this atom-coupled microring resonator serves as a “transistor” for photons. They can use these trapped atoms to carry the flow of light through the circuit. If the atoms are in the correct state, the photons can transmit through the circuit. Photons are entirely blocked if the atoms are in another state. The stronger the atoms interact with the photons, the more efficient this gate is.

“We trapped up to 70 atoms that could collectively couple to photons and gate their transmission on an integrated photonic chip. This has not been done before,” says Xinchao Zhou, graduate student in Purdue Physics and Astronomy. Zhou is also the recipient of this year’s Bilsand Dissertation Fellowship.

The entire research team is based out of Purdue University in West Lafayette, Indiana. Hung served as principal investigator and oversaw the project. Zhou performed the experiment to trap atoms in the integrated circuit, which was designed and manufactured in-house by Tzu-Han Chang, a former postdoc now working with Prof. Sunil Bhave at the Birck Nanotechnology Center. Critical portions of the experiment were set up by Zhou and Hikaru Tamura, a former postdoc at Purdue at the time of the research and now an assistant professor at the Institute of Molecular Science in Japan.

“Our technique, which we have detailed in the paper, allows us to very efficiently laser many atoms on an integrated photonic circuit. Once many atoms are trapped, they can interact collectively with the light that propagates on the guide d ‘photonic wave,’ says Zhou.

“This is unique for our system, because all the atoms are the same and indistinguishable, so they could couple to the light in the same way and build the phase coherence, which allows the atoms to interact with the light collectively with a stronger force. Just imagine a boat moving faster. when all rowers are rowing the boat in sync compared to unsynchronized movement,” says Hung.

“In contrast, the solid-state emitters embedded in a photonic circuit are hardly ‘the same’ because of the slightly different environments that influence each emitter. It is much more difficult for many solid-state emitters to build coherence of phases and interacts collectively with photons like cold atoms We will be able to use cold atoms trapped on the circuit to study new collective effects,” continues Hung.

The platform demonstrated in this research could provide a photonic link for future distributed quantum computing based on neutral atoms. It could also serve as a new experimental platform to study collective light-matter interactions and to synthesize degenerate quantum trapped gases or ultracold molecules.

“Unlike the electronic transistors used in everyday life, our atom-integrated photonic circuit obeys the principles of quantum superposition,” explains Hung. “This allows us to manipulate and store quantum information in trapped atoms, which are quantum bits known as qubits. Our circuit can also efficiently transfer the quantum information stored in photons that could ‘fly’ through the photonic wire is a fiber network to communicate with each other Atom-coupled integrated circuits or atom-photon interfaces Our research demonstrates a potential to build a quantum network based on nanophotonic integrated circuits at cold atoms.

The team has been working on this area of ​​research for several years and plans to pursue it vigorously. His past research findings related to this work include recent innovations such as the implementation of the “tractor beam” method in 2023 listed Zhou as first author, and therealization of a highly efficient optical fiber coupling to a photonic chip by 2022 with a US patent application pending. New directions of research are opened due to the team’s successful demonstration of atoms that are cooled very effectively and trapped in a circuit. The future of this research is bright with many avenues to explore.

“There are many promising steps to explore,” says Hung. “We can arrange the trapped atoms in an organized array along the photonic waveguide. These atoms can collectively couple to the waveguide by constructive interference, but they cannot radiate photons into the surrounding free space by destructive interference .We aim to build the first nanophotonic platform to realize the so-called “selective radiation” proposed by theorists in recent years to improve the fidelity of photon storage in a quantum system.

“We can also try to form new states of quantum matter on an integrated photonic circuit to study few- and many-body physics with atom-photon interactions. We can cool atoms closer to absolute zero temperature to reach quantum degeneracy so that the trapped atoms could form a Bose-Einstein condensate gas that interacts strongly We can also try to synthesize cold molecules from the trapped atoms with the radiative coupling enhanced by the microring resonator.