Quantum computers could rapidly solve complex problems that would take the most powerful classical supercomputers decades to unravel. But they'll need to be large and stable enough to efficiently perform operations. To meet this challenge, researchers at MIT and elsewhere are developing quantum computers based on ultra-compact photonic chips. These chip-based systems offer a scalable alternative to some existing quantum computers, which rely on bulky optical equipment.
These quantum computers must be cooled to extremely cold temperatures to minimize vibrations and prevent errors. So far, such chip-based systems have been limited to inefficient and slow cooling methods.
Now, a team of researchers at MIT and MIT Lincoln Laboratory has implemented a much faster and more energy-efficient method for cooling these photonic chip-based quantum computers. Their approach achieved cooling to about 10 times below the limit of standard laser cooling.
Key to this technique is a photonic chip that incorporates precisely designed antennas to manipulate beams of tightly focused, intersecting light.
The researchers' initial demonstration takes a key step toward scalable chip-based architectures that could someday enable quantum computing systems with greater efficiency and stability.
"We were able to design polarization-diverse integrated-photonics devices, utilize them to develop a variety of novel integrated-photonics-based systems, and apply them to show very efficient ion cooling. However, this is just the beginning of what we can do using these devices. By introducing polarization diversity to integrated-photonics-based trapped-ion systems, this work opens the door to a variety of advanced operations for trapped ions that weren't previously attainable, even beyond efficient ion cooling - all research directions we are excited to explore in the future," says Jelena Notaros, the Robert J. Shillman Career Development Associate Professor of Electrical Engineering and Computer Science (EECS) at MIT, a member of the Research Laboratory of Electronics, and senior author of a paper on this architecture.
She is joined on the paper by lead authors Sabrina Corsetti, an EECS graduate student; Ethan Clements, a former postdoc who is now a staff scientist at MIT Lincoln Laboratory; John Chiaverini, senior member of the technical staff at Lincoln Laboratory and a principal investigator in MIT's Center for Quantum Engineering; Felix Knollmann, a graduate student in the Department of Physics; as well as others at Lincoln Laboratory and MIT. The research appears today in two joint publications in Light: Science and Applications and Physical Review Letters .
Seeking scalability
While there are many types of quantum systems, this research is focused on trapped-ion quantum computing. In this application, a charged particle called an ion is formed by peeling an electron from an atom, and then trapped using radio-frequency signals and manipulated using optical signals.
Researchers use lasers to encode information in the trapped ion by changing its state. In this way, the ion can be used as a quantum bit, or qubit. Qubits are the building blocks of a quantum computer.
To prevent collisions between ions and gas molecules in the air, the ions are held in vacuum, often created with a device known as a cryostat. Traditionally, bulky lasers sit outside the cryostat and shoot different light beams through the cryostat's windows toward the chip. These systems require a room full of optical components to address just a few dozen ions, making it difficult to scale to the large numbers of ions needed for advanced quantum computing. Slight vibrations outside the cryostat can also disrupt the light beams, ultimately reducing the accuracy of the quantum computer.
To get around these challenges, MIT researchers have been developing integrated-photonics-based systems. In this case, the light is emitted from the same chip that traps the ion. This improves scalability by eliminating the need for external optical components.
"Now, we can envision having thousands of sites on a single chip that all interface up to many ions, all working together in a scalable way," Knollmann says.
But integrated-photonics-based demonstrations to date have achieved limited cooling efficiencies.
Keeping their cool
To enable fast and accurate quantum operations, researchers use optical fields to reduce the kinetic energy of the trapped ion. This causes the ion to cool to nearly absolute zero, an effective temperature even colder than cryostats can achieve.
But common methods have a higher cooling floor, so the ion still has a lot of vibrational energy after the cooling process completes. This would make it hard to use the qubits for high-quality computations.
The MIT researchers utilized a more complex approach, known as polarization-gradient cooling, which involves the precise interaction of two beams of light.
Each light beam has a different polarization, which means the field in each beam is oscillating in a different direction (up and down, side to side, etc.). Where these beams intersect, they form a rotating vortex of light that can force the ion to stop vibrating even more efficiently.
Although this approach had been shown previously using bulk optics, it hadn't been shown before using integrated photonics.
To enable this more complex interaction, the researchers designed a chip with two nanoscale antennas, which emit beams of light out of the chip to manipulate the ion above it.
These antennas are connected by waveguides that route light to the antennas. The waveguides are designed to stabilize the optical routing, which improves the stability of the vortex pattern generated by the beams.
"When we emit light from integrated antennas, it behaves differently than with bulk optics. The beams, and generated light patterns, become extremely stable. Having these stable patterns allows us to explore ion behaviors with significantly more control," Clements says.
The researchers also designed the antennas to maximize the amount of light that reaches the ion. Each antenna has tiny curved notches that scatter light upward, spaced just right to direct light toward the ion.
"We built upon many years of development at Lincoln Laboratory to design these gratings to emit diverse polarizations of light," Corsetti says.
They experimented with several architectures, characterizing each to better understand how it emitted light.
With their final design in place, the researchers demonstrated ion cooling that was nearly 10 times below the limit of standard laser cooling, referred to as the Doppler limit. Their chip was able to reach this limit in about 100 microseconds, several times faster than other techniques.
"The demonstration of enhanced performance using optics integrated in the ion-trap chip lays the foundation for further integration that can allow new approaches for quantum-state manipulation, and that could improve the prospects for practical quantum-information processing," adds Chiaverini. "Key to achieving this advance was the cross-Institute collaboration between the MIT campus and Lincoln groups, a model that we can build on as we take these next steps."
In the future, the team plans to conduct characterization experiments on different chip architectures and demonstrate polarization-gradient cooling with multiple ions. In addition, they hope to explore other applications that could benefit from the stable light beams they can generate with this architecture.
Other authors who contributed to this research are Ashton Hattori (MIT), Zhaoyi Li (MIT), Milica Notaros (MIT), Reuel Swint (Lincoln Laboratory), Tal Sneh (MIT), Patrick Callahan (Lincoln Laboratory), May Kim (Lincoln Laboratory), Aaron Leu (MIT), Gavin West (MIT), Dave Kharas (Lincoln Laboratory), Thomas Mahony (Lincoln Laboratory), Colin Bruzewicz (Lincoln Laboratory), Cheryl Sorace-Agaskar (Lincoln Laboratory), Robert McConnell (Lincoln Laboratory), and Isaac Chuang (MIT).
This work is funded, in part, by the U.S. Department of Energy, the U.S. National Science Foundation, the MIT Center for Quantum Engineering, the U.S. Department of Defense, an MIT Rolf G. Locher Endowed Fellowship, and an MIT Frederick and Barbara Cronin Fellowship.
