Cornell Chip Cuts Trapped-Ion Laser Cooling to 150 Microseconds

The work, led by Karan Mehta, assistant professor of electrical and computer engineering, appeared on Nov. 9 in the journal PRX Quantum. Interesting Engineering describes the result as about three times faster cooling. The same method also cooled all three measured motion modes of the ion together, across a range of about 5 megahertz, where conventional schemes often need a separate cooling step for each mode.

Cooling matters because a trapped-ion quantum computer holds each qubit in a single charged atom suspended in electromagnetic fields. Even tiny unwanted motion of the atom disturbs a calculation, so lasers must repeatedly cool it to near its lowest energy state. Mehta said the logic operations might look like the slow part, but cooling to near the ground state "ends up being a major bottleneck in leading systems," and Cornell says it can eat a large share of operating time.

How the chip cools the ion

The team sent two laser beams through the chip in opposite directions to form a standing wave, a fixed pattern of bright and dark bands, much like the still nodes on a plucked guitar string. They held the ion at a dark point, a node, which suppresses the light interactions that would otherwise heat it. Combined with a method called electromagnetically induced transparency (EIT) cooling, this brought the targeted motion to an average of about 0.05 phonons, the quanta of vibration, according to the authors' abstract on arXiv.

The team also cooled the ion below the conventional Doppler limit, which Cornell says confirms a long-standing theoretical prediction. The optical parts are built into the chip, which routes wavelengths from ultraviolet to near-infrared and was packaged to run at cryogenic temperatures. The chips were fabricated commercially in the Netherlands through a collaboration with ETH Zurich.

Why it matters for bigger machines

Putting the optics on the chip is one way groups are trying to add many more qubits without a sprawling arrangement of external lasers. Mehta's point is that the same integration that helps scaling also speeds a basic operation. Voltgrid News has followed other quantum hardware work, such as the 120-qubit error correction platform from IBM, AMD and Oak Ridge, and the Stanford chip that sorts laser light shows the same push to control light on a chip.

What is not confirmed yet

The experiment used a single ion, not a multi-ion processor. Cornell says only that the approach is applicable to larger trapped-ion computers; no multi-ion result or end-to-end speedup of a quantum algorithm has been shown. The 150 and 500 microsecond figures come from the researchers' own comparison on one ion and one set of modes. The journal page itself was not readable when this was written, so the technical numbers here are taken from the authors' arXiv abstract, Cornell's release and Interesting Engineering's report.

The work was supported by the National Science Foundation, the Army Research Office and the Alfred P. Sloan Foundation.