A new cooling method that employs a single species of trapped ion for both computing and cooling has the potential to simplify the use of quantum charge-coupled devices (QCCDs), potentially advancing quantum computing closer to practical applications.
Researchers from the Georgia Tech Research Institute (GTRI) have demonstrated a process called prompt ion exchange cooling, illustrating the ability to cool a calcium ion by moving a cold ion of the same species into close proximity. Following the transfer of energy from the hot ion to the cold one, the refrigerant ion is placed back in a nearby reservoir to be cooled for additional use.
The study is detailed in the journal Nature Communications.
Traditional ion cooling for QCCDs requires the use of two different ion species, with cooling ions coupled to lasers of a different wavelength that do not affect the ions used for quantum computing. However, this sympathetic cooling method involves additional lasers to trap and control the refrigerant ions, complicating and slowing quantum computing operations.
“In this promising QCCD architecture, we have presented a new technique for cooling ions more rapidly and simply,” explained Spencer Fallek, a GTRI research scientist. “Rapid exchange cooling can be faster and simpler as it requires less time than laser cooling two different species and operating and controlling additional lasers.”
The movement of ions occurs in a trap governed by precisely controlling voltages that generate an electrical potential between gold contacts. However, moving a cold atom from one part of the trap resembles moving a bowl with a marble resting at the bottom.
“When we’re done moving the confining potential to the final location in the trap, we don’t want the ion moving around inside the potential,” said Kenton Brown, a GTRI principal research scientist.
When the hot ion and cold ion are in close proximity, an energy exchange occurs, allowing the original cold ion—warmed due to its interaction with a computational ion—to be separated and returned to a nearby reservoir of cooled ions.
The researchers have demonstrated a proof-of-concept system using two ions, and suggest that their technique is applicable to multiple computing and cooling ions, as well as other ion species.
More than 96% of the heat—equivalent to 102(5) quanta—was eliminated from the computing ion through a single energy exchange, which was an unexpected result for Brown. The researchers found that the technique is effective regardless of the initial temperature and demonstrated that the energy exchange operation can be repeated.
Given that heat is absorbed by the computing ion from sources like computational activity and incidental heating, removing over 96% of the energy will require further enhancements.
In an operational system, cooled atoms would be available in a reservoir adjacent to the QCCD operations and maintained at a consistent temperature. As laser-cooling the computing ions would erase the quantum data they hold, it is not a feasible method for cooling them directly.
Excessive heat in a QCCD system can adversely impact the accuracy of the quantum gates, leading to errors. While a QCCD using this cooling technique has not been constructed yet, it is a planned step in the research. Future work includes streamlining the cooling process and studying its effectiveness in cooling motion in other spatial directions.
The research was guided by simulations to predict various factors, including ion pathways within the trap. “Based on the theory and simulations we had, we understood what we were looking for and how we should go about achieving it,” explained Brown.
The specialized ion trap used in the study was built by collaborators from Sandia National Laboratories. The GTRI researchers employed computer-controlled voltage generation cards to produce specific waveforms in the 154-electrode trap (of which 48 were used in the experiment). The research was conducted in a cryostat maintained at about 4 degrees Kelvin.
GTRI’s Quantum Systems Division (QSD) investigates quantum computing systems based on individual trapped atomic ions and novel quantum sensor devices based on atomic systems. GTRI researchers have designed, fabricated, and demonstrated a number of ion traps and state-of-the-art components to support integrated quantum information systems. Among the technologies developed is the ability to precisely transport ions to where they are needed.
“We have very fine control of how the ions move, the speed at which they can be brought together, the potential they’re in when they are near one another, and the timing that’s necessary to do experiments like this,” said Fallek.
Other GTRI researchers involved in the project included Craig Clark, Holly Tinkey, John Gray, Ryan McGill, and Vikram Sandhu. The research was carried out in collaboration with Los Alamos National Laboratory.
Spencer D. Fallek et al, Rapid exchange cooling with trapped ions, Nature Communications (2024). DOI: 10.1038/s41467-024-45232-z