A new cooling method using a single species of trapped ion for both computing and cooling purposes might make it easier to use quantum charge-coupled devices (QCCDs), potentially bringing practical quantum computing applications closer. The findings were published on February 5, 2024 in Nature Communications.
Scientists from the Georgia Tech Research Institute (GTRI) demonstrated that through a method known as fast ion exchange cooling, they were able to cool a calcium ion that accumulates vibrational energy while conducting quantum calculations. They did this by bringing a cold ion of the same species into close proximity. After transferring the energy from the hot ion to the cold one, the refrigerant ion is returned to a nearby reservoir for further cooling.
Traditional ion cooling for QCCDs involves the use of two different ion species, with cooling ions linked to lasers of a different wavelength that do not affect the ions used for quantum computing. This sympathetic cooling technique requires additional lasers to trap and control the refrigerant ions, beyond the lasers needed to operate the quantum computing operations, resulting in increased complexity and slower quantum operations.
“We have demonstrated a new, faster and simpler method for cooling ions in this promising QCCD architecture,” said Spencer Fallek, a GTRI research scientist. “Rapid exchange cooling can be faster because transporting the cooling ions requires less time than laser cooling two different species. And it’s simpler because using two different species requires operating and controlling more lasers.”
The ion movement occurs in a trap maintained by precisely controlling voltages that create an electrical potential between gold contacts. Kenton Brown, a GTRI principal research scientist who has worked on quantum computing issues for more than 15 years, explained that moving a cold atom from one part of the trap is similar to moving a bowl with a marble sitting in the bottom. When the bowl stops moving, the marble must also become stationary – not rolling around in the bowl.
Once the hot ion and cold ion are in close proximity, a straightforward energy exchange occurs, allowing the original cold ion – now heated by its interaction with a computing ion – to be split off and returned to a nearby cooled ion reservoir. The researchers have so far demonstrated a two-ion proof-of-concept system, but they state that their technique is applicable to the use of multiple computing and cooling ions, as well as other ion species.
More than 96 percent of the heat (measured as 102(5) quanta) was removed from the computing ion through a single energy exchange, which was a pleasant surprise to Brown. The researchers also discovered that the technique is effective regardless of the initial temperature of the computational ions, and it can be performed multiple times.
Excessive heat in a QCCD system affects the fidelity of the quantum gates and introduces errors. The GTRI researchers envision that in an operating system, cooled atoms would be available in a reservoir off to the side of the QCCD operations and maintained at a steady temperature.
The next steps in the research include building a QCCD that uses their cooling technique and studying its effectiveness at cooling motion in other spatial directions.
The unique ion trap was fabricated by collaborators at Sandia National Laboratories. GTRI researchers used computer-controlled voltage generation cards to produce specific waveforms in the trap, which has a total of 154 electrodes, and the experiments were 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. The 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 conducted in collaboration with Los Alamos National Laboratory.