New ion cooling technique could simplify quantum computing devices
Image shows the ion trap used to control the location of computational and refrigerant ions. The device was produced by Sandia National Laboratories. Credit: Sandia National Laboratories.
A new cooling technique that utilizes a single species of trapped ion for both computing and cooling could simplify the use of quantum charge-coupled devices (QCCDs), potentially moving quantum computing closer to practical applications.
Using a technique called rapid ion exchange cooling, scientists at the Georgia Tech Research Institute (GTRI) have shown that they could cool a calcium ion—which gains vibrational energy while doing quantum computations—by moving a cold ion of the same species into close proximity. After transferring energy from the hot ion to the cold one, the refrigerant ion is returned to a nearby reservoir to be cooled for further use.
The research is reported in the journal Nature Communications.
Conventional ion cooling for QCCDs involves 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. Beyond the lasers needed to control the quantum computing operations, this sympathetic cooling technique requires additional lasers to trap and control the refrigerant ions, and that both increases complexity and slows quantum computing operations.
"We have shown a new method for cooling ions faster and more simply 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."
Video shows how a computational ion can be cooled by bringing it near a refrigerant ion of the same atomic species. Credit: Georgia Tech Research Institute
The ion movement takes place in a trap maintained by precisely controlling voltages that create an electrical potential between gold contacts. But moving a cold atom from one part of the trap is a bit like moving a bowl with a marble sitting in the bottom.
When the bowl stops moving, the marble must become stationary—not rolling around in the bowl, explained Kenton Brown, a GTRI principal research scientist who has worked on quantum computing issues for more than 15 years.
"That's basically what we're always trying to do with these ions when we're moving the confining potential, which is like the bowl, from one place to another in the trap," he said. "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."
Once the hot ion and cold ion are close to each other, a simple energy swap takes place and the original cold ion—heated now by its interaction with a computing ion—can be split off and returned to a nearby reservoir of cooled ions.
The GTRI researchers have so far demonstrated a two-ion proof-of-concept system, but say their technique is applicable to the use of multiple computing and cooling ions, and other ion species.
A single energy exchange removed more than 96% of the heat—measured as 102(5) quanta—from the computing ion, which came as a pleasant surprise to Brown, who had expected multiple interactions might be necessary. The researchers tested the energy exchange by varying the starting temperature of the computational ions and found that the technique is effective regardless of the initial temperature. They have also demonstrated that the energy exchange operation can be done multiple times.
Heat—essentially vibrational energy—seeps into the trapped ion system through both computational activity and from anomalous heating, such as unavoidable radio-frequency noise in the ion trap itself. Because the computing ion is absorbing heat from these sources even as it is being cooled, removing more than 96% of the energy will require more improvements, Brown said.
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