A new cooling technique that utilizes a single kind of trapped ions for computation and cooling can simplify the use of quantum charge-coupled devices (QCCDs) and has the potential to bring quantum computing closer to real-world applications.
Scientists at the Georgia Institute of Technology Research Institute (GTRI) used a technique called rapid ion exchange cooling, showing that they could cool calcium ions (calcium ions get their vibrational energy when performing quantum calculations) by moving cold ions of the same species nearby. After transferring the energy from hot ions to cold ions, the refrigerant ions are returned to a nearby reservoir for cooling for further use.
The study was published in the journal Nature Communications.
Conventional ion cooling for QCCD involves the use of two different ion species, the cooling ions coupled to different wavelengths of lasers without affecting the ions used for quantum computing. In addition to the lasers required to control quantum computing operations, this sympathetic cooling technique requires additional lasers to capture and control refrigerant ions, which both adds complexity and slows down quantum computing operations.
Spencer Fallek, research scientist at GTRI, said, "We have demonstrated a new way to cool ions faster and more simply in this promising QCCD architecture. Rapid exchange cooling can be faster because it takes less time to transport cooling ions than a laser to cool two different substances. And it's simpler because using two different substances requires more lasers to operate and control. ”
The ion movement occurs in a trap maintained by a precisely controlled voltage that creates an electric potential between the gold contacts. But moving cold atoms from part of a trap is a bit like moving a bowl with marble at the bottom.
Kenton Brown, lead research scientist at GTRI, explains that when the bowl stops moving, the marble must become stationary, rather than rolling in the bowl, and he has worked on quantum computing problems for more than 15 years.
That's basically what we've been trying to do with these ions when we move the limiting potential (like a bowl) from one place in the trap to another," he said. "When we move the confinement potential to the final position in the trap, we don't want the ions to move within the potential. ”
Once the hot and cold ions are in close proximity to each other, a simple energy exchange occurs, and the original cold ions (now heated by interaction with the computed ions) can be separated and returned to a nearby library of cooled ions.
GTRI researchers have demonstrated a two-ion proof-of-concept system to date, but say their technology is suitable for using multiple computed and cooled ions, as well as other ion species.
A single energy exchange removes more than 96% of the heat (measured in 102(5) quantum) of the computed ions, much to Brown's surprise, who had expected multiple interactions to be required. The researchers tested the energy exchange by varying the starting temperature of the calculated ions and found that the technique was effective regardless of the initial temperature. They also demonstrated that energy exchange operations can be performed multiple times.
Heat (essentially vibrational energy) infiltrates the trapped ion system through computational activity and abnormal heating, such as the inevitable RF noise in the ion trap itself. Since the calculated ions also absorb heat from these ** as they cool, eliminating more than 96% of the energy will require more improvements, Brown said.
The researchers envision that in the operating system, the cooling atoms will be located in a reservoir on the side of the QCCD operation and kept at a stable temperature. Computational ions cannot be directly laser cooled because doing so erases the quantum data they hold.
Excessive heat in a QCCD system can adversely affect the fidelity of quantum gates, introducing errors in the system. GTRI researchers have not yet constructed a QCCD using its cooling technology, although this is a future step in the study. Other future work includes accelerating the cooling process and studying its effectiveness in cooling movements along other spatial directions.
The experimental part of the fast exchange cooling experiment is guided by simulations, taking into account factors such as the path taken by the ions during their journey within the ion trap. "We do understand what we're looking for and how we should implement it based on the theories and simulations that we have," Brown said. ”
This unique ion trap was made by collaborators at Sandia National Laboratories. GTRI researchers used a computer-controlled voltage generator card that was able to generate specific waveforms in a trap with a total of 154 electrodes, of which 48 were used in the experiment. Experiments were performed in a cryostat maintained at around 4 degrees Kelvin.
GTRI's Quantum Systems Division (QSD) studies quantum computing systems based on individual capture of atomic ions and novel quantum sensor devices based on atomic systems. GTRI researchers designed, manufactured, and demonstrated a number of ion traps and state-of-the-art components to support integrated quantum information systems. One of the technologies developed is the ability to deliver ions precisely where they are needed.
"We can have very fine control over how the ions move, how quickly they come together, the potential when they're close to each other, and the time it takes to do these kinds of experiments," Farek said. ”
Other GTRI researchers involved in the project include Craig Clark, Holly Tinkey, John Gray, Ryan McGill, and Vikram Sandhu. The study was done in collaboration with Los Alamos National Laboratory.