April 18, 2024

New ion cooling technique could simplify quantum computing devices

A new cooling technique that uses a single trapped ion species for both computing and cooling could simplify the use of quantum charge-coupled devices (QCCDs), potentially bringing quantum computing closer to practical applications. The research was published on February 5, 2024 in the journal. Nature Communications.

Using a technique called ion exchange rapid cooling, scientists at the Georgia Tech Research Institute (GTRI) have shown that they can cool a calcium ion, which gains vibrational energy while performing quantum calculations, moving a cold ion of the same species a distance close. . After transferring energy from the hot to the cold ion, the coolant ion returns to a nearby reservoir to be cooled for later use.

Conventional ion cooling for QCCD 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 quantum computing operations, this sympathetic cooling technique requires additional lasers to trap and control cooling ions, increasing the complexity and slowing down quantum computing operations.

“We have demonstrated a new method to cool ions more quickly and easily in this promising QCCD architecture,” said Spencer Fallek, GTRI research scientist. “Rapid exchange cooling can be faster because the transport of the cooling ions requires less time than laser cooling of two different species. And it is simpler because using two different species requires operating and controlling more lasers.”

The movement of the ions takes place in a trap maintained by precisely controlling voltages that create an electrical potential between the gold contacts. But moving a cold atom from one part of the trap is a bit like moving a bowl with a marble in the bottom. When the bowl stops moving, the marble should remain stationary, not rolling around in the bowl, explained Kenton Brown, a senior research scientist at GTRI who has worked on quantum computing issues for more than 15 years.

“That’s basically what we always try to do with these ions when we move the confinement potential, which is like the container, from one place to another in the trap,” he said. “When we finish moving the confinement potential to the final location in the trap, we don’t want the ion to move into the potential.”

Once the hot ion and the cold ion are close to each other, a simple energy exchange occurs and the original cold ion (now heated by its interaction with a computing ion) can be separated and returned to a nearby pool of cooled ions. . The GTRI researchers have so far demonstrated a two-ion proof-of-concept system, but say their technique is applicable to computing and cooling using multiple ions, and other ion species.

A single energy exchange removed more than 96 percent of the heat (measured as 102(5) quanta) from the computer ion, which was a pleasant surprise to Brown, who expected that multiple interactions might be necessary. The researchers tested the energy exchange by varying the initial temperature of the computational ions and found that the technique is effective regardless of the initial temperature. They have also shown that the energy exchange operation can be performed multiple times.

Heat (essentially vibrational energy) leaks into the trapped ion system through computational activity and anomalous heating, such as the inevitable radio frequency noise in the ion trap itself. Because the computing ion absorbs heat from these sources even as it cools, removing more than 96 percent of the energy will require further improvements, Brown said.

The researchers imagine that in an operational system, the cooled atoms would be available in a reservoir next to the QCCD operations and kept at a constant temperature. Computing ions cannot be cooled directly with lasers because doing so would erase the quantum data they contain.

Excessive heat in a QCCD system negatively affects the fidelity of the quantum gates, introducing errors into the system. GTRI researchers have not yet built a QCCD that uses their cooling technique, although that is a future step in the research. Other work ahead includes accelerating the cooling process and studying its effectiveness in cooling movement along other spatial directions.

The experimental component of the rapid exchange cooling experiment was guided by simulations performed to predict, among other factors, the paths that the ions would follow as they travel within the ion trap. “We definitely understood what we were looking for and how we should achieve it based on the theory and simulations we had,” Brown said.

The unique ion trap was manufactured by collaborators at Sandia National Laboratories. The GTRI researchers used computer-controlled voltage generation cards capable of producing specific waveforms in the trap, which has a total of 154 electrodes, of which the experiment used 48. The experiments were carried out 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, manufactured and demonstrated a series of next-generation ion traps and components to support integrated quantum information systems. Among the technologies developed is the ability to transport ions precisely to where they are needed.

“We have very precise control of how ions move, the speed at which they can gather together, the potential they have when they are close to each other, and the timing needed to perform experiments like this,” Fallek said.

Other GTRI researchers involved in the project were Craig Clark, Holly Tinkey, John Gray, Ryan McGill and Vikram Sandhu. The research was carried out in collaboration with the Los Alamos National Laboratory.

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