March 1, 2024

New ion cooling technique could simplify quantum computing devices

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The image shows the ion trap used to control the location of computational ions and refrigerants. The device was produced by Sandia National Laboratories. Credit: Sandia National Laboratories.

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The image shows the ion trap used to control the location of computational ions and refrigerants. The device was produced by Sandia National Laboratories. Credit: Sandia National Laboratories.

A new cooling technique that uses a single trapped ion species for both computation and cooling could simplify the use of quantum charge-coupled devices (QCCDs), potentially bringing quantum computing closer to practical applications.

Using a technique called rapid ion exchange cooling, scientists at the Georgia Tech Research Institute (GTRI) showed they could cool a calcium ion – which gains vibrational energy when doing quantum calculations – by moving a cold ion of the same species into close proximity. . After transferring energy from the hot to the cold ion, the coolant ion returns to a nearby reservoir to be cooled for later use.

The research is published in the journal Nature Communications.

Conventional ion cooling for QCCDs involves the use of two different species of ions, with cooling ions coupled to different wavelength lasers that do not affect the ions used for quantum computing. In addition to the lasers needed to control quantum computing operations, this sympathetic cooling technique requires additional lasers to capture and control the cooling ions, and this increases the complexity and slows down quantum computing operations.

“We have shown a new method to cool ions more quickly and simply in this promising QCCD architecture,” said Spencer Fallek, research scientist at GTRI. “Quick-switch cooling can be faster because 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 video shows how a computational ion can be cooled by bringing it closer to a refrigerant ion of the same atomic species. Credit: Georgia Technological Research Institute

The movement of ions occurs 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 at the bottom.

When the bowl stops moving, the marble should stay still — and not roll around in the bowl, explained Kenton Brown, GTRI’s 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 move the confining potential, which is like the bowl, from one place in the trap to another,” he said. “When we’re done moving the confining potential to the final trap location, we don’t want the ion to move within the potential.”

Once the hot ion and cold ion are close to each other, a simple exchange of energy occurs and the original cold ion – now heated by its interaction with a computational ion – can be separated and returned to a nearby reservoir of cooled ions. .

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 computational ion, which was a pleasant surprise for Brown, who had expected that multiple interactions might be necessary. The researchers tested the energy exchange by varying the initial temperature of the computational ions and found the technique to be effective regardless of the initial temperature. They also demonstrated that the energy exchange operation can be performed multiple times.

Heat – essentially vibrational energy – penetrates the trapped ion system through computational activity and anomalous heating, such as the inevitable radio frequency noise in the ion trap itself. Because the compute ion absorbs heat from these sources while it is being cooled, removing more than 96% of the energy will require further improvements, Brown said.

The researchers envision that in an operational system, cooled atoms would be available in a reservoir alongside QCCD operations and kept at a constant temperature. Computing ions cannot be cooled directly with a laser 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 this is a future step in research. Other future work includes accelerating the cooling process and studying its effectiveness in moving cooling along other spatial directions.

The experimental component of the rapid exchange cooling experiment was guided by simulations designed to predict, among other factors, the paths that ions would take on their journey within the ion trap. “We definitely understood what we were looking for and how we should go about achieving 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 boards capable of producing specific waveforms in the trap, which has a total of 154 electrodes, of which the experiment used 48. The experiments took place in a cryostat kept at about 4 degrees Kelvin.

Researchers Spencer Fallek (left) and Kenton Brown are shown with equipment used to develop a new technique for cooling ions in quantum devices. Credit: Sean McNeil, GTRI

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Researchers Spencer Fallek (left) and Kenton Brown are shown with equipment used to develop a new technique for cooling ions in quantum devices. Credit: Sean McNeil, GTRI

GTRI’s Quantum Systems Division (QSD) investigates quantum computing systems based on trapped individual 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 precisely transport ions to where they are needed.

“We have very precise control over how ions move, the speed at which they can come together, the potential they present when they are close to each other, and the time required to do experiments like this,” Fallek said.

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

More information:
Spencer D. Fallek et al, Fast exchange cooling with trapped ions, Nature Communications (2024). DOI: 10.1038/s41467-024-45232-z

Diary information:
Nature Communications

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