New ion cooling technique could simplify quantum computing devices

A new cooling technique that uses a single trapped ion species for 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 that they could cool a calcium ion (which gains vibrational energy during quantum calculations) by moving an ion cold of the same species nearby. . After transferring energy from the hot ion to the cold ion, the coolant ion is returned to a nearby reservoir to be cooled for later use.

The research is reported in the journal Natural 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 quantum computing operations, this sympathetic cooling technique requires additional lasers to trap and control the coolant ions, increasing complexity and slowing down quantum computing operations.

“We have shown a new method to cool ions faster and simpler in this promising QCCD architecture,” said GTRI researcher Spencer Fallek. “Rapid exchange cooling can be faster because transporting the cooling ions requires less time than laser cooling of two different species. And it is simpler because using two different species requires using and control more lasers.”

The movement of the ions takes place in a trap maintained by precise control of the voltages which 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 ball should become stationary and not roll around in the bowl, explained Kenton Brown, a principal investigator at GTRI 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 confinement potential, which is like the bowl, from one place to another in the trap,” he said. “When we’re done moving the confinement potential to the final trap location, we don’t want the ion to move inside the potential.”

Once the hot ion and cold ion are close to each other, a simple energy exchange takes place and the original cold ion, now heated by its interaction with a computer ion, can be separated and returned to a nearby cooled ion reservoir.

The GTRI researchers have so far demonstrated a two-ion proof-of-concept system, but say their technique is applicable to using multiple computing and cooling ions, as well as other ion species.

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

Heat – essentially vibrational energy – seeps into the trapped ion system both through computing activity and through abnormal heating, such as unavoidable radio frequency noise in the ion trap itself. Because the IT ion absorbs heat from these sources even when cooled, removing more than 96 percent of the energy will require more improvements, Brown said.

The researchers envision that in an operating system, cooled atoms would be available in a reservoir next to the QCCD operations and kept at a constant temperature. Computer ions cannot be directly laser cooled, as this would erase the quantum data they contain.

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

The experimental component of the rapid exchange cooling experiment was guided by simulations performed to predict, among other factors, the paths the ions would take during their journey through the ion trap. “We fully understood what we were looking for and how to 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 boards capable of producing specific waveforms in the trap, which has a total of 154 electrodes, including 48 for the experiment. The experiments took place in a cryostat maintained at approximately 4 degrees Kelvin.

GTRI’s Quantum Systems Division (QSD) studies quantum computing systems based on individual trapped atomic ions and new quantum sensor devices based on atomic systems. GTRI researchers have designed, fabricated and demonstrated a number of cutting-edge ion traps and components to support integrated quantum information systems. Among the technologies being developed is the ability to precisely transport ions to where they are needed.

“We have very fine control over how ions move, how quickly they can be packed together, what potential they are in 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 included Craig Clark, Holly Tinkey, John Gray, Ryan McGill and Vikram Sandhu. The research was carried out in collaboration with Los Alamos National Laboratory.