By Kortny Rolston-Duce
When it comes to transporting ions, researchers at Quantinuum have turned a corner. Both literally and figuratively.
The Quantinuum team can now move two different types of ions through a junction in a surface trap, a tiny electrode-filled device at the heart of trapped ion quantum computers.
In a pre-print publication posted on arXiv, Quantinuum researchers outlined how they developed new waveforms that can guide a pair of ytterbium and barium ions through an intersection without the charged particles becoming overly excited or flying out of the trap.
The team tested the technique on a prototype trap with a grid-like architecture that Quantinuum has designed and microfabricated. This trap design will be a central part of future quantum computers such as the System Model H3.
This feat is an important breakthrough in the world of trapped ion quantum computing and for Quantinuum.
The ability to transport paired ions through a junction at the same time and intact is critical for scaling trapped ion systems. It’s also a longstanding technical challenge that trapped ion researchers in academia, government and industry have sought to solve for years.
“What Quantinuum has accomplished is a significant breakthrough for the field of trapped ion research and for our technology,” said Tony Uttley, president, and chief operating officer at Quantinuum. “This will enable us to deliver faster quantum computers with more qubits and fewer errors.”
Smooth transport of ions
Quantinuum’s technologies are based on the Quantum Charged Coupled Device (QCCD) architecture, a concept first introduced by the Ion Storage Group at the National Institute of Standards and Technology (NIST) in the early 2000s.
Like other trapped ion technologies, this architecture relies on traps to capture ions in electric fields - or wells. Gates are performed on small chains of ions, which can be reordered and reconfigured within the architecture, enabling all-to-all connectivity.
In Quantinuum’s System Model H1 technologies, each well contains an ytterbium ion and a barium ion. The ytterbium ion functions as a qubit while the barium is cooled with a laser to reduce the motions of both ions, a technique known as sympathetic cooling. This cooling makes it possible to maintain low error rates in quantum computing operations for long calculations.
The H1-1 and H1-2 machines currently use a trap with a simple geometry or design that resembles railroad tracks. Wells of ions are moved back and forth along these linear tracks and swapped as needed to run an algorithm.
This linear design works well with fewer qubits. But it has limitations that make scaling difficult. Adding hundreds, much less thousands of qubits, would require the tracks to be much longer. It also would take more time to reposition and reset qubits.
To overcome these challenges, Quantinuum researchers have proposed moving to traps with more complex geometries. The System Model H2 will incorporate a racetrack-like design. The System Model H3 and beyond will use two-dimensional traps that resemble a city street grid with multiple railroad lines and intersections.
This design, however, also poses challenges. Getting those tracks to behave well at intersections is difficult and can jar ions and cause unwanted motion – especially those with different masses. It is somewhat like maneuvering a bullet train and allowing it to turn left or right at 90 degrees, or continue moving straight, without causing the cars to rock.
Quantinuum researchers were able to turn an ytterbium-barium ion pair around sharp corners with little motion. Until now, researchers envisioned having to separate paired ions and move them through junctions one a time, which would dramatically slow the operation. “To our knowledge, this is the first time any team has simultaneously moved two different species of ions through a junction in a surface trap,” said Dr. Cody Burton, a senior advanced physicist who worked on the project and lead author of the arXiv paper.
Researchers will continue to test and refine this new method.
Their goal is to expand from moving a single well to transporting several through multiple junctions at the same time. From there, they plan to incorporate this methodology into the System Model H3, which is expected to be the first Quantinuum quantum technology with the two-dimensional, grid-like trap.
“This new configuration will be key for scaling quantum computers in the hundreds, and then thousands, of high-fidelity qubits,” Uttley said. “While scaling, the qubits will maintain the high-quality characteristics such as low gate errors, long coherence times, and low cross-talk for which Quantinuum’s technologies are known.”