99.9904% SPAM Fidelity with barium-137 sets the standard and creates a further step towards solving some of the world’s most intractable problems.
By Kortny Rolston-Duce
Quantinuum researchers have set a record for the number of times they were able to place qubits into a quantum state and then measure the results, beating the previously stated best in class many times over.
The team led by Alex An, Tony Ransford, Andrew Schaffer, Lucas Sletten, John Gaebler, James Hostetter, and Grahame Vittorini achieved a state preparation and measurement, or SPAM, fidelity of 99.9904 percent — the highest of any quantum technology to date — using qubits formed from non-radioactive barium-137. The results, which are detailed here, have been submitted to arXiv.
This work has major implications for the quantum industry and trapped-ion technologies.
Improving SPAM fidelity helps reduce errors that accumulate in today’s “noisy” quantum machines, which is critical for moving to “fault tolerant” systems that prevent errors from cascading through a system and corrupting circuits.
In addition, being able to form qubits from barium-137 and place them into a quantum state with high fidelity is advantageous for scaling trapped-ion hardware systems. Researchers can use lasers in the visible spectrum, a more mature and readily available technology, to initialize and manipulate qubits.
“This is a major step forward for the Quantinuum team and our high-performing trapped-ion quantum hardware,” said Tony Uttley, Quantinuum president and chief operating officer. “The advancement of the quantum computing industry as a whole is going to come from lots of individual technological achievements like this one, paving the way for future fault-tolerant systems.”
What is SPAM?
For most people, the word “spam” conjures images of unwanted emails flooding an inbox or of chopped pork in a can.
In quantum computing, SPAM stands for state preparation and measurement - two of the five conditions identified by theoretical physicist David DiVincenzo as necessary for the operation of quantum computer. It refers to initializing qubits (placing them in a quantum state) and then measuring the output. SPAM is measured in terms of fidelity, or the ability to complete these tasks at a high rate of success. The higher the fidelity the better because it means a quantum computer is performing these critical tasks with fewer errors.
Researchers at Quantinuum believe SPAM fidelity will need to hit 99.97 to 99.99 percent to reach the point at which the logical error rate beats the leading order physical error rate.
Neutral ytterbium atoms have long been a source of ions in trapped-ion quantum computers. Charged by lasers, ytterbium ions are transformed into qubits. But using ytterbium presents challenges. Expensive ultraviolet lasers are needed to manipulate ytterbium ions and the results can be difficult to measure.
Barium ions, however, are easier to measure and can be manipulated with less expensive and more stable lasers in the green range. But until this work with non-radioactive barium-137, researchers have only been able to achieve low SPAM errors with barium-133 atoms, which are radioactive and require special handling.
“Nobody thought you could do quick, robust SPAM with non-radioactive barium-137,” said Dr. Anthony Ransford, a Quantinuum physicist and technical lead. “We were able to devise a scheme that enabled us to initialize the qubits and measure them better than any other qubits. We are the first to do it.”
Being able to initialize non-radioactive barium-137 ions is just the first step. The goal is to incorporate these ions into future Quantinuum hardware technologies.
“We believe using non-radioactive barium-137 ions as qubits is an attractive path to increasingly robust, scalable, quantum hardware,” Uttley said.