Quantum computers that recycle their qubits can limit errors

Quantum computers, made from qubits based on extremely cold atoms, are getting larger at an impressive rate, which could soon make them computationally powerful – but errors occur at a rate that limits their usefulness. Now researchers have figured out how to replenish and reuse these qubits to make their calculations more practical and reliable.

All existing quantum computers are too error-prone to perform calculations that simultaneously useful and give them an advantage over traditional computers, but researchers have made great strides in developing error correction schemes that could solve this problem.

In one such scheme, the building blocks of a quantum computer, called qubits, are divided into two key groups: qubits tasked with manipulating data and used to perform calculations, and others called “auxiliary qubits” that monitor errors.

Creation many high quality qubits for any purpose is a big technical problem, so Matt Norcia at the American firm Atom Computing and his colleagues have developed a way to reuse or replace auxiliary qubits, reducing their number. They have now shown that their error tracking qubits can be recycled 41 times in a row.

“Any usage calculation is likely to be a very long calculation, so you'll have to do many rounds of measurements. Ideally, you want to be able to reuse qubits over multiple rounds so you don't have to keep providing more qubits to the system,” says Norcia.

He and his colleagues used qubits made from electrically neutral ytterbium atoms, cooled to temperatures very close to absolute zero using lasers and electromagnetic pulses. They could manipulate the quantum state and quantum properties that encode information for each atom using lasers designed like “optical tweezers.” The team used this technique to divide their quantum computer into three different zones.

In the first zone, 128 optical tweezers guided qubits to perform calculations, and in the second zone, 80 tweezers contained qubits that could be used to measure errors and replace any erroneous qubit with them. The third zone served as storage, housing another 75 qubits that had just been converted into a usable state. Having these last two zones allowed the researchers to either reset and reuse the auxiliary qubits or replace them with new ones.

Norcia says getting this system to work has been difficult because any stray light from one laser that touches a nearby qubit can disrupt its operation. Because of this, researchers have had to develop precise controls over their lasers, as well as ways to tune the states of data qubits so that they remain “hidden” from or undisturbed by certain types of harmful light, he says.

“Reusing Ancilla is fundamental to the progress of quantum computing,” says Yuval Boger at the American quantum computing company QuEra. Without this capability, even very modest calculations would require millions or billions of qubits, which is simply unrealistic for any existing or future quantum computing hardware, he says.

This problem is recognized throughout the atomic qubit research community. “I think everyone is in a neutral atom [quantum computing] Space understands the need to reset and reboot atoms during computation,” says Norcia.

For example, Boger notes that a team of researchers from Harvard University and the Massachusetts Institute of Technology used a similar method to preserve a quantum computer made from 3000 ultracold rubidium atoms running for several hours. Some quantum computers with qubits made of ions driven by light, e.g. Helios machine which Quantinuum recently debuted can also reuse qubits.