Controlling light while measuring trapped ion qubits

Quantum information is fragile and often difficult to protect during experiments. Protecting qubits from accidental measurements is essential for performing controlled quantum operations, especially when protocols such as quantum error correction involve measurements or resets that corrupt the state of neighboring qubits.

Current methods for protecting atomic qubits from disturbances waste coherence time or extra qubits, which can introduce errors.

Researchers at the University of Waterloo have successfully demonstrated the ability to measure and reset trapped ion qubits to a known state without disturbing neighboring qubits just a few micrometers away – a distance smaller than the width of a human hair, which is about 100 micrometers thick.

This demonstration could have profound implications for future research in this field, including advances in quantum processors, improving the speed and capability of tasks such as quantum simulation on existing machines, and implementing error correction.

The breakthrough was achieved by a team led by Institute for Quantum Computing (IQC) faculty member and professor in the Department of Physics and Astronomy Rajbul Islam, along with postdoctoral researcher Sainath Motlakunta and students from his research group. The research is published in the journal Nature Communications.

By precisely controlling the laser light used for these operations, they overcame a challenge once thought impossible: protecting a qubit while measuring other qubits at very close ranges.

Since 2019, Islam and his team have been trapping ions to be used in quantum simulations at the Quantum Information Institute. This new demonstration is the next step after a breakthrough the group achieved in 2021 using programmable holographic techniques, which, in combination with ion trapping, proved it is feasible to manipulate and destroy just a single quantum bit.

“We used holographic beam shaping techniques and combined it with ion trapping to demonstrate that it is indeed possible to destroy any particular qubit while preserving the quantum information in other qubits that you don’t want to destroy,” Motlakunta says.

The students in the group used quantum theory to calculate how much control light could have, demonstrating that the error is actually lower than the researchers originally thought. They focused on destructive qubit operations, which destroy the state of qubits, and used “middle circuit” measurements to measure the state of qubits in the chain, a difficult process due to the close proximity of the ions.

A laser beam is then fired to manipulate a target qubit in the chain of qubits. The researchers need to be extremely precise to ensure that the laser light does not affect nearby ions just a few micrometres away, minimising a variety of interference effects known as crosstalk.

“The trapped ion qubit is measured using a laser beam tuned to a specific atomic transition,” says Islam. “During this process, the target ion scatters photons in all directions. Even if we had perfect control over the light, there would still be a risk that scattered photons would disturb the quantum state of nearby qubits, limiting how well we can protect the qubits.”

That’s where the group’s holographic technology, one of the most precise techniques used to control light, comes in, allowing laser light to be precisely aimed and controlled.

The group achieved greater than 99.9% fidelity preserving an “asset” ion qubit while an adjacent “process” qubit was reset, and greater than 99.6% preservation fidelity while applying a detection beam to the same adjacent qubit for 11 microseconds, the shortest measurement period demonstrated by another research group.

The process of measuring one qubit without disturbing the other qubits is very fragile, so in some other experiments, where this is possible, the other qubits are moved hundreds of microns away to protect them. The process of moving the qubits adds delay and noise to the experiment.

“That was something that was thought to be impossible,” Islam says. “If you think about it, why can’t you just measure a single qubit without moving anything? Almost everyone in our field said it was a bad idea, and that it would be so fragile that we shouldn’t even try.”

“Part of this research is about breaking away from the mindset that this process is too destructive to even be attempted. What we’ve realized is that at any practical level of error, how well we can control this light, and how much intensity we can suppress with the surrounding qubits, is the bottleneck in all these measurements.”

The group’s approach, using controlled light intermediate circuit measurements and resets, can be combined with other strategies, such as keeping critical qubits away from active qubits or hiding quantum information in states that the measurement laser cannot affect, to further reduce errors.