Quantum information is fragile and often difficult to protect during experiments. Protecting qubits from accidental measurements is essential for controlled quantum operations, especially during state-destroying measurements or resets on adjacent qubits in protocols such as quantum error correction.

Although current methods to preserve atomic qubits against disturbances exist, these techniques can waste coherence time, extra qubits, and introduce errors.

Researchers from the University of Waterloo have demonstrated a method to measure and reset a trapped ion qubit to a known state without disturbing neighboring qubits just a few micrometers away. The distance is smaller than the width of a human hair, which is ~100 µm s thick. According to lead researcher Rajibul Islam, the researchers combined an ion trap with holographic beam shaping technology, precisely controlling laser light to overcome the bottleneck.

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Lead researcher Rajibul Islam (second from left) and his team demonstrated a method to measure a trapped qubit while preserving another just a few micrometers away. The technique combines an ion trap with holographic beam shaping technology. Courtesy of the University of Waterloo.

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The researchers said that the demonstration has the potential to influence future research in advancing quantum processors, enhancing speed and capabilities for tasks like quantum simulations in machines that already exist today, and in implementing error correction.

The researchers used quantum theory to calculate how well light can be controlled and demonstrated that the error is in fact lower than originally thought. Focusing on destructive qubit manipulation, which destroys the state of a qubit, the researchers used mid-circuit measurement to measure the state of qubits in a chain — a challenging process due to the proximity of the ions. Next, a laser beam was directed to manipulate the target qubit in a chain of qubits. The researchers ensured that laser light did not affect nearby ions just a few micrometers away, requiring extreme precision to minimize a range of interfering effects known as crosstalk.

“Trapped ion qubits are measured by using laser beams tuned to specific atomic transitions,” Islam said. “The target ion scatters photons in all directions during this process. Even with perfect control over light, there is still a risk that these scattered photons could disturb the quantum states of nearby qubits, which limits how well we can protect them.”

The researchers achieved more than 99.9% fidelity in preserving an asset ion-qubit while a neighboring process qubit is reset, and more than 99.6% preservation fidelity while applying a detection beam on the same neighboring qubit for 11 µs, which is the shortest measurement duration that was demonstrated by a separate research group.

The process of measuring a qubit without disturbing others is so fragile that, in other experiments, scientists are required to move the other qubits many hundreds of microns away to protect them. The process of moving qubits adds delay and noise to experiments.

“What we realized that for all practical levels of errors, it's how well you can control this light and how much intensity you can suppress at the surrounding qubit — the bottleneck in all these measurements,” Islam said.

The researchers said that the approach using mid-circuit measurements and resets with controlled light can be combined with other strategies. These include moving the important qubits away from the active ones or hiding quantum information in states that the measurement laser does not affect, to further reduce errors.

The research was published in Nature Communications (www. doi.org/10.1038/s41467-024-50864-2).

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