Scientists achieve ‘impossible task’ to control light while measuring trapped ion qubits

One of the most complex problems scientists face while working with quantum information is ensuring qubits remain protected. This is because every time they reset, measure, or delete even a single qubit, neighboring qubits can be damaged, leading to loss of information.

A new study from the University of Waterloo researchers proposes a solution to this problem. The study authors have devised a way to precisely control the laser light used to manipulate qubits.

They even performed an experiment to perform this almost impossible task. The experiment involved measuring and resetting a trapped ion qubit to a known state without causing any damage or disturbance to adjacent qubits located only a few micrometers away.

“This demonstration has the potential to significantly impact future research in the field, including advancing quantum processors, enhancing speed and capabilities for tasks like quantum simulations in machines that already exist today, and implementing error correction.” the study authors note .

Why do we often fail to save the qubits?

The current methods for protecting qubits have several limitations. For instance, they often require additional resources like extra qubits or redundant processes to correct errors.

This can lead to wasted coherence time , the time qubits remain in their quantum state without disturbance. Such methods can also introduce new errors during the correction process, reducing overall efficiency and reliability.

These limitations make it very challenging to manipulate or even access qubits. The study authors came up with an interesting solution to this problem. They have been working on two separate technologies; ion trap qubits and beam holographic shaping .

The former is employed to readout, reset, and manipulate qubits using laser light. During this process, qubits are represented by ions that are confined in electromagnetic fields. The latter is used for shaping and manipulating laser light through optical elements such as holograms .

They decided to use both technologies together to control laser light such that it doesn’t disturb any other qubits in a system when they are working on a particular qubit.

The trick to deal with qubits without damaging them

The study authors decided to alter the quantum state of a qubit. So first, they calculated its quantum state using mid-circuit measurement, a process in quantum computing where the state of a qubit is measured while other operations are still ongoing.

They then used a laser that was precisely controlled in a trapped-ion qubit setup, utilizing the holographic beam shaping technology. This combined approach kept the light focused on its target and prevented it from reaching other qubits.

However, “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,” Rajibul Islam, senior study author and a professor at the University of Waterloo, said.

The holographic approach allowed the researchers to control and restrict the scattered photons. As a result, the experiment was successful and didn’t lead to any disturbance or damage to the neighboring qubits .

“We demonstrate the feasibility of in-situ state-reset and state-measurement of trapped ions, achieving  >99.9% fidelity in preserving an ‘asset’ ion-qubit while a neighboring ‘process’ qubit is reset, and  >99.6% preservation fidelity while applying a detection beam for 11 μs on the same neighbor at a distance of 6 μm,” the study authors note.

This is an incredible achievement because until now, it was considered almost impossible to measure a single qubit without shaking everything around it. “Pretty much everybody in our field said it was a bad idea and to not even try because it’s so fragile,” Islam added.

However, we showed, “It is indeed possible to destroy any specific qubit you want while maintaining the quantum information in the other qubits that you don’t want to destroy,” Sainath Motlakunta, first author of the study and a postdoctoral fellow at the University of Waterloo, said.

The study is published in the journal Nature Communications .