Reviewed by Danielle Ellis, B.Sc.Sep 24 2024
Researchers from the University of Waterloo have shown in a study published in Nature Communications that they can measure and reset a trapped ion qubit to a known state without interfering with nearby qubits that are only a few micrometers away—a distance that is less than the thickness of a human hair, which is roughly 100 micrometers.
Rajibul Islam. Image Credit: University of Waterloo
Since quantum information is brittle, it can be challenging to safeguard during experiments. Controlled quantum operations require protecting qubits from unintentional measurements, particularly when state-destroying measurements or resets on neighboring qubits occur in protocols such as quantum error correction. Existing techniques to protect atomic qubits from perturbations can introduce errors, waste coherence time, and require additional qubits.
This demonstration could greatly impact future research in the field. It could lead to advancements in quantum processors, improved speed and capabilities for tasks like quantum simulations in machines already in use, and the implementation of error correction.
This breakthrough was made possible by a team headed by Rajibul Islam, a faculty member at the Institute for Quantum Computing (IQC) and professor in the Department of Physics and Astronomy, along with postdoctoral fellow Sainath Motlakunta and students from their research group.
They overcame the once-impossible task of safeguarding qubits while measuring others at such close ranges by carefully regulating the laser light used in these operations.
Islam and his colleagues have been ensnaring ions for use in quantum simulation in the Laboratory for Quantum Information since 2019. This new demonstration builds on a breakthrough the group made in 2021 with programmable holographic technology, which, when paired with an ion trap, demonstrated that it is feasible to manipulate and destroy a single qubit.
We have used the holographic beam shaping technology, combined it with our ion trap and demonstrated 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, Postdoctoral Fellow, University of Waterloo
Students in the group showed that the error is actually smaller than previously believed by using quantum theory to compute how well light can be controlled. "Mid-circuit" measurement is used to measure the state of qubits in a chain, concentrating on destructive qubit manipulation, which destroys a qubit's state. This is a difficult process because of the ions' close proximity.
The target qubit in a chain of qubits is then manipulated by a laser beam. In order to minimize a range of interfering effects known as crosstalk, researchers must make sure that laser light does not affect nearby ions just a few micrometers away. This requires incredibly precise measurements.
Trapped ion qubits are measured by using laser beams tuned to specific atomic transitions. 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, Professor, Department of Physics and Astronomy, University of Waterloo
This is where the group's holographic technology, which is among the most accurate light-controlling technologies, entered the picture, enabling the laser light to be precisely targeted and controlled.
When a neighboring "process" qubit is reset, the group achieves more than 99.9% fidelity in preserving an "asset" ion-qubit. When a detection beam is applied to the same neighboring qubit for 11 microseconds—the shortest measurement duration ever recorded by another research group—they achieve more than 99.6% fidelity in preservation.
The process of measuring one qubit without disturbing others is so delicate that in a few experiments where this is possible, the other qubits are moved hundreds of microns away to protect them. The process of moving qubits introduces delay and noise into experiments.
Islam added, “It's something that was considered to be impossible. When I thought about it, why can I not just go and measure one qubit without moving anything? Pretty much everybody in our field said it was a bad idea and to not even try because it's so fragile. Part of this work is getting out of this mindset, that it is so destructive that this process cannot be attempted. 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.”
The study was co-authored by Sainath Motlakunta, Nikhil Kotibhaskar, Chung-You Shih, Anthony Vogliano, Darian McLaren, Lewis Hahn, Jingwen Zhu, Roland Hablützel and Rajibul Islam.
Journal Reference:
Motlakunta, S., et. al. (2024) Preserving a qubit during state-destroying operations on an adjacent qubit at a few micrometers distance. Nature Communications. doi.org/10.1038/s41467-024-50864-2