Sandia National Laboratories and the University of New Mexico researchers recently published a study in the journal PRX Quantum that proved, for the first time, a viable approach to detecting these “leakage errors” for neutral atom platforms.
Matthew Chow, center, and Bethany Little discuss with Yuan-Yu Jau, off camera, the first practical way to detect atom loss for neutral atom quantum computing at Sandia National Laboratories. Image Credit: Craig Fritz
Quiet quitting is not limited to burned-out employees. Atoms containing information inside quantum computers, known as qubits, occasionally depart silently from their positions. This undesirable condition, known as atom loss, corrupts data and disrupts calculations.
The accomplishment removes a fundamental impediment to one part of quantum computing, moving scientists closer to achieving the technology’s true potential. Many researchers believe quantum computers will help disclose truths about the universe that are impossible to discover using existing technology.
We can now detect the loss of an atom without disturbing its quantum state.
Yuan-Yu Jau, Study Principal Investigator and Atomic Physicist, Sandia National Laboratories
In the study, the team stated that its circuit-based method obtained 93.4% accuracy. The detection method allows researchers to identify and fix errors.
The research was funded by Sandia’s Laboratory Directed Research and Development program.
Detection Heads Off a Looming Crisis
Atoms are tricky little things. In some quantum computers, scientists manage them by freezing them at temperatures just above absolute zero, around -460 degrees Fahrenheit. They spring free when the temperature is a thousandth of a degree above normal. Even at the optimal temperature, they can escape by chance.
If an atom disappears amid a calculation, “The result can be completely useless. It’s like garbage,” Jau added.
A detection strategy can notify researchers whether they can trust the results and may lead to a method for fixing errors by filling in identified gaps.
According to Matthew Chow, the study’s lead, atom loss is a manageable inconvenience in small-scale machines since they have fewer qubits and hence have a smaller likelihood of losing one at any given time.
However, the future looks dismal. Millions of qubits are required to build useful quantum computers. With that many, the chances of losing them mid-program increase. Atoms would silently wander away from the jobsite in large numbers, leaving scientists with the useless task of attempting to utilize a computer that is actually vanishing in front of their eyes.
Jau stated, “This is super important because if we don’t have a solution for this, I don’t think there is a way to keep moving forward.”
Researchers discovered strategies to identify atom loss and other types of leakage defects in various quantum computing systems, such as those that use electrically charged atoms, known as trapped ion qubits, rather than neutral ones.
The New Mexico-based team is the first to identify atom loss in neutral atom systems without using damaging methods. By adopting basic circuit-based strategies to detect leakage problems, the team is assisting in avoiding the crisis of uncontrollable future leaking.
Just Don’t Look
The difficulty in detecting atom loss stems from the fact that scientists are unable to examine the atoms that must be preserved throughout the computation.
Jau further added, “Quantum calculations are extremely fragile.”
The operation fails if researchers attempt to observe the status of a qubit while it is in operation.
Erwin Schrödinger, an Austrian physicist, famously equated this concept to putting a cat in a box with something that would randomly kill it. Schrödinger noted that in quantum physics, the cat can be considered both dead and living until the box is opened.
“It is very easy to have a mathematical description of everything in terms of quantum computing. But to visualize entangled quantum information, it is hard,” Jau noted.
So, how can one determine whether an atom is in the processor without observing it?
The idea is analogous to having Schrödinger’s cat in a box, and putting that box on a scale, where the weight of the box tells you whether or not there’s a cat, but it doesn’t tell you whether the cat’s dead or alive.
Matthew Chow, Sandia National Laboratories
Surprise Finding Fuels Breakthrough
Chow, who was an intern at Sandia Labs during the project and a PhD candidate at the University of New Mexico, said he never anticipated this discovery.
Chow added, “This was certainly not a paper that we had planned to write.”
At Sandia, he was working on his dissertation by troubleshooting a small portion of the code for quantum computing. By repeatedly performing an operation and comparing the outcomes when two atoms interact versus when only one atom is present, the code diagnoses the entangling interaction, a special quantum phenomenon that connects the states of atoms.
The atoms alternate between entangled and disentangled states as a result of the operation’s recurrent application during the interaction. He noticed a significant pattern in this comparison.
The result for the two-atom scenario differed significantly from the solo-atom instance on each other run when the atoms were disentangled.
Without trying, Chow recognized he had discovered a faint signal indicating the presence of an adjacent atom in a quantum computer without directly viewing it. The oscillating measurement was the scale for determining whether the cat was still in the box.
Chow noted, “This was the thing that got me really excited — that made me show it to Vikas.”
Vikas Buchemmavari, a PhD student at UNM and frequent collaborator, knew more about quantum theory than Chow. He works in a research group led by Ivan Deutsch, the director of UNM’s Center for Quantum Information and Control.
I was simultaneously very impressed by the gate quality and very excited about what the idea meant: we could detect if the atom was there or not without damaging the information in it.
Vikas Buchemmavari, PhD Student, University of New Mexico
Sandia Labs Verifies Technique
He began formalizing the concept into a set of codes designed to identify atom loss. It would employ a second atom, unrelated to any calculations, to indirectly identify whether an atom of interest is absent.
“Quantum systems are very error-prone. To build useful quantum computers, we need quantum error correction techniques that correct the errors and make the calculations reliable. Atom loss— and leakage errors — are some of the worst kinds of errors to deal with,” Buchemmavari added.
The two then devised methods for testing their idea.
Chow further noted, “You need to test not only your ability to detect an atom but to detect an atom that starts in many different states. And then the second part is to check that it doesn’t disturb that state of the first atom.”
Chow’s Sandia team also jumped on board, testing the new process and verifying its results by comparing them to a method for directly viewing the atoms.
“We had the capability at Sandia to verify it was working because we have this measurement where we can say the atom is in the one state or the zero state or it’s gone. A lot of people don’t have that third option,” Sandia’s Bethany Little stated.
A Guide for Correcting Atom Loss
“We hope this work serves as a guide for other groups implementing these techniques to overcome these errors in their systems. We also hope this spurs deeper research into the advantages and trade-offs of these techniques in real systems,” Buchemmavari stated.
Chow, who has since received his doctorate, said he is proud of the discovery because it demonstrates that the problem of atom loss is solved, even if future quantum computers do not employ his exact method.
Chow concluded, “If you are careful to keep your eyes open, you might spot something really useful.”
Journal Reference:
Chow, M. N. H., et. al. (2024) Circuit-Based Leakage-to-Erasure Conversion in a Neutral-Atom Quantum Processor. PRX Quantum. doi.org/10.1103/PRXQuantum.5.040343