a fresh approach to error correction.
A significant obstacle to a potent new area of computing may be removed thanks to the discovery of a brand-new method for fixing faults in quantum computer calculations.
In older computers, error correction is a well-developed topic. Each and every cellphone needs to be checked and adjusted in order to send and receive data through cluttered radio waves. The ability of quantum computers to solve complex problems that are beyond the capabilities of ordinary computers depends on their ability to control the exceedingly ephemeral behaviour of subatomic particles. These computing actions are so fleeting that even testing them for errors runs the risk of bringing the entire system to an end.
In a theoretical paper published in Nature Communications, an interdisciplinary team led by Jeff Thompson, an associate professor of electrical and computer engineering at Princeton University, and including Yue Wu, Shruti Puri, and Shimon Kolkowitz from Yale University and the University of Wisconsin-Madison showed how they could significantly increase a quantum computer’s tolerance for faults and decrease the amount of redundant information required to isolate and fix those faults. The new method, which increases the allowed error rate from 1% to 4%, makes quantum computers, which are currently being developed, viable.
The operations you wish to perform on quantum computers are noisy, according to Thompson, which means that calculations are subject to a variety of failure modes.
In a conventional computer, an error can be as simple as a bit of memory accidentally flipping from a 1 to a 0, or as messy as one wireless router interfering with another. Building in some redundancy so that each piece of data is compared with duplicate copies is a typical strategy for handling such problems. However, that strategy calls for more data and raises the likelihood of mistakes. Therefore, it only functions when the vast majority of the available information is accurate. Otherwise, comparing incorrect data to incorrect data only serves to deepen the inaccuracy.
According to Thompson, redundancy is a bad technique if your baseline error rate is too high. The biggest obstacle is lowering that barrier.
Thompson’s team simply increased the visibility of errors rather than concentrating only on lowering the amount of errors. As a result of their extensive research into the physical sources of mistake, the team developed a technique that efficiently eliminates the most frequent source of error rather than simply altering the damaged data. According to Thompson, this behaviour is an example of a specific type of error known as a “erasure error,” which is inherently simpler to weed out than corrupted data that still appears to be all the other data.
In a traditional computer, it could be dangerous to presume that the slightly more common 1s are correct and the 0s are incorrect if a packet of presumably duplicate information appears as 11001. However, the case is stronger if the information appears as 11XX1, where the damaged bits are obvious.
Because you are aware of the erasure mistakes, Thompson claimed that they are much simpler to fix. They may not participate in the majority vote. That is a significant benefit.
Erasure faults in conventional computing are well understood, but researchers hadn’t previously thought about trying to construct quantum computers to turn errors into erasures, according to Thompson.
Their device could, in fact, sustain an error rate of 4.1%, which Thompson claimed is well within the realm of possibility for existing quantum computers. The most advanced error correction in prior systems, according to Thompson, could only tolerate errors of less than 1%, which is beyond the capacity of any existing quantum system with a significant number of qubits.
The team’s ability to produce erasure errors ended up being a surprising benefit of a decision Thompson made in the past. His work examines “neutral atom qubits,” in which a single atom is used to store a “qubit” of quantum information. They were the ones who invented this application of the element ytterbium. As opposed to the majority of other neutral atom qubits, which have just one electron in their outermost layer of electrons, ytterbium possesses two in this layer, according to Thompson.
As an analogy, Thompson remarked, “I picture it as a Swiss army knife, and this ytterbium is the bigger, fatter Swiss army knife.” “You get a lot of new tools from that extra little bit of complexity you get from having two electrons.”
Eliminating errors turned out to be one use for those extra tools. The group suggested boosting ytterbium electrons from their stable “ground state” to excited levels known as “metastable states,” which can be long-lived under the appropriate circumstances but are fundamentally brittle. The researchers’ proposal to encode the quantum information using these states is counterintuitive.
The electrons seem to be walking a tightrope, Thompson remarked. Additionally, the system is designed so that the same elements that lead to inaccuracy also result in electrons slipping off the tightrope.
A collection of ytterbium qubits can be illuminated, but only the defective ones light up because, as an added bonus, the electrons scatter light extremely visibly after they reach the ground state. Those that illuminate ought to be discounted as mistakes.
This development requires merging knowledge from the theory of quantum error correction and the hardware of quantum computing, drawing on the interdisciplinary nature of the research team and their close cooperation. Although the mechanics of this configuration are unique to Thompson’s ytterbium atoms, he claimed that the idea of engineering quantum qubits to produce erasure errors could be a useful objective in other systems, many of which are currently being developed globally. He added that the group is still working on this idea.
According to Thompson, other groups have already started designing their systems to turn errors into erasures. “We see this research as laying out a kind of architecture that might be utilised in many various ways,” Thompson said. “We already have a lot of interest in discovering adaptations for this task,” said the researcher.
Thompson’s team is currently working on a demonstration of the transformation of errors into erasures in a modest operational quantum computer that integrates several tens of qubits as a next step.
Citation: Yue Wu, Shimon Kolkowitz, Shruti Puri, and Jeff D. Thompson, “Erasure conversion for fault-tolerant quantum computing in alkaline earth Rydberg atom arrays,” Nature Communications, 9 August 2022.
DOI: 10.1038/s41467-022-32094-6
The Office of Naval Research, the National Science Foundation, the Army Research Office, the Defense Advanced Research Projects Agency, and the Alfred P. Sloan Foundation all provided funding for the study.
