‘Erasure’ – A new discovery could hold the key to practical quantum computing
A new method of error correction.
Researchers have discovered an entirely new technique for correcting errors in quantum computer calculations, potentially removing a significant barrier to a powerful new field of computing.
Error correction is a well-developed topic in traditional computers. To transmit and receive data over disordered waves, each mobile phone requires checks and adjustments. Quantum computers have immense potential to solve complex problems that conventional computers cannot solve, but this ability depends on harnessing the incredibly short-lived behavior of subatomic particles. These computing behaviors are so transient that even inspecting them for faults could cause the entire system to collapse.
An interdisciplinary team led by Jeff Thompson, associate professor of electrical and computer engineering at Princeton University, and collaborators Yue Wu and Shruti Puri at Yale University and Shimon Kolkowitz at the University of Wisconsin-Madison, demonstrated in a theoretical article published in Nature Communication that they could dramatically improve a quantum computer’s fault tolerance and reduce the amount of redundant information needed to isolate and correct errors. The new approach quadruples the acceptable error rate, from 1% to 4%, making it practical for quantum computers currently under development.
“The fundamental challenge for quantum computers is that the operations you want to do are noisy,” Thompson said, which means the calculations are subject to a myriad of failure modes.
In a conventional computer, an error can be as simple as a piece of memory accidentally jumping from a 1 to a 0, or as messy as a wireless router interfering with another. A common approach to dealing with such flaws is to build in some redundancy so that each piece of data is compared against duplicate copies. However, this approach increases the amount of data needed and creates more opportunity for errors. Therefore, it only works when the vast majority of information is already correct. Otherwise, checking bad data against bad data leads deeper into a pit of error.
“If your baseline error rate is too high, redundancy is a bad strategy,” Thompson said. “Getting below that threshold is the main challenge.”
Rather than just focusing on reducing the number of errors, Thompson’s team essentially made errors more visible. The team delved into the actual physical causes of errors and designed their system so that the most common source of error effectively eliminates, rather than simply corrupts, corrupted data. Thompson said this behavior represents a special type of error known as a “wipe error,” which is fundamentally easier to eliminate than corrupted data but still looks just like any other data.
In a conventional computer, if a packet of supposedly redundant information appears as 11001, it can be risky to assume that the slightly more prevalent 1s are correct and the 0s are wrong. But if the information appears as 11XX1, where the corrupted bits are obvious, the case is more compelling.
“These erase errors are much easier to fix because you know where they are,” Thompson said. “They can be excluded from the majority vote. It’s a huge advantage.
Erasure errors are well understood in conventional computing, but researchers hadn’t previously considered trying to design quantum computers to convert errors into erasures, Thompson said.
In practice, their proposed system could sustain an error rate of 4.1%, which Thompson says is well within the realm of possibility for current quantum computers. In previous systems, state-of-the-art error correction could handle less than 1% error, which Thompson says is at the limit of the capabilities of any current quantum system with a large number of qubits.
The team’s ability to generate erasure errors turned out to be an unexpected benefit of a pick made by Thompson years ago. His research explores the “neutral atom qubits”, in which quantum information (a “qubit”) is stored in a single atom. They pioneered the use of the element ytterbium for this purpose. Thompson said the group chose ytterbium in part because it has two electrons in its outermost electron shell, compared to most other neutral atom qubits, which have only a.
“I consider it a Swiss army knife, and this ytterbium is the biggest, biggest Swiss army knife,” Thompson said. “That little bit more complexity you get from having two electrons gives you a lot of unique tools.”
A use of these additional tools has proven useful in eliminating errors. The team proposed to pump electrons into ytterbium and from their stable ‘ground state’ to excited states called ‘metastable states’, which can be long-lived under the right conditions but are inherently fragile. Against all expectations, the researchers propose to use these states to encode quantum information.
“It’s like the electrons are on a tightrope,” Thompson said. And the system is designed so that the same factors that cause the error also knock electrons off the tightrope.
As a bonus, once they fall to the ground state, electrons scatter light very visibly, so illuminating a collection of ytterbium qubits only causes the faulty ones to light up. Those that light up should be written off as errors.
This advance required the combination of knowledge both quantum computing computing hardware and quantum error correction theory, taking advantage of the interdisciplinary nature of the research team and their close collaboration. While the mechanics of this configuration are specific to Thompson’s ytterbium atoms, he said the idea of engineering quantum qubits to generate erasure errors could be a useful goal in other systems – of which there are many in development around the world – and it’s something the band continues to work on.
“We see this project as setting up a kind of architecture that could be applied in different ways,” Thompson said, adding that other groups have already started designing their systems to convert errors into erasures. “We already see a lot of interest in finding adaptations for this work.”
As a next step, Thompson’s group is now working on demonstrating the conversion of errors to erasures in a small, functional quantum computer that combines several dozen qubits.
Reference: “Erasure conversion for fault-tolerant quantum computing in fault-tolerant quantum computing in alkaline earth Rydberg atom arrays” by Yue Wu, Shimon Kolkowitz, Shruti Puri and Jeff D. Thompson, August 9, 2022, Nature Communication.
The study was funded by the National Science Foundation, Army Research Office, Defense Advanced Research Projects Agency, Office of Naval Research and the Alfred P. Sloan Foundation.