Quantum computers are a bit like librarians; they both hate noise.
Compared to classical quantum computers, quantum computers are finicky and require a calm environment to perform calculations quietly. But even the quietest spaces in the universe reverberate with quantum noise, the inevitable movement of electrons and other atomic effects. If physicists can suppress noise-induced quantum errors on large enough quantum computers, they will be able to perform calculations that are difficult to perform on classical computers, such as accurate simulations of molecules.
Hardware improvements can help, but the key element is quantum error correction (QEC), a set of techniques that protect information from this quantum noise. “We need qubits to be near perfect, and engineering alone won’t get us there,” said Michael Newman, a quantum computing researcher at Google.
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On Monday, Google published its latest research on error correction in the journal nature And we showed for the first time that errors can be suppressed exponentially as the size of a quantum computer increases. “The bigger you make a system, the better it is at correcting errors, but the more errors you make,” said Daniel Gottesman, a quantum information theorist at the University of Maryland who was not involved in the study. speak “As we go through this transition period, we’ll be able to fix errors faster than they occur, and the system will get bigger and bigger, and the system will get better.”
Google researchers have created a silicon chip with 105 qubits, the quantum equivalent of classical bits. They then combined multiple physical qubits to form a collection called a logical qubit. A logical qubit lasted more than twice as long as the individual qubits that made it up, and had a 1 in 1,000 chance of error per computational cycle. (For comparison, a typical classical computer has an error rate of about 1 in 1,000,000,000,000,000,000, which is effectively zero.)
The results were first posted on the preprint server arXiv.org in August, but today Google has revealed the technology that made this advance possible: a new quantum processor called Willow (its arboreal-named predecessor, Sycamore). Shared additional details regarding the upgrade. “A really good qubit is one that enables quantum error correction,” says Julian Kelly, director of quantum hardware at Google and co-author of the new paper.
Google isn’t the only company to make strides in error correction. In September, a joint research team from Microsoft and Quantinuum, a quantum computing company based in Broomfield, Colorado, used qubits made from ions captured by lasers to achieve an error rate of 2 in 1,000.
Even with advances in error correction, it is unlikely that quantum computers will become practical in the near future. Estimates vary, but to solve useful algorithms or run robust simulations in chemistry, quantum computers require hundreds of logical qubits with an error rate of less than about 1 in a million. This is the consensus opinion of many researchers.
all that noise
Two main types of errors plague quantum computers: bit flips and dephasing. Bit flips also occur in classical computers, switching a qubit from 0 to 1 or vice versa. Dephasing takes a qubit out of its delicate quantum state, much like taking a pie out of the oven before it’s ready. Both errors can mess up your calculations.
Traditional error correction often preserves information through redundancy. If Alice wants to send the message “1” to Bob, she can send the message 3 times and copy 1 twice to send “111.” This way, even if the bit flips and becomes “101”, Bob can infer that Alice meant to send “1”. However, copying information in this way is prohibited by the laws of quantum mechanics. So in the 1990s, researchers needed to develop error correction for quantum computers. “Although there is redundancy, we need to disseminate information in such a way that there are no copies,” Gottsman said. Because the information is distributed as logical qubits, it can be stored even if one physical qubit is lost due to an error.
Researchers have been implementing code that can detect and correct errors for decades, but until recently there weren’t enough high-quality qubits. Now, hardware has finally reached a level worthy of good software. In 2022, Google used error correction in its Sycamore processor to reduce overall error rates. However, the rate had not yet reached a critical threshold, so adding physical qubits to logical qubits resulted in diminishing returns. “As logical qubits get larger, there are more opportunities for errors to occur,” says Newman, a co-author of the new study and co-author of a preprint paper on the results in 2022.
The latest advances, largely thanks to Willow, improve upon Sycamore in three important ways. First, Willow simply has more physical qubits, 105 compared to Sycamore’s 72. Increasing the number of physical qubits means that the logical qubits become larger. “It’s not just the number of qubits,” Kelly said. “Everything has to work at the same time.” By refining the manufacturing process, Kelly and his colleagues were able to improve the quality of the individual qubits. Willow qubits are more robust than Sycamore qubits. It maintains delicate quantum states five times longer and has lower error rates.
To test error correction, Google researchers encoded larger logical qubits. Logical qubits first consisted of a 3×3 grid of physical qubits, then a 5×5 grid, and finally a 7×7 grid. As the number of logical qubits increased, the error rate decreased rapidly. “I looked at these numbers and thought, ‘Oh my God, this is going to work really well,'” Newman says.
sense of scale
Experts were widely impressed with Google’s results. scientific american They reviewed peer review reports from four anonymous reviewers. “I think this is a great accomplishment that has excited the community,” one person concluded. Another agreed, writing, “This is one of the most important results of the year (if not the decade) in experimental quantum information.”
Graham Smith, a quantum information researcher at the University of Waterloo in Ontario, is impressed by the simplicity of the results. “Focusing on error correction is the right thing to do,” he says. “That’s really an advance.” Many previous error correction results relied on post-selection, a technique that artificially lowers the error rate by discarding runs that have errors.
Even with Google’s results, there are still things to be aware of. Christa Svore, a quantum computing researcher at Microsoft, points out that by another standard, the error would have been 1 in 100, not 1 in 1,000. In response to the criticism, a Google spokesperson said, “The exact numbers are…not that kind.” This is important because performance increases as size increases. That’s the key to making this scalable. ”
What everyone seems to agree on is that recent advances in error correction are a game changer. “What’s really exciting right now is the advances in quantum error correction,” Svoir said. For Gottsman and others who helped develop the theory behind error correction decades ago, the long wait is over. “The time has come for us to finally see proof of fault tolerance,” he says.
The hype around quantum computers is huge. The most extreme cases include claims that the device cures cancer, or solves climate change, or even creates wormholes. Responsible researchers frequently lament that the hype can lead to unreasonably high expectations and a “quantum winter” where funding dries up. The latest error correction results reveal another potential victim. Truly impressive advances like this one can be summarily ignored.
