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Willow: What Google’s error-correcting chip means for quantum computing

By, New Delhi
Dec 12, 2024 05:15 AM IST

Willow improves on Google’s earlier work, published in early 2023, when it described an array of 49 qubits in its Sycamore quantum processor

Google has just announced a new quantum chip, Willow, and the breakthrough needs to be read right in order to understand its implications. Yes, it is an exciting fact that Willow took under 5 minutes to perform a benchmark computation for which an existing supercomputer would need 10 septillion years, which is 1 followed by 25 zeros. The real achievement, however, is not just the computation, but also about a long-standing challenge that was conquered in getting Willow to perform it.

Willow is packed with logical qubits consisting of 105 physical qubits. (AFP/Google) PREMIUM
Willow is packed with logical qubits consisting of 105 physical qubits. (AFP/Google)

The keyword is “error correction below threshold”, something that headlines Google’s paper in Nature on the chip. This effectively means that a higher number of qubits (short for quantum bit, quantum computing’s equivalent of the bit) increases error correction exponentially. The concept already existed in theory, and Google’s demonstration holds the promise of scaling it up to quantum computers of the future.

What the achievement foretells, however, is best understood if one looks first at the challenge that preceded it.

What it means

Qubits, the basic units of quantum information, are prone to errors caused by interactions with the environment, with each other, and random fluctuations. These can cause qubits to lose their properties and hamper the quest to build mass-use quantum computers that can ultimately replace classical computers. The immediate objective, therefore, has been to find ways to correct those errors.

The idea is to incorporate error correction codes in “logical qubits”, each created across a number of “physical qubits”. When we talk of a physical qubit, we are referring to a piece of physical hardware in the form of an electron or a photon. A logical qubit is an abstraction rather than a single physical object, a system that is implemented using the physical qubits that it is built across. As such, it works in a way that is more complex than any of these physical qubit does.

We know how qubits work, and how they differ from the bits of classical computing. A classical bit stores information in either of two forms, represented by the digits 0 and 1. In quantum computing, a qubit can be 0 and 1 at the same time. While two classical bits can be any one of 00, 01, 10 and 11, a pair of qubits can be all these four simultaneously. Information thus increases exponentially with the number of qubits.

We are, of course, talking of physical qubits. Logical qubits make quantum computing even more complex since they are built across a number of physical qubits. If all these physical qubits exist in multiple states, the logical qubit built across them exists in a collective state that derives from these already complex individual states. The idea is based on the understanding even if a few physical qubits experience errors, the overall quantum state of the logical qubit can be preserved.

What Google showed is that the more logical qubits you have, error correction rises exponentially.

How it was done

Willow is packed with logical qubits consisting of 105 physical qubits. It improves on Google’s earlier work, published in early 2023, when it described an array of 49 qubits in its Sycamore quantum processor. Willow was developed in a fabrication laboratory that Google built at its quantum-computing campus in California in 2021.

Google tested Willow with larger and larger arrays of physical qubits, scaling up from a 3x3 grid of 5x5, and then 7x7.

“Each time, using our latest advances in quantum error correction, we were able to cut the error rate in half. In other words, we achieved an exponential reduction in the error rate. This historic accomplishment is known in the field as ‘below threshold’ — being able to drive errors down while scaling up the number of qubits,” Hartmut Neven, founder and Google Quantum AI, said in the announcement.

One challenge in error correction is the speed at which it is done. If the errors take place faster than they are corrected, they will accumulate, leading to failure of the system. Willow has reportedly conquered this, too.

“It’s also one of the first compelling examples of real-time error correction on a superconducting quantum system — crucial for any useful computation, because if you can’t correct errors fast enough, they ruin your computation before it’s done. And it’s a ‘beyond breakeven’ demonstration, where our arrays of qubits have longer lifetimes than the individual physical qubits do, an unfakable sign that error correction is improving the system overall,” Neven added.

Why it matters

Dr Aditi Sen De, a professor in the quantum information and computation (QIC) group with the Harish Chandra Research Institute, Prayagraj, acknowledged the importance of the breakthrough Google has announced.

“We know that current quantum computers suffer heavily from noise (errors), so any implementation that shows quantum computers outperforming classical computers gets disturbed by noise and errors. Error correction is an important tool for obtaining logical qubits from physical qubits,” she told HT.

“Firstly, in superconducting quantum chips, Google has demonstrated the realisation of an error correction protocol known theoretically, which implies that we are heading towards fault-tolerant (error-free) quantum computers. Secondly, it shows that with their chip, the logical error rate gets suppressed with the physical error rate, provided errors in physical qubits are below some critical value,” said De, who won the Shanti Swarup Bhatnagar prize in 2018 and the GD Birla Award for Scientific Excellence earlier this year.

Google, of course, is excited about scalability. “As the first system below threshold, this is the most convincing prototype for a scalable logical qubit built to date. It’s a strong sign that useful, very large quantum computers can indeed be built. Willow brings us closer to running practical, commercially-relevant algorithms that can’t be replicated on conventional computers,” Neven said.

De agreed that more qubits leading to fewer errors points to the scalability of fault-tolerant quantum computers. She noted, however, that more needs to be done. “The question remains whether the scalability of errors that Google claims will remain the same even when one has a 3000-qubit quantum computer — this has to be found out,” she said.

Yet, she acknowledged that this is “ the first breakthrough result towards a fault-tolerant quantum computer, which is beyond the noisy intermediate scale quantum computers currently available on different platforms.”

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