Google demonstrates essential step towards large-scale quantum computers

Google demonstrates essential step towards large-scale quantum computers

Google has demonstrated that its Sycamore quantum computer can find and fix computational errors, an important step for large-scale quantum computing, but its current system generates more errors than it solves.

Error-correction is a typical feature for ordinary, or classical, computers, which store data using bits with two possible states: 0 and 1. Transmitting data with extra “parity bits” that warn if a 0 has flipped to at least one 1, or vice versa, means such errors can be found and fixed.

In quantum computing the problem is far more complex as each quantum bit, or qubit, exists in a mixed state of 0 and 1, and any try to measure them directly destroys the data. One longstanding theoretical solution to the has gone to cluster many physical qubits into a single “logical qubit”. Although such logical qubits have already been created previously, they hadn’t been used for error correction as yet.


Julian Kelly at Google AI Quantum and his colleagues have demonstrated the idea on Google’s Sycamore quantum computer, with logical qubits ranging in size from five to 21 physical qubits, and discovered that logical qubit error rates dropped exponentially for each additional physical qubit. The team could make careful measurements of the extra qubits that didn’t collapse their state but, when taken collectively, still gave enough information to deduce whether errors had occurred.

Kelly says that means it is possible to create practical, reliable quantum computers later on. “That is basically our first half step along the path to show that,” he says. “A viable method of addressing really large-scale, error-tolerant computers. It’s type of a look ahead for the devices that we want to make in the future.”

Read more: China beats Google to claim the world’s most powerful quantum computer

The team has were able to demonstrate this solution conceptually   but a vast engineering challenge remains. Adding more qubits to each logical qubit brings its problems as each physical qubit is itself susceptible to errors. The opportunity of a logical qubit encountering an error rises as the number of qubits within it increases.

There exists a breakeven point in this technique, referred to as the threshold, where the error correction features catch more problems compared to the increase in qubits bring. Crucially, Google’s error correction doesn’t yet meet the threshold. To do so will demand less noisy physical qubits that encounter fewer errors and larger amounts of them specialized in each logical qubit. The team believes that mature quantum computers will require 1000 qubits to make each logical qubit – Sycamore currently has just 54 physical qubits.

Peter Knight at Imperial College London says Google’s research is progress towards something needed for future quantum computers. “If we couldn’t do this we’re not likely to have a sizable scale machine,” he says. “I applaud the actual fact they’ve done it, due to the fact without this, without this advance, you will still have uncertainty about whether the roadmap towards fault tolerance was feasible. They removed those doubts.”

But he says it’ll be a vast engineering challenge to really meet up with the threshold and build effective error correction, which would mean creating a processor with a lot more qubits than has been demonstrated as yet.

Journal reference: Nature , DOI: 10.1038/s41586-021-03588-y

More on these topics:

  • computing
  • Google
  • quantum computing
  • quantum physics

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