Google’s Sycamore quantum computer is an essential step towards Quantum Computing. Google’s Sycamore quantum computer can detect and fix computational errors, though the current system generates more errors than it resolves.
For regular computers, error correction is a standard feature. Data is stored using bits ie 0 and 1. Data is transmitted with extra parity bits which warn if 0 has flipped to 1 and vice versa. This means errors can be found and fixed.
In the case of Quantum computing, things get more complex since each quantum bit or qubit exists in a mixed state of 0 and 1. These cannot be measured directly as it can destroy the data. Clustering many physical qubits into a single “logical qubit” is the theoretical solution to this problem. Logical qubits have been used for a long time, though they have never been used for error correction.
Julian Kelly at Google AI Quantum and his colleagues used Google’s Sycamore quantum computer to demonstrate the concept with logical qubits ranging in size from five to 21 physical qubits. Results showed the logical qubit error rates dropped exponentially for each additional physical qubit. The researchers were able to measure the extra qubits that didn’t collapse their state. The study also revealed when they are collectively taken they still manage to give enough information to understand if errors had occurred.
According to Kelly, “This is basically our first half step along the path to demonstrate that. A viable way of getting to really large-scale, error-tolerant computers. It’s sort of a look ahead for the devices that we want to make in the future.”
Though the team has managed to demonstrate the solution conceptually, it still remains a challenge to add more qubits to each logical qubit, as each physical qubit is itself susceptible to errors. Also the chances of a logical qubit running into an error rise as the number of qubits inside it increase.
Currently, Google’s error correction does not meet the threshold, it will require less noisy physical qubits that encounter fewer errors and larger numbers of them devoted to each logical qubit.
Google engineers believe mature quantum computers will need 1000 qubits to make each logical qubit – Sycamore currently has just 54 physical qubits.
According to Peter Knight at Imperial College London, “If we couldn’t do this we’re not gonna have a large scale machine. I applaud the fact they’ve done it, simply because without this, without this advance, you will still have uncertainty about whether the roadmap towards fault tolerance was feasible. They removed those doubts.”
He believes it will be an engineering challenge to build a processor with many more qubits than ever demonstrated to actually meet the threshold and build effective error correction.