Luka Skoric, Senior Quantum Scientist at Riverlane presents 'Parallel window decoding enables scalable fault tolerant quantum computation' at the Quantum Computing Theory in Practice (QCTiP) Conference, 19 April, 2023.
Luka explains a new and patented method devised by Riverlane, which enables the decoding of vast amounts of data in real time. This parallelisation method is a solution to the backlog problem, where if quantum error correction data is not processed fast enough, you are forced to exponentially slow down your quantum computation. This prevents quantum computers from being useful.
The parallelisation method works across every qubit type – this paper focusses on superconducting qubits because they are the most challenging (fastest) systems for real-time decoding.
Explaining parallelisation
To carry out parallel window decoding, we break up the decoding task into chunks of work that we call “windows”. But we don’t wait for the first window to finish processing the data before moving to the next window. Instead, we decode multiple non-overlapping windows in parallel.
The reason that we use non-overlapping windows is to avoid the data processing bottlenecks that occur between adjacent windows. This is known as parallelisation in time, meaning that we don’t have to decode measurement results in the order that they were measured.
It’s analogous to parallel computing in classical computers: we are breaking down larger problems into smaller, independent parts that can be executed simultaneously by multiple processors communicating via shared memory with the goal of reducing the overall computation time.
At each decoding step, a number of syndrome rounds (a window) is selected for decoding (orange region in left columns in the diagram below), and tentative corrections are acquired. The corrections in the older part of the window (green region in right columns below) are of high confidence and are committed to. The window is then moved up to the edge of the commit region and the process repeated. Finally, we commit everything to complete the calculation.
This paper is a significant step forward towards useful quantum computing.
Full details are available in the Nature Comms paper: Parallel window decoding enables scalable fault tolerant quantum computation. Co-authored by Luka Skoric, Dan Browne, Kenton Barnes, Neil Gillespie and Earl Campbell.
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