NEW YORK, Sept. 24 -- The announcement that changed IonQ's trading session Tuesday was not about a qubit. It was about a CPU.

IonQ Inc. demonstrated what it described as the industry's first end-to-end real-time quantum error-correction decoder running on a single, commercially available processor, the company said in its press release. The decoder's significance is in the hardware constraint it eliminates: quantum computers produce errors in every computation cycle, and the engineering problem has always been whether those errors can be caught and corrected fast enough to keep a calculation valid. A decoder that runs on a standard CPU does not require custom silicon, meaning the correction layer is cheap to replicate and straightforward...