Fast storage and processing of qubit measurements*

The gate set of a logical qubit will include operations that are conditional on the measurement result of another logical qubit. For this result to be correctly interpreted, the computer must promptly determine the current Pauli frame, based on previous syndrome measurements.
We implement the operations necessary for a logical measurement by enhancing the BBN APS2/QDSP system for qubit control and readout. The new capabilities include: runtime distributed classical memory, internal data message between channels, majority vote computation, and branching based on such vote.
We first employ these features in the repeated measurement of a single transmon qubit. In this simpler setup, the controller stores all the measurement results in memory, and uses them to update its knowledge of the qubit state. Next, we will apply these functionalities to a distance-3 repetition code, where syndrome results will be stored and later retrieved for real-time error correction.