LANCER: Low-Overhead, Accurate, and Non-Destructive Calibration for Real-World Fault-Tolerant Quantum Applications

The ultimate goal of fault-tolerant quantum computing (FTQC) is to run practical applications. Due to the long execution time of practical workloads, an FTQC system must operate reliably for multiple days by correcting the errors of noisy qubits. However, drifts of error sources increase qubit error rates during execution (i.e., error drift), limiting the reliable execution time. Even worse, existing error-drift-handling methods cannot execute long-running workloads as they collapse the qubit states or fail to suppress errors. In this paper, we propose LANCER, a novel accurate and non-destructive calibration method for reliable execution under error drifts. We observe that only a subset of qubits store quantum states during execution. Based on the observation, we periodically stall the program and migrate the quantum states temporarily to idle qubits, enabling accurate calibrations without losing the states. However, this idea faces two major challenges: (1) crosstalk between running and calibrating qubits and (2) huge latency overhead due to stalls when the quantum states are migrated to idle qubits. We propose three solutions to resolve these challenges. First, we mitigate the crosstalk by toggling the frequencies of running qubits to separate them from the frequencies of calibrating qubits. Second, we reduce the latency overhead by utilizing the inherent idle times in the fault-tolerant quantum gate. Lastly, we further reduce the latency overhead by re-designing qubit layout to enable the execution even when the quantum states are migrated to idle qubits. The evaluation shows that LANCER enables the execution of 95 times larger programs (i.e., larger number of gates) compared to the baseline, with negligible latency and qubit overhead (4.3% and 4.0%, respectively).