QPUs Are Not GPUs: Why Quantum Processors Need Constant Classical Control

Many in the HPC community expect quantum processing units (QPUs) to fit into future systems the way GPUs do: as accelerators that receive work from a host and return results. This paper argues that the analogy misses something essential. Qubits are fragile, and useful quantum computation depends on quantum error correction, in which classical decoders must interpret syndrome measurements and feed corrections back to the device continuously, often within microseconds. If the decoder falls behind, delays compound and running time can grow exponentially. Using the contrast between an inherently stable airliner and an unstable fly-by-wire fighter, the paper compares the GPU offload model with the continuous control a QPU requires, describes the layers of classical computing that surround a quantum device, and outlines where QPUs are expected to matter for scientific computing. It closes with implications for system architects and facility planners: integrating quantum computing into HPC is as much a classical systems problem as a quantum one.