# Hybrid Quantum-Classical Computing **Hybrid quantum-classical computing** is the model in which a quantum processor does **not replace** classical computers but works *alongside* them — handling the specific subroutines where it wins, while classical machines orchestrate everything else. ## First principles A quantum processor (QPU) is not a general-purpose machine. It excels at a narrow set of tasks (e.g. simulating quantum systems, certain optimization and algebra problems) and is poor or wasteful at the rest. The sensible architecture is therefore **division of labour**: - The **classical computer** runs the program, prepares inputs, controls timing, reads results, and applies [[Quantum Error Correction|error-correction]] decoding. - The **QPU** is *invoked* for the hard quantum subroutine, returns its answer, and hands control back. > [!intuition] QPUs are GPUs, not CPUs > A GPU didn't replace the CPU — it was absorbed into the computer as an **accelerator** the CPU calls for the jobs it's good at (graphics, then AI). A QPU plays the same role: a specialized accelerator living inside a classical system, not a standalone rival. $ \text{classical host (CPU/GPU)} \;\xrightarrow{\text{calls}}\; \text{QPU subroutine} \;\xrightarrow{\text{returns}}\; \text{classical host} $ ## Why it matters - **Realistic deployment.** Quantum processors are expected to sit *inside* classical high-performance-computing and AI datacenters, accelerating chosen workloads — not to stand apart as a parallel universe of computing. - **The interface is as important as the qubits.** Making this work demands orchestration layers: compilers, schedulers, and system-management software that move work between classical and quantum hardware. Maturity is increasingly judged by this integration, not by qubit count alone. - It reframes progress: the goal is **heterogeneous compute** — CPUs, GPUs, and QPUs cooperating — rather than a single winning machine. ## Related - [[Hardware Accelerator]] - [[Quantum Error Correction]] - [[Physical vs Logical Qubits]] - [[Quantum Computing MOC]]