A Framework for Hybrid Analog-Digital Quantum Processors: A Pathway to Mid-Scale Fault Tolerance

The immense resource overhead of full quantum error correction (QEC) presents a formidable barrier to achiev- ing fault-tolerant quantum computation. This paper introduces a hybrid analog-digital quantum processor architecture designed as a pragmatic strategy for achieving ”mid-scale fault tolerance” (MSFT). We formally define MSFT as a hardware-level error suppression scheme that achieves a fidelity gain by replacing deep, error-prone digital gate sequences with single, high-fidelity analog evolution blocks. The architecture comprises a programmable digital core for state preparation and measurement, coupled to an analog co-processor that executes computationally intensive Hamiltonian evolutions. This approach reduces the effective logical depth and error accumulation for algorithms like VQE. We validate this claim with statistically robust simulations on standard molecular Hamiltonians (LiH and BeH), demonstrating a significant performance advantage over purely digital systems based on an aggregation of 50 independent simulation trials. The analysis includes scalability and a discussion of the critical digital-analog interface, proposing trapped-ion and neutral-atom systems as viable physical platforms