The QCAS 2026 Workshop addresses the systems engineering challenges of building scalable, fault-tolerant quantum computers. The field has reached an inflection point: multiple hardware platforms, including superconducting, neutral atom, and trapped-ion systems, have demonstrated below-threshold quantum error correction performance with real-time decoding. Industry roadmaps now project modular multi-chip quantum processors within the next few years, and national programs such as DARPA HARQ are investing in heterogeneous architectures that combine distinct qubit types for processing, memory, and communication. These developments collectively shift the central question from whether fault-tolerant quantum computing is possible to how systems can be engineered to achieve it at useful scale. QCAS 2026 is organized around four tracks spanning the full system stack: quantum error correction at scale, fault-tolerant compilation and resource estimation, distributed and modular architectures, and quantum control and hardware-software co-design. A distinguishing emphasis of this edition is the elevation of resource estimation as a unifying design methodology, serving as the quantum analog of design space exploration in classical EDA.
