A Practical Architecture for Reliable Quantum Computers

Q uantum computers offer the prospect of computation that scales exponentially with data size. Unfortunately, a single bit error can corrupt an exponential amount of data. Quantum mechanics can seem more suited to science fiction than system engineering, yet small quantum devices of 5 to 7 bits have nevertheless been built in the laboratory, 100-bit devices are on the drawing table now, and emerging quantum technologies promise even greater scalability. More importantly, improvements in quantum error-correction codes have established a threshold theorem, according to which scalable quantum computers can be built from faulty components as long as the error probability for each quantum operation is less than some constant (estimated to be as high as 10). The overhead for quantum error correction remains daunting: Current well-known codes require tens of thousands of elementary operations to provide a single fault-tolerant logical operation. But proof of the threshold theorem fundamentally alters the prospects for quantum computers. No principle of physics prevents their realization—it is an engineering problem. Empirical studies of practical quantum architectures are just beginning to appear in the literature. Elementary architectural concepts are still lacking: How do we provide quantum storage, data paths, classical control circuits, parallelism, and system integration? And, crucially, how can we design architectures to reduce error-correction overhead?