Compiling quantum software in the NISQ era

While the promise of quantum computers is extremely great, the current generation of quantum devices can do a small number of things, badly, and this will improve only incrementally in the coming years.  Therefore to achieve anything it is necessary to make the absolute maximum use of the limited hardware available in a Noisy Intermediate-Scale Quantum (NISQ) device.  I’ll discuss two aspects of this problem — circuit optimisation and qubit routing — in the context of the t|ket> compiler system by Cambridge Quantum Computing.