  Quantum architecture search (QAS) is the process of automating architecture
engineering of quantum circuits. It has been desired to construct a powerful
and general QAS platform which can significantly accelerate current efforts to
identify quantum advantages of error-prone and depth-limited quantum circuits
in the NISQ era. Hereby, we propose a general framework of differentiable
quantum architecture search (DQAS), which enables automated designs of quantum
circuits in an end-to-end differentiable fashion. We present several examples
of circuit design problems to demonstrate the power of DQAS. For instance,
unitary operations are decomposed into quantum gates, noisy circuits are
re-designed to improve accuracy, and circuit layouts for quantum approximation
optimization algorithm are automatically discovered and upgraded for
combinatorial optimization problems. These results not only manifest the vast
potential of DQAS being an essential tool for the NISQ application
developments, but also present an interesting research topic from the
theoretical perspective as it draws inspirations from the newly emerging
interdisciplinary paradigms of differentiable programming, probabilistic
programming, and quantum programming.
