  "Qubit routing" refers to the task of modifying quantum circuits so that they
satisfy the connectivity constraints of a target quantum computer. This
involves inserting SWAP gates into the circuit so that the logical gates only
ever occur between adjacent physical qubits. The goal is to minimise the
circuit depth added by the SWAP gates.
  In this paper, we propose a qubit routing procedure that uses a modified
version of the deep Q-learning paradigm. The system is able to outperform the
qubit routing procedures from two of the most advanced quantum compilers
currently available, on both random and realistic circuits, across near-term
architecture sizes.
