Compiler tools for hybrid quantum-classical algorithms

We describe the Rigetti compilation toolchain and in particular how it supports optimized implementations of certain hybrid quantum-classical algorithms. Programs written in Quil are transpiled into a restricted subset of Quil instructions that are realizable on the available control hardware and target chip topology. These transpiled programs are further compiled into binary executables for custom FPGA pulse sequencers. The toolchain provides two key features that enable high performance hybrid computing: (1) gate parameters from the original input Quil program are translated to sequencer instructions that load from classical memory shared between the sequencer and classical host computer; (2) compiled programs can contain arbitrary control flow that branches off of single-qubit measurement results. The first feature enables the compilation of Quil into binaries that can be updated at run-time and the second, enables active reset of qubit states. Together these allow for rapid iteration in applications such as the optimization of a variational quantum algorithm, because these binaries can be re-executed many times for different input parameters without need for re-compiling or waiting for qubits to relax. We provide quantitative benchmarks of the improved wall-clock performance.