Near-term quantum computers will soon reach sizes that are challenging to directly simulate, even when employing the most powerful supercomputers. Yet, the ability to simulate these early devices using classical computers is crucial for calibration, validation, and benchmarking. In order to make use of the full potential of systems featuring multi- and many-core processors, we use automatic code generation and optimization of compute kernels, which also enables performance portability. We apply a scheduling algorithm to quantum supremacy circuits in order to reduce the required communication and simulate a 45-qubit circuit on the Cori II supercomputer using 8,192 nodes and 0.5 petabytes of memory. To our knowledge, this constitutes the largest quantum circuit simulation to this date. Our highly-tuned kernels in combination with the reduced communication requirements allow an improvement in time-to-solution over state-of-the-art simulators by more than an order of magnitude at every scale.
Simulation of a 49-qubit computer should be possible in the near future.
They demonstrated simulations of up to 45 qubits using up to 8,192 nodes. With the same amount of compute resources, the simulation of 46 qubits is feasible when using single-precision floating point numbers to represent the complex amplitudes. The presented optimizations are general and our code generator improves performance portability across a wide range of processors. Extending the range of the code generator to the domain of GPUs is an ongoing project.
Additional optimizations on the quantum circuit description allows to reduce the required communication by an order of magnitude. As a result, the simulation of a 49-qubit quantum supremacy circuit would require only two global to-local swap operations. While the memory requirements to simulate such a large circuit are beyond what is possible today, the low amount of communication may allow the use of, e.g., solid-state drives. The simulation results may then be used for verification and calibration of near-term quantum devices.