Title: Time-Parallel Control of Quantum Computing Devices

Abstract:

Advances in the design of quantum technologies has led to rapidly increasing numbers of qubits in current quantum computing hardware. However, accurately controlling these large quantum systems remains a fundamental challenge in the current Noisy Intermediate-Scale Quantum (NISQ) era. Analog control pulses provide the fundamental interface between the quantum compiler and the quantum hardware, and significant progress has been made in the development of numerical methods and computational tools to optimally design pulses that realize quantum operations with high fidelity. However, solving the optimal control problem becomes more and more difficult when the number of qubits increase in the system. This is due to an exponentially increasing computational costs for solving the underlying quantum dynamical equations, as well as an increasingly complicated optimization landscape.

In recent efforts, we have improved the parallel scalability of quantum optimal control by employing a time-parallel multiple-shooting optimization algorithm. Here, intermediate quantum states are introduced as additional optimization variables, while continuity of the state evolution is enforced through equality constraints. This allows the time-evolution to be decoupled into multiple time windows that can be evolved concurrently in time, using many (classical) compute cores. Since the state evolution only becomes continuous upon convergence, the multiple-shooting approach also allows for shortcuts through the optimization landscape that potentially can accelerate optimization convergence.