Abstract: For typical quantum subroutines in the gate-based model of quantum computing,
explicit decompositions of circuits in terms of single-qubit and two-qubit
entangling gates may exist. However, they often lead to large-depth circuits
that are challenging for noisy intermediate-scale quantum (NISQ) hardware.
Additionally, exact decompositions might only exist for some modular quantum
circuits. Therefore, it is essential to find gate combinations that approximate
these circuits to high fidelity with potentially low depth, for example, using
gradient-based optimization. Traditional optimizers often run into problems of
slow convergence requiring many iterations, and perform poorly in the presence
of noise. Here we present iteratively preconditioned gradient descent (IPG) for
optimizing quantum circuits and demonstrate performance speedups for state
preparation and implementation of quantum algorithmic subroutines. IPG is a
noise-resilient, higher-order algorithm that has shown promising gains in
convergence speed for classical optimizations, converging locally at a linear
rate for convex problems and superlinearly when the solution is unique.
Specifically, we show an improvement in fidelity by a factor of $10^4$ for
preparing a 4-qubit W state and a maximally entangled 5-qubit GHZ state
compared to other commonly used classical optimizers tuning the same ansatz. We
also show gains for optimizing a unitary for a quantum Fourier transform using
IPG, and report results of running such optimized circuits on IonQ's quantum
processing unit (QPU). Such faster convergence with promise for
noise-resilience could provide advantages for quantum algorithms on NISQ
hardware, especially since the cost of running each iteration on a quantum
computer is substantially higher than the classical optimizer step.