Curriculum reinforcement learning for quantum architecture search under
  hardware errors
Yash J. Patel Akash Kundu Mateusz Ostaszewski Xavier Bonet-Monroig Vedran Dunjko Onur Danaci
The key challenge in the noisy intermediate-scale quantum era is finding
useful circuits compatible with current device limitations. Variational quantum
algorithms (VQAs) offer a potential solution by fixing the circuit architecture
and optimizing individual gate parameters in an external loop. However,
parameter optimization can become intractable, and the overall performance of
the algorithm depends heavily on the initially chosen circuit architecture.
Several quantum architecture search (QAS) algorithms have been developed to
design useful circuit architectures automatically. In the case of parameter
optimization alone, noise effects have been observed to dramatically influence
the performance of the optimizer and final outcomes, which is a key line of
study. However, the effects of noise on the architecture search, which could be
just as critical, are poorly understood. This work addresses this gap by
introducing a curriculum-based reinforcement learning QAS (CRLQAS) algorithm
designed to tackle challenges in realistic VQA deployment. The algorithm
incorporates (i) a 3D architecture encoding and restrictions on environment
dynamics to explore the search space of possible circuits efficiently, (ii) an
episode halting scheme to steer the agent to find shorter circuits, and (iii) a
novel variant of simultaneous perturbation stochastic approximation as an
optimizer for faster convergence. To facilitate studies, we developed an
optimized simulator for our algorithm, significantly improving computational
efficiency in simulating noisy quantum circuits by employing the Pauli-transfer
matrix formalism in the Pauli-Liouville basis. Numerical experiments focusing
on quantum chemistry tasks demonstrate that CRLQAS outperforms existing QAS
algorithms across several metrics in both noiseless and noisy environments.