Quantum Control Landscapes Beyond the Dipole Approximation

TL;DR

This paper explores quantum control landscapes beyond the dipole approximation by including polarizability, demonstrating numerically that such inclusion generally removes traps and restores controllability in finite-level quantum systems.

Contribution

It provides theoretical analysis and numerical evidence that adding polarizability terms eliminates traps in quantum control landscapes beyond the dipole approximation.

Findings

Control landscapes are trap-free with polarizability inclusion.

Adding polarizability restores controllability in otherwise uncontrollable systems.

Polarizability removes traps in a three-level system with known landscape traps.

Abstract

We investigate the control landscapes of closed, finite level quantum systems beyond the dipole approximation by including a polarizability term in the Hamiltonian. Theoretical analysis is presented for the $n$ level case and formulas for singular controls, which are candidates for landscape traps, are compared to their analogues in the dipole approximation. A numerical analysis of the existence of traps in control landscapes beyond the dipole approximation is made in the four level case. A numerical exploration of these control landscapes is achieved by generating many random Hamiltonians which include a term quadratic in a single control field. The landscapes of such systems are found numerically to be trap free in general. This extends a great body of recent work on typical landscapes of quantum systems where the dipole approximation is made. We further investigate the relationship between the magnitude of the polarizability and the magnitude of the controls resulting from optimization. It is shown numerically that including a polarizability term in an otherwise uncontrollable system removes traps from the landscapes of a specific family of systems by restoring controllability. We numerically assess the effect of a random polarizability term on the know example of a three level system with a second order trap in its control landscape. It is found that the addition of polarizability removes the trap from the landscape. The implications for laboratory control are discussed.