Quantum Chaos as an Essential Resource for Full Quantum State Controllability

TL;DR

This paper explores how quantum chaos properties, such as sensitivity to perturbations and ergodicity, can be harnessed as resources to achieve full controllability of quantum states, surpassing limitations of integrable systems.

Contribution

It introduces the concept that quantum chaos enables full quantum state controllability through weak perturbations, extending classical control ideas to the quantum domain.

Findings

Quantum chaos allows steering from any initial to any target state.

Full controllability is achievable beyond a logarithmic time scale.

Demonstrated with the quantum kicked rotor, including state revivals and entanglement creation.

Abstract

Using the key properties of chaos, i.e. ergodicity and exponential instability, as a resource to control classical dynamics has a long and considerable history. However, in the context of controlling "chaotic" quantum unitary dynamics, the situation is far more tenuous. The classical concepts of exponential sensitivity to trajectory initial conditions and ergodicity do not directly translate into quantum unitary evolution. Nevertheless properties inherent to quantum chaos can take on those roles: i) the dynamical sensitivity to weak perturbations, measured by the fidelity decay, serves a similar purpose as the classical sensitivity to initial conditions; and ii) paired with the fact that quantum chaotic systems are conjectured to be statistically described by random matrix theory, implies a method to translate the ergodic feature into the control of quantum dynamics. With those two properties, it can be argued that quantum chaotic dynamical systems, in principle, allow for full controllability beyond a characteristic time that scales only logarithmically with system size and $\hbar^{-1}$. In the spirit of classical targeting, it implies that it is possible to fine tune the immense quantum interference with weak perturbations and steer the system from any initial state into any desired target state, subject to constraints imposed by conserved quantities. In contrast, integrable dynamics possess neither ergodicity nor exponential instability, and thus the weak perturbations apparently must break the integrability for control purposes. The main ideas are illustrated with the quantum kicked rotor. The production of revivals, cat-like entangled states, and the transition from any random state to any other random state is possible as demonstrated.