Control landscape of measurement-assisted transition probability for a three-level quantum system with dynamical symmetry

TL;DR

This paper analyzes the control landscape of a three-level quantum system with dynamical symmetry, showing how measurement-assisted incoherent control can enhance transition probabilities beyond coherent control limits.

Contribution

It fully characterizes all critical points of the measurement-assisted control landscape for a three-level quantum system with dynamical symmetry.

Findings

All critical points are global maxima, minima, saddle points, or second order traps.

Measurement-assisted control can increase transition probability from 1/2 to about 0.687.

The study compares different transition probabilities and control scenarios.

Abstract

Quantum systems with dynamical symmetries have conserved quantities which are preserved under coherent controls. Therefore such systems can not be completely controlled by means of only coherent control. In particular, for such systems maximal transition probability between some pair of states over all coherent controls can be less than one. However, incoherent control can break this dynamical symmetry and increase the maximal attainable transition probability. Simplest example of such situation occurs in a three-level quantum system with dynamical symmetry, for which maximal probability of transition between the ground and the intermediate state by only coherent control is $1/2$, and by coherent control assisted by incoherent control implemented by non-selective measurement of the ground state is about $0.687$, as was previously analytically computed. In this work we study and completely characterize all critical points of the kinematic quantum control landscape for this measurement-assisted transition probability, which is considered as a function of the kinematic control parameters (Euler angles). This used in this work measurement-driven control is different both from quantum feedback and Zeno-type control. We show that all critical points are global maxima, global minima, saddle points and second order traps. For comparison, we study the transition probability between the ground and highest excited state, as well as the case when both these transition probabilities are assisted by incoherent control implemented by measurement of the intermediate state.