  Coherent interaction between matter and periodic light field induces both
optical Stark effect (OSE) and Bloch-Siegert shift (BSS). Observing the BSS has
been historically challenging, not only because it is weak but it is often
accompanied by a much stronger OSE. Herein, by controlling the light helicity,
we can largely restrict the OSE and BSS to different spin-transitions in CsPbI3
perovskite quantum dots, achieving room-temperature BSS as strong as 4 meV with
near-infrared pulses. The ratio between the BSS and OSE magnitudes is however
systematically higher than the prediction by the non-interacting,
quasi-particle picture. With a model that explicitly accounts for excitonic
effects, we quantitatively reproduce the experimental observations. This model
depicts a unified physical picture of the interplay between the OSE,
biexcitonic OSE and BSS in low-dimensional materials displaying strong
many-body interactions, forming the basis for the implementation of these
effects to information processing, optical modulation and Floquet engineering.
