![]() ![]() Presently, the spin decoherence mechanisms are under intense debate. ![]() There is also a lack of time-resolved PL studies on exciton coherence in perovskites probably because of the fast exciton recombination and resolution limits of the technique. However, these non-resonant steady-state PL studies do not reveal the coherence dynamics. The existence of exciton FSS in the family of facile processable, defect tolerant, bandgap-tunable colloidal perovskite NCs, provides a highly promising system for coherent optical manipulation of the excitonic degree of freedom. In CsPbBr 3 NCs, the Rashba effect flips the dark singlet and bright triplet states yielding fast radiative recombination of excitons 25, upon which coherent single-photon emission has also been evidenced in single CsPbBr 3 NCs 29. Interestingly, recent single-particle PL measurements reveal the FSS of exciton states in CsPbX 3 NCs because of the strong spin-orbit coupling (SOC) and shape anisotropy in the absence of strong quantum confinement 24, 25, 26, 27, 28. Lead halide perovskites (LHPs) and their nanostructures are intriguing candidates for functional devices exhibiting outstanding optoelectronic properties 15 and rich optical spin physics 16, 17, 18, 19, 20, 21, 22, 23. Therefore, for practical optospintronic applications, it is imperative to search for new semiconductor systems that could transcend these limitations. This typically limits the observation of such exciton coherence to extremely low temperatures and at the single-particle (nanostructure) level. Furthermore, the small energy splitting between two interacting FSS levels is highly prone to thermal energy and inhomogeneous broadening effects. Nevertheless, strong decoherence is usually triggered by increasing temperature or intense excitation because of the enhanced exciton-phonon and/or exciton-exciton Coulomb interaction 12, 13, 14. Such coherent optical control can be achieved by leveraging the exciton fine-structure splitting (FSS) in confinement-enhanced semiconductor QDs 6, 7, valley excitons in transition metal dichalcogenides (TMDs) 8, and the exciton complexes in low dimensional structures 9, 10, 11. ![]() For instance, in single quantum dots (QDs), the orthogonal photoluminescence (PL) doublets originating from the superposition of spin up and down states, can serve as a basis set for quantum computing and optical gating 5. Semiconducting nanostructures are promising platforms for logic operations and optical modulation with controllable carrier and exciton states 1, 2, 3, 4. These intrinsic exciton FSS states in perovskite NCs present fresh opportunities for spin-based photonic quantum technologies. Importantly, our results present an unambiguous full physical picture of the complex interplay of the underlying spin decoherence mechanisms. From the anomalous temperature dependence, we identify and fully parametrize all the regimes of exciton spin depolarization, finding that approaching room temperature, it is dominated by a motional narrowing process governed by the exciton multilevel coherence. The quantum beating between two exciton fine-structure splitting (FSS) levels enables coherent ultrafast optical control of the excitonic degree of freedom. ![]() Here, we uncover zero-field exciton quantum beating and anomalous temperature dependence of the exciton spin lifetimes in CsPbBr 3 perovskite nanocrystals (NCs) ensembles. However, their coherence time for incumbent semiconductors is highly susceptible to thermal decoherence and inhomogeneous broadening effects. Coherent optical manipulation of exciton states provides a fascinating approach for quantum gating and ultrafast switching. ![]()
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