Designing Coherence in Perovskite Quantum Dot Assemblies by Choice of Ligand

Loss of coherence in a quantum subsystem by coupling it to an environment limits its applications in quantum information technology. Because of large optical absorption cross sections and high single-photon emission rates, perovskite quantum dots (QDs) have garnered considerable attention for applications in quantum information, but they are limited by loss of coherence due to electron-phonon coupling. We calculated coherence times in dimers of small cesium lead bromide QDs bridged by various ligands. Typically, QD dimers exhibit longer coherence times than the corresponding monomer due to more delocalized electronic excitations, resulting in weaker electron-phonon coupling. Dimers connected by both aliphatic and aromatic ligands exhibit extended coherence times. Aromatic ligands with states close to QD band edges can enhance interdot coupling; however, the ligand frontier orbitals and the QD band edges should not hybridize, since this allows fast ligand motions to accelerate decoherence. Exciton delocalization between QDs is reduced by structural disorder and long aliphatic ligands. Generally, improved coherence times are achieved by electronic delocalization, surface passivation, and the suppression of active phonon modes. The reported results provide guidelines for engineering perovskite QD assemblies and related materials for applications in quantum information technologies.