Stimulated Generation of Indistinguishable Single Photons from a Quantum Ladder System

Due to their excellent optical properties, semiconductor quantum dots are promising systems for the on-demand generation of single photons. A wealth of different excitation schemes have been developed, each with their specific advantages and disadvantages. Resonant excitation allows for the generation of highly indistinguishable photons, while the single-photon purity is limited by reexcitation [1]. In contrast, two-photon excitation of the biexciton suppresses reexcitation, resulting in ultra-low multi-photon errors [2]. However, the indistinguishability of emitted photons is inherently limited by the cascaded decay [3].<br/><br/>Here, we theoretically model and experimentally demonstrate a single-photon generation scheme that combines all the advantages of previously established excitation methods [4]. The scheme is based on the resonant two-photon excitation of the biexciton followed by a precisely-timed stimulation of the biexciton-exciton transition, which effectively prepares the system in an exciton state. On the one hand, the two-photon excitation of the biexciton suppresses re-excitation and enables ultra-low multi-photon errors. On the other hand, the precisely timed stimulation pulse prepares the system in the exciton state at a well-defined time. This strongly reduces timing jitter in the preparation of the exciton state caused by the biexciton population lifetime, and consequently, reestablishes the high indistinguishability for photons emitted from the exciton to ground state decay.<br/>The scheme allows to deterministically program the polarization of the emitted photon (H or V) via the polarization of the stimulation pulse. Moreover, this allows to obtain all emission of interest to occur in the polarization of the detection channel enabling higher brightness than cross-polarized resonant excitation. Experiments and simulations exploring the system dynamics are in very good agreement and confirm the validity of the theoretical model.<br/><br/>[1] K. A. Fischer, et al., Nature Physics 13, 649-654 (2017)<br/>[2] L. Hanschke et al., npj Quantum Information 4, 43 (2018)<br/>[3] E. Schöll et al., Physical Review Letters 125, 233605 (2020)<br/>[4] F. Sbresny et al., arXiv:2107.03232 (2021)