Local and remote synthesis of single-photon space-time wave packets

Space-time wave packets (STWPs) are a new class of spatiotemporally structured light fields that are endowed with a host of unique properties, including diffraction-free propagation at controllable group velocity, self-healing, and dispersion-free propagation in dispersive media. These characteristics make STWPs useful candidates for free-space optical communications, particularly for quantum communications. We demonstrate here the first single-photon STWP produced in two different configurations from entangled-photon pairs produced through spontaneous parametric downconversion. In the first configuration, a spatiotemporal spectral modulator is placed in the path of the signal photon, whose arrival is heralded by detection of the idler photon, which we refer to as local STWP synthesis. In the second configuration, the signal photon after the modulator is detected to herald the arrival of the idler photon, which we refer to as remote STWP preparation. In both configurations, we observe the predicted diffraction-free propagation of single-photon STWPs. These results pave the way for quantum communications using spatiotemporally structured quantum states of light.