Plenty of quantum information protocols are enabled by manipulation and detection of photonic spectro-temporal degrees of freedom via light-matter interfaces. While present implementations are well suited for high-bandwidth photon sources such as quantum dots, they lack the high resolution required for intrinsically narrowband light-atom interactions. Here, we demonstrate far-field temporal imaging based on ac-Stark spatial spin-wave phase manipulation in a multimode gradient echo memory. We achieve a spectral resolution of 20 kHz with MHz-level bandwidth and an ultralow noise equivalent to 0.023 photons, enabling operation in the single-quantum regime. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement