  Single-photon subtraction plays important roles in optical quantum
information processing as it provides a non-Gaussian characteristic in
continuous-variable quantum information. While the conventional way of
implementing single-photon subtraction based on a low-reflectance beam splitter
works properly for a single-mode quantum state, it is unsuitable for a
multimode quantum state because a single photon is subtracted from all multiple
modes without maintaining their mode coherence. Here we experimentally
implement and characterize a mode-tunable coherent single-photon subtractor
based on sum frequency generation. It can subtract a single photon exclusively
from one desired time-frequency mode of light or from a coherent superposition
of multiple time-frequency modes. To fully characterize the implemented
single-photon subtractions, we employ quantum process tomography based on
coherent states. The mode-tunable coherent single-photon subtractor will be an
essential element for realizing non-Gaussian quantum networks necessary to get
a quantum advantage in information processing.
