  We present an implementation of photonic qubit precertification that performs
the delicate task of detecting the presence of a flying photon without
destroying its qubit state, allowing loss-sensitive quantum cryptography and
tests of nonlocality even over long distance. By splitting an incoming single
photon in two via parametric down-conversion, we herald the photon's arrival
from an independent photon source while preserving its quantum information with
up to $92.3\pm0.6$ % fidelity. With reduced detector dark counts,
precertification will be immediately useful in quantum communication.
