  A single quantum particle can be described by a wavefunction that spreads
over arbitrarily large distances, but it is never detected in two (or more)
places. This strange phenomenon is explained in quantum theory by what Einstein
repudiated as "spooky action at a distance": the instantaneous nonlocal
collapse of the wavefunction to wherever the particle is detected. We
demonstrate this single-particle spooky action, for the first time with no
efficiency loophole, by splitting a single photon between two laboratories and
experimentally testing if the choice of measurement in one lab really causes a
change in the local quantum state in the other lab. To this end, we use
homodyne measurements with six different measurement settings and
quantitatively verify Einstein's spooky action by violating an
Einstein-Podolsky-Rosen-steering inequality by $0.042 \pm 0.006$. Our
experiment also verifies the entanglement of the split single photon even when
one side is untrusted.
