  Fundamental optics such as lenses and prisms work by applying phase shifts to
incoming light via the refractive index. In these macroscopic devices, many
particles each contribute a miniscule phase shift, working together to impose a
total phase shift of many radians. In principle, even a single isolated
particle can apply a radian-level phase shift, but observing this phenomenon
has proven challenging. We have used a single trapped atomic ion to induce and
measure a large optical phase shift of $1.3 \pm 0.1$ radians in light scattered
by the atom. Spatial interferometry between the scattered light and unscattered
illumination light enables us to isolate the phase shift in the scattered
component. The phase shift achieves the maximum value allowed by atomic theory
over the accessible range of laser frequencies, validating the microscopic
model that underpins the macroscopic phenomenon of the refractive index.
Single-atom phase shifts of this magnitude open up new quantum information
protocols, including long-range quantum phase-shift-keying cryptography [1,2]
and quantum nondemolition measurement [3,4].
