  We demonstrate a new type of weak measurement based on the dynamics of
spontaneous emission. The pointer in our scheme is given by the Lorentzian
distribution characterizing atomic exponential decay via emission of a single
photon. We thus introduce weak measurement, so far demonstrated nearly
exclusively with laser beams and Gaussian statistics, into the quantum regime
of single emitters and single quanta, enabling the exploitation of a wide class
of sources that are abundant in nature. We describe a complete analogy between
our scheme and weak measurement with conventional Gaussian pointers. Instead of
a shift in the mean of a Gaussian distribution, an imaginary weak value is
exhibited in our scheme by a significantly slower-than-natural exponential
distribution of emitted photons at the postselected polarization, leading to a
large shift in their mean arrival time. The dynamics of spontaneous emission
offer a broader view of the measurement process than is usually considered
within the weak measurement formalism. Our scheme opens the path for the use of
atoms and atomlike systems as sensitive probes in weak measurements, one
example being optical magnetometry.
