  Very much like the ubiquitous quantum interference of a single particle with
itself, quantum interference of two independent, but indistinguishable,
particles is also possible. This interference is a direct result of quantum
exchange statistics, however, it is observed only in the joint probability to
find the particles in two separated detectors. Here we report the first
observation of such interference fringes between two independent and
non-interacting electrons in an interferometer proposed by Yurke et al. and
Samuelsson et al. Our experiment resembles the "Hanbury Brown and Twiss" (HBT)
experiment, which was performed with classical waves. In the experiment, two
independent and mutually incoherent electron beams were each partitioned into
two trajectories. The combined four trajectories enclosed an Aharonov-Bohm (AB)
flux (but not the two trajectories of a single electron). While individual
currents were found to be independent of the AB flux, as expected, the
cross-correlation between current fluctuations in two opposite points across
the device exhibited strong AB oscillations. This is a direct signature of
orbital entanglement between two electrons even though they never interact with
each other.
