  Photons are neutral particles that do not interact directly with a magnetic
field. However, recent theoretical work has shown that an effective magnetic
field for photons can exist if the phase of light would change with its
propagating direction. This direction-dependent phase indicates the presence of
an effective magnetic field as shown for electrons experimentally in the
Aharonov-Bohm experiment. Here we replicate this experiment using photons. In
order to create this effective magnetic field, we construct an on-chip
silicon-based Ramsey-type interferometer. This interferometer has been
traditionally used to probe the phase of atomic states, and here we apply it to
probe the phase of photonic states. We experimentally observe a phase change,
i.e. an effective magnetic field flux from 0 to 2pi. In an Aharonov-Bohm
configuration for electrons, considering the device geometry, this flux
corresponds to an effective magnetic field of 0.2 Gauss.
