  Radio-frequency phase modulation of frequency entangled photons leads to a
two-photon interference pattern in the frequency domain. In recent experiments,
the pattern was measured with narrow-band frequency filters which select
photons belonging to a given frequency bin. Here we show how photons can be
grouped into even and odd frequencies by using periodic frequency filters
called interleavers. In our theoretical analysis we show how this reduces the
high-dimensional photon state to an effective two-dimensional state. This is of
interest for applications such as quantum cryptography or low-dimensional tests
of quantum nonlocality. We then report an experimental realization of this
proposal. The observed two-photon interference pattern and violation of the
CHSH inequality -- the simplest binary-outcome Bell inequality -- are in good
agreement with the theoretical predictions.
