  We describe the design, fabrication, and spectroscopy of coupled, high
Quality (Q) factor silicon nanobeam photonic crystal cavities. We show that the
single nanobeam cavity modes are coupled into even and odd superposition modes,
and we simulate the frequency and Q factor as a function of nanobeam spacing,
demonstrating that a differential wavelength shift of 70 nm between the two
modes is possible while maintaining Q factors greater than 10^6. For both
on-substrate and free-standing nanobeams, we experimentally monitor the
response of the even mode as the gap is varied, and measure Q factors as high
as 200,000.
