  We searched for binary companions to 20 young brown dwarfs in the Upper
Scorpius association (145 pc, 5 Myr, nearest OB association) with the the Laser
Guide Star adaptive optics system and the facility infrared camera NIRC2 on the
10 m Keck II telescope. We discovered a 0.14" companion (20.9+-0.4 AU) to the
<0.1 MSun object SCH J16091837-20073523. From spectral deconvolution of
integrated-light near-IR spectroscopy of SCH1609 using the SpeX spectrograph
(Rayner et al. 2003), we estimate primary and secondary spectral types of
M6+-0.5 and M7+-1.0, corresponding to masses of 79+-17 MJup and 55+-25 MJup at
an age of 5 Myr and masses of 84+-15 MJup and 60+-25 MJup at an age of 10 Myr.
For our survey objects with spectral types later than M8, we find an upper
limit on the binary fraction of <9% (1-sigma) at separations of 10 -- 500 AU.
We combine the results of our survey with previous surveys of Upper Sco and
similar young regions to set the strongest constraints to date on binary
fraction for young substellar objects and very low mass stars. The binary
fraction for low mass (<40 MJup) brown dwarfs in Upper Sco is similar to that
for T dwarfs in the field; for higher mass brown dwarfs and very low mass
stars, there is an excess of medium-separation (10-50 AU projected separation)
young binaries with respect to the field. These medium separation binaries will
likely survive to late ages.
