  Our 50 ks Chandra observation of the small radio supernova remnant (SNR)
G1.9+0.3 shows a complete shell structure with strong bilateral symmetry, about
$100''$ in diameter. The radio morphology is also shell-like, but only about
$84''$ in diameter, based on observations made in 1985. We attribute the size
difference to expansion between 1985 and our Chandra observations of 2007.
Expansion is confirmed in comparing radio images from 1985 and 2008. We deduce
that G1.9+0.3 is of order 100 years old -- the youngest supernova remnant in
the Galaxy. Based on a very high absorbing column density of $5.5 \times
10^{22}$ cm$^{-2}$, we place G1.9+0.3 near the Galactic Center, at a distance
of about 8.5 kpc, where the mean remnant radius would be about 2 pc, and the
required expansion speed about $14,000$ km s$^{-1}$. The X-ray spectrum is
featureless and well-described by the exponentially cut off synchrotron model
{\tt srcut}. With the radio flux at 1 GHz fixed at 0.9 Jy, we find a spectral
index of 0.65 and a rolloff frequency of $1.4 \times 10^{18}$ Hz. The implied
characteristic rolloff electron energy of about $94 (B/10 \mu{\rm G})^{-1/2}$
TeV is the highest ever reported for a shell supernova remnant. It can easily
be reached by standard diffusive shock acceleration, given the very high shock
velocities; it can be well described by either age-limited or
synchrotron-loss-limited acceleration. Not only is G1.9+0.3 the youngest known
Galactic remnant, it is also only the fourth Galactic X-ray
synchrotron-dominated shell supernova remnant.
