  We demonstrate a compact technique to compress electron pulses to attosecond
length, while keeping the energy spread reasonably small. The technique is
based on Dielectric Laser Acceleration (DLA) in nanophotonic silicon
structures. Unlike previous ballistic optical microbunching demonstrations, we
use a modulator-demodulator scheme to compress phase space in the time and
energy coordinates. With a second stage, we show that these pulses can be
coherently accelerated, producing a net energy gain of $1.5\pm0.1$ keV, which
is significantly larger than the remaining energy spread of $0.88
\,_{-0.2}^{+0.0}$ keV FWHM. We show that by linearly sweeping the phase between
the two stages, the energy spectrum can be coherently moved in a periodic
manner, while keeping the energy spread roughly constant. After leaving the
buncher, the electron pulse is also transversely focused, and can be matched
into a following accelerator lattice. Thus, this setup is the prototype
injector into a scalable DLA based on Alternating Phase Focusing (APF).
