  We show that nature imposes no fundamental upper limit to the number of
information bits per expended photon that can, in principle, be read reliably
when classical data is encoded in a medium that can only passively modulate the
amplitude and phase of the probe light. We show that with a coherent-state
(laser) source, an on-off (amplitude-modulation) pixel encoding, and
shot-noise-limited direct detection (an overly-optimistic model for commercial
CD/DVD drives), the highest photon information efficiency achievable in
principle is about 0.5 bit per transmitted photon. We then show that a
coherent-state probe can read unlimited bits per photon when the receiver is
allowed to make joint (inseparable) measurements on the reflected light from a
large block of phase-modulated memory pixels. Finally, we show an example of a
spatially-entangled non-classical light probe and a receiver
design---constructable using a single-photon source, beam splitters, and
single-photon detectors---that can in principle read any number of error-free
bits of information. The probe is a single photon prepared in a uniform
coherent superposition of multiple orthogonal spatial modes, i.e., a W-state.
The code, target, and joint-detection receiver complexity required by a
coherent-state transmitter to achieve comparable photon efficiency performance
is shown to be much higher in comparison to that required by the W-state
transceiver.
