  Barcoding photons can provide a host of functionalities that could benefit
future quantum communication systems and networks beyond today's imagination.
As a significant application of barcoding photons, we introduce code division
multiple-access (CDMA) communication systems for various applications. In this
context, we introduce and discuss the fundamental principles of a novel quantum
CDMA (QCDMA) technique based on spectrally encoding and decoding of
continuous-mode quantum light pulses. In particular, we present the
mathematical models of various QCDMA modules that are fundamental in describing
an ideal and typical QCDMA system, such as quantum signal sources, quantum
spectral encoding phase operators, M$\times$M quantum broadcasting
star-coupler, quantum spectral phase decoding operators, and the quantum
receivers. In describing a QCDMA system, this paper considers a unified
approach where the input continuous-mode quantum light pulses can take on any
form of pure states such as Glauber states and quantum number states. For input
number states, one can observe features like entanglement and quantum
interference. More interestingly, due to Heisenberg's uncertainty principle,
the quantum signals sent by photon number states obtain complete phase
uncertainty at the time of measurement. Therefore, at the receiver output, the
multiaccess inter-signal interference vanishes. Due to Heisenberg's uncertainty
principle, the received signal intensity at the photodetector's output changes
from a coherent detection scheme for input Glauber states to an incoherent
detection scheme for input number states. Our mathematical model is valuable in
the signal design and data modulations of point-to-point quantum
communications, quantum pulse shaping, and quantum radar signals and systems
where the inputs are continuous mode quantum signals.
