  Optical layer attacks on communication networks are one of the weakest
reinforced areas of the network, allowing attackers to overcome security when
proper safeguards are not put into place. Here, we present our solution or
Quantum Encryption in Phase Space (QEPS), a physical layer encryption method to
secure data over the optical fiber, based on our novel round-trip
Coherent-based Two-Field Quantum Key Distribution (CTF-QKD) scheme. We perform
a theoretical study through simulation and provide an experimental
demonstration. The same encryption is used for QEPS as CTF-QKD but achieved
through a pre-shared key and one-directional transmission design. QEPS is
uniquely different from traditional technology where encryption is performed at
the optical domain with coherent states by applying a quantum phase-shifting
operator. The pre-shared secret is used to seed a deterministic random number
generator and control the phase modulator at the transmitter for encryption and
at the receiver for decryption. Using commercially available simulation
software, we study two preventative measures for different modulation formats
which will prevent an eavesdropper from obtaining any data. QEPS demonstrates
that it is secure against tapping attacks when attackers have no information of
the phase modulator and pre-shared key. Finally, an experiment with commercial
components demonstrates QEPS system integrability.
