  We show how to implement several continuous-variable coherent protocols with
linear optics. Noise can accumulate when implementing each coherent protocol
with realistic optical devices. Our analysis bounds the level of noise
accumulation. We highlight the connection between a coherent channel and a
nonlocal quantum nondemolition interaction and give two new protocols that
implement a coherent channel. One protocol is superior to a previous method for
a nonlocal quantum nondemolition interaction because it requires fewer
communication resources. We then show how continuous-variable coherent
superdense coding implements two nonlocal quantum nondemolition interactions
with a quantum channel and bipartite entanglement. We finally show how to
implement continuous-variable coherent teleportation experimentally and provide
a way to verify the correctness of its operation.
