Encoding M classical bits in the arrival time of dense-coded photons
  We present a scheme to encode M extra classical bits to a dense-coded pair of photons. By tuning the delay of an entangled pair of photons to one of 2^M time-bins and then applying one of the quantum dense coding protocols, a receiver equipped with a synchronized clock of reference is able to decode M bits (via classical time-bin encoding) + 2 bits (via quantum dense coding). This protocol, yet simple, does not dispense several special features of the used programmable delay apparatus to maintain the coherence of the two-photon state. While this type of time-domain encoding may be thought to be ideally of boundless photonic capacity (by increasing the number of available time-bins), errors due to the environmental noise and the imperfect devices and channel evolve with the number of time-bins. 
Salem F. Hegazy, Ahmed E. Morra, Salah S. A. Obayya
quant-ph physics.optics