Figure 1

Basic scheme of the DPTS protocol exploiting phase and time domain.

A train of weak coherent pulses (WCP) is emitted by a laser of repetition rate ν (2/T), and attenuated to the single photon level. A phase modulator (PM) encodes the first key bit in second-neighbor pulses with a period of T by choosing randomly either 0 or π. An intensity modulator (IM) is used to choose the position of the pulses to encode the second key bit. The number of pulses N, where the intensity modulator uses the same time instances, is defined as a block. In this way, Alice prepares a sequence of different states: |0〉, |1〉, |2〉, |3〉. Using a delay line interferometer with a delay of T between arms, Bob can simultaneously measure the phase and pulse position.