Figure 1

A single-photon implementation of BB84 suggested in Ref. 2. The kets $|\mathrm{E}⟩$ and $|\mathrm{L}⟩$ correspond, respectively, to an advanced (early) and a delayed (late) single-photon wave packet. Alice sends one of the four states listed below the diagram of the apparatus. The chart indicates which of Alice’s states are consistent with a given measurement event at Bob’s side. As described in the text, Bob’s apparatus does not require active change of measurement basis.Reuse & Permissions

Figure 2

A new scheme for quantum key distribution that combines OWA with passive detection. Two time-bin qubits are sent from Alice to Bob in one of the four quantum states on the left of the figure. The chart on the right uses two levels of structure to describe the detection pattern at Bob’s side. The coarse structure is defined by the bold lines. Each of the nine bold-lined rectangles corresponds to a specification of the joint time of detection of the two photons. The fine structure is defined by the thin lines. Each of the four thin-lined rectangles within the bold-lined rectangles corresponds to a specification of which detector fired for each of the two photons (this coding is illustrated by an example at the bottom left of the figure). The numbers in the curly brackets in each thin-lined rectangle indicates which (if any) of the four quantum states on the left are consistent with the corresponding detection pattern.Reuse & Permissions

Figure 3

A proposed implementation for the source employed in Fig. 2. “SPDC” is a nonlinear crystal pumped by a brief pulse to produce a noncollinear, polarization-entangled two-photon state via spontaneous parametric down-conversion. The action of elements “M” and “P” is described in the text.Reuse & Permissions