Light can be used to encode information in a variety of ways. Polarization, for example: a ‘0’ bit could be represented by a pulse of horizontally polarized light, and a ‘1’ bit could be vertically polarized. This generally works well for transmissions over free-space. Also, by allowing superposition states and reducing the intensity to single-photon levels, one can start to access interesting quantum protocols such as quantum key distribution (QKD). You can do this with other encodings, too—“time bin”, for example, where you encode information in the arrival time, early or late, relative to a reference. But because of the way the superposition state (that is, the early “and” late state) is measured, it doesn't generally work well over air because of turbulence.
A recently discovered enhancement of the measurement device by my colleagues intrinsically bypasses the turbulence problem, and in this paper, we couple this improved apparatus with a QKD system to demonstrate a real quantum protocol with time-bin encoded light transmitted over long-distance (1.2 km) free space. The approach we take here could in future be used as a bridge between optic fiber (where turbulence isn't an issue) and free-space for quantum protocols.
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