What are single photons good for?

Abstract

In a long-held preconception, photons play a central role in present-day quantum technologies. But what are sources producing photons one by one good for precisely? Well, in opposition to what many suggest, we show that single-photon sources are not helpful for point to point quantum key distribution because faint laser pulses do the job comfortably. However, there is no doubt about the usefulness of sources producing single photons for future quantum technologies. In particular, we show how single-photon sources could become the seed of a revolution in the framework of quantum communication, making the security of quantum key distribution device-independent or extending quantum communication over many hundreds of kilometers. Hopefully, these promising applications will provide a guideline for researchers to develop more and more efficient sources, producing narrowband, pure and indistinguishable photons at appropriate wavelengths.

Keywords:

• single photons
• QKD
• quantum optics (inc. quantum information)

Acknowledgments

We thank Ci Wen Lim, Clara Osorio, Bruno Sanguinetti and Rob Thew for interesting discussions. We gratefully acknowledge support by the EU projects Qessence and Qscale, and from the Swiss NCCRs QP and QSIT.

Notes

Notes

1. A single-photon source is defined as a device delivering photons one at a time. Theorists would prefer to say that it creates Fock states filled with one excitation Experimentalists would emphasize that a single-photon source is characterized by a second-order autocorrelation function See 1,2 for recent reviews on single-photon sources.

2. The loss of a typical telecommunication optical fiber is 0.2 dB/km for the optimal wavelength around 1.5 µm. Note also that optical fibers are not the only way of implementing a quantum channel. The long-distance distribution of photons through free space is also an active field of investigation (see for example 21 or 22 for nice illustrations). We emphasize that the quantum repeater principle can be applied to any kind of channel to overcome the problem of loss.

3. For a review of quantum repeaters based on atomic ensembles and linear optics, see 26.