Single and Entangled Photon Sources

This paper explores the fundamentals and applications of single and entangled photon sources. These sources are the ground works for advances in quantum communications, ranging from quantum cryptography to topics as peculiar as quantum teleportation. Single Photon Sources I. What is a Single Photon Source? A single photon source is an emitter of light that has all of its photons separated in time. It is important to distinguish between an attenuated light source and a single photon source—laser light may be attenuated down to a level such that a single photon is found per meter, but laser light consists of coherent light, which obeys classical statistics. Because of this, attenuated laser light will be contaminated with pairs or triplets of photons. Attenuated lasers are a good approximation of a single photon source, but they are by no means a real single photon source. What is meant by a real single photon source? As mentioned above, a real single photon source has all of its photons separated in time, that is, it emits antibunched light [1]. This means that when it emits light, it emits a single quanta of light at a time (i.e. single photons all separated in time). Many photons are produced at the same time in lasers. This is why even after attenuation a laser cannot be considered a single photon source—photons that pass through the source of attenuation may have been produced simultaneously and are thus correlated (bunched). II. Measurement of Single Photon Sources To quantify the degree of antibunching for a particular source of photons, the second order coherence function is used [1]: Where I is the intensity of the light, and are the positions of two separate detectors, and is the time between detection of photons. This function is a measure of the joint photo-detection probability for the two detectors at different times [1]. For classical light, , meaning that it is more probable for the detectors to register photons at the same time. For antibunched light, , meaning that there is zero probability that two photons will be detected at the same time. To measure the coherence between photons, a Hanbury-Brown Twiss setup can be used, shown below in figure 1. Fluorescent light enters the setup  g(2)(r1,t;r2,t )  I(r1,t)I(r2,t ) I(r1,t) I(r2,t )