Sources of Single and Entangled Photons

The ability to produce single photons and entangled photon pairs in desired quantum states is essential in any system intended to handle quantum information. Single photons serve as qubits in quantum computers, and the most powerful operations in those computers are performed by entangling them. Further, entangled pairs of photons are the carriers for signals in quantum-encrypted communications. Traditionally, single photons could only be produced by the aggressive attenuation of stronger signals. Entangled photons were traditionally produced by spontaneous parametric down conversion. In the latter process, a nonlinear optical crystal annihilates a high-frequency photon and creates two lower-frequency photons. This is a random process with a very low probability of happening, so it requires a very high-intensity source at the input, but this is relatively easy to supply, so SPDC remains a proven method of generating entangled photons. As random processes, these two methods share a pair of distinct but related disadvantages: First, it seriously limits the system’s ability to produce a photon at a specific time, which is an impediment to creating any kind of clock-based quantum system. Second, it means that there may be multiple photons created at a given time, which destroys the absolute security of quantum cryptography. The condition that photons should be separated in time may be called antibunching or sub-Poisson behavior. Important early work in this area was done by Kimble, Dagenais and Mandel [1]. These are the photon producers that may properly be called sources of single and entangled photons, and the desire to create effective implementations of them has driven much of the recent research in quantum optics. In the last decade in particular, many novel sources have been developed to address this need.