As widely assumed single photon sources (SPSs) represent a essentials/building blocks in present and future quantum technologies.
They consist in devices able to emit one photon per time, on demand, i.e. when excited by optical electrical pulses.
Combining nanocrystal quantum dots, efficient quantum emitters, and photonic cavities we achieve state-of-the-art efficiencies both in photon purity and extraction efficiencies.
To understand at best the performances of these devices we describe here the parameters taken into account:
Brightness: the probability that each light pulse contains a single photon. It is clear that for an ideal deterministic SPS this probability is 1.
0For clarity we should then distinguish in between brightness at the first lens and at the end of a single mode fiber, most preferable parameter for end-users. The first relate to the probability of collecting a photon,per excitation pulse, at the output of the semiconductor structure where the collection setup is placed. The process of extraction of photons from semiconductor structures it is indeed not trivial and amount to a factor inferior of 1%. One of the strategies to overcome this limitation is to modify the electromagnetic field vacuum around the QD accelerating it’s spontaneous emission into the collection mode and suppressing at the same time all other possible radiative recombination channels. This enhancement is defined by the factor “beta”. The total extraction efficiency from the structure must also include the probability for a photon in this mode to be externally collected in an optical lens, factor defined as “eta”. The product of the two (“eta” x “beta”) is a figure of merit proper of the photonic structure and specifically describes the probability for each photon emitted by the QD, to be collected at the first lens, placed on top of the device.
Brightness at the first lens:
Purity: it defines the probability of having only one photon per pulse. It is studied by sending the emitted light in a so-called Haibury-Brown-Twist (HBT) interferometer and counting the total coincidences at the two exits. Reconstructing the second order correlation function (g2) we extract the coincidences at zero delay which define the probability of having emission of multiple photons.