Quantum Dot Single-Photon Source Lab

An epitaxial quantum dot is a nanometer-scale semiconductor island (such as InAs inside a GaAs matrix) grown via Stranski-Krastanov self-assembly. Due to three-dimensional carrier confinement, its electronic density of states becomes atomic-like with discrete delta-function energy levels. When resonantly driven by a short laser pi-pulse, the quantum dot creates a single electron-hole pair (exciton) that radiatively recombines, deterministically emitting exactly one photon per pulse.

Embedding the quantum dot at the antinode of a distributed Bragg reflector (DBR) micropillar optical microcavity with quality factor Q and small mode volume V enhances the local vacuum field density of states. This yields a Purcell factor FP = (3 / (4 * pi^2)) * (lambda / n)^3 * (Q / V). Purcell enhancement accelerates spontaneous emission by over 10 times (shortening lifetime from 1.1 ns to under 100 ps) and funnels over 60% of emitted photons into a collimated single-mode spatial beam.

In a Hanbury Brown-Twiss (HBT) interferometer, the emitted light is split by a 50:50 beamsplitter onto two single-photon detectors. The normalized coincidence rate g(2)(tau) measures the photon statistics. For a true single-photon source, an emitted photon cannot split, leading to zero simultaneous coincidences at zero delay (g(2)(0) approaching 0, strictly under 0.05). In contrast, Poissonian laser light gives g(2)(0) = 1 and thermal chaotic light gives g(2)(0) = 2.

Hong-Ou-Mandel (HOM) two-photon interference tests whether successively emitted photons are quantum-mechanically identical across polarization, spatial mode, and spectral wavepacket envelope. When two indistinguishable photons simultaneously arrive at a symmetric beamsplitter, destructive quantum interference causes both photons to bunch and exit the same output port together. Achieving HOM visibility above 95% is the foundational benchmark required for linear optical quantum computing, photonic cluster state generation, and quantum repeaters.

A commercial quantum dot single-photon foundry requires approximately $12.8 million in initial capital expenditure. Major investments include solid-source molecular beam epitaxy (MBE) with in-situ wafer monitoring ($4.2M), electron-beam lithography and deep cryogenic ICP-RIE etching for smooth micropillar sidewalls ($3.4M), low-vibration closed-cycle 4K confocal cryostats with superconducting nanowire detectors ($3.0M), and automated fiber-pigtailed optical packaging stations ($2.2M).