A Photon Pair Factory
Producing single photons on demand has become routine in quantum physics laboratories over the past decade. Reliably producing exactly two photons at the same time from a single solid-state device has remained extraordinarily difficult — until now. Researchers at the Beijing Academy of Quantum Information Sciences have developed a quantum dot device that emits photon pairs with 98.3 percent purity, one of the highest figures ever achieved with a solid-state emitter.
The achievement matters because photon pairs are essential building blocks for quantum technologies. When two photons are produced together through certain quantum processes, they can be entangled — meaning their quantum states are correlated in ways that classical physics cannot explain. Entangled photon pairs are the fundamental resource for quantum key distribution, quantum teleportation, and advanced quantum sensing applications.
Why Pairs Are Harder Than Singles
Single-photon sources based on semiconductor quantum dots have achieved remarkable performance in recent years. These nanoscale structures, often described as artificial atoms, can be engineered to emit one photon at a time with high efficiency and purity. The physics is relatively straightforward: excite an electron in the quantum dot, wait for it to relax and emit a photon, repeat.
Producing pairs is fundamentally more challenging. The desired process involves creating a biexciton state — a condition where two electrons are simultaneously excited in the quantum dot. When this biexciton state decays, it releases two photons in rapid succession through a cascade process. The problem is that in practice, the first excited electron typically emits its photon and relaxes before the second electron can arrive, preventing the biexciton state from forming reliably.
Previous attempts to generate photon pairs from quantum dots suffered from low efficiency and contamination by unpaired single photons. Traditional photon-pair sources using nonlinear crystals can produce pairs through a process called spontaneous parametric down-conversion, but these are inherently probabilistic — sometimes producing one pair, sometimes two, sometimes none — introducing noise that limits their usefulness for practical quantum applications.
