A single point in a material that gives out light one – one indivisible packet of light – at a time, and never two at once: a building block for quantum communication and . A 2D host is attractive because the emitter sits right at the surface, so almost all of its light gets out.
Going deeper
A quantum emitter releases photons one at a time. In a monolayer, excitons roll into a strain well – for instance where the layer is draped over a pillar – and emit from that one spot; the proof is a dip in coincidences at zero delay, well below one half.
What makes an emitter single
A single two-level system can hold only one excitation at a time, so after emitting a photon it must be re-excited before it can emit another. Two photons therefore never leave together. The standard test splits the light onto two detectors and histograms the delay between clicks: a single emitter shows a dip at zero delay, quantified by g(2)(0). A value below 0.5 rules out two independent emitters, and good sources reach well below 0.1.
This is what distinguishes a quantum emitter from an ordinary bright spot. A laser gives g(2)(0) = 1, thermal light gives 2, and any ensemble of emitters gives something close to 1.
Emitters in 2D hosts
emit single photons from localised : an exciton trapped in a local potential minimum caused by , a defect, or both. Strain is controllable – draping a monolayer over nanopillars or features creates wells at chosen positions, which addresses the usual problem that emitters appear randomly. Hexagonal boron nitride hosts a different class: bright, stable emitters that work at room temperature, deep in its , generally attributed to defect centres whose exact chemical identity is still argued over.
Being one layer deep has practical advantages. Almost all the emitted light escapes rather than being trapped in a high-index bulk, the emitter sits within nanometres of a cavity, or electrode, and strain, electric field and magnetic field all reach it directly.
What still limits them
The environment that makes these emitters tunable also makes them noisy. Charges moving in nearby traps shift the transition energy from shot to shot – spectral diffusion – which broadens lines far beyond their lifetime limit and spoils the indistinguishability that quantum interference requires. in hBN, careful and resonant excitation all help.
TMDC emitters mostly need cryogenic temperatures; hBN emitters work warm but are harder to place deliberately, and their brightness and stability vary from site to site. For applications, what matters is not a single good spectrum but an emitter that is at a known position, at a known wavelength, bright, stable for hours and, ideally, coupled to a photonic structure – and no host yet offers all of that at once.
For specialists
A localised optical transition emitting antibunched light, identified by g(2)(0) well below 0.5. In monolayer TMDCs the emitters are excitons trapped at strain wells or defects; hBN hosts bright emitters that work at room temperature and are attributed to defect centres. Being one layer deep helps extraction and makes the emitter responsive to strain, field and cavity coupling, at the cost of spectral diffusion.