Researchers from CEA-IRIG/PHELIQS, in collaboration with Institut Néel-CNRS, have introduced a calibration-free method to measure the absolute temperature from the Bose-Einstein distribution of acoustic phonons around a single quantum emitter. The approach relies on analyzing the phonon sidebands in the emitter's photoluminescence spectrum, offering a direct probe of the temperature of the environment of a quantum dot emitter at the nanoscale.

Our method exploits a fundamental interaction between the quantum emitter and its surroundings. When an emitter (like a quantum dot) absorbs or emits light, it also interacts with phonons—vibrations in the crystal lattice that carry heat. These interactions create distinctive features in the light emitted by the quantum dot:

• A sharp peak called the zero-phonon line (ZPL), representing light emitted without phonon interaction.
• Broader "wings" on either side, called phonon sidebands, which result from photon emission processes that are mediated by the simultaneous creation (Stokes process) or absorption (anti-Stokes process) of a phonon.
  

At low temperatures, the anti-Stokes sideband (higher energy) is very weak because there are few phonons to absorb, while at higher temperatures, it grows stronger.  We show that the ratio of the Stokes to anti-Stokes intensities directly encodes the local temperature. The plot of the logarithm of this ratio against the energy shift from the ZPL is a straight line whose slope gives the temperature without any calibration.