Single Quantum Dot as an Optical Thermometer for Millikelvin Temperatures

Resonant laser spectroscopy of a negatively charged self-assembled quantum dot is utilized to measure the temperature of a three-dimensional fermionic reservoir down to 100 mK. With a magnetic field applied to the quantum dot, the single-charged ground state is split by the Zeeman energy. As the quantum dot is in tunnel contact with a thermal electron reservoir, a thermal occupation of the quantum-dot spin states is enforced by cotunneling processes. Resonant laser-induced fluorescence is used in order to measure the thermal quantum-dot spin-state population.