Quantum information processing protocols require light sources emitting single indistinguishable photons that have exactly the same spectral, spatial and polarization properties. For instance, some quantum cryptography techniques use strongly attenuated lasers as single photon sources but the probability to send more than one photon at the same time being not zero, the security of the information transfer deteriorates. In this context, the semiconductor quantum dots, which are known to emit photons one by one, are excellent candidates for the development of such sources in the solid state, with the great advantage of being integrated in on-chip devices. Nevertheless, the quantum dot emission has a certain spectral linewidth that makes the emitted photons partially distinguishable over time. Therefore, a time constant that characterizes well the photon indistinguishability is related to the emitter’s coherence time, which is typically of a few hundreds of picoseconds in the usual operating regimes, limiting the use of conventional photo-detectors with response time of the order of the nanosecond.
Based on the comprehensive analysis of the temporal profile of the photon coalescence phenomenon, LPA researchers measured the time during which two photons emitted at distinct moments by a single quantum dot remain indistinguishable. In the specific regime of resonant elastic scattering where photons are scattered one by one by the dot along with the spectral properties of the excitation laser, photon indistinguishability times of more than a dozen of nanoseconds can be obtained and simply tuned by varying the spectral width of the excitation laser. This opens the way for the use of quantum dots as indistinguishable single photon sources for applications in quantum optics with conventional detectors.
These results were published in Physical Review Letters in 2015.
Illustration: scheme of the resonant excitation experimental setup used to generate single indistinguishable photons from a single semiconductor quantum dot.