Transform-limited single photons from a single quantum dot

Abstract

Developing a quantum photonics network requires a source of very-high-fidelity single photons. An outstanding challenge is to produce a transform-limited single-photon emitter to guarantee that single photons emitted far apart in the time domain are truly indistinguishable. This is particularly difficult in the solid-state as the complex environment is the source of noise over a wide bandwidth. A quantum dot is a robust, fast, bright and narrow-linewidth emitter of single photons; layer-by-layer growth and subsequent nano-fabrication allow the electronic and photonic states to be engineered. This represents a set of features not shared by any other emitter but transform-limited linewidths have been elusive. Here, we report transform-limited linewidths measured on second timescales, primarily on the neutral exciton but also on the charged exciton close to saturation. The key feature is control of the nuclear spins, which dominate the exciton dephasing via the Overhauser field.

Franson Interference Generated by a Two-Level System

“…The measured Franson visibility of 66% indicates a clear violation over the classical limit and approaches the limit for a violation of Bell's inequalities. We speculate that the visibility in our experiments is limited by properties intrinsic to the source, perhaps originating in influences related to phonon scattering and/or spectral diffusion, which are processes often causing deviations from ideal twolevel system behavior in InAs epitaxial QDs [30,46,47]. Nonetheless, our method based on single photon pairs can be potentially used in future quantum communication schemes, with entanglement preserved through optical fibers.…”

Excitation with quantum light. II. Exciting a two-level system

“…For this reason, there is a race to build always better sources, in particular in the semiconductor community where such devices would furthermore have a large economic and technological potential. [44][45][46] The Fourier transform limit for single photon emission has already been reached 47 and there is now much efforts to combine and enhance other features such as brigthness, efficiency and, of course, antibunching. [48][49][50] Our proposal takes a new direction and rather than bettering engineering and implementation, we turn to a different mechanism to increase the quality of SPS by magnitudes not accessible only with a better technology.…”

Imaginary echoes

“…In addition to phonon induced processes, charge noise and spin noise can also affect emitted photon coherence 18,19 . However, our purpose here is assess the ultimate limits of a QD based source, and note that charge and spin noise can be heavily suppressed in suitably engineered samples 12,13 , while coupling to phonons can ever be completely quenched, as even at T = 0 K phonon emission can still take place.…”

“…Highly directional emission is crucial to the overall efficiency of the source, and is typically achieved by either placing the QD in a waveguide with low out-of-plane scattering 15,16 , or by coupling resonantly to an optical cavity mode 6-9,12,13 . Nevertheless, the solid-state nature of QDs leads to strong coupling between the electronic degrees of freedom and their local environment; fluctuating charges 17 , nuclear spins 18,19 , and lattice vibrations 20-23 all lead to a suppression of photon coherence and a resulting reduction in indistinguishability 11,[24][25][26][27] . While early experiments were indeed limited by these factors 6-9 , improvements in fabrication and resonant excitation techniques have steadily increased photon indistinguishability to levels now exceeding 99% in resonantly coupled QD-cavity systems 12,13 .…”