Experiments show that a quantum dots ensemble's emission is partially mutually coherent under a laser excitation pulses with duration shorter than 600 fs. A possible pulse induced coherent superradiance predicted by Dicke is discussed.
The possibility of realizing the superradiant regime of electromagnetic emission by the assembly of quantum dots is considered. The overall dynamical process is analyzed in detail. It is shown that there can occur several qualitatively different stages of evolution. The process starts with dipolar waves triggering the spontaneous radiation of individual dots. This corresponds to the fluctuation stage, when the dots are not yet noticeably correlated with each other. The second is the quantum stage, when the dot interactions through the common radiation field become more important, but the coherence is not yet developed. The third is the coherent stage, when the dots radiate coherently, emitting a superradiant pulse. After the superradiant pulse, the system of dots relaxes to an incoherent state in the relaxation stage. If there is no external permanent pumping, or the effective dot interactions are weak, the system tends to a stationary state during the last stationary stage, when coherence dies out to a low, practically negligible, level. In the case of permanent pumping, there exists the sixth stage of pulsing superradiance, when the system of dots emits separate coherent pulses.
We report a study on coherence of excitation of single quantum dots. We address the coherent excitation of biexcitons, the process that is indispensable for deterministic photon pair generation in quantum dots. Based on theoretical modeling we optimized the duration of the excitation pulse in our experiment to minimize the laser-induced dephasing and increase the biexciton-to-background single-exciton occupation probability. An additional effect of this approach is a high degree of time-bin entanglement with a concurrence of up to 0.78(6) and a 0.88(3) overlap with a maximally entangled state.
We have studied the photoluminescence correlation from a single InAs/GaAs self-assembled Stranski–Krastanow quantum dot under continuous, as well as under pulsed excitation. Under weak continuous excitation, where the single dot luminescence is due primarily to single exciton recombinations, antibunching is observed in the single dot emission correlation. Under weak pulsed excitation, the number of photons emitted by the quantum dot per pulse is close to one. We present data obtained under both conditions and are able to show that devices based on single quantum dots can be used to generate single photons.
Summary form only given. The spectroscopy of individual quantum dots (QDs) using optical micro-probing techniques is opening up possibilities of various applications based on the physics of the zero-dimensional (0D) semiconductor structures. Self-assembled quantum dots (SAQDs) are also attractive 0D structures, where strong confinement on a no-meter scale can be realized. Here, we carried out time-correlation measurements of excited carriers in single SAQDs in order to realize coherent optical control. The sharpness of the resonance lines in single dot photoluminescence allows us to observe a long phase relaxation time.
Inhomogeneous broadening in ensembles of semiconductor quantum dots (QDs) has hindered coherent operations due to the detuning effects, caused by the large fluctuation in the QD transition energy due to size dispersion. This difficulty is especially evident when using femtosecond laser pulses for excitation. Here, the authors successfully measure detuning-dependent coherent evolution for a QD ensemble by employing prepulse two-dimensional coherent spectroscopy. The dephasing mechanism is found to be significantly different from that observed using picosecond lasers. This work presents a novel method to explore collective phenomena, such as superfluorescence, as well as motivating future studies of coherent interactions in the femtosecond regime.
A pulsed laser scenario, designed to prepare a variety of self-assembled quantum dots with nearly 100% population in a biexciton state, is proposed. We use realistic parameters for typical self-assembled quantum dots and show that two-colour sequential as well as concurrent excitation of the biexciton state can virtually eliminate unwanted exciton population and prepare a quantum dot in the biexciton state with near-deterministic accuracy. The ensuing radiative emission would be a prerequisite for a high rate, on-demand, source of entangled photons.