We propose a scheme to control cavity quantum electrodynamics in the single photon limit by delayed feedback. In our approach a single emitter-cavity system, operating in the weak coupling limit, can be driven into the strong coupling-type regime by an external mirror: The external loop produces Rabi oscillations directly connected to the electron-photon coupling strength. As an expansion of typical cavity quantum electrodynamics, we treat the quantum correlation of external and internal light modes dynamically and demonstrate a possible way to implement a fully quantum mechanical time-delayed feedback. Our theoretical approach proposes a way to experimentally feedback control quantum correlations in the single photon limit.

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In a solid-state platform for quantum information science, the biexciton cascade is an important source of entangled photons. However, the entanglement is usually reduced considerably by the fine-structure splitting of the exciton levels. We show how to counteract this loss of entanglement by applying optical feedback. Substantial control and enhancement of photon entanglement can be achieved by coherently feeding back a part of the emitted signal, e.g., by a mirror, and by tuning the feedback phase and delay time. We present full quantum-mechanical calculations, which include the external photon mode continuum, and discuss the mechanisms leading to the above effects.

We develop a full quantum-optical approach for optical self-feedback of a microcavity laser. These miniaturized devices work in a regime between the quantum and classical limit and are test-beds for the differences between a quantized theory of optical self-feedback and the corresponding semiclassical theory. The light intensity and photon statistics are investigated with and without an external feedback: We show that in the low-gain limit, where relaxation oscillations do not appear, the recently observed photon bunching in a quantum dot microcavity laser with optical feedback can be accounted for only by the fully quantized model. By providing a description of laser devices with feedback in the quantum limit we reveal novel insights into the origin of bunching in quantized and semiclassical models.PACS numbers: 42.55. Sa, 42.50.Ar, 42.65.Sf, 42.55.Px Introduction-Lasers are a cornerstone of modern technology. They also constitute ideal systems to study a variety of non-linear effects which open possible routes to the exploitation of complex dynamics in applications as well as in fundamental research. Especially semiconductor lasers with external optical feedback can exhibit rich dynamics which depends strongly on the feedback strength or phase and is under intense investigation [1][2][3][4][5][6]. Most experimental and theoretical investigations of feedback have focussed on semiconductor lasers involving high numbers of active emitters and photons with output powers in the mW (high gain) regime. In this regime, a semiclassical treatment of the light field, as in the well established Lang-Kobayashi-model [1], is capable of reproducing rich dynamics observed experimentally [7]. Recent experiments have also considered miniaturized systems such as microcavity lasers which allow for exploring the regime of much lower output intensity and gain, where only a few dozens emitters are involved, and have observed a modified influence of optical self-feedback on the laser statistics [8]. In general, feedback in semiconductor lasers typically induces chaotic emission resulting in a bunched photon statistics, i.e., a photon-photon correlation g (2) (0) > 1 compared to the pure lasing limit g (2) (0) = 1. Such classical radiation with g (2) (0) ≥ 1 can usually be described by semiclassical models using quantum theory for the emitters but treating the field classically. On the other hand, a low intensity/low gain situation, where only a small number of emitters are involved, typically requires a full quantum description [9]. Therefore, the range of validity of the semiclassical description is not clear. In this Rapid Communication, we develop a fully quan- * schulze@itp.tu-berlin.de tized theory of optical self-feedback in a low-gain regime characteristic of a microcavity laser operating between the quantum and the semiclassical limit. We compare it to a semiclassical approach and find qualitative differences in the light field statistics above the lasing threshold in the low-gain regime. Here, we define the low-gain regim...

A quantum-kinetic approach to the ultrafast dynamics of carrier multiplication in semiconductor quantum dots is presented. We investigate the underlying dynamics in the electronic subband occupations and the time-resolved optical emission spectrum, focusing on the interplay between the light-matter and the Coulomb interaction. We find a transition between qualitatively differing behaviors of carrier multiplication, which is controlled by the ratio of the interaction induced time scale and the pulse duration of the exciting light pulse. On short time scales, i.e., before intraband relaxation, this opens the possibility of detecting carrier multiplication without refering to measurements of (multi-)exciton lifetimes.