We investigate the influence of electron-phonon interactions on the dynamical properties of a quantum-dot-cavity QED system. We show that non-Markovian effects in the phonon reservoir lead to strong changes in the dynamics, arising from photon-assisted dephasing processes, not present in Markovian treatments. A pronounced consequence is the emergence of a phonon induced spectral asymmetry when detuning the cavity from the quantum-dot resonance. The asymmetry can only be explained when considering the polaritonic quasi-particle nature of the quantum-dot-cavity system. Furthermore, a temperature induced reduction of the light-matter coupling strength is found to be relevant in interpreting experimental data, especially in the strong coupling regime. The emergent field of quantum information technology [1] has spurred major research activities on controlling the fundamental interaction between a semiconductor quantum-dot (QD) and a cavity. Solid-state cavity QED (cQED) systems are inherently coupled to the environment, since the emitter is embedded in a solid. This is in contrast to atomic cQED where the atom can be effectively isolated and only few discrete energy levels are sufficient in the description. Remarkably dephasing from solid-state environments cannot simply be seen as a nuisance, but can in fact lead to enhanced coupling of QDs to a detuned cavity mode of importance for efficient single-photon sources [2-4] and nanolasers [5]. Modeling the continuum of reservoir modes of solid-state systems constitutes a considerable challenge. The coupling of the QD-cavity system to its solid-state environment has almost exclusively been described using Markovian theories [2,6], neglecting memory effects of the reservoirs. While the Markovian approximation is well justified for some reservoirs, this is not in general true for the reservoir consisting of quantized lattice vibrations. Such phonon reservoirs dephase the QD-cavity system, whereby the entanglement between light and matter in general is destroyed. Notably the first experimental demonstrations of the strong coupling regime in solid-state cQED [3] revealed features in the emission spectra for large QDcavity detuning that could not be explained by standard Markovian theory [6]. Since then there has been a lively debate [2,4,7] on the origin of the deviations. We demonstrate that non-Markovian phonon processes play an important role for solid-state cQED.Here, using a simple physical model we show that photon-assisted dephasing processes are of great importance in describing the effect of phonons in a cQED setting. The underlying physical picture is that the polariton quasi-particle, formed by dressing the QD with the cavity photon, is dephased by phonon processes. We focus on the regime of relatively small QD-cavity detunings and pulsed excitation conditions where dephasing processes mediated by longitudinal acoustic (LA) phonons are expected to be important, and investigate the consequences on the dynamical properties of the cQED system. Pulsed excitation is r...

A solid-state single-photon source emitting indistinguishable photons on-demand is an essential component of linear optics quantum computing schemes. However, the emitter will inevitably interact with the solid-state environment causing decoherence and loss of indistinguishability. In this paper, we present a comprehensive theoretical treatment of the influence of phonon scattering on the coherence properties of single photons emitted from semiconductor quantum dots. We model decoherence using a full microscopic theory and compare with standard Markovian approximations employing Lindblad-type relaxation terms. Significant differences between the two approaches are found.

We study the fundamental limit on single-photon indistinguishability imposed by decoherence due to phonon interactions in semiconductor quantum dot-cavity QED systems. Employing an exact diagonalization approach we find large differences compared to standard methods. An important finding is that short-time non-Markovian effects limit the maximal attainable indistinguishability. The results are explained using a polariton picture that yields valuable insight into the phononinduced dephasing dynamics. PACS numbers: 78.67.Hc, 03.65.Yz, 42.50.Pq The study of the coherence properties of single photons emitted from semiconductor cavity QED (cQED) systems is important for applications in quantum information technology [1] and provides insight into the fundamental decoherence effects induced by the environment. For all-solid-state cQED systems, such as a quantum dot (QD) embedded in a photonic crystal cavity [2] [ Fig. 1(a)] or a micropillar cavity [3], the main decoherence mechanism at low temperatures is the electron-phonon interaction [4][5][6], as many recent studies show [2,[7][8][9][10][11][12][13][14].Decoherence limits the degree of indistinguishability of single photons emitted from cQED systems [ Fig. 1(a)], thus diminishing their applicability for scalable linear optical quantum computing [1], where an all-solid-state single-photon source is a key element. Furthermore, recent experimental results [2,8,9,11,12,14] necessitate a departure from the well understood paradigms of atomic cQED, since the strong interaction with reservoirs in the solid state calls for new basic models and physical interpretations. A better understanding of phonon-induced decoherence thus leads to insight into the fundamental physics of nanostructured solids, and can help ushering novel quantum technological devices. However, thus far only little attention has been given to the influence of phonon interactions on the indistinguishability. Only few experiments have been reported [3,15,16] and previous theoretical studies have employed a Markovian pure dephasing approximation [17][18][19][20][21][22][23][24] or phenomenological descriptions of finite-memory dephasing processes [25], none of them treating the phonon interaction microscopically while taking into account the cavity.In this Letter we show that the non-Markovian nature of the phonon reservoir has a large effect on singlephoton indistinguishability: short-time virtual processes occurring on time scales much shorter than a typical "dephasing time", must be considered. Also, it is essential to treat the phonon interaction microscopically and on equal footing with the electron-photon interaction. The analysis is based on an exact diagonalization (ED) technique, retaining the inherent non-Markovian nature of the phonon interaction to all orders in the phonon coupling. Our findings are contrasted to standard approximate approaches for including phonon interactions [13,[26][27][28][29], namely second order expansions and phenomenological pure dephasing descriptions. Figures 1(c)...

We investigate the influence of the electron-phonon interaction on the decay dynamics of a quantum dot coupled to an optical microcavity. We show that the electron-phonon interaction has important consequences on the dynamics, especially when the quantum dot and cavity are tuned out of resonance, in which case the phonons may add or remove energy leading to an effective nonresonant coupling between quantum dot and cavity. The system is investigated using two different theoretical approaches: (i) a second-order expansion in the bare phonon coupling constant, and (ii) an expansion in a polaron-photon coupling constant, arising from the polaron transformation which allows an accurate description at high temperatures. In the low temperature regime we find excellent agreement between the two approaches. An extensive study of the quantum dot decay dynamics is performed, where important parameter dependencies are covered. We find that in general the electron-phonon interaction gives rise to a greatly increased bandwidth of the coupling between quantum dot and cavity. At low temperature an asymmetry in the quantum dot decay rate is observed, leading to a faster decay when the quantum dot has a larger energy than to the cavity. We explain this as due to the absence of phonon absorption processes. Furthermore, we derive approximate analytical expressions for the quantum dot decay rate, applicable when the cavity can be adiabatically eliminated. The expressions lead to a clear interpretation of the physics and emphasizes the important role played by the effective phonon density, describing the availability of phonons for scattering, in quantum dot decay dynamics. Based on the analytical expressions we present the parameter regimes where phonon effects are expected to be important. Also, we include all technical developments in appendices.

We show that the resonance fluorescence spectrum of a quantum dot excited by a strong optical pulse contains multiple peaks beyond those of the Mollow triplet. We show that as the area of the optical pulse is increased, new side peaks split off the central peak and shift in frequency. A simple analytical theory has been derived, which quantitatively accounts for the appearance and position of the peaks. This theory explains the physics responsible for the multiple peaks. By considering the time-dependent spectrum we demonstrate a time ordering of the side peaks, which is further evidence for the suggested physical explanation.