Microscopic theory of indistinguishable single-photon emission from a quantum dot coupled to a cavity: The role of non-Markovian phonon-induced decoherence

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 phonon-induced dephasing dynamics.Comment: published version, comments are very welcom

Measuring the effective phonon density of states of a quantum dot

We employ detuning-dependent decay-rate measurements of a quantum dot in a photonic-crystal cavity to study the influence of phonon dephasing in a solid-state quantum-electrodynamics experiment. The experimental data agree with a microscopic non-Markovian model accounting for dephasing from longitudinal acoustic phonons, and identifies the reason for the hitherto unexplained difference between non-resonant cavity feeding in different nanocavities. From the comparison between experiment and theory we extract the effective phonon density of states experienced by the quantum dot. This quantity determines all phonon dephasing properties of the system and is found to be described well by a theory of bulk phonons.Comment: 5 pages, 3 figures, submitte

Quenching of phonon-induced processes in quantum dots due to electron-hole asymmetries

Differences in the confinement of electrons and holes in quantum dots are shown to profoundly impact the magnitude of scattering with acoustic phonons in materials where crystal deformation shifts the conduction and valence band in the same direction. Using an extensive model that includes the non-Markovian nature of the phonon reservoir, we show how the effect may be addressed by photoluminescence excitation spectroscopy of a single quantum dot. We also investigate the implications for cavity QED, i.e. a coupled quantum dot-cavity system, and demonstrate that the phonon scattering may be strongly quenched. The quenching is explained by a balancing between the deformation potential interaction strengths and the carrier confinement and depends on the quantum dot shape. Numerical examples suggest a route towards engineering the phonon scattering.Comment: 5 pages, 5 figures, submitted for peer review, comments are welcom

Microscopic theory of phonon-induced effects on semiconductor quantum dot decay dynamics in cavity QED

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 non-resonant 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.Comment: published PRB version, comments are very welcom

Non-markovian model of photon-assisted dephasing by electron-phonon interactions in a coupled quantum-dot-cavity system

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.Comment: 4 pages, 4 figures, published in Physical Review Letters, changes according to referees comments, comments are welcom

Overcoming phonon-induced dephasing for indistinguishable photon sources

Reliable single photon sources constitute the basis of schemes for quantum communication and measurement based quantum computing. Solid state single photon sources based on quantum dots are convenient and versatile but the electronic transitions that generate the photons are subject to interactions with lattice vibrations. Using a microscopic model of electron-phonon interactions and a quantum master equation, we here examine phonon-induced decoherence and assess its impact on the rate of production, and indistinguishability, of single photons emitted from an optically driven quantum dot system. We find that, above a certain threshold of desired indistinguishability, it is possible to mitigate the deleterious effects of phonons by exploiting a three-level Raman process for photon production

The Role of Autophagy in iNKT Cell Development

CD1d-restricted invariant natural killer T (iNKT) cells are innate-like T cells that express an invariant T cell receptor (TCR) α-chain and recognize self and foreign glycolipid antigens. They can rapidly respond to agonist activation and stimulate an extensive array of immune responses. Thymic development and function of iNKT cells are regulated by many different cellular processes, including autophagy, a self-degradation mechanism. In this mini review, we discuss the current understanding of how autophagy regulates iNKT cell development and effector lineage differentiation. Importantly, we propose that iNKT cell development is tightly controlled by metabolic reprogramming

Quantitative analysis of quantum dot dynamics and emission spectra in cavity quantum electrodynamics:Paper

We present detuning-dependent spectral and decay-rate measurements to study the difference between spectral and dynamical properties of single quantum dots embedded in micropillar and photonic-crystal cavities. For the micropillar cavity, the dynamics is well described by the dissipative Jaynes-Cummings model, while systematic deviations are observed for the emission spectra. The discrepancy for the spectra is attributed to coupling of other exciton lines to the cavity and interference of different propagation paths towards the detector of the fields emitted by the quantum dot. In contrast, quantitative information about the system can readily be extracted from the dynamical measurements. In the case of photonic crystal cavities we observe an anti crossing in the spectra when detuning a single quantum dot through resonance, which is the spectral signature of strong coupling. However, time-resolved measurements reveal that the actual coupling strength is significantly smaller than anticipated from the spectral measurements and that the quantum dot is rather weakly coupled to the cavity. We suggest that the observed Rabi splitting is due to cavity feeding by other quantum dots and/or multiexcition complexes giving rise to collective emission effects.Comment: 14 pages, 5 figures, submitte