Alexander Carmele

Few-photon model of the optical emission of semiconductor quantum dots.

The Jaynes-Cummings model provides a well established theoretical framework for single electron two level systems in a radiation field. Similar exactly solvable models for semiconductor light emitters such as quantum dots dominated by many particle interactions are not known. We access these systems by a generalized cluster expansion, the photon-probability cluster expansion: a reliable approach for few-photon dynamics in many body electron systems. As a first application, we discuss vacuum Rabi oscillations and show that their amplitude determines the number of electrons in the quantum dot.

Formation dynamics of an entangled photon pair { a temperature dependent analysis

We theoretically study the polarization entanglement of photons generated by the biexciton cascade in a GaAs/InAs semiconductor quantum dot (QD) located in a nanocavity. A detailed analysis of the complex interplay between photon and carrier coherences and phonons which occurs during the cascade allows us to clearly identify the conditions under which entanglement is generated and destroyed. A quantum state tomography is evaluated for varying exciton fine-structure splittings. Also, by constructing an effective multiphonon Hamiltonian which couples the continuum of the QD-embedding wetting layer states to the quantum confined states, we investigate the relaxation of the biexciton and exciton states. This consistently introduces a temperature dependence to the cascade. Considering typical Stranski-Krastanov grown QDs for temperatures around 80 K the degree of entanglement starts to be affected by the dephasing of the exciton states and is ultimately lost above 100 K.

Two-dimensional electron gases: Theory of ultrafast dynamics of electron-phonon interactions in graphene, surfaces, and quantum wells

Many-particle electron-phonon interaction effects in two-dimensional electron gases are investigated within a Born–Markov approach. We calculate the electron-phonon interaction on a microscopic level to describe relaxation processes of quantum confined electrons on ultrafast time scales. Typical examples, where two-dimensional electron gases play a role, are surfaces and two-dimensional nanostructures such as graphene and quantum wells. In graphene, we find nonequilibrium phonon generation and ultrafast cooling processes after optical excitation. Electron relaxation dynamics at the silicon (001) 2×1 surface exhibits two time scales, corresponding to intrasurface and inside bulk-scattering processes. For GaAs quantum wells, we present broad emission spectra in the terahertz range assisted by LO-phonons of the barrier material.

Bunching and anti-bunching in electronic transport

-function for electronic transport and use it to investigate the bunching and anti-bunching of electron currents. Importantly, we show that super-Poissonian electron statistics do notnecessarily imply electron bunching, and that sub-Poissonian statistics do not imply anti-bunching.We discuss the information contained in g

A bright triggered twin-photon source in the solid state

The realization of integrated light sources capable of emitting non-classical multi-photon states, is a fascinating, yet equally challenging task at the heart of quantum optics [1]. One example of such light-states are photon twins, which up till now have mostly been generated with low emission rates using probabilistic parametric down-conversion sources [2] or atoms [3].

Feedback-Induced Steady-State Light Bunching Above the Lasing Threshold

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 insights into the origin of bunching in quantized and semiclassical models.

Boundary-driven Heisenberg chain in the long-range interacting regime: Robustness against far-from-equilibrium effects

We investigate the Heisenberg XXZ-chain with long-range interactions in the Z-dimension. By applying two magnetic boundary reservoirs we drive the system out of equilibrium and induce a non-zero steady state current. The long-range coupled chain shows nearly ballistic transport and linear response for all potential differences of the external reservoirs. In contrast, the common isotropic nearest-neighbor coupling shows negative differential conductivity and a transition from diffusive to subdiffusive transport for a far from equilibrium driving. Adding disorder, the change in the transport for nearest neighbor coupling is therefore highly dependent on the driving. We find for the disordered long-range coupled XXZ-chain, any change in the transport behavior is independent of the potential difference and the coupling strengths of the external reservoirs.

Unraveling coherent quantum feedback for Pyragas control [Invited]

We present a Heisenberg-operator-based formulation of coherent quantum feedback and Pyragas control. This model is easy to implement and allows for an efficient and fast calculation of the dynamics of feedback-driven observables as the number of contributing correlations grows in systems with a fixed number of excitations only linearly in time. Furthermore, our model unravels the quantum kinetics of entanglement growth in the system by explicitly calculating non-Markovian multi-time correlations, e.g., how the emission of a photon is correlated with an absorption process in the past. Therefore, the time-delayed differential equations are expressed in terms of insightful physical quantities. Another considerable advantage of this method is its compatibility with typical approximation schemes, such as factorization techniques and the semi-classical treatment of coherent fields. This allows the application on a variety of setups, ranging from closed quantum systems in the few excitation regimes to open systems and Pyragas control in general.

Theory of single quantum dot lasers: Pauli-blocking-enhanced anti-bunching

We present a theoretical model to describe the dynamics of a single semiconductor quantum dot interacting with a microcavity system. The confined quantum dot levels are pumped electrically via a carrier reservoir. The investigated dynamics includes semiconductor-specific, reservoir-induced Pauli-blocking terms in the equations of the photon probability functions. This enables a direct study of the photon statistics of the quantum light emission in dependence on the different pumping rates.

Superradiant to subradiant phase transition in the open system Dicke model: dark state cascades

Collectivity in ensembles of atoms gives rise to effects like super- and subradiance. While superradiance is well studied and experimentally accessible, subradiance remains elusive since it is difficult to track experimentally as well as theoretically. Here we present a new type of phase transition in the resonantly driven, open Dicke model that leads to a deterministic generation of subradiant states. At the transition the system switches from a predominantly superradiant to a predominantly subradiant state. Clear experimental signatures for the effect are presented and entanglement properties are discussed. Letting the system relax into the ground state generates a cascade of dark Dicke states, with dark state populations up to unity. Furthermore we introduce a collectivity measure that allows to quantify collective behavior.