We present a theoretical study of a many-emitter phonon laser based on optically driven semiconductor quantum dots placed within an acoustic nanocavity. A transformation of the phonon laser Hamiltonian leads to a Tavis-Cummings type interaction with an unexpected additional many-emitter energy shift. This many-emitter interaction with the cavity mode results in a variety of phonon resonances which dependent strongly on the number of participating emitters. These collective resonances show the highest phonon output. Furthermore, we show that the output can be increased even more via lasing at the two phonon resonance.
We investigate the spin transport properties of an isotropic quantum Heisenberg spin-chain. Driving the system out of equilibrium via two different reservoirs at the boundaries, the system exhibits negative differential conductivity for strong driving. We describe the system-reservoir interaction with a Lindblad approach. We show that the interplay between Lindblad dynamics and system dynamics influences highly the spin current. For weak driving, equal rates maximize the current while strong driving shows a counter intuitive behavior. Our findings could guide to an understanding of the transport properties which are dependent on the external driving.
In this proceeding we show how to formulate the quantum stochastic Schrodinger equation (QSSE) of the Jaynes-Cummings-model, where the cavity mode is subject to coherent feedback as control mechanism. We start from the Jaynes-Cummings Hamiltonian and the Hamiltonian for feedback on the cavity mode and derive the QSSE and show how to represent the system state as a matrix product state in order to derive a numerically tractable model for the case of the Jaynes-Cummings model subject to feedback. We compare the dynamics for stabilization of the Rabi oscillation with the solution of the Schrodinger equation in the single excitation regime. Furthermore, we compare this with the behavior for the case with two excitations in the system.
We investigate an optomechanical system with an unstable steady state, which can be stabilized via Pyragas control. We will demonstrate this for low and high pump rates. The system contains a pumped cavity, where one mirror is movable. To obtain time delayed feedback, we feed back the cavity field with an external mirror. This way, we achieve a maximal cavity field in the low pumping regime, which also corresponds to a large displaced movable mirror.
Quantum information science relies on the feature of distant quantum entities (mostly "qubits") to form non-local states. A main challenge consists of generating such non-local entangled states between qubits. We exploit the fact that for coupled qubits, the eigenstates of the coupled system are usually highly entangled, and of different excitation energies. This allows to address the different entangled eigenstates by frequency-dependent control schemes.
In our proposal, we present such a control mechanism, and demonstrate how it can be used to create entanglement from a fully separable initial state. The mechanism of our choice is time-delayed quantum-coherent feedback. If a qubit occupation decays via the emission of a photon, one can store this photon for a delay time τ and couple the radiation back into the qubit afterwards. Through the choice of τ, one can set the phase of the feedback, which will then lead to either an increased or decreased qubit decay. Since this phase depends on sin(ωτ), this effect strongly depends on the qubit frequency ω. In particular, it can be used to separate different entangled states in a quantum network by enhancing the decay of all entangled eigenstates except one.
We discuss this protocol on the example of two coupled qubits, and analyze in detail its effectiveness depending on the feedback delay time τ.
We propose to use a time-delayed quantum-coherent feedback mechanism to increase and control the entanglement of photon pairs emitted by a quantum dot biexciton cascade. The quantum dot biexciton cascade is a well-known source of entangled photons on demand, however excitonic fine-structure splitting decreases the achievable polarization entanglement. We demonstrate that feedback can change the spectrum of the emitted photons in a way that the entanglement is either strongly increased or decreased, depending on the feedback time and phase. We analyze the dependence on parameters such as the delay time and the robustness of the proposed mechanism.
We propose a scheme to achieve cavity-assisted strong coupling between the internal degrees of a Rydberg superatom to a moving membrane in the single photon and phonon limit. Our set-up allows for efficient transfer between electronic excitation and single phonons by combining the collective enhancement effect of the superatom Rabi frequency with typical cavity-optomechanics schemes in the strong coupling limit.
The investigation of the intersubband dynamics occurring in the conduction band of doped semiconductor quantum
dots (QD's) has emerged as a powerful tool to probe their electronic structure and properties. Here, the
temperature, the dephasing and the relaxation dynamics are of interest, both from a fundamental point of view
but also regarding the performance of QD-based optoelectronic devices. Whereas the lineshape of QD interband
transitions is well described by the independent boson model treating a diagonal electron-phonon coupling.
For intraband transitions both diagonal and non-diagonal electron-phonon coupling are relevant. Typically,
such problems involving diagonal and non-diagonal coupling are treated within approximate or perturbative
approaches. Here, we present a fully quantized theory to calculate the coupled electron-phonon dynamics based
on a non-perturbative equation of motion approach reproducing also correct results for limits of known exactly
The biexciton cascade within semiconductor quantum dots is known to be a useful source of polarizationentangled
photon pairs. An analytical solution in the weak-coupling regime for quantum-state tomography
matrix elements, i.e. the two-photon correlation function, is presented. It is demonstrated that with the
inclusion of pure dephasing and considering degenerate exciton levels, the entanglement is not affected,
and a convenient formula to calculate the degree of entanglement in an experiment is derived.
We present a microscopic theory for the time resolved light emission from a semiconductor quantum dot,
interacting with LO-phonons. We investigate the emission properties of the QD under strong external
laser powers, leading to strongly correlated electron phonon dynamics. We employ an equation of motion
approach, which treats the electron phonon interaction with necessary accuracy far beyond the typical
bath approximation. We can describe several new non-equilibrium features, like phonon assisted Mollowtriplets
and phonon related anti-crossings, which occur in the spectrum in the strong excitation regime.
We present a microscopic description of the photon statistics and phonon signatures in the quantum
light emission from semiconductor quantum dots. Using higher-order Born approximation, time-resolved
optical emission and scattering spectra are calculated in the weak coupling regime. In the strong coupling
regime, a mathematical induction method is presented to calculate the longitudinal optical (LO) phonon-assisted
quantum dot cavity-QED. The phonon interaction at room temperature has a strong impact on
the quantum correlation of the cavity field and enhances the non-classicality of the cavity field, initially
prepared in a thermal state.
Quantum dot based light emitters can be used as sources of nonclassical light. We focus on the theory of InAs/GaAs quantum dots embedded in a two dimensional wetting layer at low electrical pump currents but strongly correlated electron-photon dynamics. For this purpose a self-consistent theory of transport and emission is developed. A substantial carrier heating in such devices is predicted and the relation of electrical pumping and single photon emission is analyzed within the photon-probability-cluster-expansion.