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.
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 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.
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.
The correct understanding of the electronic structure and relaxation behavior in nanosystems is essential for technical applications. We propose a spectroscopic method to measure the dipole-forbidden electronic transitions of quantum dots and trace their relaxation behavior. Therefore, we utilize two-dimensional coherent spectroscopy, which is an advantageous tool to get information about the dynamics of exciton densities and coherences in nanoscopic structures. In combination with nanoplasmonics, it enables excitation of dipole-forbidden states. A nanoplasmonic dolmen structure allows us to dynamically excite either dipole-allowed and dipole forbidden states selectively. In combination with two-dimensional spectroscopy, this gives us additional control over excitation and tracing relaxation involving dipole-forbidden states in nanoscopic systems.