Quantum technologies in the terahertz (THz) require a coherent interface between addressable qubits and THz quantum channels -- a capacity that so far, remains largely underdeveloped. Here, we propose and demonstrate the generation of steady-state entanglement between polar quantum emitters, mediated by THz photons. We exploit strong visible-light driving of the emitters to create Rabi-split dressed eigenstates whose energy separation can be optically tuned into the THz regime. The polar nature of the emitters activates THz transitions within these eigenstates, allowing them to couple to a THz photonic mode that induces collective dissipative dynamics. A coherent driving and control of these effective THz emitters is achieved by using a sideband optical drive with detuning close to the THz transition frequency. The resulting interplay of collective dissipation and driving activates a mechanism to generate steady-state entanglement with high values of the concurrence (
C>0.9), attainable under experimentally feasible parameters. Crucially, both coherent manipulation and quantum state tomography are implemented entirely through optical means, avoiding direct THz control and detection. This establishes a hybrid visible-THz quantum interface in which a THz channel mediates qubit-qubit entanglement (a key operational requirement for THz quantum technologies) while remaining optically accessible.
Feedback is a fundamental and well-understood method of control used since ancient times; this thesis investigates coherent feedback control in a quantum Hall regime. Quantum Feedback requires a level of isolation from measurement in order to maintain the coherence of the electron wavefunction, unlike the classical variant of feedback where measurement is commonplace as information is easily duplicated. Here we build upon a Landauer Büttiker model for electrons as flying qubits in a closed feedback arrangement which uses scattering matrices to represent and connect the elements of the feedback device to better analyse the qualities that modify feedback effects in a variety of systems. Here we will also investigate the effects of finite bias at each stage of the thesis, with a focus on what effect it has on the function of the feedback system and the extent of the effect depending on how feedback is employed.