Quantum feedback control of a solid-state two-level system

We have studied theoretically the basic operation of a quantum feedback loop designed to maintain the desired phase of quantum coherent oscillations in a two-level system. Such feedback can suppress the dephasing of oscillations due to interaction with environment. Prospective experiments can be realized using metallic single-electron devices or GaAs technology.Comment: 4 pages, 4 figure

Quantum Nondemolition Squeezing of a Nanomechanical Resonator

We show that the nanoresonator position can be squeezed significantly below the ground state level by measuring the nanoresonator with a quantum point contact or a single-electron transistor and applying a periodic voltage across the detector. The mechanism of squeezing is basically a generalization of quantum nondemolition measurement of an oscillator to the case of continuous measurement by a weakly coupled detector. The quantum feedback is necessary to prevent the ``heating'' due to measurement back-action. We also discuss a procedure of experimental verification of the squeezed state.Comment: 9 pages, 3 figure

Co2 emission reduction in Germany - from a public perspective

Following the Paris Agreement, Germany set targets and interim goals in its Climate Action Plan to become climate neutral by 2045, but the implementation needs to catch up. This thesis analyzes CO2 reduction measures that Germany takes in the public sector by examining the study 'Climate Protection Potentials in Municipalities' and comparing it with the other levels of science, private cooperations, start-ups, and NGOs. Based on a grounded theory approach and interviews, a comparative analysis of the examination reveals that systemic change is required. One important recommendation is that new laws and regulations are needed to achieve climate neutralit

On-chip cavity quantum phonodynamics with an acceptor qubit in silicon

We describe a chip-based, solid-state analogue of cavity-QED utilizing acoustic phonons instead of photons. We show how long-lived and tunable acceptor impurity states in silicon nanomechanical cavities can play the role of a matter non-linearity for coherent phonons just as, e.g., the Josephson qubit plays in circuit-QED. Both strong coupling (number of Rabi oscillations ~ 100) and strong dispersive coupling (0.1-2 MHz) regimes can be reached in cavities in the 1-20 GHz range, enabling the control of single phonons, phonon-phonon interactions, dispersive phonon readout of the acceptor qubit, and compatibility with other optomechanical components such as phonon-photon translators. We predict explicit experimental signatures of the acceptor-cavity system.Comment: 6 pages, 2 figures, PDFLaTeX. New version improves clarit

Crossover of phase qubit dynamics in presence of negative-result weak measurement

Coherent dynamics of a superconducting phase qubit is considered in the presence of both unitary evolution due to microwave driving and continuous non-unitary collapse due to negative-result measurement. In the case of a relatively weak driving, the qubit dynamics is dominated by the non-unitary evolution, and the qubit state tends to an asymptotically stable point on the Bloch sphere. This dynamics can be clearly distinguished from conventional decoherence by tracking the state purity and the measurement invariant (``murity''). When the microwave driving strength exceeds certain critical value, the dynamics changes to non-decaying oscillations: any initial state returns exactly to itself periodically in spite of non-unitary dynamics. The predictions can be verified using a modification of a recent experiment.Comment: 5 pages, 4 eps figure