339 research outputs found
Quantum-limited amplification and parametric instability in the reversed dissipation regime of cavity optomechanics
Cavity optomechanical phenomena, such as cooling, amplification or
optomechanically induced transparency, emerge due to a strong imbalance in the
dissipation rates of the parametrically coupled electromagnetic and mechanical
resonators. Here we analyze the reversed dissipation regime where the
mechanical energy relaxation rate exceeds the energy decay rate of the
electromagnetic cavity. We demonstrate that this regime allows for
mechanically-induced amplification (or cooling) of the electromagnetic mode.
Gain, bandwidth, and added noise of this electromagnetic amplifier are derived
and compared to amplification in the normal dissipation regime. In addition, we
analyze the parametric instability, i.e. optomechanical Brillouin lasing, and
contrast it to conventional optomechanical phonon lasing. Finally, we propose
an experimental scheme that realizes the reversed dissipation regime using
parametric coupling and optomechanical cooling with a second electromagnetic
mode enabling quantum-limited amplification. Recent advances in high-Q
superconducting microwave resonators make the reversed dissipation regime
experimentally realizable.Comment: 5+3 pages, 5 figures, 1 tabl
Level attraction in a microwave optomechanical circuit
Level repulsion - the opening of a gap between two degenerate modes due to
coupling - is ubiquitous anywhere from solid state theory to quantum chemistry.
In contrast, if one mode has negative energy, the mode frequencies attract
instead. They converge and develop imaginary components, leading to an
instability; an exceptional point marks the transition. This, however, only
occurs if the dissipation rates of the two modes are comparable. Here we expose
a theoretical framework for the general phenomenon and realize it
experimentally through engineered dissipation in a multimode superconducting
microwave optomechanical circuit. Level attraction is observed for a mechanical
oscillator and a superconducting microwave cavity, while an auxiliary cavity is
used for sideband cooling. Two exceptional points are demonstrated that could
be exploited for their topological properties.Comment: 5 pages, 4 figures; includes Supplementary informatio
Leveraging Ensemble Diversity for Robust Self-Training in the Presence of Sample Selection Bias
Self-training is a well-known approach for semi-supervised learning. It
consists of iteratively assigning pseudo-labels to unlabeled data for which the
model is confident and treating them as labeled examples. For neural networks,
softmax prediction probabilities are often used as a confidence measure,
despite the fact that they are known to be overconfident, even for wrong
predictions. This phenomenon is particularly intensified in the presence of
sample selection bias, i.e., when data labeling is subject to some constraint.
To address this issue, we propose a novel confidence measure, called
-similarity, built upon the prediction diversity of an ensemble of
linear classifiers. We provide the theoretical analysis of our approach by
studying stationary points and describing the relationship between the
diversity of the individual members and their performance. We empirically
demonstrate the benefit of our confidence measure for three different
pseudo-labeling policies on classification datasets of various data modalities
Synthesis of dumbbell-like DBATT dimers**
Two-dimensional acenes are promising candidates for applications in single-molecule spectroscopy due to the electronic properties caused by their zig-zag periphery. However, synthetic access to their dimers remains unexplored because of low solubility and reduced stability. Herein we report a facile approach towards two-dimensional acenes dimers on the example of 2.3,8.9-dibenzanthanthrene (DBATT) connected via rigid linker. The exploitation of dehydrative π-extension on the last step of the synthesis allows avoiding difficulties connected with cumbersome purification of low soluble products. The technique was shown to be applicable for the synthesis of DBATT dimers connected rigidly with conjugated and non-conjugated linkers, containing alternating para-phenylene and bicyclo[2.2.2]octane moieties.Publikationsfonds ML
Experiments on flux qubits with pi-shifters
The results of the research re-ported in this work show that tunable gap flux qubits have a potential for building quantum registers. Cavities coupled to flux qubits can be used for in-formation storage and transfer between qubits. SFS π-shifters provide a simple approach to bias multi-qubit circuits. A possibility to change the qubit resonance frequency while preserving qubit coherence enables implementation of switchable coupling between qubits and cavities
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