Quantum back-action evasion and filtering in optomechanical systems

The measurement precision of optomechanical sensors reached sensitivity levels such that they have to be described by quantum theory. In quantum mechanics, every measurement will introduce a back-action on the measured system itself. For optomechanical force sensors, a trade-off between back-action and measurement precision exists through the interplay of quantum shot noise and quantum radiation pressure noise. Finding the optimal power to balance these effects leads to the standard quantum limit (SQL), which bounds the sensitivity of force sensing. To overcome the SQL and reach the fundamental bound of parameter estimation, the quantum Cramér-Rao bound, techniques called quantum smoothing and quantum back-action evasion are required. The first part of this thesis explores quantum smoothing in the context of optomechanical force sensing. Quantum smoothing combines the concepts of prediction and retrodiction to estimate the parameters of a system in the past. To illustrate the intricacies of these estimations in the quantum setting, two filters, the Kalman and Wiener filters, are introduced. Their prediction and retrodiction estimates are given for a simple optomechanical setup, and resulting differences are analyzed concerning the available quantum smoothing theories in the literature. In the second part of this thesis, a back-action evasion technique called coherent quantum-noise cancellation (CQNC) is explored. In CQNC, an effective negative-mass oscillator is coupled to an optomechanical sensor to create destructive interference of quantum radiation pressure noise. An all-optical realization of such an effective negative-mass oscillator is introduced, and a comprehensive study of its performance in a cascaded CQNC scheme is given. We determine ideal CQNC conditions, analyze non-ideal noise cancellation and provide a case study. Under feasible parameters, the case study shows a possible reduction of radiation pressure noise of 20% and that the effective negative-mass oscillator as the first subsystem in the cascade is the preferable order.Die Messgenauigkeit optomechanischer Sensoren hat eine Sensitvität erreicht, sodass sie im Rahmen der Quantentheorie beschrieben werden müssen. Quantenmechanik besagt, dass jede Messung eine Rückkopplung auf das vermessene System induziert. Bei optomechanischen Kraftsensoren is ein Kompromiss zwischen Rückkopplung und Messgenauigkeit durch die Verzahnung von Schrotrauschen und Strahlungsdruckrauschen begründet. Die Verwendung der optimalen Leistung, derart dass diese beiden Prozesse in Waage liegen, führt zum Standardquantenlimit (SQL). Hierdurch wird die Messgenauigkeit begrenzt. Um das SQL zu überwinden und die fundamentale Grenze der Parameterschätzung zu erreichen, welche durch Quanten-Cramér-Rao-Ungleichung bestimmt ist, werden die Methoden der Quantenglättung und Rückkopplungsumgehung benötigt. Im ersten Teil dieser Arbeit wird das Gebiet der Quantenglättung im Kontext von optomechanischer Kraftmessung untersucht. Die Quantenglättung kombiniert die Methoden der Vorhersage und Retrodiktion, um Abschätzungen an die Parameter eines Quantensystems zu tätigen, welche in der Vergangenheit liegen. Um die Feinheiten dieser Abschätzungen für Quantensysteme zu demonstrieren, werden zwei Filter, der Kalman- und der Wiener-Filter eingeführt. An einem einfachen optomechanischen System, werden deren Ergebnisse für die Vorhersage und Retrodiktion berechnet. Mögliche Diskrepanzen werden im Kontext der verfügbaren Theorien der Quantenglättung beleuchtet. Im zweiten Teil dieser Dissertation wird eine Rückkopplungsumgehungsmethode, die kohärente Quantenrauschunterdrückung (coherent quantum-noise cancellation, CQNC) untersucht. Bei CQNC wird ein Oszillator mit effektiver negativer Masse an einen optomechanischen Sensor gekoppelt, um destruktiv mit dem Strahlungsdruckrauschen zu interferieren. Eine mögliche optische Realisierung eines solchen negativen Masse Oszillators wird vorgestellt und mit einem optomechanischem Kraftsensor kaskadiert. Dieser Aufbau wird hinsichtlich seiner Rauschünterdrückungfähigkeit untersucht. Diesbezüglich ermitteln wir die Bedingungen für eine vollständige Abwendung von Strahlungsdruckrauschen und analysieren den Einfluss von möglichen Abweichungen von diesen Bedingungen auf die Rauschünterdrückung. Zuletzt präsentieren wir eine Fallstudie eines möglichen experimentellen Aufbaus. Die Fallstudie zeigt eine mögliche Strahlungsdrückreduzierung von 20% und dass der Oszillator mit effektiver negativer Masse als erstes System in der Kaskade zu bervorzugen ist

All-optical coherent quantum-noise cancellation in cascaded optomechanical systems

Coherent quantum noise cancellation (CQNC) can be used in optomechanical sensors to surpass the standard quantum limit (SQL). In this paper, we investigate an optomechanical force sensor that uses the CQNC strategy by cascading the optomechanical system with an all-optical effective negative mass oscillator. Specifically, we analyze matching conditions, losses and compare the two possible arrangements in which either the optomechanical or the negative mass system couples first to light. While both of these orderings yield a sub-SQL performance, we find that placing the effective negative mass oscillator before the optomechanical sensor will always be advantageous for realistic parameters. The modular design of the cascaded scheme allows for better control of the sub-systems by avoiding undesirable coupling between system components, while maintaining similar performance to the integrated configuration proposed earlier. We conclude our work with a case study of a micro-optomechanical implementation.Comment: 9 pages, 6 figures, Appendix A and

Domestic mould exposure and invasive aspergillosis-air sampling of Aspergillus spp. spores in homes of hematological patients, a pilot study

Aspergillus spp.-related morbidity and mortality remains a major challenge in the management of neutropenic patients. Little is known about the impact of domestic Aspergillus spp. exposure. In this controlled prospective study, fungal spores were collected from homes of neutropenic patients. Cases were defined as patients with probable or proven controls as patients with no invasive pulmonary aspergillosis, while patients with possible disease were evaluated as a third group. Forty patients were enrolled and returned questionnaires on high-risk activities and mould exposure. A. fumigatus was detected in concentrations of 0 to 76 cfu/m(3) in every home. A. terreus was detected in nine (18%) homes. Mean Aspergillus spp. cfu/m(3) according to EORTC criteria were: proven/probable IA (15 patients) - 36; possible IA (12 patients) - 42; no IA (13 patients) - 42. Of the seven patients with self-reported moulded walls at home, four had probable and three had possible aspergillosis; the risk ratio of developing IA was 1.65 (95% CI: 1.25-2.17). In conclusion self-reported domestic mould exposure was associated with a high incidence of IA and may be a feasible tool for identifying high-risk patients. There was no correlation between domestic ambient-air spore counts and IA

Confirmation of the topology of the Wendelstein 7-X magnetic field to better than 1:100,000

Fusion energy research has in the past 40 years focused primarily on the tokamak concept, but recent advances in plasma theory and computational power have led to renewed interest in stellarators. The largest and most sophisticated stellarator in the world, Wendelstein 7-X (W7-X), has just started operation, with the aim to show that the earlier weaknesses of this concept have been addressed successfully, and that the intrinsic advantages of the concept persist, also at plasma parameters approaching those of a future fusion power plant. Here we show the first physics results, obtained before plasma operation: that the carefully tailored topology of nested magnetic surfaces needed for good confinement is realized, and that the measured deviations are smaller than one part in 100,000. This is a significant step forward in stellarator research, since it shows that the complicated and delicate magnetic topology can be created and verified with the required accuracy