Christian Gräf

The GEO 600 squeezed light source

The next upgrade of the GEO 600 gravitational-wave detector is scheduled for 2010 and will, in particular, involve the implementation of squeezed light. The required non-classical light source is assembled on a 1.5 m 2 breadboard and includes a full coherent control system and a diagnostic balanced homodyne detector. Here, we present the first experimental characterization of this setup as well as a detailed description of its optical layout. A squeezed quantum noise of up to 9 dB below the shot-noise level was observed in the detection band between 10 Hz and 10 kHz. We also present an analysis of the optical loss in our experiment and provide an estimation of the possible non-classical sensitivity improvement of the future squeezed light enhanced GEO 600 detector.

Long-term stable squeezed vacuum state of light for gravitational wave detectors

Currently, the German/British gravitational wave (GW) detector GEO 600 is being upgraded within the GEO-HF program. One part of this upgrade consists of the integration of a squeezed-light laser to non-classically improve the detection sensitivity at frequencies where the instrument is limited by shot noise. This has been achieved recently (Abadie et al 2011 Nature Phys. 7 962). The permanent employment of squeezed light in GW observatories requires long-term stability of the generated squeezed state. In this paper, we discuss an unwanted mechanism that can lead to a varying squeezing factor along with a changing phase of the squeezed field. We present an extension of the implemented coherent control scheme (Vahlbruch et al 2006 Phys. Rev. Lett. 97 011101) that allowed an increase in the long-term stability of the GEO 600 squeezed-light laser. With it, a quantum noise reduction of more than 9 dB within the detection band of todays and next-generation GW observatories was observed up to 20 h with a duty cycle of more than 99%.

The AEI 10 m prototype interferometer

A 10 m prototype interferometer facility is currently being set up at the AEI in Hannover, Germany. The prototype interferometer will be housed inside a 100 m 3 ultra-high vacuum envelope. Seismically isolated optical tables inside the vacuum system will be interferometrically interconnected via a suspension platform interferometer. Advanced isolation techniques will be used, such as inverted pendulums and geometrical anti-spring filters in combination with multiple-cascaded pendulum suspensions, containing an all-silica monolithic last stage. The light source is a 35 W Nd:YAG laser, geometrically filtered by passing it through a photonic crystal fibre and a rigid pre-modecleaner cavity. Laser frequency stabilisation will be achieved with the aid of a high finesse suspended reference cavity in conjunction with a molecular iodine reference. Coating thermal noise will be reduced by the use of Khalili cavities as compound end mirrors. Data acquisition and control of the experiments is based on the AdvLIGO digital control and data system. The aim of the project is to test advanced techniques for GEO 600 as well as to conduct experiments in macroscopic quantum mechanics. Reaching standard quantum-limit sensitivity for an interferometer with 100 g mirrors and subsequently breaching this limit, features most prominently among these experiments. In this paper we present the layout and current status of the AEI 10 m Prototype Interferometer project.

Broadband squeezing of quantum noise in a Michelson interferometer with Twin-Signal-Recycling

Twin-Signal-Recycling (TSR) builds on the resonance doublet of two optically coupled cavities and efficiently enhances the sensitivity of an interferometer at a dedicated signal frequency. We report on what we believe to be the first experimental realization of a TSR Michelson interferometer and also its broadband enhancement by squeezed light injection. The complete setup was stably locked, and a broadband quantum noise reduction of the interferometers shot noise by a factor of up to 4 dB was demonstrated. The system was characterized by measuring its quantum noise spectra for several tunings of the TSR cavities. We found good agreement between the experimental results and numerical simulations.

GEO 600 and the GEO-HF upgrade program: successes and challenges

The German–British laser-interferometric gravitational wave detector GEO 600 is in its 14th year of operation since its first lock in 2001. After GEO 600 participated in science runs with other first-generation detectors, a program known as GEO-HF began in 2009. The goal was to improve the detector sensitivity at high frequencies, around 1 kHz and above,with technologically advanced yet minimally invasive upgrades. Simultaneously, the detector would record science quality data in between commissioning activities. As of early 2014, all of the planned upgrades have been carried out and sensitivity improvements of up to a factor of four at the high-frequency end of the observation band have been achieved. Besides science data collection, an experimental program is ongoing with the goal to further improve the sensitivity and evaluate future detector technologies. We summarize the results of the GEO-HF program to date and discuss its successes and challenges.

Design of a speed meter interferometer proof-of-principle experiment

The second generation of large scale interferometric gravitational wave (GW) detectors will be limited by quantum noise over a wide frequency range in their detection band. Further sensitivity improvements for future upgrades or new detectors beyond the second generation motivate the development of measurement schemes to mitigate the impact of quantum noise in these instruments. Two strands of development are being pursued to reach this goal, focusing both on modifications of the well-established Michelson detector configuration and development of different detector topologies. In this paper, we present the design of the worldʼs first Sagnac speed meter (SSM) interferometer, which is currently being constructed at the University of Glasgow. With this proof-of-principle experiment we aim to demonstrate the theoretically predicted lower quantum noise in a Sagnac interferometer compared to an equivalent Michelson interferometer, to qualify SSM for further research towards an implementation in a future generation large scale GW detector, such as the planned Einstein telescope observatory.

Quantum noise of non-ideal Sagnac speed meter interferometer with asymmetries

The speed meter concept has been identified as a technique that can potentially provide laser-interferometric measurements at a sensitivity level which surpasses the standard quantum limit (SQL) over a broad frequency range. As with other sub-SQL measurement techniques, losses play a central role in speed meter interferometers and they ultimately determine the quantum noise limited sensitivity that can be achieved. So far in the literature, the quantum noise limited sensitivity has only been derived for lossless or lossy cases using certain approximations (for instance that the arm cavity round trip loss is small compared to the arm cavity mirror transmission). In this article we present a generalized, analytical treatment of losses in speed meters that allows accurate calculation of the quantum noise limited sensitivity of Sagnac speed meters with arm cavities. In addition, our analysis allows us to take into account potential imperfections in the interferometer such as an asymmetric beam splitter or differences of the reflectivities of the two arm cavity input mirrors. Finally, we use the examples of the proof-of-concept Sagnac speed meter currently under construction in Glasgow and a potential implementation of a Sagnac speed meter in the Einstein Telescope to illustrate how our findings affect Sagnac speed meters with metre- and kilometre-long baselines.

Design of the 10 m AEI prototype facility for interferometry studies

The AEI 10 m prototype interferometer facility is currently being constructed at the Albert Einstein Institute in Hannover, Germany. It aims to perform experiments for future gravitational wave detectors using advanced techniques. Seismically isolated benches are planned to be interferometrically interconnected and stabilized, forming a low-noise testbed inside a 100 m^3 ultra-high vacuum system. A well-stabilized high power laser will perform differential position readout of 100 g test masses in a 10 m suspended arm-cavity enhanced Michelson interferometer at the crossover of measurement (shot) noise and backaction (quantum radiation pressure) noise, the so-called Standard Quantum Limit (SQL). Such a sensitivity enables experiments in the highly topical field of macroscopic quantum mechanics. In this article we introduce the experimental facility and describe the methods employed, technical details of subsystems will be covered in future papers.

Local-oscillator noise coupling in balanced homodyne readout for advanced gravitational wave detectors

The second generation of interferometric gravitational wave detectors are quickly approaching their design sensitivity. For the first time these detectors will become limited by quantum backaction noise. Several backaction evasion techniques have been proposed to further increase the detector sensitivity. Since most proposals rely on a flexible readout of the full amplitude- and phase-quadrature space of the output light field, balanced homodyne detection is generally expected to replace the currently used DC readout. Up to now, little investigation has been undertaken into how balanced homodyne detection can be successfully transferred from its ubiquitous application in tabletop quantum optics experiments to large-scale interferometers with suspended optics. Here we derive implementation requirements with respect to local-oscillator noise couplings and highlight potential issues with the example of the Glasgow Sagnac Speed Meter experiment, as well as for a future upgrade to the Advanced LIGO detectors.

Smart Charging Technologies for Portable Electronic Devices

In this article we describe our efforts of extending demand-side control concepts to the application in portable electronic devices, such as laptop computers, mobile phones and tablet computers. As these devices feature built-in energy storage (in the form of batteries) and the ability to run complex control routines, they are well-suited for the implementation of smart charging concepts. We developed simple hardware and software based prototypes of smart charging controllers for a laptop computer that steer the charging process depending on the frequency of the electricity grid and in case of the software implementation also based on the battery charge status. If similar techniques are incorporated into millions of devices in UK households, this can contribute significantly to the stability of the electricity grid, help to mitigate the short-term power production fluctuations from renewable energy sources and avoid the high cost of building and maintaining conventional power plants as standby reserve.