J. Harms

Density of superfluid helium droplets

The classical integral cross sections of large superfluid 4 He N droplets and the number of atoms in the droplets (N = 10 3 − 10 4 ) have been measured in molecular beam scattering experiments. These measurements are found to be in good agreement with the cross sections predicted from density functional calculations of the radial density distributions with a 10–90% surface thickness of 5.7u By using a simple model for the density profile of the droplets a thickness of about 6–8u is extracted directly from the data.

Status of the GEO600 detector

Of all the large interferometric gravitational-wave detectors, the German/British project GEO600 is the only one which uses dual recycling. During the four weeks of the international S4 data-taking run it reached an instrumental duty cycle of 97% with a peak sensitivity of 7 × 10−22 Hz−1/2 at 1 kHz. This paper describes the status during S4 and improvements thereafter.

Squeezed-input, optical-spring, signal-recycled gravitational-wave detectors

We theoretically analyze the quantum noise of signal-recycled laser interferometric gravitational-wave detectors with additional input and output optics, namely frequency-dependent squeezing of the vacuum entering the dark port and frequency-dependent homodyne detection. We combine the work of Buonanno and Chen on the quantum noise of signal-recycled interferometers with ordinary input-output optics, and the work of Kimble el al. on frequency-dependent input-output optics with conventional interferometers. Analytical formulas for the optimal input and output frequency dependencies are obtained. It is shown that injecting squeezed light with the optimal frequency-dependent squeezing angle into the dark port yields an improvement on the noise spectral density by a factor of exp(-2r) (in power) over the entire squeezing bandwidth, where r is the squeezing parameter. It is further shown that frequency-dependent (variational) homodyne read-out leads to an additional increase in sensitivity which is significant in the wings of the doubly resonant structure. The optimal variational input squeezing in case of an ordinary output homodyne detection is shown to be realizable by applying two optical filters on a frequency-independent squeezed vacuum. Throughout this paper, we take as example the signal-recycled topology currently being completed at the GEO600 site. However, theoretical results obtained here are also applicable to the proposed topology of Advanced LIGO.

High resolution infrared spectroscopy of single SF6 molecules in helium droplets. II. The effect of small amounts of 4He in large 3He droplets

The rotationally resolved infrared spectra of single SF6 molecules embedded in large 3He droplets have been studied as a function of mean sizes between N3=2×103 and 105 atoms by the cluster beam depletion technique. The observed frequency shifts and line broadening of the dopant spectra are interpreted in terms of the small impurity of about 30–50 4He atoms, which because of their lower zero point energy completely surround the SF6 molecule. For the largest droplets, the observed rotational structure is similar to that observed in pure 4He droplets but the rotational temperature as determined from the relative line intensities is Trot=0.15(1) K, which is about a factor 2.5 lower than in pure 4He droplets. The lower temperature is expected on the basis of the lower heat of evaporation of the 3He atoms on the outside of the droplet. Mixed 3He/4He droplets produced by increasing the 4He concentration in the source gas to 1.2 and 4% of the 3He were found to have temperatures between that for the almost pure ...

The status of GEO 600

The GEO 600 laser interferometer with 600m armlength is part of a worldwide network of gravitational wave detectors. GEO 600 is unique in having advanced multiple pendulum suspensions with a monolithic last stage and in employing a signal recycled optical design. This paper describes the recent commissioning of the interferometer and its operation in signal recycled mode.

Realistic filter cavities for advanced gravitational wave detectors

The ongoing global effort to detect gravitational waves continues to push the limits of precision measurement while aiming to provide a new tool for understanding both astrophysics and fundamental physics. Squeezed states of light offer a proven means of increasing the sensitivity of gravitational wave detectors, potentially increasing the rate at which astrophysical sources are detected by more than an order of magnitude. Since radiation pressure noise plays an important role in advanced detectors, frequency dependent squeezing will be required. In this paper we propose a practical approach to producing frequency dependent squeezing for Advanced LIGO and similar interferometric gravitational wave detectors.

Droplets formed in helium free-jet expansions from states near the critical point

Droplets formed in 4He free jets expanded from source stagnation states in the vicinity of the critical point (Tc=5.2 K, Pc=2.3 bar) are investigated using a mass-spectrometer time-of-flight (TOF) technique. Depending on the source conditions, three different TOF peaks are identified: (a) atoms, (b) droplets formed by condensation from the gas phase, and (c) droplets formed by disintegration of the liquid phase. The latter show the lowest ever observed 4He cluster speeds, about 50 m/s, at a source pressure of 1.5 bar and a source temperature of 4–5 K, just below the critical pressure and temperature. The TOF distributions at the critical point are very broad and this may be due to critical-point fluctuations during the averaging period (2–10 min) of the individual measurements.

Subtraction of Newtonian noise using optimized sensor arrays

Fluctuations in the local Newtonian gravitational field present a limit to high precision measurements, including searches for gravitational waves using laser interferometers. In this work, we present a model of this perturbing gravitational field and evaluate schemes to mitigate the effect by estimating and subtracting it from the interferometer data stream. Information about the Newtonian noise is obtained from simulated seismic data. The method is tested on causal as well as acausal implementations of noise subtraction. In both cases it is demonstrated that broadband mitigation factors close to 10 can be achieved removing Newtonian noise as a dominant noise contribution. The resulting improvement in the detector sensitivity will substantially enhance the detection rate of gravitational radiation from cosmological sources.

Low-Frequency Terrestrial Gravitational-Wave Detectors

Direct detection of gravitational radiation in the audio band is being pursued with a network of kilometer-scale interferometers (LIGO, Virgo, KAGRA). Several space missions (LISA, DECIGO, BBO) have been proposed to search for sub-hertz radiation from massive astrophysical sources. Here we examine the potential sensitivity of three ground-based detector concepts aimed at radiation in the 0.1--10 Hz band. We describe the plethora of potential astrophysical sources in this band and make estimates for their event rates and thereby, the sensitivity requirements for these detectors. The scientific payoff from measuring astrophysical gravitational waves in this frequency band is great. Although we find no fundamental limits to the detector sensitivity in this band, the remaining technical limits will be extremely challenging to overcome.

Prompt gravity signal induced by the 2011 Tohoku-Oki earthquake

Transient gravity changes are expected to occur at all distances during an earthquake rupture, even before the arrival of seismic waves. Here we report on the search of such a prompt gravity signal in data recorded by a superconducting gravimeter and broadband seismometers during the 2011 Mw 9.0 Tohoku-Oki earthquake. During the earthquake rupture, a signal exceeding the background noise is observed with a statistical significance higher than 99% and an amplitude of a fraction of μGal, consistent in sign and order of magnitude with theoretical predictions from a first-order model. While prompt gravity signal detection with state-of-the-art gravimeters and seismometers is challenged by background seismic noise, its robust detection with gravity gradiometers under development could open new directions in earthquake seismology, and overcome fundamental limitations of current earthquake early-warning systems imposed by the propagation speed of seismic waves.