The detection of gravitation waves (GW) produced by astrophysical sources was predicted by Albert Einstein in 1916 in his theory of general relativity. Such measurement was long thought impossible because of the high strain sensitivity it requires in the typical 10Hz–10kHz spectral range. The typical required relative sensitivity is some 10^(-20)/√Hz, as calculated for the detection of stellar explosion using earth-based detectors. Ever since the pioneer work of Joseph Weber in the late 1950’s, who claimed to have detected GW using high Q-factor resonant bar detectors, there has been an increasing number of similar projects such as the ALLEGRO at Baton Rouge, USA, AURIGA at Legnaro, Italy or EXPLORER at CERN in Switzerland, which are running examples of this technology. Those detectors nevertheless are too narrowband because of their typical sub-100Hz and sharp resonance frequency. Highly sensitive and broadband detectors have emerged with the construction of optical-based interferometric detectors as first proposed by Gerstenshtein and Putovoit in 1963. Since the 70’s, multiple small-scale prototype benches such as the 30m-long Garching interferometer, the 10m one in Glasgow or the 40m one at Caltech have flourished, but GW detection requires interferometric arm length on the order of a few km long. To this end, the 4km-long LIGO US project was founded in 1992 and set up in Hanford and Livingston, the 3km-long VIRGO French-Italian project started in 1993 and set up in Pisa, and the KAGRA project started in 1995 in Tokyo. The GEO project was founded in 1989 through a collaboration between the Glasgow and Garching groups. With its 600m long arm, the GEO 600 interferometer serves as a test bench to improved detection technologies and its sensitivity compares to that of the larger LIGO generation of detectors. Since 2008, improvement of both VIRGO and LIGO strain sensitivity throughout a large bandwidth has skyrocketed. The new generation of Advanced VIRGO (aVIRGO) and Advanced LIGO (aLIGO) have allowed to reach an unprecedented strain level of detection on the order of 10^(-23)/√Hz. Those major upgrades over the original interferometers allowed, for the first time, the simultaneous detection of gravitational waves for a collision of black holes on September 14, 2015.
After all external sources of technical noise are reduced, the key to a further improved sensitivity is the reduction of the remaining and dominating quantum noise of the probe laser. For instance, aLIGO, aVIRGO and GEO600 are today limited by quantum noise throughout most of their detection bandwidth. It is this specific noise source that we address in this QNOIWA project. Quantum shot noise prevails at high frequencies (>100Hz), while an interplay between shot noise and radiation pressure noise has to be accounted for at lower frequencies (30-100Hz). The latter effect first pointed out by Braginsky in the 1960s [13] is known as the Quantum Back Action (QBA). It can be understood as follows: an ideal detection implies that the interferometer end-mirror is only put in motion by a gravitational wave. In such case, the detected optical signal is shot-noise limited, and the signal-to-noise ratio (SNR) increases as the square root of laser intensity. Following this logic, we could in principle reach unlimited detection sensitivity simply by increasing the laser power. In practice however, light acts on the end mirror through the radiation pressure force, a random force due to the Poissonian statistics of photons which causes unwanted random motion. This QBA effect leads to an increase in detection noise. Because of the very low resonance frequency of the end mirror (typically around 1Hz), the effect is mostly visible bellow~100Hz, and decreases quadratically with the GW detection frequency. The objective of this QNOIWA project was to experimentally beat quantum noise, i.e. demonstrate the simultaneous reduction of QBA in the lower part of the detection bandwidth and the reduction of shot noise in the upper part in the kHz frequency range. Our experiment have been successful, and will certainly pave the road to the pending objective of a future integration in large scale GW detectors, in lower frequency range.
