Quantum noise reduction using squeezed states inLIGO; Sensitivity improvement of a LIGO gravitational Waynedetector through squeezed state injection
S. E. Dwyer
Direct detection of gravitational waves will require
earth based detectors to measure strains of the order 10-21, at
frequencies of 100 Hz, a sensitivity that has been accomplished
with the initial generation of LIGO interferometric gravitational
wave detectors. A new generation of detectors currently under
construction is designed improve on the sensitivity of the initial
detectors by about a factor of 10. The quantum nature of light will
limit the sensitivity of these Advanced LIGO interferometers at
most frequencies; new approaches to reducing the quantum noise will
be needed to improve the sensitivity further. This quantum noise
originates from the vacuum fluctuations that enter the unused port
of the interferometer and interfere with the laser light. Vacuum
fluctuations have the minimum noise allowed by Heisenberg's
uncertainty principle, [Delta]X1 [Delta]X2 >/=1, where the two
quadratures X1 and X2 are non-commuting observables responsible for
the two forms of quantum noise, shot noise and radiation pressure
noise. By replacing the vacuum fluctuations entering the
interferometer with squeezed states, which have lower noise in one
quadrature than the vacuum state, we have reduced the shot noise of
a LIGO interferometer. The sensitivity to gravitational waves
measured during this experiment represents the best sensitivity
achieved to date at frequencies above 200 Hz, and possibly the
first time that squeezing has been measured in an interferometer at
frequencies below 700 Hz. The possibility that injection of
squeezed states could introduce environmental noise couplings that
would degrade the crucial but fragile low frequency sensitivity of
a LIGO interferometer has been a major concern in planning to
implement squeezing as part of baseline interferometer operations.
These results demonstrate that squeezing is compatible with the low
frequency sensitivity of a full scale gravitational wave
interferometer. We also investigated the limits to the level of
squeezing observed, including optical losses and fluctuations of
the squeezing angle. The lessons learned should allow for
responsible planning to implement squeezing in Advanced LIGO,
either as an alternative to high power operation or an early
upgrade to improve the sensitivity. This thesis is available at
DSpace@MIT and has LIGO document number P1300006.
•Jan 1, 2013•0 citations