Bayesian Inference for Gravitational Waves from Binary Neutron Star Mergers in Third Generation Observatories.

Third generation (3G) gravitational-wave detectors will observe thousands of coalescing neutron star binaries with unprecedented fidelity. Extracting the highest precision science from these signals is expected to be challenging owing to both high signal-to-noise ratios and long-duration signals. We demonstrate that current Bayesian inference paradigms can be extended to the analysis of binary neutron star signals without breaking the computational bank. We construct reduced-order models for ∼90-min-long gravitational-wave signals covering the observing band (5-2048 Hz), speeding up inference by a factor of ∼1.3×10^{4} compared to the calculation times without reduced-order models. The reduced-order models incorporate key physics including the effects of tidal deformability, amplitude modulation due to Earth's rotation, and spin-induced orbital precession. We show how reduced-order modeling can accelerate inference on data containing multiple overlapping gravitational-wave signals, and determine the speedup as a function of the number of overlapping signals. Thus, we conclude that Bayesian inference is computationally tractable for the long-lived, overlapping, high signal-to-noise-ratio events present in 3G observatories.

Observation of Squeezed Light in the 2 µm Region.

We present the generation and detection of squeezed light in the 2 µm wavelength region. This experiment is a crucial step in realizing the quantum noise reduction techniques that will be required for future generations of gravitational-wave detectors. Squeezed vacuum is generated via degenerate optical parametric oscillation from a periodically poled potassium titanyl phosphate crystal, in a dual resonant cavity. The experiment uses a frequency stabilized 1984 nm thulium fiber laser, and squeezing is detected using balanced homodyne detection with extended InGaAs photodiodes. We have measured 4.0±0.1 dB of squeezing and 10.5±0.5 dB of antisqueezing relative to the shot noise level in the audio frequency band, limited by photodiode quantum efficiency. The inferred squeezing level directly after the optical parametric oscillator, after accounting for known losses and phase noise, is 10.7 dB.

Arm-length stabilisation for interferometric gravitational-wave detectors using frequency-doubled auxiliary lasers.

Residual motion of the arm cavity mirrors is expected to prove one of the principal impediments to systematic lock acquisition in advanced gravitational-wave interferometers. We present a technique which overcomes this problem by employing auxiliary lasers at twice the fundamental measurement frequency to pre-stabilise the arm cavities' lengths. Applying this approach, we reduce the apparent length noise of a 1.3 m long, independently suspended Fabry-Perot cavity to 30 pm rms and successfully transfer longitudinal control of the system from the auxiliary laser to the measurement laser.

Control and tuning of a suspended Fabry-Perot cavity using digitally enhanced heterodyne interferometry.

We present the first demonstration of real-time closed-loop control and deterministic tuning of an independently suspended Fabry-Perot optical cavity using digitally enhanced heterodyne interferometry, realizing a peak sensitivity of ~10 pm/√Hz over the 10-1000 Hz frequency band. The methods presented are readily extensible to multiple coupled cavities. As such, we anticipate that refinements of this technique may find application in future interferometric gravitational-wave detectors.

Large dynamic range, high resolution optical heterodyne readout for high velocity slip events.

We present a free-space optical displacement sensor for measuring geological slip event displacements within a laboratory setting. This sensor utilizes a fiberized Mach-Zehnder based optical heterodyne system coupled with a digital phase lock loop, providing a large dynamic range (multiple centimeters), high displacement resolution (with an amplitude spectral density of <10-10 m/Hz for frequencies above 100 Hz), and high velocity tracking capabilities (up to 4.96 m/s). This displacement sensor is used to increase the displacement and the time sensitivity for measuring laboratory-scale earthquakes induced in geological samples by using a triaxial compression apparatus. The sensor architecture provides an improved displacement and time resolution for the millisecond-duration slip events, at high containment and loading pressure and high temperatures. Alternatively, the sensor implementation can be used for other non-contact displacement readouts that required high velocity tracking with low noise and large dynamic range sensing. We use 13 high-velocity slip events in Fontainebleau sandstone to show the large dynamic range displacement tracking ability and displacement amplitude spectral densities to demonstrate the optical readout's unique sensing capabilities.

Stable transfer of an optical frequency standard via a 4.6 km optical fiber.

We present a technique for the stable transfer of an optical frequency reference over a kilometer-scale optical fiber link. This technique implements phase measurements and laser feedback to cancel out the phase fluctuations that are introduced to an optical frequency standard as it passes through the fiber. We also present results for a bench top experiment, developed for the Advanced LIGO lock acquisition system, where this technique is implemented to phase-lock two Nd:YAG lasers, through a 4.6 km optical fiber. The resulting differential optical frequency noise reaches a level as low as 0.5 mHz/ radical Hz for Fourier frequencies between 5 Hz and 20 Hz, which is equal to a fractional frequency stability of 1.7 x 10(-18)/ radical Hz.

Picometer level displacement metrology with digitally enhanced heterodyne interferometry.

Digitally enhanced heterodyne interferometry is a laser metrology technique employing pseudo-random codes phase modulated onto an optical carrier. We present the first characterization of the technique's displacement sensitivity. The displacement of an optical cavity was measured using digitally enhanced heterodyne interferometry and compared to a simultaneous readout based on conventional Pound-Drever-Hall locking. The techniques agreed to within 5 pm/ radicalHz at 1 Hz, providing an upper bound to the displacement noise of digitally enhanced heterodyne interferometry. These measurements employed a real-time signal extraction system implemented on a field programmable gate array, suitable for closed-loop control applications. We discuss the applicability of digitally enhanced heterodyne interferometry for lock acquisition of advanced gravitational wave detectors.

A squeezed light source operated under high vacuum.

Non-classical squeezed states of light are becoming increasingly important to a range of metrology and other quantum optics applications in cryptography, quantum computation and biophysics. Applications such as improving the sensitivity of advanced gravitational wave detectors and the development of space-based metrology and quantum networks will require robust deployable vacuum-compatible sources. To date non-linear photonics devices operated under high vacuum have been simple single pass systems, testing harmonic generation and the production of classically correlated photon pairs for space-based applications. Here we demonstrate the production under high-vacuum conditions of non-classical squeezed light with an observed 8.6 dB of quantum noise reduction down to 10 Hz. Demonstration of a resonant non-linear optical device, for the generation of squeezed light under vacuum, paves the way to fully exploit the advantages of in-vacuum operations, adapting this technology for deployment into new extreme environments.

Tip-tilt mirror suspension: beam steering for advanced laser interferometer gravitational wave observatory sensing and control signals.

We describe the design of a small optic suspension system, referred to as the tip-tilt mirror suspension, used to isolate selected small optics for the interferometer sensing and control beams in the advanced LIGO gravitational wave detectors. The suspended optics are isolated in all 6 degrees of freedom, with eigenmode frequencies between 1.3 Hz and 10 Hz. The suspended optic has voice-coil actuators which provide an angular range of ±4 mrad in the pitch and yaw degrees of freedom.