A 6D interferometric inertial isolation system

We present a novel inertial-isolation scheme based on six degree-of-freedom (6D) interferometric sensing of a single reference mass. It is capable of reducing inertial motion by more than two orders of magnitude at 100\,mHz compared with what is achievable with state-of-the-art seismometers. This will enable substantial improvements in the low-frequency sensitivity of gravitational-wave detectors. The scheme is inherently two-stage, the reference mass is softly suspended within the platform to be isolated, which is itself suspended from the ground. The platform is held constant relative to the reference mass and this closed-loop control effectively transfers the low acceleration-noise of the reference mass to the platform. A high loop gain also reduces non-linear couplings and dynamic range requirements in the soft-suspension mechanics and the interferometric sensing

Towards the Design of Gravitational-Wave Detectors for Probing Neutron-Star Physics

The gravitational waveform of merging binary neutron stars encodes information about extreme states of matter. Probing these gravitational emissions requires the gravitational-wave detectors to have high sensitivity above 1 kHz. Fortunately for current advanced detectors, there is a sizeable gap between the quantum-limited sensitivity and the classical noise at high frequencies. Here we propose a detector design that closes such a gap by reducing the high-frequency quantum noise with an active optomechanical filter, frequency-dependent squeezing, and high optical power. The resulting noise level from 1 kHz to 4 kHz approaches the current facility limit and is a factor of 20 to 30 below the design of existing advanced detectors. This will allow for precision measurements of (i) the post-merger signal of the binary neutron star, (ii) late-time inspiral, merger, and ringdown of low-mass black hole-neutron star systems, and possible detection of (iii) high-frequency modes during supernovae explosions. This design tries to maximize the science return of current facilities by achieving a sensitive frequency band that is complementary to the longer-baseline third-generation detectors: the10 km Einstein Telescope, and 40 km Cosmic Explorer. We have highlighted the main technical challenges towards realizing the design, which requires dedicated research programs. If demonstrated in current facilities, the techniques can be transferred to new facilities with longer baselines.Comment: 14 pages, 15 figures, published versio

Quantum-enhanced interferometry for axion searches

We propose an experiment to search for axions and axion-like-particles in the galactic halo using quantum-enhanced interferometry. This proposal is related to the previously reported ideas (Phys. Rev. D 98, 035021, Phys. Rev. Lett. 121, 161301, Phys. Rev. D 100, 023548) but searches for axions in the mass range from $10^{-16}$ eV up to $10^{-8}$ eV using two coupled optical cavities. We also show how to apply squeezed states of light to enhance the sensitivity of the experiment similar to the gravitational-wave detectors. The proposed experiment has a potential to be further scaled up to a multi-km long detector. We show that such an instrument has a potential to set constrains of the axion-photon coupling coefficient of $\sim 10^{-18}$ GeV$^{-1}$ for axion masses of $10^{-16}$ eV or detect the signal

Gravitationally induced phase shift on a single photon

The effect of the Earth's gravitational potential on a quantum wave function has only been observed for massive particles. In this paper we present a scheme to measure a gravitationally induced phase shift on a single photon travelling in a coherent superposition along different paths of an optical fiber interferometer. To create a measurable signal for the interaction between the static gravitational potential and the wave function of the photon, we propose a variant of a conventional Mach-Zehnder interferometer. We show that the predicted relative phase difference of $10^{-5}$ radians is measurable even in the presence of fiber noise, provided additional stabilization techniques are implemented for each arm of a large-scale fiber interferometer. Effects arising from the rotation of the Earth and the material properties of the fibers are analysed. We conclude that optical fiber interferometry is a feasible way to measure the gravitationally induced phase shift on a single-photon wave function, and thus provides a means to corroborate the equivalence of the energy of the photon and its effective gravitational mass.Comment: 13 pages, 5 figure

Sensing and control scheme for the inteferometer configuration with an L-shaped resonator

The detection of high-frequency gravitational waves around kHz is critical to understanding the physics of binary neutron star mergers. A new interferometer design has been proposed in [Phys. Rev. X {\bf 13}, 021019 (2023)], featuring an L-shaped optical resonator as the arm cavity, which resonantly enhances kHz gravitational-wave signals. This new configuration has the potential to achieve better high-frequency sensitivity than the dual-recycled Fabry-Perot Michelson. In this article, we propose a sensing and control scheme for this configuration. Despite having the same number of length degrees of freedom as the dual-recycled Fabry-Perot Michelson, the new configuration requires one less degree of freedom to be controlled due to the degeneracy of two length degrees of freedom at low frequencies. We has also shown that introducing the Schnupp asymmetry is ineffective for controlling the signal-recycling cavity length. Therefore, we propose adding control fields from the dark port to control this auxiliary degree of freedom.Comment: 19 pages,9 figure

A High-Finesse Suspended Interferometric Sensor for Macroscopic Quantum Mechanics with Femtometre Sensitivity

We present an interferometric sensor for investigating macroscopic quantum mechanics on a table-top scale. The sensor consists of a pair of suspended optical cavities with finesse over 350,000 comprising 10 g fused silica mirrors. The interferometer is suspended by a four-stage, light, in-vacuum suspension with three common stages, which allows for us to suppress common-mode motion at low frequency. The seismic noise is further suppressed by an active isolation scheme, which reduces the input motion to the suspension point by up to an order of magnitude starting from 0.7 Hz. In the current room-temperature operation, we achieve a peak sensitivity of 0.5 fm/Hz in the acoustic frequency band, limited by a combination of readout noise and suspension thermal noise. Additional improvements of the readout electronics and suspension parameters will enable us to reach the quantum radiation pressure noise. Such a sensor can eventually be utilized for demonstrating macroscopic entanglement and for testing semi-classical and quantum gravity models

Nonlinearities in Fringe-Counting Compact Michelson Interferometers

Compact Michelson interferometers are well positioned to replace existing displacement sensors in the readout of seismometers and suspension systems, such as those used in contemporary gravitational-wave detectors. Here, we continue our previous investigation of a customised compact displacement sensor built by SmarAct that operates on the principle of deep frequency modulation. The focus of this paper is the linearity of this device and its subsequent impact on sensitivity. We show the three primary sources of nonlinearity that arise in the sensor: residual ellipticity, intrinsic distortion of the Lissajous figure, and distortion caused by exceeding the velocity limit imposed by the demodulation algorithm. We verify the theoretical models through an experimental demonstration, where we show the detrimental impact that these nonlinear effects have on device sensitivity. Finally, we simulate the effect that these nonlinearities are likely to have if implemented in the readout of the Advanced LIGO suspensions and show that the noise from nonlinearities should not dominate across the key sub-10 Hz frequency band