The toolbox to study the Universe grew on 14 September 2015 when the LIGO–Virgo collaboration heard a signal from two colliding black holes between 30 and 250 Hz. Since then, many more gravitational waves have been detected as detectors continue to increase sensitivity. However, the current and future interferometric detectors will never be able to detect gravitational waves below a few Hz due to oceanic activity on Earth. An interferometric space mission, the laser interferometer space antenna, will operate between 1 mHz and 0.1 Hz, leaving a gap in the decihertz band. To detect gravitational-wave signals also between 0.1 and 1 Hz, the Lunar Gravitational-wave Antenna will use an array of seismic stations. The seismic array will be deployed in a permanently shadowed crater on the lunar south pole, which provides stable ambient temperatures below 40 K. A cryogenic superconducting inertial sensor is under development that aims for fm/√Hz sensitivity or better down to several hundred mHz, and thermal noise limited below that value. Given the 106 m size of the Moon, strain sensitivities below 10−20 1/√Hz can be achieved. The additional cooling is proposed depending on the used superconductor technology. The inertial sensors in the seismic stations aim to make a differential measurement between the elastic response of the Moon and the inertial sensor proof-mass motion induced by gravitational waves. Here, we describe the current state of research toward the inertial sensor, its applications, and additional auxiliary technologies in the payload of the lunar gravitational-wave detection mission.

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To achieve the expected level of sensitivity of third-generation
gravitational-wave observatories, more accurate and sensitive instruments than those of the second generation must be used to reduce all sources of noise.
Amongst them, one of the most relevant is seismic noise, which will require the
development of a better isolation system, especially at low frequencies (below 10
Hz), the operation of large cryogenic silicon mirrors, and the improvement of
optical wavelength readouts. In this framework, this article presents the activities
of the E-TEST (Einstein Telescope Euregio Meuse-Rhine Site & Technology) to
develop and test new key technologies for the next generation of GW observatories.
A compact isolator system for a large silicon mirror at a low frequency is proposed. The design of the isolator allows the overall height
of the isolation system to be significantly compact and also suppresses seismic
noise at low frequencies. To minimize the effect of thermal noise, the isolation
system is provided with a 100-kg silicon mirror which is suspended in a vacuum
chamber at cryogenic temperature (25-40 K). To achieve this temperature without
inducing vibrations to the mirror, a radiation-based cooling strategy is employed.
In addition, cryogenic sensors and electronics are being developed as part of the
E-TEST to detect vibrational motion in the penultimate cryogenic stage. Since
the used silicon material is not transparent below the wavelengths
typically used for GW detectors, new optical components and
lasers must be developed in the range above 1500 nm to reduce absorption and
scattering losses. Therefore, solid-state and fiber lasers with a wavelength of 2090
nm, matching high-efficiency photodiodes, and low-noise crystalline coatings are
being developed. Accordingly, the key technologies provided by E-TEST serve
crucially to reduce the limitations of the current generation of GW observatories
and to determine the technical design for the next generation.


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