Quantum noise modeling and detector configurations
Quantum noise, together with thermal noise, are the two fundamental noises limiting detector sensitivity. It originates from a quantum description of the state of light, or more colloquially, from the fact that the photodetection is a photon-counting process described by a counting distribution, e.g., a Poissonian distribution. Quantum noise, if expressed as equivalent GW-strain noise, depends on various parameters of the interferometer (light power inside arm cavities, finesse of arm cavities, optical loss), it greatly depends on the interferometer configuration (signal recycling, speedmeter, broadband vs tuned), and finally can also be mitigated with quantum technologies that prepare the light in favorable states (squeezing). Members of this group have contributed to all of these approaches. A review article summarizing some of the basics can be found here [Barsotti et al (2019)].
Our group has contributed to the development of a new approach to squeezed-light exploitation for quantum-noise reduction based on the so-called EPR-entanglement scheme [Ma et al (2017)]. The idea inherits elements of the secure communication protocols, where the GW signal would take the role of the "secret" to be communicated between two parties. From the GW detection perspective, it is simply a method to use the detector itself to provide a frequency-dependent preparation of squeezed states for a broadband quantum-noise reduction without the need of additional optical systems like a filter cavity. A possible implementation of this technology is currently being prepared for the Virgo detector.
Detector concepts different from large-scale laser interferometers have been proposed to detect GWs mostly at frequencies well below the LIGO/Virgo observation band. This includes torsion-bar detectors (Ando et al, Slagmolen et al), superconductive-levitation sensors (Paik et al), and atom interferometers (Kasevich et al, Bouyer et al). A recent paper presents a more detailed concept of atom interferometers for GW detection [Canuel et al (2020)].
Decoherence / optical loss
A major limitation of the squeezed-light based mitigation of quantum noise is optical loss, which causes a mixing of the squeezed state with coherent vacuum states. One of the contributions to optical loss comes from imperfect beam shapes. This issue was investigated with the numerical simulation Finesse, which is able to represent coherent and squeezed quantum states in higher-order modes [Töyrä et al (2017)].