Gravitational wave astronomy reached a milestone in 2015 with the detection of GW150914 , the collision of two roughly 30 solar-mass black holes by the Advanced LIGO detectors. Subsequent detections of binary black hole mergers and, in 2017, the detection of the first binary neutron star (BNS) coalescence marking the birth of the field of multi-messenger astronomy , have cemented gravitational wave astronomy as an essential tool in physics.
The ability to detect gravitational waves with good SNR is almost entirely dependent on the strain sensitivity of the detectors. We are currently improving the strain sensitivity of Advanced LIGO (aLIGO) and hope to achieve the design sensitivity (around 180Mpc for a “standard” BNS) around 2020. However, the time horizon for major upgrades to LIGO or the construction of next generation facilities is at least 2027 or beyond. The period between aLIGO reaching design sensitivity and the commencement of next generation detector construction presents an opportunity to improve detector performance and observation with some modest upgrades and relatively little downtime.
A+ is a design for a roughly 1.7x improvement in the sensitivity of aLIGO. This design focuses on lowering the coating thermal noise and quantum noise by a factor of approximately 2. The quantum noise reduction (radiation pressure and shot noise) will be achieved with frequency dependent squeezing via the use of a 300m filter-cavity. This talk will discuss the motivation, design, sensitivity and installation timeline for the Advanced LIGO A+ upgrade.
 Abbott e. al., PRL 116, 061102 (2016)
 Abbott et al., Astr. Phys. Lett. 848, (2) L12 (2017)