 

% The current LIGO facilities performance limits~\cite{LIGO-SRD} are set at high frequencies ($f \gtrsim 50~ \mathrm{Hz}$) by path-length fluctuations due to the residual gas~\cite{Zucker:1994ix} at a strain noise level of a a few parts in $10^{-25} \big/\sqrt{\mathrm{Hz}}$, and at low frequencies ($f \lesssim 50~ \mathrm{Hz}$) by Newtonian gravity gradient noise. In principle, both of these can be further  reduced; the former by introducing additional pumping along the interferometer arms, and the latter by using distributed arrays of seismic sensors arranged around the test masses to sense local ground motion and thereby estimate the gravity gradient noise in real-time~\cite{Coughlin:2016tmu}. 
The planned \Asharp{} upgrade and possible future enhancements to this upgrade can be accommodated by the existing LIGO facilities at least until the next-generation detectors become fully operational, thereby ensuring overlap and continuity of gravitational wave astrophysics in the USA. However, while enhancements to \Asharp{} may be pursued, no technical path is currently seen to significantly improve sensitivity beyond \Asharp{} given the limitation imposed by the 4~km arm lengths of the LIGO Observatories.

In addition, LIGO Laboratory is pursuing other avenues of research should some aspect of the \Asharp{} technology described above prove not to be realizable. A similar strain sensitivity improvement could be pursued through the so-called ``Voyager'' technology of cryogenic silicon mirrors and \qty{2}{\um} lasers~\cite{voyager}, although this technology is not as mature as the A$^\sharp$ technology and implementation would have a greater cost and downtime impact.

The operation of LIGO at its current sites is stable for the foreseeable future. The NSF holds a permit to operate LIGO Hanford Observatory on DOE-managed land. In 2018, this permit was extended an additional 25 years, now valid until Aug 25, 2043. Similarly, the NSF leases land from Louisiana State University on which to operate LIGO Livingston Observatory; that lease is valid until Oct 11, 2044. LIGO Laboratory remains deeply invested in community outreach in the regions where the two observatories are located in order ensure continued stable operations. This relationship with local community, including economic leadership, serves to ensure that the relatively quiet seismic environments in which the two detectors are situated can be protected as the local communities continue to grow; LIGO is regularly consulted for its input into decisions by the local governments when new construction permits are under consideration. Long-term success will require the continued, vigorous engagement of the NSF.

%\dhs{Short Paragraph on the facility maintenance program, describing campaigns over past and future years to ensure infrastructure operations well into the 2040s through a program on lifecycle maintenance and stewardship. Maybe hint at a limited lifetime despite responsible stewardship\\}

A renewal proposal for the continuation of LIGO Laboratory operations by Caltech and MIT through 2028 was reviewed in February 2023 and approved by the NSF. LIGO Laboratory is committed to a continued facilities maintenance and upgrade program into the 2040s that will ensure its unique observatory infrastructure can accommodate the future upgrades discussed above. This includes, most importantly, stewardship of the vacuum system as well as the scientific and engineering facilities required to fabricate, assemble and install the upgraded hardware. In order to ensure operations into the 2040s to at least overlap with the commencement of CE operations in the U.S., continued funding commitment from the NSF for facility and instrument improvements and maintenance will be required.

