Since the first detection by Advanced LIGO in 2015, ground-based gravitational wave (GW)
detectors have observed many compact-binary merger signals around the 100 Hz
band. Access to lower frequencies is one of the important targets for next-generation GW
observations, motivated by early warnings of upcoming mergers, observations of massive
binaries, and searches for stochastic background GWs from the early Universe. However,
at low frequencies, seismic noise and suspension thermal noise are dominant, making
sensitivity improvement below 10 Hz difficult. Although space-based detectors can avoid
these noises, they require technically demanding missions with significant cost and long
development timescale; therefore, it is also important to explore complementary
approaches for low-frequency observations with ground-based detectors.

We investigate two approaches through design, analysis, and proof-of-principle
experiments: a neutron displacement-noise-free interferometer (neutron DFI) and a
jiggled interferometer (JIGI). Neutron DFI was proposed to avoid a limitation of a laser
DFI: its sensitive frequency band is too high for astrophysical GW sources, even for a
kilometer-sized interferometer. In a neutron DFI, neutrons are used instead of
light. Since neutrons have velocities much lower than the speed of light, and since
neutron speed can be chosen within experimental constraints, the propagation time can be
matched to the period of low-frequency GWs. In the proof-of-principle experiment at
J-PARC, the setup successfully canceled simulated displacement noise while preserving
simulated GW signals.  JIGI is based on the same basic idea as the Juggled
Interferometer (JIFO), which seeks to replicate the free-fall environment of space
within a terrestrial setup. Like JIFO, JIGI uses repeatedly free-falling test masses to
eliminate seismic and suspension thermal noise during free fall. The distinctive feature
of JIGI is that it significantly shortens the duration of each free fall, typically to
0.01 s, corresponding to a fall distance of approximately 0.1 mm. This shortened
free-fall duration provides two key advantages: improved angular stability of the test
masses and the elimination of laser tracking requirements. We evaluated the effect of
shortened free-fall duration and acquired Michelson interferometer signals during
repetitive 0.02 s free falls.

These results provide an experimental basis for pursuing sub-10 Hz observations with
ground-based interferometers by mitigating displacement-noise limitations and clarify
implementation issues toward future low-frequency detector designs.