Squashed Pyramid Interferometer Network (SPIN): Direct Access to Chirality of Cosmological Gravitational Waves

The cosmological gravitational wave background provides a powerful window on parity-violating physics at energies far beyond the reach of terrestrial experiments. However, any colocated planar detector network is insensitive to isotropic circular polarization, independent of its relative orientation. In this Letter, we show that this no-go result can be evaded by a new class of colocated 3D interferometer designs, which we call Squashed Pyramid, whose non-coplanar configuration geometrically isolates chirality. The design can be viewed as a minimal extension of the Einstein Telescope geometry, obtained by introducing a slightly tilted arm relative to the ET planar configuration. The coplanar correlation channel is blind to circular polarization, whereas the colocated non-coplanar channel is insensitive to the unpolarized background and acquires a response only in the presence of nonzero net helicity. Squashed Pyramid interferometer networks therefore furnish a unique probe of cosmological gravitational wave chirality, opening a realistic terrestrial pathway to test parity violation and fundamental symmetry breaking in the early Universe.