Finite $N$ Bulk Hilbert Space in ETH Matrix Model for double-scaled SYK

We extend the notion of chord number in the strict large $N$ double-scaled Sachdev-Ye-Kitaev (DSSYK) model to the corresponding finite $N$ ETH matrix model. The chord number in the strict large $N$ DSSYK model is known to correspond to the discrete length of the Einstein-Rosen bridge in the gravity dual, which reduces to the renormalized geodesic length in JT gravity at weak coupling. At finite $N$, these chord number states form an over-complete basis of the non-perturbative Hilbert space, as the structure of the inner product gets significantly modified due to the Cayley-Hamilton theorem: There are infinitely many null states. In this paper, by considering ``EFT for gravitational observables'' or a version of ``non-isometric code'', we construct a family of chord number operators at finite $N$. While the constructed chord number operator depends on the reference chord number state, it realizes approximate $q$-deformed oscillator algebra and reproduces semiclassical bulk geometry around the reference state. As a special case, we will show that when the reference is chosen to be the chord number zero state, the chord number operator precisely matches with the Krylov state complexity, leading to the ``ramp-slope-plateau'' behavior at late times, implying the formation of ``grey hole''.