From twelve to three active qubits: Ancilla-recycled rodeo filtering for trapped neutron-proton scattering

Abstract: Rodeo filtering applies (R) ancilla-assisted energy interrogations. If all measurements are deferred, a static realization requires (n_s+R) active qubits for an (n_s)-qubit system, whereas mid-circuit measurement and reset allow one ancilla to be recycled and reduce the width to (n_s+1) without changing the ideal filter. We demonstrate this compression for a trapped-spectrum input to model neutron--proton scattering. A static (R=10) circuit on IonQ Forte-1 uses 12 active qubits, while a dynamic circuit on IBM Aachen uses only three, a 75\% reduction. The controlled interleaved IBM scan gives (ΔE_c=-0.017\pm0.507\keV), comparable to the static result (-0.568\pm0.694\keV). Mapping these centers through the finite-confinement modified effective range expansion (MERE) gives (\mathcal K_{3.7}=p\cotδ_{0,3.7} =0.13419\pm0.00020~\mathrm{fm}^{-1}) for IonQ and (0.13435\pm0.00015~\mathrm{fm}^{-1}) for the interleaved IBM scan, both consistent with the exact value (0.13436~\mathrm{fm}^{-1}). Both implementations retain complete four-configuration support and therefore reproduce the exact (4\times4) effective-space level by sample-based quantum diagonalization. Three IBM batches nevertheless expose run-dependent center variations beyond finite-shot fluctuations, while the postselection attenuation is more stable. Ancilla recycling therefore makes the rodeo width independent of (R), freeing qubits for the nuclear register while preserving a finite-confinement scattering input, but exchanges spatial resources for mid-circuit latency and repeatability requirements.