From twelve to three active qubits: Ancilla-recycled rodeo filtering for trapped neutron-proton scattering
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 \(\Delta E_c=-0.017\pm0.507\keV\), comparable to the static result \(-0.568\pm0.694\keV\). Mapping these centers through the finite-confinement modified effective range expansion (MERE) gives \(\mathcal K_{3.7}=p\cot\delta_{0,3.7} =0.13419\pm0.00020~\mathrm{fm}^{-1}\) 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.