Certifying Nonlocal Spin Quantum Entanglement from Local Tunneling Spectra

Abstract

Quantum entanglement is intrinsically nonlocal, whereas tunneling spectroscopy measures one site at a time. We show that this apparent mismatch can be overcome: local tunneling spectra can certify spin entanglement without coherent control or absolute intensity calibration. An exact spin sum rule turns the high-bias conductance into an internal normalization, allowing the inelastic spin-flip weight to place a lower bound on the local quantum Fisher information. Total-spin conservation then fixes the otherwise inaccessible intersite fluctuation, converting two local spectra into a bound on a collective entanglement witness. We incorporate coupling to the substrate through a spectroscopic bound on spin leakage, while a hierarchy of thresholds distinguishes entanglement between the spins from entanglement with their environment. Including instrumental broadening and noise, the resulting certificate holds at a stated confidence level. These results establish a route for extracting nonlocal quantum information from equilibrium local spectroscopy.