Discriminating between Lüders and von Neumann measuring devices: An NMR investigation

Highlights: • Reformulation of a recent protocol to discriminate between Lüders and von Neumann measuring devices. • Experimental discrimination of state-update rules using a four-qubit NMR system. • Experimental support for Lüders state-update rule. - Abstract: Quantum state after measuring a degenerate observable is given by Lüders and von Neumann state update rules. While the former preserves superpositions, the latter does not. Even though both rules are valid and realizable, which rule a given measuring device ("Black Box") obeys, depends on its internal details. Recently Hegerfeldt and Mayato (2012) [5] had formulated a protocol to discriminate between the two kinds of measuring devices. Here we have reformulated this protocol for system and measuring qubits. We then experimentally investigated this protocol on an NMR spectrometer, and found that Lüders rule is favoured.