Qubits break long-held quantum limit by evolving in superposed time paths

Researchers from India and Poland have demonstrated that a fundamental quantum limit on temporal correlations, long thought unbreakable, can be surpassed using qubits evolving in superpositions of different time paths. Traditionally, the Leggett–Garg inequality tests whether an object behaves classically or quantum mechanically over time, with quantum systems known to violate this inequality only up to the temporal Tsirelson’s bound (TTB). The new study shows that by allowing a qubit to simultaneously follow two incompatible time evolutions—enabled by quantum superposition—this bound can be exceeded, revealing stronger-than-expected quantum correlations across time. The team implemented their experiment using three qubits in a molecule studied via nuclear magnetic resonance (NMR). One qubit acted as a controller in a superposition state, directing the target qubit to evolve along two different time paths simultaneously, while the third qubit read out temporal correlations. This setup led to violations of the Leggett–Garg inequality well beyond the TTB,