Researchers from India and Poland used a three-qubit NMR experiment to place a target qubit into a coherent superposition of two incompatible time evolutions, producing temporal correlations that exceed the long-assumed temporal Tsirelson's bound. The degree of LGI violation grew with the strength of the superposition and was tunable. Remarkably, the superposed time evolutions made temporal quantum correlations roughly five times more resilient to decoherence, suggesting potential applications in quantum computing and sensing while highlighting scaling challenges.
Qubits Beat a Long-Standing Temporal Quantum Limit by Evolving in Superposed Time Paths
For decades physicists believed there was a hard ceiling on how strongly a quantum system’s present can be correlated with its past and future. A new experiment by a team from India and Poland shows that a single qubit, when made to evolve along a superposition of distinct time evolutions, can exceed that limit and sustain temporal quantum correlations far longer than expected.
Background: Leggett–Garg Inequality and the Temporal Tsirelson Bound
The work builds on the Leggett–Garg inequality (LGI), a 1985 test devised to tell classical temporal behaviour from genuinely quantum temporal correlations. Under ordinary unitary evolution, quantum systems can violate the LGI but only up to a theoretical maximum known as the temporal Tsirelson’s bound (TTB). That bound was long treated as unbreakable within standard time-evolution frameworks.
Experiment: Superposition of Time Evolutions
Rather than forcing the system to follow a single, definite unitary evolution, the researchers prepared a control qubit in a coherent superposition that directed the target qubit to follow two incompatible unitaries at once. Intuitively, this is like placing a quantum object in a state that corresponds simultaneously to two opposite time histories — a concept with no classical analogue but permitted by quantum superposition.
Implementation Using NMR
The team implemented the idea in a nuclear magnetic resonance (NMR) experiment using a molecule that effectively hosted three qubits. Roles were assigned as follows: the first qubit acted as the controller (prepared in superposition), the second was the target whose temporal correlations were probed, and the third served as a readout qubit to sample the target at different times.
Results: Exceeding the TTB and Enhanced Robustness
Measured temporal correlations of the target qubit violated the Leggett–Garg inequality well beyond the temporal Tsirelson’s bound. The violation was tunable: as the degree of superposition between the two evolutions increased, so did the LGI violation. Crucially, the superposition-of-unitaries strategy also made the temporal quantum correlations more robust to environmental noise. In the reported experiment the target qubit retained measurable LGI violation for roughly five times longer than in the standard single-evolution scenario.
“We experimentally realize superposition of unitaries in NMR systems and demonstrate this enhanced violation. In the presence of noise, such a superposition of unitaries remarkably extends the time of LGI violation, showcasing improved robustness against decoherence,” the authors note.
Implications and Limitations
Fundamentally, the result shifts our understanding of temporal quantumness: correlations between past and future can be stronger and longer-lived than previously thought. Practically, the technique suggests new ways to prolong useful quantum behavior for applications in quantum computing and quantum metrology, where longer coherence and stronger temporal correlations can improve performance.
However, the demonstration was carried out in a tightly controlled NMR setting with three effective qubits. Scaling this approach to larger, noisy quantum processors and testing it across different hardware platforms remain significant challenges. Future work will explore other experimental platforms and more general measurement scenarios.
Publication: The study is published in Physical Review Letters.
