Quantum Coherence and Macroscopic Realism in Quantum Systems

Quantum coherence, the maintenance of fixed phase relations between quantum states, underpins phenomena from superconductivity to quantum information processing. At microscopic scales, coherent superpositions are well established, but their persistence and consequences in larger, potentially macroscopic systems remain a central challenge. Macroscopic realism posits that macroscopic objects always occupy definite states independent of observation, a view tested by inequalities and temporal‐correlation criteria designed to reveal nonclassical behaviour. Violations of these criteria—most notably Leggett–Garg inequalities—demonstrate the incompatibility of macroscopic realism with quantum theory. Research now spans from trapped ions and superconducting circuits to optomechanical resonators, probing how environmental interactions, energy constraints and measurement back‐action influence coherence and apparent classicality. Theoretical advances in open‐system dynamics and resource quantification, including refinements of quantum Fisher information and temporal correlation witnesses, have deepened understanding of when and how quantum coherence can persist or be observed at larger scales. These insights inform both foundational tests of quantum theory and emerging quantum technologies, such as high‐precision sensors and scalable qubit architectures, by elucidating the boundary between quantum superposition and classical realism.

Research from Nature Portfolio

Recent studies have investigated the robustness of quantum‐temporal correlations under realistic conditions, showing that a driven and dissipative qubit subject to energy constraints violates Leggett–Garg inequalities to varying degrees depending on dephasing and measurement coarsening. Pure coherent dynamics achieve maximal temporal‐inequality violations, whereas dissipative or driven systems display reduced nonclassical signatures. In a foundational experiment with a superconducting flux qubit, a refined protocol closed the clumsiness loophole and unambiguously demonstrated superposition of macroscopically distinct current states, ruling out broad classes of macrorealist modifications by many standard deviations. These works collectively advance the precision and conceptual clarity of tests that challenge macroscopic realism in solid‐state quantum platforms.

Quantum Coherence and Macroscopic Realism in Quantum Systems publication trend

The graph below shows the total number of articles in quantum coherence and macroscopic realism in quantum systems across all publications each year (not limited to Nature Index journals).

Technical terms

Quantum coherence: The preservation of relative phase relationships between components of a quantum superposition.

Macroscopic realism: The hypothesis that macroscopic objects possess definite properties independent of observation.

Leggett–Garg inequality: A temporal‐correlation criterion whose violation indicates incompatibility with macroscopic realism under noninvasive measurement assumptions.

Temporal correlations: Statistical dependencies between measurement outcomes at different times revealing underlying quantum dynamics.

Open quantum system: A quantum system interacting with an external environment, leading to decoherence and dissipation.

References

1. Optimization of Time-Ordered Processes in the Finite and Asymptotic Regimes. PRX Quantum (2024).
2. Mass-Independent Scheme to Test the Quantumness of a Massive Object. Physical Review Letters (2024).
3. Witnessing environment dimension through temporal correlations. Quantum (2024).
4. Quantum violation of LGI under an energy constraint for different scenarios systems. Scientific Reports (2023).
5. A strict experimental test of macroscopic realism in a superconducting flux qubit. Nature Communications (2016).