Eigenstate Thermalization Limits Observable Macroscopic Quantum Superpositions, Rendering Coherence Indistinguishable for Most Times

The enduring quest to observe quantum superposition in macroscopic objects faces a fundamental challenge, according to new research. Gabriel Dias Carvalho from Universidade de Pernambuco, Pedro S. Correia from Universidade Estadual de Santa Cruz, and Thiago R. de Oliveira from Universidade Federal Fluminense demonstrate that even in perfect isolation, the natural evolution of quantum systems actively suppresses the observable signatures of macroscopic coherence. Using a well-known quantum state as a model, the team reveals that while initial measurements can distinguish a macroscopic superposition from a classical equivalent, standard quantum evolution quickly renders them indistinguishable. This work establishes that inherent quantum thermalization, independent of external disturbances, represents a key limitation to observing macroscopic quantum phenomena and fundamentally impacts our understanding of the boundary between the quantum and classical worlds.

Limits to Macroscopic Quantum Superpositions

The quest to observe macroscopic quantum superpositions faces fundamental challenges stemming from equilibration and the Eigenstate Thermalization Hypothesis. This work investigates how quickly quantum systems lose coherence due to interactions with their environment, determining the conditions under which quantum behaviour transitions to classical behaviour, destroying observable superpositions. The approach analyses the dynamics of quantum systems subject to weak and strong coupling to a thermal bath, modelling environmental interactions as a series of collisions. By examining the time evolution of the system’s wavefunction, researchers determine the rate of quantum coherence loss and approach to thermal equilibrium.

The analysis considers both integrable and non-integrable systems, revealing how inherent properties influence decoherence. Specifically, the team investigates many-body localisation, a phenomenon that can inhibit thermalisation and potentially preserve quantum coherence. The results demonstrate that even weak coupling to a thermal bath can rapidly destroy quantum coherence in non-integrable systems, precluding the observation of macroscopic superpositions. Furthermore, the research establishes a connection between the Eigenstate Thermalization Hypothesis and the timescale for decoherence, showing that systems satisfying the ETH exhibit particularly rapid loss of quantum coherence. The findings provide valuable insights into the fundamental limits on observing macroscopic quantum phenomena and guide strategies for mitigating decoherence in quantum experiments.

Self-Induced Decoherence in Closed Quantum Systems

This research argues that macroscopic quantum superpositions, such as those found in GHZ states, inevitably become indistinguishable from classical mixtures due to the natural process of equilibration within an isolated quantum system, even without external environmental influences. This occurs through the suppression of off-diagonal coherence via unitary evolution, effectively causing the system to decohere itself through its own internal dynamics. Equilibration describes a closed quantum system reaching equilibrium, representing a broader loss of quantum coherence beyond simple thermalisation. The GHZ state serves as a benchmark for demonstrating quantum non-locality and macroscopic superposition.

Off-diagonal coherence refers to the quantum mechanical phase relationships between states in a superposition, and its loss is a key step towards classical behaviour. Unitary evolution, the natural time evolution of a closed quantum system, surprisingly leads to decoherence. The research demonstrates that the system and the mixture become operationally indistinguishable, meaning no measurement can distinguish between them. The difference decays polynomially with system size but is ultimately overwhelmed by the system’s dynamics. The indistinguishability is more pronounced in larger systems.

This work suggests that observing true macroscopic quantum phenomena is incredibly difficult, even in principle, setting a fundamental limit on how long a macroscopic superposition can be maintained. The findings have implications for quantum computing and other quantum technologies, suggesting that isolating and protecting quantum systems from decoherence is even more challenging than previously thought. The research challenges the traditional view that decoherence requires an external environment, demonstrating that internal dynamics can be sufficient to suppress quantum coherence. It sheds light on the question of where the quantum world ends and the classical world begins, suggesting that equilibration plays a crucial role in defining this boundary.

Equilibration Suppresses Macroscopic Quantum Superpositions

This research demonstrates that even in perfectly isolated quantum systems, the natural process of equilibration suppresses macroscopic superpositions, limiting the emergence of macroscopic quantum effects. Scientists investigated this phenomenon using the GHZ state as a model, showing that initial distinctions between a superposition and its classical counterpart vanish over time due to intrinsic unitary dynamics. The team found that equilibration not only obscures coherence from standard measurements but actively diminishes macroscopic superpositions themselves, confirmed through analysis of both distinguishability measures and quantifiers of macroscopic quantumness. Further investigation employed state and commutator indices to assess the distribution of quantum coherence within many-body systems, revealing that long-term states of both the GHZ state and its classical mixture lack macroscopic quantumness. This indicates that equilibration does not generate new macroscopic superpositions but progressively erases existing ones, highlighting a fundamental limit to observing macroscopic quantum phenomena.