Abstract Identifying which quantum states qualify as macroscopic superpositions remains a fundamental challenge in quantum physics, as no universally accepted operational criterion currently exists. In this work we introduce a dynamical criterion for macroscopic quantum superposition based on the continuous-measurement-based measure of quantum superposition together with the principle of macrorealism. The key idea is to characterize macroscopicity through the rate of purity decay of the quantum state under continuous monitoring, which quantifies the susceptibility of quantum coherence to classicalizing mechanisms. Two classes of macrorealistic dynamics are considered: environmental decoherence described by a Lindblad master equation (class A) and objective collapse dynamics in the continuous spontaneous localization model (class B). Applying this framework to an optomechanical oscillator, we show that the emergence of macroscopic superposition depends on the competition between measurement-induced decoherence and the macrorealistic mechanism. In class A the relevant contributions increase with excitation number, whereas in class B large excitation numbers can suppress macroscopicity. Unlike many existing macroscopicity measures based solely on phase-space structure or decoherence times, the present criterion directly relates macroscopicity to experimentally accessible parameters in continuous quantum measurement.
Abstract Identifying which quantum states qualify as macroscopic superpositions remains a fundamental challenge in quantum physics, as no universally accepted operational criterion currently exists. In this work we introduce a dynamical criterion for macroscopic quantum superposition based on the continuous-measurement-based measure of quantum superposition together with the principle of macrorealism. The key idea is to characterize macroscopicity through the rate of purity decay of the quantum state under continuous monitoring, which quantifies the susceptibility of quantum coherence to classicalizing mechanisms. Two classes of macrorealistic dynamics are considered: environmental decoherence described by a Lindblad master equation (class A) and objective collapse dynamics in the continuous spontaneous localization model (class B). Applying this framework to an optomechanical oscillator, we show that the emergence of macroscopic superposition depends on the competition between measurement-induced decoherence and the macrorealistic mechanism. In class A the relevant contributions increase with excitation number, whereas in class B large excitation numbers can suppress macroscopicity. Unlike many existing macroscopicity measures based solely on phase-space structure or decoherence times, the present criterion directly relates macroscopicity to experimentally accessible parameters in continuous quantum measurement.
We establish an objective scheme to determine the macroscopicity of quantum superposition tests with mechanical degrees of freedom. It is based on the Bayesian hypothesis falsification of a class of macrorealist modifications of quantum theory, such as the model of continuous spontaneous localization. The measure uses the raw data gathered in an experiment, taking into account all measurement uncertainties, and can be used to directly assess any conceivable quantum mechanical test. We determine the resulting macroscopicity for three recent tests of quantum physics: double-well interference of Bose-Einstein condensates, Leggett-Garg tests with atomic random walks, and entanglement generation and read-out of nanomechanical oscillators.
Optomechanics is currently believed to provide a promising route towards the achievement of genuine quantum effects at the large, massive-system scale. By using a recently proposed figure of merit that is well suited to address continuous-variable systems, in this paper we analyze the requirements needed for the state of a mechanical mode (embodied by an end-cavity cantilever or a membrane placed within an optical cavity) to be qualified as macroscopic. We show that according to the phase-space-based criterion that we have chosen for our quantitative analysis, the state achieved through strong single-photon radiation-pressure coupling to a quantized field of light and conditioned by measurements operated on the latter might be interpreted as macroscopically quantum. In general, though, genuine macroscopic quantum superpositions appear to be possible only under quite demanding experimental conditions.
The quantum measurement problem—why observation appears to collapse superposition states—has resisted resolution despite decades of progress in decoherence theory. While environmental decoherence explains the emergence of classicality, it lacks a quantitative criterion distinguishing "measured" from "merely decohered" systems. This work proposes such a criterion: measurement occurs when environmental coupling strength, quantified by the dimensionless parameter W = (I * tau) / tau0, exceeds a universal threshold W0. Here I is path distinguishability (Englert’s D), tau is information retention time, and tau0 is system coherence time. The framework predicts continuous visibility decay V(W) = 1 / [1 + exp(kappa * (W - W0))] rather than binary collapse, with threshold W0 ≈ 1–3 across platforms after normalization. Testable predictions include: (1) I × tau trade-off (different combinations of distinguishability and retention yielding identical visibility at matched W); (2) quantum eraser timing dependence (recovery window ~ tau0); and (3) finite collapse propagation speed, formalized as the Causal Limit Postulate (v = c in vacuum). The model predicts a measurable delay (Delta t ≈ L / c) and introduces Causal Viscosity, where n_eff ~ Gamma^(1/2), to describe propagation drag in decohering media. A cavity-QED toy model with 87Rb atoms validates the framework with realistic parameters, independently confirmed by dispersive regression.
Macroscopic quantum superpositions are widely believed to be unobservable because large systems cannot be perfectly isolated from their environments. Here, we show that even under perfect isolation, intrinsic unitary dynamics in generic many-body systems, as analyzed through the eigenstate thermalization hypothesis and random matrix theory, can suppress the observable signatures of macroscopic coherence. Using the Greenberger-Horne-Zeilinger (GHZ) state as a representative example, we demonstrate that while fully correlated measurements can initially distinguish a macroscopic superposition from its corresponding classical mixture, generic many-body evolution renders them operationally indistinguishable for most times. By analyzing both distinguishability measures and established quantifiers of macroscopic quantumness, we find that equilibration not only hides coherence from accessible observables but also suppresses the corresponding signatures of macroscopic quantumness, in particular within the additive-local framework considered here. These results identify unitary thermalization, independent of environmental decoherence, as a fundamental mechanism that limits the observation of macroscopic quantum effects.
The quantum measurement problem remains one of the most profound unresolved issues in fundamental physics. While quantum mechanics delivers extraordinarily precise statistical predictions, it does not specify a physical mechanism by which definite classical outcomes arise from quantum superpositions. Decoherence theory accounts for the suppression of interference but does not explain outcome selection or the stability of classical reality. This paper investigates quantum measurement within the framework of Relativistic Coherent Vacuum Gravity Theory (rCVGT). In rCVGT, the vacuum is modeled as a structured relativistic medium characterized by a coherence order parameter ψ(x), a coherence magnitude Q(x) = |ψ(x)|², a vacuum-flow four-field u^μ(x), and a physical time-rate field τ(x). We propose that quantum measurement corresponds to a dynamical instability of vacuum coherence induced by macroscopic coupling to matter and radiation. Classical outcomes emerge when local vacuum coherence is driven below a critical stability threshold, resulting in an irreversible reconfiguration of the vacuum state. The collapse timescale is determined by the vacuum-coherence self-energy defined within rCVGT. This mechanism simultaneously provides a physical foundation for classicality, irreversibility, and the arrow of time. The framework preserves the formal structure and empirical predictions of quantum mechanics while offering a unified physical explanation of measurement grounded in relativistic vacuum dynamics.
The transition from the quantum to the classical realm remains one of the most profound open questions in physics. While quantum theory predicts the existence of macroscopic superpositions, their apparent absence in the everyday world is attributed either to environmental decoherence or to an intrinsic mechanism for wave-function collapse. This work presents a quantitative and experimentally grounded framework for distinguishing these possibilities. We propose a levitated optomechanical platform capable of generating controllable Schrodinger-cat states in the center of mass motion of a dielectric nanosphere. A comprehensive master equation incorporates gas collisions, black-body radiation, and photon-recoil noise, establishing a calibrated environmental baseline. The Continuous Spontaneous Localization (CSL) model is embedded within the same framework, predicting a characteristic saturation of the decoherence rate with superposition size and a quadratic scaling with mass. A Bayesian inference protocol is outlined to discriminate collapse induced excess decoherence from environmental noise. Together these elements provide a concrete experimental blueprint for a decisive test of quantum linearity, either revealing new physics beyond standard quantum mechanics or setting the most stringent bounds to date on objective-collapse parameters.