Abstract The quantum-to-classical transition remains one of the most debated open problems in modern physics. While decoherence theory successfully explains the suppression of interference in many experimental regimes, a clear operational distinction between reversible decoherence and irreversible collapse is still lacking, especially in mesoscopic and macroscopic interferometric systems. In this work, we propose a phenomenological modification of the Schrödinger dynamics that introduces an entropy-dependent non-linear term driving an irreversible transition from quantum superpositions to classical states beyond a well-defined threshold. The model predicts finite collapse times that depend on experimentally accessible parameters and naturally suppresses any revival of coherence once the transition has occurred. The theory is explicitly falsifiable: if coherence is observed to revive beyond the predicted threshold (≈10⁶ effective degrees of freedom), the model is ruled out. We compare the predicted collapse times with the observed loss of coherence in a wide range of interferometric and mesoscopic experiments, including cavity QED, optomechanical systems, superconducting circuits, and molecular interferometry. In all analyzed regimes, the model reproduces the observed orders of magnitude of the characteristic decoherence or collapse times. Finally, we discuss experimental scenarios—particularly involving large molecules such as porphyrins—where the model can be directly tested and potentially distinguished from standard decoherence-only descriptions.
Abstract The quantum-to-classical transition remains one of the most debated open problems in modern physics. While decoherence theory successfully explains the suppression of interference in many experimental regimes, a clear operational distinction between reversible decoherence and irreversible collapse is still lacking, especially in mesoscopic and macroscopic interferometric systems. In this work, we propose a phenomenological modification of the Schrödinger dynamics that introduces an entropy-dependent non-linear term driving an irreversible transition from quantum superpositions to classical states beyond a well-defined threshold. The model predicts finite collapse times that depend on experimentally accessible parameters and naturally suppresses any revival of coherence once the transition has occurred. The theory is explicitly falsifiable: if coherence is observed to revive beyond the predicted threshold (≈10⁶ effective degrees of freedom), the model is ruled out. We compare the predicted collapse times with the observed loss of coherence in a wide range of interferometric and mesoscopic experiments, including cavity QED, optomechanical systems, superconducting circuits, and molecular interferometry. In all analyzed regimes, the model reproduces the observed orders of magnitude of the characteristic decoherence or collapse times. Finally, we discuss experimental scenarios—particularly involving large molecules such as porphyrins—where the model can be directly tested and potentially distinguished from standard decoherence-only descriptions.
Quantum mechanics is an extremely successful theory that agrees with every experimental test. However, the principle of linear superposition, a central tenet of the theory, apparently contradicts a commonplace observation: macroscopic objects are never found in a linear superposition of position states. Moreover, the theory does not explain why during a quantum measurement, deterministic evolution is replaced by probabilistic evolution, whose random outcomes obey the Born probability rule. In this article a review is given of an experimentally falsifiable phenomenological proposal, known as continuous spontaneous collapse: a stochastic nonlinear modification of the Schr\"odinger equation, which resolves these problems, while giving the same experimental results as quantum theory in the microscopic regime. Two underlying theories for this phenomenology are reviewed: trace dynamics and gravity-induced collapse. As the macroscopic scale is approached, predictions of this proposal begin to differ appreciably from those of quantum theory and are being confronted by ongoing laboratory experiments that include molecular interferometry and optomechanics. These experiments, which test the validity of linear superposition for large systems, are reviewed here, and their technical challenges, current results, and future prospects summarized. It is likely that over the next two decades or so, these experiments can verify or rule out the proposed stochastic modification of quantum theory.
An open problem in modern physics is why microscopic quantum objects can be at two places at once (i.e. a superposed quantum state) while macroscpoic classical object never show such a behaviour. Collapse models provides a quantitative answer for this problem and explain how macroscopic classical world emerges out of microscopic quantum world. A universal noise field is postulated in collapse models, inducing appropriate Brownian- motion corrections to standard quantum dynamics. The strength of collapse-driven Brownian fluctuations depend on: (i) the parameters characterizing the system (e.g., mass, size, density), and (ii) two phenomenological parameters defining the statistical properties of the collapse noise. The collapse-driven Brownian motion works such that microscopic systems behave quantum mechanically, while macroscopic objects are classical. At the intermediate mesocopic scale, collapse models predict deviations from standard quantum predictions. This issue has been subject of experimental tests. All experiments to date have been at the scales where collapse effects are negligible for all practical purposes. However, recent experimental progress in revealing quantum features of larger objects, increases the hope for testing at unprecedented scales where collapse models can be falsified. Current experiments are mainly focused on the preparation of macroscopic systems in a spatial quantum superposition state. The collapse effects would then manifest as loss of visibility in the observed inference pattern. However, one needs a quantum interference with single particles of mass ∼ 10 10 amu for a decisive test of collapse models. Creating such massive superpositionsis quite challenging, and beyond currectstate-of-the-art. Quite recently, an alternative approach has been proposed where the collapse manifests in the fluctuating properties of light interacting with the quantum system. The great advantage of this new approach is that here there is no need for the preparation of a quantum superposed state. It has been discussed that promising results can be revealed in the spectrum of light interacting with a radiation pressure-driven mechanical oscillator in a cavity optomechanics setting. Here, we review the theoretical modelling of the above optomechenical proposal. We discuss how collapse-driven Brownian motion modifies the spectrum. We quantify the collapse effect and explain how it depends on the parameters of the mechanical oscillator (e.g., mass, density, geometry).
Abstract The quantum measurement problem—the absence of any dynamical mechanism connecting continuous wavefunction evolution to discrete empirical outcomes—has persisted since the foundations of quantum theory. Decoherence explains interference suppression but cannot explain outcome selection: the diagonal density matrix remains an improper mixture until one outcome is actualized. We propose that collapse is a physical phase transition in the coupled system–apparatus field. The order parameter ψ, constructed as a coarse-grained collective coordinate of apparatus degrees of freedom, evolves under Ginzburg–Landau dynamicswith a symmetry-breaking potential. When the coherence pressure γ exceeds a critical threshold γc, the symmetric phase(superposition) becomes unstable and the field crystallizes into one of the discrete stable minima (eigenstates). We derive the critical coupling from microscopic system–apparatus Hamiltonians, obtaining γc ∼ (√Ng)−1 where N is the number of apparatus degrees of freedom and g is the coupling constant. This scaling explains why macroscopic apparatus collapse wavefunctions while microscopic interactions preserve coherence. The Born rule Pn = |cn|2 is preserved through this dynamical selection mechanism: attractor basin geometry under probability-conserving flow converts probabilistic weights into definite outcomes without altering the underlying probabilities. The theory yields four falsifiable predictions absent from standard quantum mechanics: critical slowing near γc, hysteresis in the collapse–recoherence cycle, metastable supercooled superpositions, and transient Jacobian spikes at the moment of collapse. We provide quantitative estimates for superconducting qubit readout, predicting collapsetimes of order 100 ns with critical slowing observable within current experimental precision. This framework provides a concrete existence proof, within a controlled effective-theory regime, that collapse dynamics can be constructed from standard quantum mechanics plus statistical mechanics, requiring no modification to the Schr¨odinger equation.
The collapse of the wave function remains one of the deepest puzzles in quantum mechanics. The Schrödinger equation is unitary and time-reversal symmetric, yet measurement requires an additional postulate that is neither—and we have no physical explanation for why. Existing interpretations either deny that collapse is real (Many-Worlds), treat it as purely subjective (QBism), or add stochastic noise by hand without deriving it from fundamental principles (GRW, CSL). We demonstrate that collapse is a real physical process requiring fundamentally irreversible dynamics. No purely unitary, time-symmetric theory can account for the objective suppression of macroscopic superpositions—irreversibility must be built into the dynamics from the start. We introduce a minimal extension of quantum mechanics through coupling to a universal scalar field (the I-field) governed by an explicit time-asymmetric field equation. Collapse emerges objectively through deterministic dissipative dynamics, with no reference to observers, measurement devices, or stochastic noise. By integrating out the I-field degrees of freedom, we derive effective Lindblad dynamics for quantum systems. The collapse rate scales quadratically with system mass, ensuring microscopic quantum coherence remains intact while macroscopic superpositions are dynamically suppressed. Unlike stochastic collapse models (GRW/CSL), the mechanism is deterministic and produces no spontaneous heating. Unlike gravitational collapse proposals (Diósi-Penrose), it provides explicit field equations rather than heuristic arguments. Unlike environmental decoherence, the irreversibility is fundamental rather than effective. The theory respects relativistic causality and makes testable predictions for current and near-future experiments in matter-wave interferometry, optomechanical systems, and molecular interferometry. Observation of mass-dependent decoherence without accompanying stochastic heating would provide strong evidence for the I-field mechanism. The framework establishes irreversibility as a fundamental interaction rather than an emergent phenomenon, providing a unified physical origin for both wave function collapse and the quantum arrow of time.
Quantum mechanics exhibits a broad collection of theoretical results in complete agreement with experimental evidence. Besides showing unquestionable success in the description of isolated systems, it can be also successfully used to characterize non-isolated quantum system. In such a case, phenomena like dissipation, diffusion or decoherence. The theory of open quantum systems provides the framework where such features can be conveniently explained. \nThis thesis is about decoherence and collapse models. Albeit conceptually they are very far from each other, they both belong the framework of open quantum systems. Indeed, they consider systems interacting with an external entity: an environment for decoherence models, a noise for collapse models. Although the external influence has different origin, they can be described by similar dynamical equations and, to confirm or falsify them, the same experimental tests can be performed. \nThe quantum Brownian motion model can be considered the paradigm of decoherence models. We provide an exact and analytic equation for the time evolution of the operators and we show that the corresponding equation for the states is equivalent to well-known results in the literature. Our derivation allows to compute the time evolution of physically relevant quantities in a much easier way than previous formulations. Moreover, we are not bound to compute the time evolution of the state of the system, which in general is a complicated task. The explicit dependence on the initial state appears only in the initial expectation values and not in the dynamics. This makes possible the derivation of expectation values also for nontrivial states. \nAnother decoherence model we considered is a recently proposed model, based on the mass-energy equivalence. The model describes a decoherence source acting universally on every system whose superposition is extended on positions experiencing different gravitational potentials. We studied the conditions under which this mechanism becomes the dominant decoherence effect in typical interferometric experiments. We show that current experiments are off by several orders of magnitude. New ideas are needed to achieve the necessary requirements. \nThe second part of this thesis concerns collapse models, in particular the Continuous Spontaneous Localization (CSL) model. We focus on experimental tests that can probe it. In this respect, experiments can be grouped in two classes: interferometric tests and non-interferometric ones. The first class includes those experiments, which directly create and detect quantum superpositions of the center of mass of massive systems. The strongest bounds on the CSL parameters come from the second class of non-interferometric experiments, which are sensitive to small position displacements and detect CSL-induced diffusion in position. \nWe investigate how we can benefit from the non-interferometric perspective given by optomechanical setups, which have reached high sensitivities as force and position sensors. Three examples are considered.\nFirst, we compute the upper bounds on the CSL parameters, which can be inferred by the gravitational wave detectors LIGO, LISA Pathfinder and AURIGA.\nSecond, we report new results from an experiment based on a high-quality cantilever cooled to millikelvin temperature. High accuracy measurements of the cantilever thermal fluctuations reveal a nonthermal force noise of unknown origin. This excess noise is compatible with the CSL heating predicted by Adler. Several physical mechanisms able to explain the observed noise have been ruled out.\nThird, we propose an unattempted non-interferometric test aimed to investigate the still unexplored region of the CSL parameter space. Our proposal relies on torsional degrees of freedom rather than the usual vibrational ones. We believe that the test that has been put forward here, will eventually probe the unexplored CSL parameter space.
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.