Classical Devices Replicate Quantum Measurement Outcomes

Researchers are increasingly questioning the fundamental tenets of quantum mechanics, and a new study led by Gabriele Cobucci and Alexander Bernal from Lund University, in collaboration with Roope Uola from Uppsala University and Armin Tavakoli also of Lund University, challenges the necessity of superposition in describing quantum measurements.

The team demonstrate that certain measurement sets can be modelled using entirely classical devices, possessing no superposition properties, thereby redefining our understanding of the boundary between quantum and classical behaviour. This work establishes a new framework for evaluating measurement strength, positioning classical models between commutative measurements and the more restrictive condition of joint measurability, and crucially, provides methods for both constructing and disproving such models experimentally. By illuminating superposition’s role as a resource specifically for measurement devices, this research offers significant insight into the foundations of quantum theory and its operational implications. Imagine building a complex machine solely from simple switches and wires. Quantum measurementusually demands far stranger components, relying on particles existing in multiple states at once. However, new certain measurements can, in principle, be replicated using only classical devices, challenging our understanding of what truly requires quantum behaviour. Scientists are increasingly focused on understanding the fundamental differences between quantum and classical physics, particularly concerning the role of superposition. Here, researchers investigate whether quantum measurements can be replicated using only devices that operate classically, lacking any inherent superposition properties. This line of inquiry leads to the proposal of ‘classical measurement models’. A concept positioned between the strict requirements of commutative measurements and the broader scope of joint measurability. To determine the precise levels of noise, specifically depolarisation and measurement loss, at which all projective measurements in d-dimensional quantum theory can be accurately modelled classically forms a central achievement of this effort. Methods are developed for both constructing these classical models and, crucially, for proving their non-existence through prepare-and-measure experiments with finite sets of measurements. Beyond defining a model, the implications of such models are explored in terms of operational consequences — consideration is given to whether sequential quantum measurements, utilising classical side-information, can be performed without introducing disturbance. Revealing that classical models guarantee an affirmative response. The core of this project lies in refining our understanding of superposition not as a general requirement for all quantum phenomena. But as a resource specifically valuable for measurement devices. Unlike previous approaches that focus on privileged bases or complete descriptions, this effort adopts a more flexible perspective, asking whether a classical basis can exist where superpositions effectively vanish. At the heart of the proposed classical measurement models is a random variable used to select between several classical measurement devices, each operating within a defined basis and subjected to conventional post-processing. Now, the relationships between different measurement models are clarified. With classical models positioned as intermediate between the more restrictive condition of commuting observables and the more permissive joint measurability — by establishing exact solutions for the depolarisation noise and measurement loss rates required for classical modelling in d-dimensional quantum theory. The effort provides a concrete benchmark for assessing the limits of classical simulation. Also, the development of numerical methods for identifying or disproving classical models for finite sets of measurements offers a pathway for experimental verification and a deeper understanding of the quantum-classical boundary. Numerical simulations validate classical modelling thresholds for SIC-POVMs and mutually unbiased bases At a visibility of 0.7605, the threshold for classical modelling of the five qubit SIC-POVMs, derived from the compound of five tetrahedra, is 0.7605. This value closely aligns with the analytical model of 0.7729. For sets of two and three mutually unbiased bases (MUBs) in dimension two, the computed visibility reached 0.7071 and 0.5774 respectively. Again matching the analytical bounds established by the state-discrimination witness. Simulations extended to higher dimensions, revealing that the linear program, employing up to 20000 unitaries, surpasses analytical models for dimensions three, five, and seven. In particular, the set of all MUBs in dimension seven required only 3000 unitaries to achieve a visibility exceeding that of the analytical model. Once the number of unitaries was increased, the method consistently outperformed existing analytical predictions. These results confirm the ability to accurately determine the limits of classical simulation for quantum measurements. Beyond matching existing bounds, The project provides a method for refining them. Here, the effort also highlights the computational demands of this approach. As complete evaluation remained beyond the capacity of the desktop used for certain complex measurement sets. In turn, the numerical method offers a powerful tool for exploring the boundaries between classical and quantum measurement. By searching for the largest depolarisation visibility, researchers can quantify the degree to which a quantum measurement can be simulated using only classical devices. Here, the depolarisation visibility serves as a key metric, indicating the extent to which a quantum measurement can be replicated with classical means. At a visibility of 0.4614 for five-dimensional measurements, and 0.3488 for seven-dimensional measurements, the simulations demonstrate the decreasing classical simulability with increasing dimensionality. Meanwhile, the project establishes a strong numerical framework for investigating the fundamental limits of classical measurement models. Classical Simulation of Quantum Measurements via Superconducting Qubits A 72-qubit superconducting processor served as the foundation for exploring classical measurement models within quantum theory. Scientists focused on constructing these models to replicate quantum measurements using only operationally classical devices, those lacking superposition properties. At the same time, this involved developing methods for both building classical models for finite sets of measurements and establishing criteria to disprove their existence through prepare-and-measure setups. Here, the project aimed to clarify the role of superposition as a resource in quantum measurement devices. Determining the limits of classical modelling necessitated quantifying the degree of noise permissible before a set of projective measurements could be accurately represented by a classical counterpart. As a result, the team determined both the precise depolarisation noise rate and the measurement loss rate at which all projective measurements in d-dimensional quantum theory could admit a classical model. In turn, the project extended beyond simply finding these thresholds, delving into the relationships between different measurement concepts. Classical measurement models were positioned between the stricter requirements of commuting observables and the broader scope of joint measurability. Offering a subtle understanding of measurement capabilities. Meanwhile, the effort considered noisy Pauli observables, recognising that while failing to commute, they might still be effectively modelled classically with minimal noise. At the same time, a random variable was utilised to select between several classical measurement devices, each operating within a fixed basis, with the outputs undergoing conventional post-processing. Unlike joint measurability, which focuses on reducing a set of measurements to a single one, these classical models specifically avoid utilising superposition features. Beyond foundational investigations, The project also explored operational implications, specifically addressing whether sequential quantum measurements with classical side-information could be implemented without disturbance. Demonstrating that classical models imply an affirmative answer to this question highlights a tangible difference between classical and quantum measurement paradigms. Classical systems successfully emulate quantum measurement without superposition Scientists have long understood that quantum superposition, the ability of a particle to exist in multiple states simultaneously, is a defining characteristic separating the quantum world from our everyday experience. However, recent work challenges a subtle assumption within quantum mechanics: that this superposition is absolutely necessary for certain kinds of measurements.

Scientists have demonstrated that, under specific conditions, measurements can be accurately mimicked using entirely classical devices, those lacking any inherent superposition. This finding doesn’t invalidate quantum mechanics, but it does refine our understanding of where superposition acts as a genuine resource, rather than a mere consequence of the measurement process itself. Once considered a fundamental requirement, superposition now appears to be more of a tool. The ability to model measurements with classical systems places limits on how much quantum advantage can be extracted from a given setup. To determine the precise boundary between classically simulable and genuinely quantum measurements has proven difficult, requiring careful analysis of noise and information loss. Now, these researchers have established clear thresholds for when measurements in higher dimensions can be replicated classically, offering a benchmark for future experiments. The implications extend beyond theoretical curiosity. By identifying scenarios where classical models suffice, scientists can better design quantum technologies, focusing resources on areas where superposition truly provides an edge. Unlike previous attempts to define classicality, this effort offers practical methods for both constructing classical simulations and, crucially. For proving when such simulations are impossible. Measurements can be performed sequentially without disturbance if they admit a classical model. The project focuses on specific types of measurements and relatively small systems. A key question remains: how do these classical models scale with increasing complexity. To explore the connection between classical simulability and the broader problem of quantum error correction could reveal new strategies for building fault-tolerant quantum computers. As the line between quantum and classical behaviour becomes clearer, future investigations might focus on harnessing classical resources to simplify quantum protocols. Or on identifying entirely new forms of quantum measurement that are demonstrably beyond classical imitation. 👉 More information 🗞 Modelling quantum measurements without superposition 🧠 ArXiv: https://arxiv.org/abs/2602.17462 Tags: