How Quantum and Beyond-Quantum Theories Constrain Correlations in Systems with Bounded Spin Under Spatial Rotations

Centrala begrepp

Quantum theory perfectly describes the rotational correlations in systems with spin up to 1, but for higher spins (≥ 3/2), more general, beyond-quantum correlations exist, offering advantages in metrological tasks while still respecting fundamental physical principles.

Sammanfattning

• Bibliographic Information: Aloy, A., Galley, T.D., Jones, C.L., Ludescher, S.L., & Müller, M.P. (2024). Spin-bounded correlations: rotation boxes within and beyond quantum theory. arXiv preprint arXiv:2312.09278v2.

• Research Objective: This paper investigates how the probabilities of measurement outcomes in physical systems are constrained by spatial rotations, particularly when the system's spin is bounded, exploring both within and beyond the framework of quantum theory.

• Methodology: The researchers employ the concept of "rotation boxes," analogous to non-local boxes in Bell scenarios, to analyze the correlations arising from rotating a preparation device relative to a measurement device. They mathematically define sets of correlations for both quantum (QJ) and more general (RJ) spin-J systems, comparing their properties and boundaries.

• Key Findings:

  • For spins J = 0, 1/2, and 1, quantum theory encompasses all possible rotational correlations.
  • For J ≥ 3/2, a gap emerges between quantum and general correlations, implying the existence of beyond-quantum resources.
  • A metrological task is presented where a hypothetical spin-3/2 system exceeding quantum limits achieves a higher success probability than any quantum system with the same spin.
  • The study establishes a Tsirelson-type inequality for J ≥ 3/2, demarcating the boundary of quantum correlations within the broader set.
  • General spin-J correlations (RJ) are shown to provide an efficient outer semidefinite programming (SDP) approximation for the quantum set (QJ).

• Main Conclusions: The research reveals that while quantum theory accurately captures rotational behavior for lower spins, higher-spin systems permit correlations beyond the quantum limit. This finding has significant implications for understanding the interplay between spacetime symmetries and quantum theory, potentially leading to novel semi-device-independent quantum information protocols and insights into the fundamental structure of physical theories.

• Significance: This work pushes the boundaries of quantum foundations by exploring correlations beyond the scope of quantum theory while adhering to fundamental physical principles. It opens avenues for developing new semi-device-independent protocols and offers a fresh perspective on the relationship between spacetime and quantum mechanics.

• Limitations and Future Research: The paper primarily focuses on single systems with bounded spin under SO(2) rotations. Future research could explore more complex scenarios involving multiple systems, higher spacetime symmetries, and their implications for quantum information processing and foundational questions in physics.

Statistik

Quantum systems with spin J = 3/2 can achieve a success probability of approximately 85.36% in the presented metrological task.
Hypothetical beyond-quantum systems with spin J = 3/2 can reach a success probability of approximately 88.28% in the same task.

Citat

"How can we characterize the set of quantum spin-J correlations in the space of general spin-J correlations?"
"Assumptions on the response of physical systems to spacetime symmetries can be used directly in semi-DI protocols for certification."
"In particular, such assumptions are sometimes physically simpler or more meaningful (corresponding to e.g. energy or particle number bounds [10, 18]) than abstract assumptions often made in the field, such as upper bounds on the Hilbert space dimension of the physical system."

Viktiga insikter från

Spin-bounded correlations: rotation boxes within and beyond quantum theory

by Albert Aloy,... på arxiv.org 11-19-2024

https://arxiv.org/pdf/2312.09278.pdf

Djupare frågor

How could the concept of rotation boxes be extended to incorporate other spacetime symmetries beyond SO(2) rotations, and what implications might this have for quantum field theory?

The concept of rotation boxes, which analyze the response of physical systems to spatial rotations around a fixed axis, can be naturally extended to encompass other spacetime symmetries. This generalization could provide deeper insights into the interplay between quantum theory and spacetime structure, potentially even shedding light on the foundations of quantum field theory. Here's how such an extension could be approached:
1. Generalization to other Lie Groups:

From SO(2) to SU(2): The most immediate extension is to consider the full rotation group SU(2) instead of just rotations around a fixed axis (SO(2)). This would involve analyzing how probabilities transform under arbitrary three-dimensional rotations.
Beyond Rotations: The framework can be further generalized to other Lie groups, each representing a different spacetime symmetry:

Translations: Instead of rotations, consider boxes whose input corresponds to spatial translations. This could be particularly relevant for studying quantum field theories on a lattice.
Poincaré Group: Ultimately, one could aim to incorporate the full Poincaré group, which includes rotations, translations, and boosts. This would directly probe the relativistic structure of quantum theory.
2. Implications for Quantum Field Theory:

Constraints on Probabilistic Theories: By studying how different spacetime symmetries constrain the allowed correlations in generalized probabilistic theories, we can gain a deeper understanding of why quantum theory takes the form it does. For instance, we might find that certain symmetries inherently limit the strength of correlations, potentially explaining the limitations imposed by Tsirelson's bound in quantum theory.
Emergent Spacetime: Conversely, this approach could offer insights into how spacetime itself might emerge from a more fundamental pre-geometric theory. If we start with a generalized probabilistic theory without assuming any particular spacetime structure, certain symmetries of the theory might naturally give rise to an effective notion of space and time.
Quantum Gravity: Ultimately, a better understanding of the interplay between quantum theory and spacetime symmetries could provide valuable hints for developing a consistent theory of quantum gravity.
3. Mathematical Formalism:

Representations of Lie Groups: The mathematical framework for this generalization would involve studying representations of the relevant Lie groups on the state spaces of generalized probabilistic theories.
Constraints on Correlations: Analogous to the spin-bounded correlations in rotation boxes, we would analyze how bounds on the "charges" associated with other symmetries (e.g., momentum, energy) constrain the allowed probabilities.

Could there be practical limitations or unforeseen consequences in harnessing these beyond-quantum correlations in real-world applications, even if they are theoretically possible?

While the existence of beyond-quantum correlations, like those exhibited by rotation boxes with higher spins, is fascinating from a theoretical perspective, their practical harnessing for real-world applications presents significant challenges and potential limitations:
1. Fundamental Limits:

Unknown Physical Realizations: Currently, we lack concrete physical systems that demonstrably exhibit these beyond-quantum correlations. The theoretical models, like the generalized probabilistic theories, provide a framework for their description but don't guarantee their existence in nature.
Resource Requirements: Even if such systems exist, accessing and manipulating them might require extraordinary resources or conditions that are currently beyond our technological capabilities.
2. Technological Challenges:

Noise and Decoherence: Real-world systems are inherently noisy, and these beyond-quantum correlations might be extremely fragile to decoherence. Maintaining their coherence long enough for practical applications could be a formidable obstacle.
Control and Measurement: Precisely controlling and measuring these hypothetical systems to exploit their unique correlations would likely demand entirely new experimental techniques and technologies.
3. Unforeseen Consequences:

Computational Complexity: While some aspects of rotation boxes admit efficient characterizations using semidefinite programming, the computational complexity of more general tasks involving beyond-quantum correlations remains largely unexplored. There's a possibility that these correlations might not offer any computational advantages over quantum resources.
Unintended Side Effects: Introducing such exotic correlations into our existing technological infrastructure could have unforeseen and potentially detrimental consequences. For instance, they might interfere with the security of quantum cryptographic protocols or introduce new vulnerabilities in other quantum information processing tasks.
4. Theoretical Uncertainties:

Consistency with Other Physical Laws: It's crucial to ensure that these beyond-quantum correlations, if they exist, are consistent with other fundamental physical laws, such as thermodynamics and general relativity. Violations of these laws could have profound implications for our understanding of the universe.

If we view the universe itself as a giant "rotation box" subject to cosmic symmetries, does this perspective offer any insights into the emergence of quantum phenomena or the limits of our understanding of physical laws?

Viewing the universe as a colossal "rotation box" subject to cosmic symmetries is a captivating thought experiment that could potentially offer intriguing, albeit speculative, insights into the nature of quantum phenomena and the boundaries of our current physical understanding.
1. Emergent Quantumness:

Symmetries as Foundation: Just as specific symmetries within the rotation box framework give rise to the allowed correlations, perhaps the fundamental symmetries of the universe, such as those described by the Poincaré group or even more fundamental groups, play a crucial role in the emergence of quantum phenomena.
Quantumness from Cosmic Play: Imagine the universe as constantly "rotating" and transforming under these cosmic symmetries. The observed quantum behavior of particles and fields could then be a manifestation of these grander, underlying transformations, much like how the probabilities in a rotation box are determined by the applied rotation.
2. Limits of Our Understanding:

Beyond the Standard Model: The Standard Model of particle physics, while remarkably successful, is known to be incomplete. It doesn't incorporate gravity and leaves several fundamental questions unanswered. If the universe operates as a "rotation box" governed by symmetries beyond our current grasp, it could explain the limitations of the Standard Model and point towards new physics.
The Illusion of Locality: Quantum entanglement, with its seemingly "spooky action at a distance," challenges our classical intuitions about locality. Perhaps, within the "cosmic rotation box" paradigm, our notion of locality is merely an emergent property, an approximation that breaks down at the most fundamental levels of reality.
3. Challenges and Open Questions:

Identifying the "Cosmic Input": If the universe is indeed a "rotation box," what constitutes the "input"? Is it related to cosmic time, the expansion of the universe, or some other fundamental process?
Observational Signatures: What observational signatures or experimental tests could we devise to support or refute this "cosmic rotation box" hypothesis?
4. Philosophical Implications:

The Universe as Information: This perspective aligns with the idea that information might be a fundamental constituent of reality. The universe, viewed as a "rotation box," processes information according to its inherent symmetries, giving rise to the physical world we observe.
Redefining Observers: Our role as observers within this "cosmic rotation box" takes on a new meaning. We become participants in the grand cosmic "experiment," our observations intricately intertwined with the fundamental symmetries governing the universe.