arXiv subjects

Adrian Kent

Publications and source records attributed to Adrian Kent.

At least 19 recordsLinked to original sources

arXiv2026

Fundamental Physics, Existential Risks and Human Futures

Over the past 25 years, I have been involved in some intriguing developments in the foundations of physics, exploring the quantum reality problem, the relationship between quantum theory and gravity and the interplay between consciousness and physical laws. These investigations make it plausible that we will find physics beyond quantum theory, potentially including both new evolution laws and new types of measurement. There is also a significant chance they could have potentially transformative impact on information processing and on the development of and our future with AI.

quant-ph↗

arXiv2026

The Grasshopper Problem on the Sphere

The spherical grasshopper problem is a geometric optimization problem that arises in the context of Bell inequalities and can be interpreted as identifying the best local hidden variable approximation to quantum singlet correlations for measurements along random axes separated by a fixed angle. In a parallel publication [arXiv:2504.20953], we presented numerical solutions for this problem and explained their significance for singlet simulation and testing. In this companion paper, we describe in detail the geometric and computational framework underlying these results. We examine the role of spherical discretization and compare three natural variants of the problem: antipodal complementary lawns, antipodal independent lawns, and non-antipodal complementary lawns. We analyze the geometric structure of the corresponding optimal lawn configurations and interpret it in terms of a spherical harmonics expansion. We also discuss connections to other physical models and to classical problems in geometric probability.

quant-ph↗

arXiv2025

The thermodynamics of readout devices and semiclassical gravity

We analyse the common claim that nonlinear modifications of quantum theory necessarily violate the second law of thermodynamics. We focus on hypothetical extensions of quantum theory that contain readout devices. These black boxes provide a classical description of quantum states without perturbing them. They allow quantum state cloning, though in a way consistent with the relativistic no-signalling principle. We review the existence of such devices in the context of Moller-Rosenfeld semiclassical gravity, which postulates that the gravitational field remains classical and is sourced by the expectation value of a quantum energy-momentum tensor. We show that the definition of information in the models examined in this paper deviates from that given by von Neumann entropy, and that claims of second law violations based on the distinguishability of non-orthogonal states or on violations of uncertainty principles fail to hold in such theories.

gr-qc↗

arXiv2025

A Spin-Based Pathway to Testing the Quantum Nature of Gravity

A key open problem in physics is the correct way to combine gravity (described by general relativity) with everything else (described by quantum mechanics). This problem suggests that general relativity and possibly also quantum mechanics need fundamental corrections. Most physicists expect that gravity should be quantum in character, but gravity is fundamentally different to the other forces because it alone is described by spacetime geometry. Experiments are needed to test whether gravity, and hence space-time, is quantum or classical. We propose an experiment to test the quantum nature of gravity by checking whether gravity can entangle two micron-sized crystals. A pathway to this is to create macroscopic quantum superpositions of each crystal first using embedded spins and Stern-Gerlach forces. These crystals could be nanodiamonds containing nitrogen-vacancy (NV) centres. The spins can subsequently be measured to witness the gravitationally generated entanglement. This is based on extensive theoretical feasibility studies and experimental progress in quantum technology. The eventual experiment will require a medium-sized consortium with excellent suppression of decoherence including vibrations and gravitational noise. In this white paper, we review the progress and plans towards realizing this. While implementing these plans, we will further explore the most macroscopic superpositions that are possible, which will test theories that predict a limit to this.

quant-ph↗

arXiv2025

Optimal Local Simulations of a Quantum Singlet

Bell's seminal work showed that no local hidden variable (LHV) model can fully reproduce the quantum correlations of a two-qubit singlet state. His argument and later developments by Clauser et al. effectively rely on gaps between the anticorrelations achievable by classical models and quantum theory for projective measurements along randomly chosen axes separated by a fixed angle. However, the size of these gaps has to date remained unknown. Here we numerically determine the LHV models maximizing anticorrelations for random axes separated by any fixed angle, by mapping the problem onto ground state configurations of fixed-range spin models. We identify angles where this gap is largest and thus best suited for Bell tests. These findings enrich the understanding of Bell non-locality as a physical resource in quantum information theory and quantum cryptography.

quant-ph↗

arXiv2024

Mixture equivalence principles and post-quantum theories of gravity

We examine the mixture equivalence principle (MEP), which states that proper and improper mixed states with the same density matrix are always experimentally indistinguishable, and a weaker version, which states that this is sometimes true in gravity theories. We point out that Moller-Rosenfeld semiclassical gravity violates the weak MEP and that nonlinear extensions of quantum mechanics violate the MEP. We further demonstrate that modifications of the Born rule in quantum theory also typically violate the MEP. We analyse such violations in the context of thermal baths, where proper and improper thermal states induce different physical situations. This has significant implications in the context of black hole physics. We argue that Moller-Rosenfeld semiclassical gravity is not the semiclassical limit of quantum gravity in the context of black hole spacetimes, even in the presence of $N\gg1$ matter fields.

gr-qc↗

arXiv2024

Experimental practical quantum tokens with transaction time advantage

Quantum money is the first invention in quantum information science, promising advantages over classical money by simultaneously achieving unforgeability, user privacy, and instant validation. However, standard quantum money relies on quantum memories and long-distance quantum communication, which are technologically extremely challenging. Quantum "S-money" tokens eliminate these technological requirements while preserving unforgeability, user privacy, and instant validation. Here, we report the first full experimental demonstration of quantum S-tokens, proven secure despite errors, losses and experimental imperfections. The heralded single-photon source with a high system efficiency of 88.24% protects against arbitrary multi-photon attacks arising from losses in the quantum token generation. Following short-range quantum communication, the token is stored, transacted, and verified using classical bits. We demonstrate a transaction time advantage over intra-city 2.77 km and inter-city 60.54 km optical fibre networks, compared with optimal classical cross-checking schemes. Our implementation demonstrates the practicality of quantum S-tokens for applications requiring high security, privacy and minimal transaction times, like financial trading and network control. It is also the first demonstration of a quantitative quantum time advantage in relativistic cryptography, showing the enhanced cryptographic power of simultaneously considering quantum and relativistic physics.

quant-ph↗

arXiv2024

Do small massive superpositions necessarily significantly entangle with gravity?

Christodoulou and Rovelli (CR) [1] have argued that a Bose et al.-Marletto-Vedral (BMV) experiment that confirmed the quantum nature of gravity would give laboratory evidence for a quantum superposition of spacetime geometries created in the course of the experiment. Hanif et al. [2] have argued that mass interferometers can be used to test whether gravity acts as a quantum entity when measured. We note that not all quantum models of gravity imply that mass superpositions necessarily become significantly entangled with any degrees of freedom of the gravitational field during the experiments discussed.

gr-qc↗

arXiv2024

Should we necessarily treat masses as localized when analysing tests of quantum gravity?

Recently proposed ``table-top tests of quantum gravity'' involve creating, separating and recombining superpositions of masses at non-relativistic speeds. The general expectation is that these generate superpositions of gravitational fields via the Newtonian potential. Analyses suggest that negligible gravitational radiation is generated if the interference experiments involve sufficiently small accelerations. One way of thinking about this is that matter and the static gravitational field are temporarily entangled and then disentangled. Another is that the static gravitational field degrees of freedom are dependent on the matter and do not belong to a separate Hilbert space, and that there is always negligible entanglement between matter and dynamical gravitational degrees of freedom. In this last picture, localized masses effectively become infinitely extended objects, inseparable from their Newtonian potentials. While this picture seems hard to extend to a fully relativistic theory of non-quantum gravity, it has significant implications for analyses of how or whether BMV and other non-relativistic experiments might test the quantum nature of gravity. If the masses in a BMV experiment are regarded as occupying overlapping regions (or indeed all of space), explaining how they become entangled does not require that their gravitational interaction involves quantum information exchange. On this view, while the experiments test gravity in a regime where quantum theory describes all relevant matter degrees of freedom, they do not necessarily test its quantum nature. It might be argued that no plausible explanation other than quantum gravity could be consistent both with these experiments and with relativity. But this relies on further theoretical assumptions and is weaker than claiming direct evidence for quantum gravitational interactions from the experiments alone.

gr-qc↗

arXiv2024

Time and distance constraints for mass and charge interferometry

We reanalyse and extend constraints on mass and charge interferometry identified by Mari et al. (2016). We show that their constraint on the time required for coherent interference can be extended by a factor of two. We extend their analysis to consider experiments in which one interferometer measures gravitational or electric fields generated by another. We note that these analyses imply a maximum separation between a mass or charge interferometer and a decohering gravitational or electric field measurement that can be carried out without backreaction.

quant-ph↗

arXiv2023

Origin of Symmetry Breaking in the Grasshopper Model

The planar grasshopper problem, originally introduced in (Goulko & Kent 2017 Proc. R. Soc. A 473, 20170494), is a striking example of a model with long-range isotropic interactions whose ground states break rotational symmetry. In this work we analyze and explain the nature of this symmetry breaking with emphasis on the importance of dimensionality. Interestingly, rotational symmetry is recovered in three dimensions for small jumps, which correspond to the non-isotropic cogwheel regime of the two-dimensional problem. We discuss simplified models that reproduce the symmetry properties of the original system in N dimensions. For the full grasshopper model in two dimensions we obtain quantitative predictions for optimal perturbations of the disk. Our analytical results are confirmed by numerical simulations.

math-ph↗

arXiv2023

Towards practical quantum position verification

We discuss protocols for quantum position verification schemes based on the standard quantum cryptographic assumption that a tagging device can keep classical data secure [Kent, 2011]. Our schemes use a classical key replenished by quantum key distribution. The position verification requires no quantum communication or quantum information processing. The security of classical data makes the schemes secure against non-local spoofing attacks that apply to schemes that do not use secure tags. The schemes are practical with current technology and allow for errors and losses. We describe how a proof-of-principle demonstration might be carried out.

quant-ph↗

arXiv2023

The measurement postulates of quantum mechanics are not redundant

Masanes, Galley and M\"uller [1] argue that the measurement postulates of non-relativistic quantum mechanics follow from the structural postulates together with an assumption they call the "possibility of state estimation". Their argument also relies on what they term a "theory-independent characterization of measurements for single and multipartite systems". We refute their conclusion, giving explicit examples of non-quantum measurement and state update rules that satisfy all their assumptions. We also show that their "possibility of state estimation" assumption is neither necessary nor sufficient to ensure a sensible notion of state estimation within a theory whose states are described by the quantum formalism. We further show their purportedly "theory-independent" characterization assumes several properties of quantum measurements that exclude plausible alternative types of measurement. We illustrate all these points with specific alternative measurement postulates and post-measurement state update rules. We conclude that, contrary to some folklore, quantum mechanics is by no means an island in theory-space. It can consistently be extended by rules for obtaining information about quantum states other than via POVMs. Whether such rules are realised in nature, for example in linking quantum theory and gravity, is an empirical question that cannot be resolved by theoretical analysis alone.

quant-ph↗

arXiv2023

Friendly thoughts on thoughtful friendliness

We discuss Wiseman, Cavalcanti and Rieffel's "thoughtful" local friendliness no-go theorem and the experimental programme they propose to test local friendliness inequalities. We argue that, to prove the theorem, the assumptions need to be strengthened to exclude the possibility of variable numbers of thoughtful agents existing in different phases of the experiment. We argue further that this possibility may arise naturally, even in one-world versions of quantum theory. We also query whether the motivations they give for their assumptions hold up well under their definition of "thoughtfulness" as displaying human-level cognitive ability, and suggest that their justification requires replacing "thoughtfulness" by "consciousness" or "conscious thoughtfulness".

quant-ph↗

arXiv2022

Comparing Singlet Testing Schemes

We compare schemes for testing whether two parties share a two-qubit singlet state. The first, standard, scheme tests Braunstein-Caves (or CHSH) inequalities, comparing the correlations of local measurements drawn from a fixed finite set against the quantum predictions for a singlet. The second, alternative, scheme tests the correlations of local measurements, drawn randomly from the set of those that are $\theta$-separated on the Bloch sphere, against the quantum predictions. We formulate each scheme as a hypothesis test and then evaluate the test power in a number of adversarial scenarios involving an eavesdropper altering or replacing the singlet qubits. We find the `random measurement' test to be superior in most natural scenarios.

quant-ph↗

arXiv2022

Environmental Collapse Models

We propose dynamical collapse models in which the stochastic collapse terms affect only photons and/or gravitons. In principle, isolated systems comprising only massive particles could evolve unitarily indefinitely in such models. In practice, since photons and gravitons are ubiquitous and scatter from massive particles, dynamical collapses of the former will effectively induce collapses of the latter. In non-relativistic models in which particle number is conserved and interactions are modelled by classical potentials, massive systems can be modelled as collections of elementary massive particles bound by potentials, interacting with an environment of photons and gravitons. In this picture, although the photon and/or graviton collapse dynamics effectively localize massive systems, these collapses take the effective form of approximate measurements on the environment whose effect on the massive systems is indirect. We argue that these environmental collapse models, like standard mass-dependent spontaneous localisation models, may be consistent with quantum experiments on microscopic systems while predicting very rapid effective collapse of macroscopic massive systems, and hence a potential solution to the quantum measurement problem. However, the models considered here have different experimental signatures from standard mass-dependent spontaneous localisation models. For example, they predict no deviations from standard quantum interferometry for mesoscopic systems of massive particles isolated from a decohering environment, nor do they predict anomalous spontaneous emission of radiation from isolated matter of the type prediction by standard mass-dependent spontaneous localization models. New experiments and analyses are required to obtain empirical bounds on the decoherence rate in our models.

quant-ph↗

arXiv2021

Testing the nonclassicality of spacetime: what can we learn from Bell-Bose et al.-Marletto-Vedral experiments?

The Bose et al.-Marletto-Vedral (BMV) experiment [S. Bose et al., Phys. Rev. Lett. 119, 240401 (2017); C. Marletto and V. Vedral, Phys. Rev. Lett. 119, 240402 (2017)] aims to prove that spacetime is nonclassical by observing entanglement generated by gravity. However, local hidden variable theories (LHVTs) can simulate the entangled correlations. We propose to extend the entanglement generated by the BMV experiment to distant quantum particles in a Bell experiment. Violating a Bell inequality would rule out LHVTs, providing a stronger proof of the nonclassicality of spacetime than the BMV proposal.

quant-ph↗

arXiv2021

Practical quantum tokens without quantum memories and experimental tests

Unforgeable quantum money tokens were the first invention of quantum information science, but remain technologically challenging as they require quantum memories and/or long distance quantum communication. More recently, virtual 'S-money' tokens were introduced. These are generated by quantum cryptography, do not require quantum memories or long distance quantum communication, and yet in principle guarantee many of the security advantages of quantum money. Here, we describe implementations of S-money schemes with off-the-shelf quantum key distribution technology, and analyse security in the presence of noise, losses, and experimental imperfection. Our schemes satisfy near instant validation without cross-checking. We show that, given standard assumptions in mistrustful quantum cryptographic implementations, unforgeability and user privacy could be guaranteed with attainable refinements of our off-the-shelf setup. We discuss the possibilities for unconditionally secure (assumption-free) implementations.

quant-ph↗