arXiv subjects

Adrian Kent

Publications and source records attributed to Adrian Kent.

At least 55 records · Page 3Linked to original sources

arXiv2018

Semi-quantum Gravity and Testing Gravitational Bell Non-locality

Semi-classical gravity attempts to define a hybrid theory in which a classical gravitational field is coupled to a unitarily evolving quantum state. Although semi-classical gravity is inconsistent with observation, a viable theory of this type might be appealing, since it potentially might preserve the basic features of our two most successful theories while unifying them. It might also offer a natural solution to the quantum measurement problem. I explore the scope for such "semi-quantum" hybrid theories, and note some interesting, though daunting, constraints. Consistency with observation generally requires pyschophysical parallelism with the classical gravitational field rather than the quantum matter. Solvability suggests the gravitational field at a point should be determined by physics in its past light cone, which requires local hidden variables and predicts anomalously non-Newtonian gravitational fields. These predictions could be tested by low energy, although technologically challenging, experiments in which the Bell non-locality of the gravitational field is verified by direct measurement.

gr-qc↗

arXiv2018

Are There Testable Discrete Poincaré Invariant Physical Theories?

In a model of physics taking place on a discrete set of points that approximates Minkowski space, one might perhaps expect there to be an empirically identifiable preferred frame. However, the work of Dowker, Bombelli, Henson, and Sorkin might be taken to suggest that random sprinklings of points in Minkowski space define a discrete model that is provably Poincaré invariant in a natural sense. We examine this possibility here. We argue that a genuinely Poincaré invariant model requires a probability distribution on sprinklable sets -- Poincaré orbits of sprinklings -- rather than individual sprinklings. The corresponding $σ$-algebra contains only sets of measure zero or one. This makes testing the hypothesis of discrete Poincaré invariance problematic, since any local violation of Poincaré invariance, however gross and large scale, is possible, and cannot be said to be improbable. We also note that the Bombelli-Henson-Sorkin argument, which rules out constructions of preferred timelike directions for typical sprinklings, is not sufficient to establish full Lorentz invariance. For example, once a pair of timelike separated points is fixed, a preferred spacelike direction {\it can} be defined for a typical sprinkling, breaking the remaining rotational invariance.

gr-qc↗

arXiv2018

Beable-Guided Quantum Theories: Generalising Quantum Probability Laws

We introduce the idea of a {\it beable-guided quantum theory}. Beable-guided quantum theories (BGQT) are generalisations of quantum theory, inspired by Bell's concept of beables. They modify the quantum probabilities for some specified set of fundamental events, histories, or other elements of quasiclassical reality by probability laws that depend on the realised configuration of beables. For example, they may define an additional probability weight factor for a beable configuration, independent of the quantum dynamics. BGQT can be fitted to observational data to provide foils against which to compare explanations based on standard quantum theory. For example, a BGQT could, in principle, characterise the effects attributed to dark energy or dark matter, or any other deviation from the predictions of standard quantum dynamics, without introducing extra fields or a cosmological constant. The complexity of the beable-guided theory would then parametrise how far we are from a standard quantum explanation. Less conservatively, we give reasons for taking suitably simple beable-guided quantum theories as serious phenomenological theories in their own right. Among these are that cosmological models defined by BGQT might in fact fit the empirical data better than any standard quantum explanation, and that BGQT suggest potentially interesting non-standard ways of coupling quantum matter to gravity.

quant-ph↗

arXiv2018

Lorentzian Quantum Reality: Postulates and Toy Models

We describe postulates for a novel realist version of relativistic quantum theory or quantum field theory in Minkowski space or other background spacetimes with suitable asymptotic properties. We illustrate their application in toy models.

quant-ph↗

arXiv2018

Quanta and Qualia

I sketch a line of thought about consciousness and physics that gives some motivation for the hypothesis that conscious observers deviate - perhaps only very subtly and slightly - from quantum dynamics. Although it is hard to know just how much credence to give this line of thought, it does motivate a stronger and more comprehensive programme of quantum experiments involving quantum observers.

quant-ph↗

arXiv2018

Dualities and Twins: Reflections on Hapgood

These programme notes were written for a production of "Hapgood" at the Hampstead Theatre, London in December 2015. I thank Sir Tom Stoppard for helpful suggestions.

physics.pop-ph↗

arXiv2017

Quantum Reality via Late Time Photodetection

We further investigate postulates for realist versions of relativistic quantum theory and quantum field theory in Minkowski space and other background space-times. According to these postulates, quantum theory is supplemented by local variables that depend on possible outcomes of hypothetical measurements on the late time electromagnetic field in spacelike separated regions. We illustrate the implications in simple examples using photon wave mechanics, and discuss possible extensions to quantum field theory.

quant-ph↗

arXiv2017

Knowledge-Concealing Evidencing of Knowledge about a Quantum State

Bob has a black box that emits a single pure state qudit which is, from his perspective, uniformly distributed. Alice wishes to give Bob evidence that she has knowledge about the emitted state while giving him little or no information about it. We show that zero-knowledge evidencing of such knowledge is impossible in quantum relativistic protocols, extending a previous result of Horodecki et al.. We also show that no such protocol can be both sound and complete. We present a new quantum relativistic protocol which we conjecture to be close to optimal in security against Alice and which reveals little knowledge to Bob, for large dimension $d$. We analyse its security against general attacks by Bob and restricted attacks by Alice.

quant-ph↗

arXiv2017

The grasshopper problem

We introduce and physically motivate the following problem in geometric combinatorics, originally inspired by analysing Bell inequalities. A grasshopper lands at a random point on a planar lawn of area one. It then jumps once, a fixed distance $d$, in a random direction. What shape should the lawn be to maximise the chance that the grasshopper remains on the lawn after jumping? We show that, perhaps surprisingly, a disc shaped lawn is not optimal for any $d>0$. We investigate further by introducing a spin model whose ground state corresponds to the solution of a discrete version of the grasshopper problem. Simulated annealing and parallel tempering searches are consistent with the hypothesis that for $ d < π^{-1/2}$ the optimal lawn resembles a cogwheel with $n$ cogs, where the integer $n$ is close to $ π( \arcsin ( \sqrtπ d /2 ) )^{-1}$. We find transitions to other shapes for $d \gtrsim π^{-1/2}$.

cond-mat.stat-mech↗

arXiv2016

Secure Quantum Signatures Using Insecure Quantum Channels

Digital signatures are widely used in modern communication to guarantee authenticity and transferability of messages, The security of currently used classical schemes relies on computational assumptions. We present a quantum signature scheme that does not require trusted quantum channels. We prove that it is unconditionally secure against the most general coherent attacks, and show that it requires the transmission of significantly fewer quantum states than previous schemes. We also show that the quantum channel noise threshold for our scheme is less strict than for distilling a secure key using quantum key distribution. This shows that direct quantum signature schemes can be preferable to signature schemes relying on secret shared keys generated using quantum key distribution.

quant-ph↗

arXiv2015

A Quantum Paradox of Choice and Purported Classical Analogues

We recently considered the task of summoning an unknown quantum state and proved necessary and sufficient conditions for Alice to be able to guarantee to complete the task when there may be several possible calls, of which she need only respond to one. We showed that these are strictly stronger conditions than those previously established by Hayden and May for the case where Alice knows there will only be one call. We introduced the concept of a {\it quantum paradox of choice} to summarize the implications of these results: Alice is given more options to complete our version of the task, yet one can easily construct examples where our version is impossible and the apparently simpler version considered by Hayden-May is possible. Finkelstein has argued that one can identify analogous classical paradoxes of choice in a relativistic setting. We examine Finkelstein's proposed classical tasks and explain why they seem to us disanalogous.

quant-ph↗

arXiv2015

A Quantum Paradox of Choice: More Freedom Makes Summoning a Quantum State Harder

The properties of quantum information in space-time can be investigated by studying operational tasks. In one such task, summoning, an unknown quantum state is supplied at one point, and a call is made at another for it to be returned at a third. Hayden-May recently proved necessary and sufficient conditions for guaranteeing successful return of a summoned state for finite sets of call and return points when there is a guarantee of at most one summons. We prove necessary and sufficient conditions when there may be several possible summonses and complying with any one constitutes success. We show there is a "quantum paradox of choice" in summoning: the extra freedom in completing the task makes it strictly harder. This intriguing result has practical applications for distributed quantum computing and cryptography and also implications for our understanding of relativistic quantum information and its localization in space-time.

quant-ph↗

arXiv2015

A critical look at risk assessments for global catastrophes

Recent papers by Busza et al. (BJSW) and Dar et al. (DDH) argue that astrophysical data can be used to establish small bounds on the risk of a "killer strangelet" catastrophe scenario in the RHIC and ALICE collider experiments. DDH and other commentators (initially including BJSW) suggested that these empirical bounds alone do give sufficient reassurance. This seems unsupportable when the bounds are expressed in terms of expected cost -- a good measure, according to standard risk analysis arguments. For example, DDH's main bound, $p_{\rm catastrophe} < 2 \times 10^{-8}$, implies only that the expectation value of the number of deaths is bounded by 120. This paper reappraises the DDH and BJSW risk bounds by comparing risk policy in other areas. For example, it is noted that, even if highly risk tolerant assumptions are made and no value is placed on the lives of future generations, a catastrophe risk no higher than $\approx 10^{-15}$ per year would be required for consistency with established policy for radiation hazard risk minimization. It is concluded that the costs of small risks of catastrophe have been significantly underestimated by BJSW (initially), by DDH and by other commentators. Lessons for future policy are proposed.

hep-ph↗

arXiv2015

Device-Independent Relativistic Quantum Bit Commitment

We examine the possibility of device-independent relativistic quantum bit commitment. We note the potential threat of {\it location attacks}, in which the behaviour of untrusted devices used in relativistic quantum cryptography depends on their space-time location. We describe relativistic quantum bit commitment schemes that are immune to these attacks, and show that these schemes offer device-independent security against hypothetical post-quantum adversaries subject only to the no-signalling principle. We compare a relativistic classical bit commitment scheme with similar features, and note some possible advantages of the quantum schemes.

quant-ph↗

arXiv2015

Deterministic Relativistic Quantum Bit Commitment

We describe new unconditionally secure bit commitment schemes whose security is based on Minkowski causality and the monogamy of quantum entanglement. We first describe an ideal scheme that is purely deterministic, in the sense that neither party needs to generate any secret randomness at any stage. We also describe a variant that allows the committer to proceed deterministically, requires only local randomness generation from the receiver, and allows the commitment to be verified in the neighbourhood of the unveiling point. We show that these schemes still offer near-perfect security in the presence of losses and errors, which can be made perfect if the committer uses an extra single random secret bit. We discuss scenarios where these advantages are significant.

quant-ph↗

arXiv2014

Bloch sphere colourings and Bell inequalities

We consider here the predictions of quantum theory and local hidden variables for the correlations obtained by measuring a pair of qubits by projections defined by randomly chosen axes separated by a given angle θ. The predictions of local hidden variable models for projective measurements on qubits correspond to binary colourings of the Bloch sphere with antipodal points oppositely coloured. We prove Bell inequalities separating the predictions of all local hidden variable models from the singlet correlations predicted by quantum theory for all θin the range 0 < θ< π/3. We raise and explore the possibility of proving stronger Bell inequalities directly from optimization results on sphere colourings. In particular, we explore strong and weak forms of the hemispherical colouring maximality hypothesis (HCMH) that, for a continuous range of θ> 0, the maximum LHV anti-correlation is obtained by assigning to each qubit a colouring with one hemisphere black and the other white. Our results show that hemispherical colourings are near-optimal for small θ; we also describe numerical tests consistent with the HCMH that bound the range of θ. Finally, we note proofs of related results for binary colourings of R^n.

quant-ph↗

arXiv2014

Does it Make Sense to Speak of Self-Locating Uncertainty in the Universal Wave Function? Remarks on Sebens and Carroll

Following a proposal of Vaidman, Sebens and Carroll have argued that in Everettian (i.e. purely unitary) quantum theory, observers are uncertain, before they complete their observation, about which Everettian branch they are on. They argue further that this solves the problem of making sense of probabilities within Everettian quantum theory, even though the theory itself is deterministic. We note some problems with these arguments.

gr-qc↗

arXiv2014

Quantum digital signatures with quantum key distribution components

Digital signatures guarantee the authenticity and transferability of messages, and are widely used in modern communication. The security of currently used classical digital signature schemes, however, relies on computational assumptions. In contrast, quantum digital signature (QDS) schemes offer information-theoretic security guaranteed by the laws of quantum mechanics. We present two QDS protocols which have the same experimental requirements as quantum key distribution, which is already commercially available. We also present the first security proof for any QDS scheme against coherent forging attacks.

quant-ph↗