Weekly Papers on Quantum Foundations (38)

Authors: Mohamed HatifiRalph WilloxSamuel ColinThomas Durt

Recently, the properties of bouncing oil droplets, also known as “walkers”, have attracted much attention because they are thought to offer a gateway to a better understanding of quantum behaviour. They constitute indeed a macroscopic realization of wave-particle duality, in the sense that their trajectories are guided by a self-generated surrounding wave. The aim of this paper is to develop a phenomenological theory for the behavior of walkers in terms of de Broglie-Bohm and Nelson dynamics. We study in particular how modifications of the de Broglie pilot-wave theory, \`a la Nelson, affect the process of relaxation to quantum equilibrium, and prove an H-theorem for the relaxation to quantum equilibrium under Nelson dynamics. We compare the onset of equilibrium in the Nelson and de Broglie-Bohm approaches and we also propose some simple experiments by which one can test the applicability of our theory to the context of bouncing oil droplets.

Authors: John S. Briggs

An assessment is given as to the extent to which pure unitary evolution, as distinct from environmental decohering interaction, can provide the transition necessary for an observer to interpret perceived quantum dynamics as classical. This has implications for the interpretation of quantum wavefunctions as a characteristic of ensembles or of single particles and the related question of wavefunction collapse.

Crowther, Karen (2018) When do we stop digging? Conditions on a fundamental theory of physics. [Preprint]

Author(s): Ding Jia (贾丁)

There has been a body of work deriving the complex Hilbert-space structure of quantum theory from axioms/principles/postulates to deepen our understanding of quantum theory and to reveal ways to go beyond it to resolve foundational issues. Recent progress in incorporating indefinite causal structure…

[Phys. Rev. A 98, 032112] Published Wed Sep 19, 2018

Publication date: Available online 24 August 2018

Source: Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics

Author(s): R. Hermens

Authors: Yuri BonderCristóbal Corral

It is well known that a theory with explicit Lorentz violation is not invariant under diffeomorphisms. On the other hand, for geometrical theories of gravity, there are alternative transformations, which can be best defined within the first-order formalism, and that can be regarded as a set of improved diffeomorphisms. These symmetries are known as local translations and, among other features, they are Lorentz covariant off shell. It is thus interesting to study if theories with explicit Lorentz violation are invariant under local translations. In this work, an example of such a theory, known as the minimal gravity sector of the Standard Model Extension, is analyzed. Using a robust algorithm, it is shown that local translations are not a symmetry of the theory. It remains to be seen if local translations are spontaneously broken under spontaneous Lorentz violation, which are regarded as a more natural alternative when spacetime is dynamic.

Authors: Alexander M. DalzellAram W. HarrowDax Enshan KohRolando L. La Placa

Quantum computational supremacy arguments, which describe a way for a quantum computer to perform a task that cannot also be done by a classical computer, typically require some sort of computational assumption related to the limitations of classical computation. One common assumption is that the polynomial hierarchy (PH) does not collapse, a stronger version of the statement that P $\neq$ NP, which leads to the conclusion that any classical simulation of certain families of quantum circuits requires time scaling worse than any polynomial in the size of the circuits. However, the asymptotic nature of this conclusion prevents us from calculating exactly how many qubits these quantum circuits must have for their classical simulation to be intractable on modern classical supercomputers. We refine these quantum computational supremacy arguments and perform such a calculation by imposing fine-grained versions of the non-collapse assumption. Each version is parameterized by a constant $a$ and asserts that certain specific computational problems with input size $n$ require $2^{an}$ time steps to be solved by a non-deterministic algorithm. Then, we choose a specific value of $a$ for each version that we argue makes the assumption plausible, and based on these conjectures we conclude that Instantaneous Quantum Polynomial-Time (IQP) circuits with 180 qubits, Quantum Approximate Optimization Algorithm (QAOA) circuits with 360 qubits and boson sampling circuits (i.e. linear optical networks) with 90 photons are large enough for the task of producing samples from their output distributions up to constant multiplicative error to be intractable on current technology.

Authors: David L. Bartley

The significance of the Bohm/de Broglie hidden-particle position in the relativistic regime is addressed, seeking connection to the (orthodox) single-particle Newton-Wigner position. The effect of non-positive excursions of the ensemble density for extreme cases of positive-energy waves is easily computed using an integral of the equations of motion developed here for free spin-0 particles in 1+1 dimensions and is interpreted in terms of virtual-like pair creation and annihilation beneath the Compton wavelength. A Bohm-theoretic description of the acausal explosion of a specific Newton-Wigner-localized state is presented in detail. The presence of virtual pairs found is interpreted as the Bohm picture of the spatial extension beyond single point particles proposed in the 1960s as to why space-like hyperplane dependence of the Newton-Wigner wavefunctions may be needed to achieve Lorentz covariance. For spin-1/2 particles the convective current is speculatively utilized for achieving parity with the spin-0 theory. The spin-0 improper quantum potential is generalized to an improper stress tensor for spin-1/2 particles.

Authors: Marcel ReginattoMichael J. W. Hall

We consider the coupling of quantum fields to classical gravity in the formalism of ensembles on configuration space, a model that allows a consistent formulation of interacting classical and quantum systems. Explicit calculations show that there are solutions for which two quantum fields are in an entangled state, even though their interaction occurs solely via a common classical gravitational field, and that such entangled solutions can evolve from initially unentangled ones. These results support the observation of a previous paper that an observed generation of entanglement would not provide a definitive test of the nonclassical nature of gravity.

Authors: Sina SalekDaniel EblerGiulio Chiribella

Quantum mechanics allows for situations where the relative order between two processes is entangled with a quantum degree of freedom. Here we show that such entanglement can enhance the ability to transmit quantum information over noisy communication channels. We consider two completely dephasing channels, which in normal conditions are unable to transmit any quantum information. We show that, when the two channels are traversed in an indefinite order, a quantum bit sent through them has a 25\% probability to reach the receiver without any error. For partially dephasing channels, a similar advantage takes place deterministically: the amount of quantum information that can travel through two channels in a superposition of orders can be larger than the amount of quantum information that can travel through each channel individually.

Authors: Koji Yasuda

The measurement apparatus proposed in the titled paper, “Energy-Time Uncertainty Relations in Quantum Measurements” [Found. Phys. 46, 1522-1550 (2016)] was examined. A simple proof was presented for the non-existence of the apparatus.


Three recent arguments seek to show that the universal applicability of unitary quantum theory is inconsistent with the assumption that a well-conducted measurement always has a definite physical outcome. In this paper I restate and analyze these arguments. The import of the first two is diminished by their dependence on assumptions about the outcomes of counterfactual measurements. But the third argument establishes its intended conclusion. Even if every well-conducted quantum measurement we ever make will have a definite physical outcome, this argument should make us reconsider the objectivity of that outcome.


We review the argument that latent image formation is a measurement in which the state vector collapses, requiring an enhanced noise parameter in objective reduction models. Tentative observation of a residual noise at this level, plus several experimental bounds, imply that the noise must be colored (i.e., non-white), and hence frame dependent and non-relativistic. Thus a relativistic objective reduction model, even if achievable in principle, would be incompatible with experiment; the best one can do is the non-relativistic CSL model. This negative conclusion has a positive aspect, in that the non-relativistic CSL reduction model evades the argument leading to the Conway–Kochen “Free Will Theorem”.

Standard quantum theory explains the behaviour of microscopic things like electrons and atoms. It should also, in principle apply to larger objects – but it might not
Azhar, Feraz and Loeb, Abraham (2018) Gauging Fine-Tuning. [Preprint]
de Ronde, Christian and Massri, Cesar (2018) A New Objective Definition of Quantum Entanglement as Potential Coding of Intensive and Effective Relations. [Preprint]

Reimagining of Schrödinger’s cat breaks quantum mechanics — and stumps physicists

Reimagining of Schrödinger’s cat breaks quantum mechanics — and stumps physicists, Published online: 18 September 2018; doi:10.1038/d41586-018-06749-8

In a multi-‘cat’ experiment, the textbook interpretation of quantum theory seems to lead to contradictory pictures of reality, physicists claim.

Quantum theory cannot consistently describe the use of itself

Quantum theory cannot consistently describe the use of itself, Published online: 18 September 2018; doi:10.1038/s41467-018-05739-8

Quantum mechanics is expected to provide a consistent description of reality, even when recursively describing systems contained in each other. Here, the authors develop a variant of Wigner’s friend Gedankenexperiment where each of the current interpretations of QM fails in giving a consistent description.

An inconsistent friend

An inconsistent friend, Published online: 18 September 2018; doi:10.1038/s41567-018-0293-7

Are there limits to the applicability of textbook quantum theory? Experiments haven’t found any yet, but a new theoretical analysis shows that treating your colleagues as quantum systems might be a step too far.

Roberts, Bryan W. (2018) Time Reversal. [Preprint]

Author(s): Tatsuma Nishioka

In this review the entanglement and Renyi entropies in quantum field theory are described from different points of view, including the perturbative approach and holographic dualities. The applications of these results to constraining renormalization group flows are presented effectively and illustrated with a variety of examples.

[Rev. Mod. Phys. 90, 035007] Published Mon Sep 17, 2018

Calosi, Claudio and Morganti, Matteo (2018) Interpreting Quantum Entanglement: Steps Towards Coherentist Quantum Mechanics. The British Journal for the Philosophy of Science.
French, Steven (2018) Between Factualism and Substantialism: Structuralism as a Third Way. [Preprint]

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