# Weekly Papers on Quantum Foundations (3)

Classical and Quantum Measurement Theory. (arXiv:2201.04667v1 [quant-ph])

Classical and quantum measurement theories are usually held to be different because the algebra of classical measurements is commutative, however the Poisson bracket allows noncommutativity to be added naturally. After we introduce noncommutativity into classical measurement theory, we can also add quantum noise, differentiated from thermal noise by Poincar\’e invariance. With these two changes, the extended classical and quantum measurement theories are equally capable, so we may speak of a single “measurement theory”. The reconciliation of general relativity and quantum theory has been long delayed because classical and quantum systems have been thought to be very different, however this unification allows us to discuss a unified measurement theory for geometry in physics.

Wave-particle duality of light appearing in the intensity interferometric situation. (arXiv:2201.04790v1 [quant-ph])

We show a wave-particle duality of light and its complementary relation in the context of the intensity interference measured by intensity correlation measurement, especially for the case of the second-order intensity interference observed in the Hong-Ou-Mandel interferometer. Different from the complementary relation appearing in the interference based on the phase coherence like in the Young’s double-slit interferometer, the complementary relation in the intensity interference has a gap between classical and nonclassical lights. This reveals a new nonclassical nature of light where both wave and particle properties are classically understandable. We further extend the wave-particle duality and the complementarity to higher-order intensity interferometric situations.

Towards real-world quantum networks: a review. (arXiv:2201.04802v1 [quant-ph])

Quantum networks play an extremely important role in quantum information science, with application to quantum communication, computation, metrology and fundamental tests. One of the key challenges for implementing a quantum network is to distribute entangled flying qubits to spatially separated nodes, at which quantum interfaces or transducers map the entanglement onto stationary qubits. The stationary qubits at the separated nodes constitute quantum memories realized in matter while the flying qubits constitute quantum channels realized in photons. Dedicated efforts around the world for more than twenty years have resulted in both major theoretical and experimental progress towards entangling quantum nodes and ultimately building a global quantum network. Here, we review the development of quantum networks and the experimental progress over the past two decades leading to the current state of the art for generating entanglement of quantum nodes based on various physical systems such as single atoms, cold atomic ensembles, trapped ions, diamonds with Nitrogen-Vacancy centers, solid-state host doped with rare-earth ions, etc. Along the way we discuss the merits and compare the potential of each of these systems towards realizing a quantum network.

Fundamental limits of superconducting quantum computers. (arXiv:2201.05114v1 [quant-ph])

The Continuous Spontaneous Localization (CSL) model is an alternative formulation of quantum mechanics which introduces a noise coupled non linearly to the wave function to account for its collapse. We consider CSL effects on quantum computers made of superconducting transmon qubits. As a direct effect CSL reduces quantum superpositions of the computational basis states of the qubits: we show the reduction rate to be negligibly small. However, an indirect effect of CSL, dissipation induced by the noise, also leads transmon qubits to decohere, by generating additional quasiparticles. Since the decoherence rate of transmon qubits depends on the quasiparticle density, by computing their generation rate induced by CSL, we can estimate the corresponding quasiparticle density and thus the limit set by CSL on the performances of transmon quantum computers. We show that CSL could spoil the quantum computation of practical algorithms on large devices. We further explore the possibility of testing CSL effects on superconducting devices.

An introduction to PT-symmetric quantum mechanics — time-dependent systems. (arXiv:2201.05140v1 [quant-ph])

I will provide a pedagogical introduction to non-Hermitian quantum systems that are PT-symmetric, that is they are left invariant under a simultaneous parity transformation (P) and time-reversal (T). I will explain how generalised versions of this antilinear symmetry can be utilised to explain that these type of systems possess real eigenvalue spectra in parts of their parameter spaces and how to set up a consistent quantum mechanical framework for them that enables a unitary time-evolution. In the second part I will explain how to extend this framework to explicitly time-dependent Hamiltonian systems and report in particular on recent progress made in this context. I will explain how to construct the essential key quantity in this framework, the time-dependent Dyson map and metric and solutions to the time-dependent Schr\”odinger equation, in an algebraic fashion, using time-dependent Darboux transformations, utilising Lewis-Riesenfeld invariants, point transformations and some approximation methods. I comment on the ambiguities of this metric and demonstrate that this can even lead to infinite series of metric operators. I conclude with some applications to PT-symmetrically coupled oscillators, demonstrate the equivalence of the time-dependent double wells and unstable anharmonic oscillators and show how the unphysical PT$-symmetrically broken regions in the parameter space for the time-independent theory becomes physical in the explicitly time-dependent systems. I discuss how this leads to a prolongation of the otherwise rapidly decaying von Neumann entropy. The so-called sudden death of the entropy is stopped at a finite value. Consensus between Epistemic Agents is Difficult. (arXiv:2201.04642v1 [physics.soc-ph]) We introduce an epistemic information measure between two data streams, that we term$influence$. Closely related to transfer entropy, the measure must be estimated by epistemic agents with finite memory resources via sampling accessible data streams. We show that even under ideal conditions, epistemic agents using slightly different sampling strategies might not achieve consensus in their conclusions about which data stream is influencing which. As an illustration, we examine a real world data stream where different sampling strategies result in contradictory conclusions, explaining why some politically charged topics might exist due to purely epistemic reasons irrespective of the actual ontology of the world. On the Consilience between QBism and Phenomenology. (arXiv:2201.04734v1 [quant-ph]) Authors: Hans Christian von Baeyer Two decades after its creation, the interpretation of quantum mechanics called QBism is entering a new phase. Since it shares a personalist, subjective world-view with phenomenology, the philosophical study of human experience, there is a growing interest in the relationship between the two subjects. I call attention to the little-known philosopher Samuel Todes, whose phenomenology focused on the essential role of the human body in our understanding of the world. After reviewing this radical proposal, and illustrating it with some examples, I recommend it as an interesting mediator between the communities of physicists and phenomenologists. In addition, I argue that it may prove useful for promoting the public understanding of QBism. Chris Isham: mentor, colleague, friend. (arXiv:2112.13722v2 [physics.hist-ph] UPDATED) Authors: M. J. Duff Celebrating fifty years of collaboration and friendship with Chris Isham. Symmetries Near the Horizon. (arXiv:1904.12820v2 [hep-th] UPDATED) Authors: Henry W. LinJuan MaldacenaYing Zhao We consider a nearly-AdS$_2$gravity theory on the two-sided wormhole geometry. We construct three gauge-invariant operators in NAdS which move bulk matter relative to the dynamical boundaries. In a two-sided system, these operators satisfy an SL(2) algebra (up to non-perturbative corrections). In a semiclassical limit, these generators act like SL(2) transformations of the boundary time, or conformal symmetries of the two sided boundary theory. These can be used to define an operator-state mapping. A particular large N and low temperature limit of the SYK model has precisely the same structure, and this construction of the exact generators also applies. We also discuss approximate, but simpler, constructions of the generators in the SYK model. These are closely related to the “size” operator and are connected to the maximal chaos behavior captured by out of time order correlators. Can dark energy emerge from a varying$G$and spacetime geometry?. (arXiv:2201.04629v1 [gr-qc]) The accelerated expansion of the Universe implies the existence of an energy contribution known as dark energy. Associated with the cosmological constant in the standard model of cosmology, the nature of this dark energy is still unknown. We will discuss an alternative gravity model in which this dark energy contribution emerges naturally, as a result of allowing for a time-dependence on the gravitational constant,$G$, in Einstein’s Field Equations. With this modification, Bianchi’s identities require an additional tensor field to be introduced so that the usual conservation equation for matter and radiation is satisfied. The equation of state of this tensor field is obtained using additional constraints, coming from the assumption that this tensor field represents the space-time response to the variation of$G$. We will also present the predictions of this model for the late-Universe data, and show that the energy contribution of this new tensor is able to explain the accelerated expansion of the Universe without the addition of a cosmological constant. Unlike many other alternative gravities with varying gravitational strength, the predicted$G$evolution is also consistent with local observations and therefore this model does not require screening. We will finish by discussing possible other implications this approach might have for cosmology and some future prospects. The Road to Precision Cosmology. (arXiv:2201.04741v1 [astro-ph.CO]) Authors: Michael S. Turner The past 50 years has seen cosmology go from a field known for the errors being in the exponents to precision science. The transformation, powered by ideas, technology, a paradigm shift and culture change, has revolutionized our understanding of the Universe, with the$\Lambda$CDM paradigm as its crowning achievement. I chronicle the journey of precision cosmology and finish with my thoughts about what lies ahead. A quantum state for the late Universe. (arXiv:2108.05111v2 [gr-qc] UPDATED) We consider the quantum description of a toy model universe in which the Schwarzschild-de Sitter geometry emerges from the coherent state of a massless scalar field. Although highly idealised, this simple model allows us to find clear hints supporting the conclusion that the reaction of the de Sitter background to the presence of matter sources induces i) a modified Newtonian dynamics at galactic scales and ii) different values measured for the present Hubble parameter. Both effects stem from the conditions required to have a normalisable quantum state. Science Potential for Stellar-mass Black Holes as Neighbors of Sgr A*. (arXiv:2201.03154v1 [astro-ph.HE] CROSS LISTED) Authors: Shammi TahuraZhen PanHuan Yang It has been suggested that there is possibly a class of stellar-mass black holes (BHs) residing near (distance$\le 10^3 M$) the galactic center massive black hole, Sgr A*. Possible formation scenarios include the mass segregation of massive stellar-mass black holes and/or the disk migration if there was an active accretion flow near Sgr A* within$\mathcal{O}(10)\$ Myr. In this work, we explore the application of this type of objects as sources of space-borne gravitational wave detectors, such as Laser Interferometer Space Antenna (LISA). We find it is possible to probe the spin of Sgr A* based on the precession of the orbital planes of these stellar-mass black holes moving around Sgr A*. We also show that the dynamical friction produced by accumulated cold dark matter near Sgr A* generally produces small measurable phase shift in the gravitational waveform. In the case that there is an axion cloud near Sgr A*, the dynamical friction induced modification to gravitational waveform is measurable only if the mass of the axion field is in a narrow range of the mass spectrum. Gravitational interaction between the axion cloud and the stellar-mass black holes may introduce additional precession around the spin of Sgr A*. This additional precession rate is generally weaker than the spin-induced Lense-Thirring precession rate, but nevertheless may contaminate the spin measurement in a certain parameter regime. At last, we point out that the multi-body gravitational interaction between these stellar-mass black holes generally causes negligible phase shift during the LISA lifetime.

Pinning down the proton

Nature Physics, Published online: 13 January 2022; doi:10.1038/s41567-021-01484-w

Pinning down the proton

Free Energy Pragmatics: Markov blankets don’t prescribe objective ontology, and that’s okay

Hipolito, Ines and va es, Thomas (2022) Free Energy Pragmatics: Markov blankets don’t prescribe objective ontology, and that’s okay. [Preprint]

On Two Different Kinds of Computational Indeterminacy

Papayannopoulos, Philippos and Fresco, Nir and Shagrir, Oron (2021) On Two Different Kinds of Computational Indeterminacy. [Preprint]

Detecting Entanglement Structure in Continuous Many-Body Quantum Systems

Author(s): Philipp Kunkel, Maximilian Prüfer, Stefan Lannig, Robin Strohmaier, Martin Gärttner, Helmut Strobel, and Markus K. Oberthaler

A prerequisite for the comprehensive understanding of many-body quantum systems is a characterization in terms of their entanglement structure. The experimental detection of entanglement in spatially extended many-body systems describable by quantum fields still presents a major challenge. We develo…

[Phys. Rev. Lett. 128, 020402] Published Mon Jan 10, 2022

Wavefunction Realism Does Not ‘Privilege Position’

Schroeren, David (2021) Wavefunction Realism Does Not ‘Privilege Position’. [Preprint]

The rise of logical empiricist philosophy of science and the fate of speculative philosophy of science

Katzav, Joel and Vaesen, Krist (2022) The rise of logical empiricist philosophy of science and the fate of speculative philosophy of science. [Preprint]

QBISM: AN ECO-PHENOMENOLOGY OF QUANTUM PHYSICS

Bitbol, Michel and De la Tremblaye, Laura (2022) QBISM: AN ECO-PHENOMENOLOGY OF QUANTUM PHYSICS. [Preprint]

Brownian motion from a deterministic system of particles

Ardourel, Vincent (2022) Brownian motion from a deterministic system of particles. [Preprint]

Particles in Quantum Field Theory

Fraser, Doreen (2017) Particles in Quantum Field Theory. [Preprint]