# Weekly Papers on Quantum Foundations (9)

Measurement of Entangled State Before, During, and After a Proposed Entangled Two-Photon Molecular Excitation. (arXiv:2202.11764v1 [quant-ph])

Entangled photon pairs are predicted to linearize and increase the efficiency of multiphoton absorption, allowing continuous wave laser diodes to drive ultrafast time-resolved spectroscopy, nonlinear photonics, and nonlinear biological microscopy at low fluxes. However, despite a range of theoretical studies and experimental measurements of entangled two-photon absorption cross sections in molecular systems, inconsistencies persist about the value of the linearized and enhanced cross section. We present an entangled two-photon spectrometer that is capable of characterizing entangled photon states before, during, and after a proposed two-photon excitation event. The spectrometer uses a custom <100 nW, 20 fs, broadband entangled photon source that theoretically optimizes any possible entangled two-photon absorption. The spectrometer is then used to measure Rhodamine 6G, a common fluorescent molecular dye with a virtual-state-mediated two-photon absorption. The proposed entangled two-photon interaction is found to be equal to or lower than that of a classical single photon scattering event, providing further bounds for proposed theoretical and experimental measurements. The entangled linearization of nonlinear, multi-photon, and ultrafast spectroscopies could have broad scientific aspects, but the results of this paper suggest that molecules with near-resonant, real intermediate states are necessary for technologically significant applications.

Bohmian mechanics is not deterministic. (arXiv:2202.12279v1 [quant-ph])

The aim of this paper is to argue that Bohmian mechanics (or any similar pilot-wave theory) cannot reasonably be claimed to be a deterministic theory. In fact, even if one simply assumes the “quantum equilibrium distribution” in order to correctly reproduce the (Born) statistics of experimental outcomes, Bohmian mechanics requires something like an external stochastic oracle that samples this distribution. Taken by itself, Bohmian mechanics is unable to assign some value of its hidden variable (i.e. position) to a given experiment in a long run of repeated identical experiments, and hence it cannot predict its result (as might be expected of a deterministic hidden variable theory). Thus the advantages of Bohmian mechanics over other interpretations of quantum mechanics, if any, must lie either at an ontological level, or in the potential ability to explain the quantum equilibrium distribution and hence the Born rule.

Making Mistakes Saves the Single Observer’s World of the Extended Wigner’s Friend Experiment. (arXiv:1801.08537v5 [quant-ph] UPDATED)

The Extended Wigner’s Friend thought experiment, comprising a quantum system containing an agent who draws conclusions upon observing the outcome of a measurement of a quantum state prepared in two nonorthogonal versions by another agent led its authors to conclude that quantum theory cannot consistently describe the use of itself. It has also been proposed that this thought experiment is equivalent to entangled state (Bell type) experiments. It is argued in this paper that the assumption of the freedom of choice of the first Wigner’s friend regarding how to prepare a quantum state in one of the two available nonorthogonal versions invalidates such equivalence.

Position and momentum operators for a moving particle in bulk. (arXiv:2202.11872v1 [hep-th])

Authors: Wu-zhong Guo

In this paper we explore how to describe a bulk moving particle in the dual conformal field theories (CFTs). One aspect of this problem is to construct the dual state of the moving particle. On the other hand one should find the corresponding operators associated with the particle. The dynamics of the particle, i.e., the geodesic equation, can be formulated as a Hamiltonian system with canonical variables. The achievements of our paper are to construct the dual CFT states and the operators corresponding to the canonical variables. The expectation values of the operators give the expected solutions of the geodesic line, and the quantum commutators reduce to the classical Poisson brackets to leading order in the bulk gravitational coupling. Our work provides a framework to understand the geodesic equation, that is gravitational attraction, in the dual CFTs.

On Generalized Theories of Varying Fine Structure Constant. (arXiv:2202.08142v2 [gr-qc] UPDATED)

Authors: Soumya Chakrabarti

We work with a class of scalar extended theory of gravity that can drive the present cosmic acceleration as well as accommodate a mild cosmic variation of the fine structure constant $\alpha$. The motivation comes from a vintage theory developed by Bekenstein, Sandvik, Barrow and Magueijo. The $\alpha$ variation is introduced by a real scalar field interacting with charged matter. We execute a cosmological reconstruction based on a parametrization of the present matter density of the Universe. Observational consistency is ensured by comparing the theoretical estimates with JLA + OHD + BAO data sets, using a Markov chain Monte Carlo simulation. An analysis of molecular absorption lines from HIRES and UVES spectrographs is considered as a reference for the variation of $\alpha$ at different redshifts. Two examples are discussed. The first explores a field-dependent kinetic coupling of the scalar field interacting with charged matter. The second example is a generalized Brans-Dicke formalism where the varying $\alpha$ is fitted in as an effective matter field. This generates a simultaneous variation of the Newtonian constant $G$ and $\alpha$. The pattern of this variation can have a crucial role in cosmic expansion history.

Thirty-six Entangled Officers of Euler: Quantum Solution to a Classically Impossible Problem

Author(s): Suhail Ahmad Rather, Adam Burchardt, Wojciech Bruzda, Grzegorz Rajchel-Mieldzioć, Arul Lakshminarayan, and Karol Życzkowski

A mathematical problem with no classical solution turns out to be solvable using quantum rules.

[Phys. Rev. Lett. 128, 080507] Published Fri Feb 25, 2022

Emergence of Constructor-Based Irreversibility in Quantum Systems: Theory and Experiment

Author(s): Chiara Marletto, Vlatko Vedral, Laura T. Knoll, Fabrizio Piacentini, Ettore Bernardi, Enrico Rebufello, Alessio Avella, Marco Gramegna, Ivo Pietro Degiovanni, and Marco Genovese

How irreversibility arises in a universe with time-reversal symmetric laws is a central problem in physics. In this Letter, we discuss a radically different take on the emergence of irreversibility, adopting the recently proposed constructor theory framework. Irreversibility is expressed as the requ…

[Phys. Rev. Lett. 128, 080401] Published Wed Feb 23, 2022