November 2023

Erik Curiel Universität Bonn Preprint: –

We present a methodological argument to refute the so-called many-worlds interpretation (MWI) of quantum theory. Several known criticisms in the literature have already pointed out problematic aspects of this interpretation, such as the lack of a satisfactory account of probabilities, or the huge ontological cost of MWI. Our criticism, however, does not go into the technical details of any version of MWI, but is at the same time more general and more radical. We show, in fact, that a whole class of theories–of which MWI is a prime example–fails to satisfy some basic tenets of science which we call facts about natural science. The problem of approaches the likes of MWI is that, in order to reproduce the observed empirical evidence about any concrete quantum measurement outcome, they require as a tacit assumption that the theory does in fact apply to an arbitrarily large range of phenomena, and ultimately to all phenomena. We call this fallacy the holistic inference loop, and we show that this is incompatible with the facts about natural science, rendering MWI untenable and dooming it to be refuted.

Quantum superposition of high-dimensional states enables both computational speed-up and security in cryptographic protocols. However, the exponential complexity of tomographic processes makes certification of these properties a challenging task. In this work, we experimentally certify coherence witnesses tailored for quantum systems of increasing dimension, using pairwise overlap measurements enabled by a six-mode universal photonic processor fabricated with a femtosecond laser writing technology. In particular, we show the effectiveness of the proposed coherence and dimension witnesses for qudits of dimensions up to 5. We also demonstrate advantage in a quantum interrogation task, and show it is fueled by quantum contextuality. Our experimental results testify to the efficiency of this novel approach for the certification of quantum properties in programmable integrated photonic platforms

Weak values of quantum observables are a powerful tool for investigating a broad spectrum of quantum phenomena. For this reason, several methods to measure them in the laboratory have been proposed. Some of these methods require weak interactions and postselection, while others are deterministic, but require statistics over a number of experiments growing exponentially with the number of measured particles. Here we propose a deterministic dimension-independent scheme for estimating weak values of arbitrary observables. The scheme, based on coherently controlled SWAP operations, does not require prior knowledge of the initial and final states, nor of the measured observables, and therefore can work with uncharacterized preparation and measurement devices. As a byproduct, our scheme provides an alternative expression for two-time states, that is, states describing quantum systems subject to pre and post-selections. Using this expression, we show that the controlled-SWAP scheme can be used to estimate weak values for a class of two-time states associated to bipartite quantum states with positive partial transpose.

We calculate the typical bipartite entanglement entropy $langle S_Arangle_N$ in systems containing indistinguishable particles of any kind as a function of the total particle number $N$, the volume $V$, and the subsystem fraction $f=V_A/V$, where $V_A$ is the volume of the subsystem. We expand our result as a power series $langle S_Arangle_N=a f V+bsqrt{V}+c+o(1)$, and find that $c$ is universal (i.e., independent of the system type), while $a$ and $b$ can be obtained from a generating function characterizing the local Hilbert space dimension. We illustrate the generality of our findings by studying a wide range of different systems, e.g., bosons, fermions, spins, and mixtures thereof. We provide evidence that our analytical results describe the entanglement entropy of highly excited eigenstates of quantum-chaotic spin and boson systems, which is distinct from that of integrable counterparts.

Spinfoams provide a framework for the dynamics of loop quantum gravity that is manifestly covariant under the full four-dimensional diffeomorphism symmetry group of general relativity. In this way they complete the ideal of three-dimensional diffeomorphism covariance that consistently motivates loop quantum gravity at every step. Specifically, spinfoam models aim to provide a projector onto, and a physical inner product on, the simultaneous kernel of all of the constraints of loop quantum gravity by means of a discretization of the gravitational path integral. In the limit of small Planck constant, they are closely related to the path integral for Regge calculus, while at the same time retaining all of the tools of a canonical quantum theory of gravity. They may also be understood as generalizations of well-understood state sum models for topological quantum field theories. In this chapter, we review all of these aspects of spinfoams, as well as review in detail the derivation of the currently most used spinfoam model, the EPRL model, calculational tools for it, and the various extensions of it in the literature. We additionally summarize some of the successes and open problems in the field.