November 2023

Christopher J. Fewster
University of YorkMeasurement in Quantum Field Theory: Problems and Solutions

Many presentations of quantum mechanics include a postulate that the state of a system undergoes an instantaneous change following a measurement. This is clearly incompatible with special and general relativity and raises questions concerning the description of measurement in quantum field theory (QFT). Attempts to extend measurement postulates to QFT by hand have produced pathologies, …

Christopher J. Fewster
University of YorkMeasurement in Quantum Field Theory: Problems and Solutions
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Hollow-core fiber loading of nanoparticles into ultra-high vacuum

Many experiments in the field of optical levitation with nanoparticles today are limited by the available technologies for particle loading. Here we introduce a new particle loading method that solves the main challenges, namely deterministic positioning of the particles and clean delivery at ultra-high vacuum levels as required for quantum experiments. We demonstrate the efficient loading, positioning, and repositioning of nanoparticles in the range of $100-755,mathrm{nm}$ diameter into different lattice sites of a standing wave optical trap, as well as direct loading of nanoparticles at an unprecedented pressure below $10^{-9},mathrm{mbar}$. Our method relies on the transport of nanoparticles within a hollow-core photonic crystal fiber using an optical conveyor belt, which can be precisely positioned with respect to the target trap. Our work opens the path for increasing nanoparticle numbers in the study of multiparticle dynamics and high turn-around times for exploiting the quantum regime of levitated solids in ultra-high vacuum.

Note on episodes in the history of modeling measurements in local spacetime regions using QFT

The formulation of a measurement theory for relativistic quantum field theory (QFT) has recently been an active area of research. In contrast to the asymptotic measurement framework that was enshrined in QED, the new proposals aim to supply a measurement framework for measurements in local spacetime regions. This paper surveys episodes in the history of quantum theory that contemporary researchers have identified as precursors to their own work and discusses how they laid the groundwork for current approaches to local measurement theory for QFT.

Proceedings 19th International Conference on Quantum Physics and Logic

This volume contains the proceedings of the 19th International Conference on Quantum Physics and Logic (QPL 2022), which was held June 27-July 1, 2022 at Wolfson College, University of Oxford, UK. QPL is an annual conference that brings together academic and industry researchers working on mathematical foundations of quantum computation, quantum physics, and related areas. The main focus is on the use of algebraic and categorical structures, formal languages, semantic methods, as well as other mathematical and computer scientific techniques applicable to the study of physical systems, physical processes, and their composition.

Random pure Gaussian states and Hawking radiation

A black hole evaporates by Hawking radiation. Each mode of that radiation is thermal. If the total state is nevertheless to be pure, modes must be entangled. Estimating the minimum size of this entanglement has been an important outstanding issue. We develop a new theory of constrained random symplectic transformations, based on that the total state is pure, Gaussian and random, and every mode thermal as in Hawking theory. From this theory we compute the distribution of mode-mode correlations, from which we bound mode-mode entanglement. We find that correlations between thinly populated modes (early-time high-frequency modes and/or late modes of any frequency) are strongly suppressed. Such modes are hence very weakly entangled. Highly populated modes (early-time low-frequency modes) can on the other hand be strongly correlated, but a detailed analysis reveals that they are nevertheless also weakly entangled. Our analysis hence establishes that restoring unitarity after a complete evaporation of a black hole does not require strong quantum entanglement between any pair of Hawking modes. Our analysis further gives exact general expressions for the distribution of mode-mode correlations in random, pure, Gaussian states with given marginals, which may have applications beyond black hole physics.

Identification of Causal Influences in Quantum Processes

Though the topic of causal inference is typically considered in the context of classical statistical models, recent years have seen great interest in extending causal inference techniques to quantum and generalized theories. Causal identification is a type of causal inference problem concerned with recovering from observational data and qualitative assumptions the causal mechanisms generating the data, and hence the effects of hypothetical interventions. A major obstacle to a theory of causal identification in the quantum setting is the question of what should play the role of “observational data,” as any means of extracting data at a certain locus will almost certainly disturb the system. Hence, one might think a priori that quantum measurements are already too much like interventions, so that the problem of causal identification trivializes. This is not the case. Fixing a limited class of quantum instruments (namely the class of all projective measurements) to play the role of “observations,” we note that as in the classical setting, there exist scenarios for which causal identification is not possible. We then present sufficient conditions for quantum causal identification, starting with a quantum analogue of the well-known “front-door criterion” and finishing with a broader class of scenarios for which the effect of a single intervention is identifiable. These results emerge from generalizing the process-theoretic account of classical causal inference due to Jacobs, Kissinger, and Zanasi beyond the setting of Markov categories, and thereby treating the classical and quantum problems uniformly.

Conservation Laws Reveal the Quantumness of Gravity

A universal framework for quantum-classical dynamics based on information-theoretic approaches is presented. Based on this, we analyze the interaction between quantum matter and a classical gravitational field. We point out that, under the assumption of conservation of momentum or energy, the classical gravitational field cannot cause the change of the momentum or energy of the quantum system, which is not consistent with the observation of existing experiments (e.g. the free fall experiment), while on the contrary the quantum gravitational field can do so. Our analysis exposes the fundamental relationship between conservation laws and the quantum properties of objects, offering new perspectives for the study of quantum gravity.