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Quantum Sensors and Actuators for Quantum Agents

Arthur O. T. Pang

Published Jun 1, 2024• 0 citations
OpenAlex

Abstract

Traditional quantum measurements describe a process in which properties of quantum states are converted and stored into some classical memory such that entities that interact classically, such as humans, can examine and interpret such information. However, these measurements are not inclusive of observers that possess quantum memory and the ``observations'' they may wish to make. Such observers (hereafter ``quantum agents'') may use sensors that, although outside the purview of traditional quantum measurement, nonetheless generate correlations between their memory and the state of interest. In this thesis, I presented work studying sensors for quantum agents as an extension of sensors given by the traditional formulation of quantum measurements. For example, the use of SWAP-like unitaries as sensors will put the state of interest directly into the quantum memory of our agent, but at the same time is not described by traditional quantum measurement. I study these quantum sensors, which can be more powerful sensors than those given by traditional quantum measurement, but at the same time give a stronger measurement disturbance than sensors described in traditional quantum measurements. Additionally, I present work studying a quantum actuator for quantum agents controlling the use of quantum channels. I studied how putting two channels in superpositions can result in noise channels being able to transmit more information than when the channels are used individually. I show that putting channels into superposition will allow access to an additional degree of freedom in the channel, which causes an increase in the information transmitted. Finally, I present a work unrelated to quantum agents exploring an ontological theory derived from Bohmian mechanics' assumptions but with momentum as the ontological variable. In Bohmian theory, the choice of position as the ontological variable is neither self-evident nor obvious, and Bohm-like theories with a different choice of the ontological variable can also be constructed. I present an experimental comparison of Bohmian theory and a momentum-ontologic Bohmian-like theory and argue for the importance and need for making and justifying the choice of ontology.

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Traditional quantum measurements describe a process in which properties of quantum states are converted and stored into some classical memory such that entities that interact classically, such as humans, can examine and interpret such information. However, these measurements are not inclusive of observers that possess quantum memory and the observations'' they may wish to make. Such observers (hereafter quantum agents'') may use sensors that, although outside the purview of traditional quantum measurement, nonetheless generate correlations between their memory and the state of interest. In this thesis, I presented work studying sensors for quantum agents as an extension of sensors given by the traditional formulation of quantum measurements. For example, the use of SWAP-like unitaries as sensors will put the state of interest directly into the quantum memory of our agent, but at the same time is not described by traditional quantum measurement. I study these quantum sensors, which can be more powerful sensors than those given by traditional quantum measurement, but at the same time give a stronger measurement disturbance than sensors described in traditional quantum measurements. Additionally, I present work studying a quantum actuator for quantum agents controlling the use of quantum channels. I studied how putting two channels in superpositions can result in noise channels being able to transmit more information than when the channels are used individually. I show that putting channels into superposition will allow access to an additional degree of freedom in the channel, which causes an increase in the information transmitted. Finally, I present a work unrelated to quantum agents exploring an ontological theory derived from Bohmian mechanics' assumptions but with momentum as the ontological variable. In Bohmian theory, the choice of position as the ontological variable is neither self-evident nor obvious, and Bohm-like theories with a different choice of the ontological variable can also be constructed. I present an experimental comparison of Bohmian theory and a momentum-ontologic Bohmian-like theory and argue for the importance and need for making and justifying the choice of ontology.
Abstract The traditional formalism of quantum measurement (hereafter ‘TQM’) describes processes where some properties of quantum states are extracted and stored as classical information. While TQM is a natural and appropriate description of how humans interact with quantum systems, it is silent on the question of how a more general, quantum , agent would do so. How do we describe the observation of a system by an agent with the ability to store not only classical information but quantum states in its memory? In this paper, we extend the idea of measurement to a more general class of sensors for quantum agents which interact with a system in such a way that the agent’s memory stores information (classical or quantum) about the system under study. For appropriate sensory interactions, the quantum agent may ‘learn’ more about the system than would be possible under any set of classical measurements—but as we show, this comes at the cost of additional measurement disturbance. We experimentally demonstrate such a system and characterize the tradeoffs by considering the channel capacity required to erase the effect of a measurement.
Abstract Quantum mechanics is usually formulated with an implicit assumption that agents who can observe and interact with the world are external to it and have a classical memory. However, there is no accepted way to define the quantum–classical cut and no a priori reason to rule out fully quantum agents with coherent quantum memories. In this work, we introduce an entirely quantum notion of measurement, called a sensation, to account for quantum agents that experience the world through quantum sensors. Sensations eschew probabilities and instead describe a deterministic flow of quantum information. We quantify the information gain and disturbance of a sensation using concepts from quantum information theory and find that sensations always disturb at least as much as they inform. Viewing measurements as sensations could lead to a new understanding of quantum theory in general and to new results in the context of quantum networks.
The paper argues that far from challenging-or even refuting-Bohm's quantum theory, the no-hidden-variables theorems in fact support the Bohmian ontology for quantum mechanics. The reason is that (i) all measurements come down to position measurements; and (ii) Bohm's theory provides a clear and coherent explanation of the measurement outcome statistics based on an ontology of particle positions, a law for their evolution and a probability measure linked with that law. What the no-hidden-variables theorems teach us is that (i) one cannot infer the properties that the physical systems possess from observables; and that (ii) measurements, being an interaction like other interactions, change the state of the measured system.
Quantum mechanics is usually formulated with an implicit assumption that agents who can observe and interact with the world are external to it and have a classical memory. Quantum measurement theory has consequently been formally defined with such external agents in mind, and its predictions match our classical experience. There is however no accepted way to define the quantum-classical cut and (if quantum theory is universal) no a priori reason to rule out fully quantum agents with coherent quantum memories. In this work we extend the definition of measurement to account for observers with quantum memories who need not experience the world in the way we do. We define measurements without probabilities, and show that information gain and uncertainty can still be quantified in a meaningful way. The results demonstrate how quantum agents observe the world in a way that cannot be accounted for in the usual language of observables and POVMs. This approach to measurements could lead to a new understanding of quantum theory in general and to new results in the context of quantum networks.
In this paper, epistemology and ontology of quantum states are discussed based on a completely new way of founding quantum theory. The fundamental notions are conceptual variables in the mind of an observer or in the joint minds of a group of observers. These conceptual variables are very often accessible, that is, it is possible to find values of the variables by doing experiments or by making measurements. An important notion is that of maximal accessibility. It is shown here that this new machinery may facilitate the discussion of when a specific quantum state can be given an ontological interpretation, and also the more speculative question whether all states can be given such an interpretation. The discussion here is general, and has implications for the basic problem of how one should look upon information from experiments and measurement, in particularly the question concerning when this information may reflect properties of the real world.
The article describes an interpretation of the mathematical formalism of standard quantum mechanics in terms of relations. In particular, the wave function ψ(x) is interpreted as a complex-valued relation between an entity (often called “particle”) and a second entity x (often called “spatial point”). Such complex-valued relations can also be formulated for classical physical systems. Entanglement is interpreted as a relation between two entities (particles or properties of particles). Such relations define the concept of “being next to each other”, which implies that entangled entities are close to each other, even though they might appear to be far away with respect to a classical background space. However, when space is also considered to be a network of relations (of which the classical background space is a large-scale continuum limit), such nearest neighbor configurations are possible. The measurement problem is discussed from the perspective of this interpretation. It should be emphasized that this interpretation is not meant to be a serious attempt to describe the ontology of our world, but its purpose is to make it obvious that, besides Bohmian mechanics, presumably many other ontological interpretations of quantum theory exist.