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arXiv:2008.10617 · New Journal of Physics

Do qubits dream of entangled sheep? Quantum measurement without classical output

Noah Lupu-Gladstein, Aharon Brodutch, Hugo Ferretti, Weng-Kian Tham, Arthur O. T. Pang, Kent Bonsma-Fisher, Aephraim M. Steinberg

Published May 1, 2024• 2 citations
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Abstract

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.

DOI
10.1088/1367-2630/ad48ad

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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.
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.
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.
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.
In this article we propose a solution to the measurement problem in quantum mechanics. We point out that the measurement problem can be traced to an a priori notion of classicality in the formulation of quantum mechanics. If this notion of classicality is dropped and instead classicality is defined in purely quantum mechanical terms the measurement problem can be avoided. We give such a definition of classicality. It identifies classicality as a property of large quantum system. We show how the probabilistic nature of quantum mechanics is a result of this notion of classicality. We also comment on what the implications of this view are for the search of a quantum theory of gravity.
In this article we propose a solution to the measurement problem in quantum mechanics. We point out that the measurement problem can be traced to an a priori notion of classicality in the formulation of quantum mechanics. If this notion of classicality is dropped and instead classicality is defined in purely quantum mechanical terms the measurement problem can be avoided. We give such a definition of classicality. It identifies classicality as a property of large quantum system. We show how the probabilistic nature of quantum mechanics is a result of this notion of classicality. We also comment on what the implications of this view are for the search of a quantum theory of gravity.
This thesis details two experiments and one theoretical manuscript that explore the nature of measurement disturbance in quantum mechanics. The first experiment exploits measurement disturbance to enhance measurement precision by two orders of magnitude, and in principle even more. The second experiment investigates a paradox where three quantum pigeons seem to occupy two pigeonholes without any pair belonging to the same hole. It finds that measurement disturbance accounts for some, but not all of the counter-intuitive phenomena at play in the paradox. The final manuscript develops a new theoretical framework for studying quantum measurement and disturbance from the viewpoint of quantum agents. Our framework reveals how agents endowed with quantum memories might view measurement not as a stochastic collapse, but as a continuous flow of quantum information.