Abstract Schrödinger’s cat is an iconic example for the problem of the transition from the microscopic quantum world to the macroscopic, classical one. Up to which system size quantum effects can be expected? How can the transition between the classical and the quantum world be characterized? The field of macroscopicity aims at providing answers to those questions, both from a theoretical and an experimental point of view. Here, we present the main concepts in macroscopicity, including macroscopicity measures, experimental realizations and the link to metrology, from a pedagogical perspective. We provide visualizations and intuitive explanations, together with a hands-on activity where students can create their own macroscopic quantum cats from cardboard cells that are in a superposition of being dead and alive.
Abstract Schrödinger’s cat is an iconic example for the problem of the transition from the microscopic quantum world to the macroscopic, classical one. Up to which system size quantum effects can be expected? How can the transition between the classical and the quantum world be characterized? The field of macroscopicity aims at providing answers to those questions, both from a theoretical and an experimental point of view. Here, we present the main concepts in macroscopicity, including macroscopicity measures, experimental realizations and the link to metrology, from a pedagogical perspective. We provide visualizations and intuitive explanations, together with a hands-on activity where students can create their own macroscopic quantum cats from cardboard cells that are in a superposition of being dead and alive.
Schrödinger’s cat thought experiment profoundly reveals the fundamental tension between the principle of quantum superposition and macroscopic classicalreality. This paper systematically elaborates six emerging mathematical-physicalframeworks aimed at reconstructing this paradox: Dynamic Time Algebra, PostMarkovian Paradigm, Temporal Layer Causal Evolution Theory, Chronogroup Framework, Chronological Fiber Category Theory, and Origin of Order Theory. Thesetheories respectively start from algebraic, logical, homological, group-theoretic, categorical, and thermodynamic structures, demonstrating the essential reasons whymacroscopic systems cannot maintain quantum superpositions. They collectivelypoint to a core viewpoint: the classical state of the cat is not determined by theexternal observer’s ”looking”, but is spontaneously completed by an internal ”selfstabilization mechanism” initiated by the system itself before the observer’s intervention, in order to repair the ”structural fracture” at the micro-macro interface.The core contribution of this paper is to reveal that these six theories are not isolated but are manifestations of the same underlying physical process under differentmathematical projections. By analyzing their unification theorems, core mechanisms, and experimentally testable predictions (such as entropy production pulses,logical afterimages, critical slowing down, etc.), this paper provides a self-consistentstructural answer to the quantum measurement problem without introducing thecollapse postulate or many-world branches.
The question whether quantum mechanics is complete and the nature of the transition between quantum mechanics and classical mechanics have intrigued physicists for decades. There have been many experimental breakthroughs in creating larger and larger quantum superposition and entangled states since Erwin Schrödinger proposed his famous thought experiment of putting a cat in a superposition of both alive and dead states in 1935. Remarkably, recent developments in quantum optomechanics and electromechanics may lead to the realisation of quantum superposition of living microbes soon. Recent evidence also suggests that quantum coherence may play an important role in several biological processes. In this review, we first give a brief introduction to basic concepts in quantum mechanics and the Schrödinger’s cat thought experiment. We then review developments in creating quantum superposition and entangled states and the realisation of quantum teleportation. Non-trivial quantum effects in photosynthetic light harvesting and avian magnetoreception are also discussed. At last, we review recent proposals to realise quantum superposition, entanglement and state teleportation of micro-organisms, such as viruses and bacteria.
This paper reviews and suggests a resolution of the problem of definite outcomes of measurement. This problem, also known as ‘Schrodinger’s cat’, has long posed an apparent paradox because the state resulting from a measurement appears to be a quantum superposition in which the detector is in two macroscopically distinct states (alive and dead in the case of the cat) simultaneously. Many alternative interpretations of the quantum mathematical formalism, and several alternative modifications of the theory, have been proposed to resolve this problem, but no consensus has formed supporting any one of them. Applying standard quantum theory to the measurement state, together with the analysis and results of decades of nonlocality experiments with pairs of entangled systems, this paper shows the entangled measurement state is not a paradoxical macroscopic superposition of states. It is instead a phase-dependent superposition of correlations between states of the subsystems. Thus Schrodinger’s cat is a non-paradoxical ‘macroscopic correlation’ in which one of the two correlated systems happens to be a detector. This insight resolves the problem of definite outcomes but it does not entirely resolve the measurement problem because the entangled state is still reversible.
In quantum theory, a particle is allowed to be in a state of coherent superposition among a set of orthogonal states. A vivid picture of this concept is the “Schrödinger cat”: a cat is alive and dead simultaneously. This would be no surprise if one means a large number of an ensemble of cats with 50% alive and 50% dead. But, “Schrödinger cat” is a cat. People are talking about a cat being alive and dead simultaneously. In mathematics, the best word to characterize the relationship between states “alive” and “dead” is perhaps “orthogonal”. In quantum mechanics, the superposition of orthogonal states is used to describe the state of a quantum object, or a particle. The superposition principle is indeed a mystery compared to our everyday classical life experience. Beyond the superposition of orthogonal single-particle states, the superposition of orthogonal multi-particle states represents an even more troubling concept in classical theory.
Quantum correlations are correlations that arise from a unique feature in quantum mechanics: quantum superposition. The most famous example of quantum superposition is the Schrodinger’s cat state, where a cat is in a state of neither dead nor alive before an observer determines the cat’s status. This is not what we expect. Nevertheless, quantum superposition and correlations are routinely created in the laboratories with microscopic systems involving atoms and light particles. Can this counterintuitive notion be realised in the macroscopic world? This thesis investigates this question by devising criteria that certify the existence of quantum correlations in mesoscopic/macroscopic systems. Powerful mathematical tools are used to simulate two quantum protocols in a mesoscopic system which have applications in quantum communication and computation.
Abstract Chapter 15 deals with various unresolved issues, such as how measurements are to be described, whether they induce collapse of linear superpositions, and if so, what the result should be. Another is whether quantum mechanics applies to macroscopic measuring devices, and if not, why not. Some of these issues are addressed via the Schrödinger cat-in-the-box experiment, in particular whether the cat is ever half alive and half dead; analogs of the corresponding Schrödinger-cat state (not involving a cat) have been studied experimentally. For Schrödinger, it was ridiculous to apply quantum mechanics to macroscopic situations, nevertheless, it has been explored by many physicists. Whether measurements induce collapse and whether collapse arises in various formulations/interpretations of quantum mechanics is examined in the context of generic measuring instruments known as pointer devices. Various conclusions have been reached, though consensus has not, despite only one interpretation having been verified experimentally.