Recently it was demonstrated, both theoretically and experimentally, how to separate a particle from its spin, or any other property, a phenomenon known as the "Quantum Cheshire Cat". We present two novel gedanken experiments, based on the quantum Zeno effect, suggesting a dynamical process thorough which this curious phenomenon occurs. We analyze, for the first time, a quantum current consisting of spin without mass. Thus, the quantum variables of pre- and post-selected particles are understood to be involved in various interactions, even in the absence of their owners. This current is shown to provide a local explanation for seemingly nonlocal interactions.
Recently it was demonstrated, both theoretically and experimentally, how to separate a particle from its spin, or any other property, a phenomenon known as the "Quantum Cheshire Cat". We present two novel gedanken experiments, based on the quantum Zeno effect, suggesting a dynamical process thorough which this curious phenomenon occurs. We analyze, for the first time, a quantum current consisting of spin without mass. Thus, the quantum variables of pre- and post-selected particles are understood to be involved in various interactions, even in the absence of their owners. This current is shown to provide a local explanation for seemingly nonlocal interactions.
Abstract We analyse the quantum Cheshire cat using contextuality theory, to see if this can tell us anything about how best to interpret this paradox. We show that this scenario can be analysed using the relation between three different measurements, which seem to result in a logical contradiction. We discuss how this contextual behaviour links to weak values, and coherences between prohibited states. Rather than showing a property of the particle is disembodied, the quantum Cheshire cat instead demonstrates the effects of these coherences, which are typically found in pre- and post-selected systems.
In a recent work, Aharonov et al suggested that a photon could be separated from its polarization in an experiment involving pre- and post-selection (Aharonov et al 2013 New J. Phys. 15 113015 ). They named the effect 'quantum Cheshire Cat', in a reference to the cat that is separated from its grin in the novel Alice ' s Adventures in Wonderland . Following these ideas, Denkmayr et al performed a neutron interferometric experiment and interpreted the results suggesting that neutrons were separated from their spin (Denkmayr et al 2014 Nat. Commun. 5 4492 ). Here we show that these results can be interpreted as simple quantum interference, with no separation between the quantum particle and its internal degree of freedom. We thus hope to clarify the phenomenon with this work, by removing these apparent paradoxes.
The Quantum Cheshire Cat experiment showed that when weak measurements are performed on pre- and post-selected system, the counterintuitive result has been obtained that a neutron is measured to be in one place without its spin, and its spin is measured to be in another place without the neutron. A generalization of this effect is presented with a massive particle whose mass is measured to be in one place with no momentum, while the momentum is measured to be in another place without the mass. The new result applies to any massive particle, independent of its spin or charge. A gedanken experiment which illustrates this effect is presented using a nested pair of Mach-Zehnder interferometers, but with some of the mirrors and beam splitters moving relative to the laboratory frame. The titular interpretation of this experiment is extremely controversial, and rests on several assumptions, which are discussed in detail. An alternative interpretation using the counterparticle model of Aharonov et al. is also discussed.
The validity of quantum-mechanical predictions has been confirmed with a high degree of accuracy in a wide range of experiments. Although the statistics of the outcomes of a measuring apparatus have been studied intensively, little has been explored and is known regarding the accessibility of quantum dynamics. For these sorts of fundamental studies of quantum mechanics, interferometry using neutron matter-waves in particular, provides almost ideal experimental circumstances. In this device quantum interference between spatially separated beams occurs on a macroscopic scale. Recently, the full determination of weak-values of neutrons 1 2 - spin adds a new aspect to the study of quantum dynamics. Moreover, a new counter-intuitive phenomenon, called quantum Cheshire Cat, is observed in an interference experiment. In this article, we present an overview of these experiments.
Many counterfactual predictions are made by quantum theory, such as quantum tunneling and violations of Bell’s inequalities. Experimental verifications of these predictions demonstrate that quantum theory is reliable for the microscopic world. Despite its widespread application, consistent theoretical interpretations of quantum theory remain a challenge. Recently, Aharonov et al. proposed several new and intriguing counterfactuals based on quantum weak measurement and interaction-free measurement, such as the quantum violation of the pigeonhole principle and the departure of physical properties from particles, also known as the quantum Cheshire cat effect. These counterfactuals garnered considerable interest. Although no common consensus has been reached, many pioneering experiments have already been conducted in light of these theoretical proposals. Furthermore, these counterfactuals and the key concepts underlying them, weak measurement and interaction-free measurement, open up a whole new way of thinking about and understanding the strange quantum theory. Herein, our aim is to provide a unified perspective on these counterfactuals. The theoretical ideas, particularly the fundamental quantum concepts underlying these counterfactuals, are the focus of our attention. First, we present concise and insightful introductions to weak measurement and interaction-free measurement, which are the two key fundamental constructions for discussing all of the following quantum counterfactuals. Next, we discuss the quantum violation of the pigeonhole principle in depth, explicitly demonstrating that the key idea leading to the violation is destructive quantum interference between the possible paths of two particles traveling along the same path. Then, two key proposals of the quantum Cheshire cat effect are examined based on weak values of certain physical properties. Both proposals are built around interaction-free measurement, and the departure of physical properties from their bodies is realized based on the quantum Zeno effect. After introducing the weak measurement and weak value, the non-locality in the quantum Cheshire cat effect can be dismissed, and the physical properties leave the body continuously with infinitesimal quantum amplitudes. Meanwhile, during the departure, the total property in the entire space remains invariant. Finally, we present our conclusion on quantum counterfactuals, arguing that when coherent processes are endowed with concrete physical properties, counterfactuals are unavoidable. Throughout our discussions, we try to elaborate on the key roles played by the quantum concepts, quantum amplitudes, and quantum coherence in these counterfactuals. The idea behind weak measurement and weak value is that information about a coherent process can be obtained with sufficiently weak coupling. In other words, a quantum coherent process becomes observable with a weak measurement and even acquires a specific property quantified by a weak value. When compared to our conventional understanding of quantum coherent processes, this is a mind-boggling viewpoint. The idea behind interaction-free measurement is that weak amplitudes can cause large system evolution even if they cannot transport the body of the system, which is also astonishing! This demonstrates the enormous power of infinitesimal quantum amplitudes in quantum communication and computation. Therefore, all quantum counterfactuals provide new perspectives to further investigate these fundamental quantum concepts.
Abstract One of the common conceptions of nature, typically derived from the experiences with classical systems, is that attributes of the matter coexist with the substance. In the quantum regime, however, the quantum particle itself and its physical property may be in spatial separation, known as the quantum Cheshire cat effect. While there have been several reports to date on the observation of the quantum Cheshire cat effect, all such experiments are based on first-order interferometry and destructive projection measurement, thus allowing simple interpretation due to measurement-induced disturbance and also subject to trivial interpretation based on classical waves. In this work, we report an experimental observation of the quantum Cheshire cat effect with noninvasive weak quantum measurement as originally proposed. The use of the weak-measurement probe has allowed us to identify the location of the single photon and that of the disembodied polarization state in a quantum interferometer. The weak-measurement probe based on two-photon interference makes our observation unable to be explained by classical physics. We furthermore elucidate the quantum Cheshire cat effect as quantum interference of the transition amplitudes for the photon and the polarization state which are directly obtained from the measurement outcomes or the weak values. Our work not only reveals the true quantum nature of Cheshire cat effect but also sheds light on a comprehensive understanding for the counter-intuitive quantum phenomena.