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arXiv:2510.10091 · Acta Physica Sinica

Spin exchange of two spin-1/2 atoms

PAN Zeming, Naiming TAN, Chao Gao, Zhihai YAO, Xiaoqian Wang

Published Jan 1, 2025• 0 citations
arXivHTMLPDF

Abstract

The quantum Cheshire cat effect is an important phenomenon in quantum mechanics that reveals the separability of physical properties from their carriers. This effect transcends the classical framework whose attributes must be inherently attached to objects, providing new perspectives for quantum information and precision measurement. According to the quantum Cheshire cat effect, we prepare a pre-selected state of a spin-1/2 atomic system composed of two particles through a pre-selection process. We conduct quantum weak measurements on the spins and positions of these two atoms and extract weak values by using the method of imaginary time evolution (ITE). Subsequently, we perform post-selection on these two atoms and design two distinct post-selected states. Initially, we calculate analytical solutions when both atoms encounter these two different post-selected states separately. Then, during the stage of obtaining weak values via ITE, we first discuss the scenario with only one post-selected state. In this case, our experimental goal is to achieve spin exchange between the two atoms. We use ITE to obtain normalized coincidence rate for the system. By linearly fitting these normalized coincidence rate, we derive numerical solutions for the weak values of the system. The comparison between the analytical solutions and numerical results indicates that they are in close agreement, demonstrating that our method promotes spin exchange between the two atoms. Next, we examine scenarios involving both post-selected states in the post-selection process. After completing weak measurements on particles, delayed-choice allows them to evolve along different paths ultimately leading to distinct post-selected states. One particular post-selected state that results in final measurement outcomes indicates that the spin exchange occurs between both particles with amplification. Conversely, the other post-selected state ensures that even after undergoing weak measurement and delayed-choice, the states of the two particles remain consistent with their pre-measurement conditions. We also compare the analytical and numerical solutions of the experiment involving delayed choice and find that they are very consistent with each other. This consistency indicates that delayed-choice indeed has a significant influence on whether the final exchange occurs. Our research theoretically confirms the feasibility of fermionic systems within bipartite quantum Cheshire cat effects and illustrates how delayed-choice influences quantum Cheshire cat effects in spin-1/2 atomic systems.

DOI
10.7498/aps.74.20241781

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The quantum Cheshire cat effect is an important phenomenon in quantum mechanics that reveals the separability of physical properties from their carriers. This effect transcends the classical framework whose attributes must be inherently attached to objects, providing new perspectives for quantum information and precision measurement. According to the quantum Cheshire cat effect, we prepare a pre-selected state of a spin-1/2 atomic system composed of two particles through a pre-selection process. We conduct quantum weak measurements on the spins and positions of these two atoms and extract weak values by using the method of imaginary time evolution (ITE). Subsequently, we perform post-selection on these two atoms and design two distinct post-selected states. Initially, we calculate analytical solutions when both atoms encounter these two different post-selected states separately. Then, during the stage of obtaining weak values via ITE, we first discuss the scenario with only one post-selected state. In this case, our experimental goal is to achieve spin exchange between the two atoms. We use ITE to obtain normalized coincidence rate for the system. By linearly fitting these normalized coincidence rate, we derive numerical solutions for the weak values of the system. The comparison between the analytical solutions and numerical results indicates that they are in close agreement, demonstrating that our method promotes spin exchange between the two atoms. Next, we examine scenarios involving both post-selected states in the post-selection process. After completing weak measurements on particles, delayed-choice allows them to evolve along different paths ultimately leading to distinct post-selected states. One particular post-selected state that results in final measurement outcomes indicates that the spin exchange occurs between both particles with amplification. Conversely, the other post-selected state ensures that even after undergoing weak measurement and delayed-choice, the states of the two particles remain consistent with their pre-measurement conditions. We also compare the analytical and numerical solutions of the experiment involving delayed choice and find that they are very consistent with each other. This consistency indicates that delayed-choice indeed has a significant influence on whether the final exchange occurs. Our research theoretically confirms the feasibility of fermionic systems within bipartite quantum Cheshire cat effects and illustrates how delayed-choice influences quantum Cheshire cat effects in spin-1/2 atomic systems.
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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.
Observing counterintuitive quantum phenomena have provided physical perspectives on quantum mechanics as well as important evidence of its validity. One of such phenomena, namely the quantum Cheshire cat effect, illustrates the situation that the physical property is disembodied from its physical carrier. Here, we experimentally demonstrate the quantum Cheshire cat effect with a nondestructive weak measuring device. A pre- and post-selected photonic quantum system is subject to a weak nondestructive measuring device to probe where the photon and its polarization property can be found. Intriguingly, a quantum pointer after the weak quantum measurement indicates that the polarization can be found at the path the photon did not take.
Intuition suggests that an object should carry all of its physical properties. However, a quantum object may not act in such a manner-it can temporarily leave some of its physical properties where it never appears. This phenomenon is known as the quantum Cheshire cat effect. It has been proposed that a quantum object can even permanently discard a physical property and obtain a new one it did not initially have. Here, we observe this effect experimentally by casting non-unitary imaginary-time evolution on a photonic cluster state to extract weak values, which reveals the counterintuitive phenomenon that two photons exchange their spins without classically meeting each other. A phenomenon presenting only in the quantum realm, our results are in stark contrast with the perception of inseparability between objects and properties, and shed new light on comprehension of the ontology of observables.
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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.