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arXiv:2201.11425 · Communications Physics

Unambiguous joint detection of spatially separated properties of a single photon in the two arms of an interferometer

Surya Narayan Sahoo, Sanchari Chakraborti, Som Kanjilal, Saumya Ranjan Behera, Dipankar Home, A. Matzkin, Urbasi Sinha

Published Aug 7, 2023• 8 citations
arXivHTMLPDF

Abstract

Abstract The quantum superposition principle implies that a particle entering an interferometer evolves by simultaneously taking both arms. If a non-destructive, minimally-disturbing interaction coupling a particle property to a pointer is implemented on each arm while maintaining the path superposition, quantum theory predicts that, for a fixed state measured at the output port, certain particle properties can be associated with only one or the other path. This phenomenon is known as the quantum Cheshire cat effect. Here we report the realization of this prediction through joint observation of the spatial and polarization degrees of freedom of a single photon in the two respective arms of an interferometer. Significant pointer shifts ( ~ 50 microns), corresponding to measured weak values, are observed in each arm. This observation, involving coupling distinct properties of a quantum system in spatially separated regions, opens new possibilities for quantum information protocols and for tests of quantumness for mesoscopic systems.

DOI
10.1038/s42005-023-01317-7

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Abstract The quantum superposition principle implies that a particle entering an interferometer evolves by simultaneously taking both arms. If a non-destructive, minimally-disturbing interaction coupling a particle property to a pointer is implemented on each arm while maintaining the path superposition, quantum theory predicts that, for a fixed state measured at the output port, certain particle properties can be associated with only one or the other path. This phenomenon is known as the quantum Cheshire cat effect. Here we report the realization of this prediction through joint observation of the spatial and polarization degrees of freedom of a single photon in the two respective arms of an interferometer. Significant pointer shifts ( ~ 50 microns), corresponding to measured weak values, are observed in each arm. This observation, involving coupling distinct properties of a quantum system in spatially separated regions, opens new possibilities for quantum information protocols and for tests of quantumness for mesoscopic systems.
We report an experimental realization of the quantum paradox of the separation of a single photon from one of its properties (the so-called "quantum Cheshire cat"). We use a modified Sagnac interferometer with displaced paths to produce appropriately pre- and postselected states of heralded single photons. Weak measurements of photon presence and circular polarization are performed in each arm of the interferometer by introducing weak absorbers and small polarization rotations and analyzing changes in the postselected signal. The absorber is found to have an appreciable effect only in one arm of the interferometer, while the polarization rotation significantly affects the signal only when performed in the other arm. We carry out both sequential and simultaneous weak measurements and find good agreement between measured and predicted weak values. In the language of Aharonov et al. and in the sense of the ensemble averages described by weak values, the experiment establishes the separation of a particle from one its properties during the passage through the interferometer.
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.
Weak measurements, which enable access to weak values with minimal disturbance to the quantum system, have significantly advanced the study of counterintuitive quantum phenomena, including the quantum Cheshire Cat (QCC). Building on this methodology, we report a single-path realization of the QCC effect by mapping the two interferometric arms onto orthogonal polarization states of a single photon within the orbital angular momentum (OAM) degree of freedom. This approach demonstrates that the pre- and post-selected quantum states yield weak measurement statistics consistent with the polarization and OAM observables behaving as although they are localized differently, effectively decoupling the OAM property from its polarization-encoded carrier. Using a polarization-addressed spatial light modulator, we implement two programmable weak couplings on the same hardware: a weak absorptive perturbation (probing particle-like localization) that yields the particle-detectability weak value from the slope of the normalized post-selection probability and a small OAM rotation (probing wave-like OAM) that yields the OAM-related weak value from the slope of the interference visibility—both operating within the linear weak-coupling regime. By comparing the statistics from these two measurements, we experimentally confirm the effective decoupling of the photon’s particle localization and its OAM. This pointer-free, single-platform architecture preserves full weak-value readout capability while substantially reducing alignment and stabilization complexity, offering a compact and resource-efficient route to QCC-based property separation well suited for high-dimensional quantum applications.
Abstract Although our every-day experience rejects it, the quantum Cheshire Cat suggests a potential spatial separation between different properties of a single particle in an interferometer. The first experiment with neutrons confirmed the quantum Cheshire Cat effect by using the path and spin degrees of freedom. The locations of each property are determined qualitatively through reactions to locally applied perturbations. Yet, no consensus on the interpretation has been reached. To clarify the origin of the effect, in the present experiment the energy degree of freedom is used as the third property; the three properties of neutrons appear to be separated in different paths in the interferometer. The analysis of the experiment suggests the strong involvement of the inner product between the state vectors, one evolved from the initial state through the perturbation and the other being the final state. The inner product results in amplitudes from two sub-beams which contribute to the intensity. The cross-term between amplitudes gives rise to the quantum Cheshire Cat.
The quantum Cheshire cat (QCC) thought experiment proposes that a quantum object's property (e.g., polarization, spin, etc.) can be separated from its physical body. This conclusion arose from an argument that interprets a zero weak value (WV) of polarization as no polarization. We show that this argument is incomplete in the sense that a zero WV reading could equally be interpreted as linear polarization. Nevertheless, through a generalization of the QCC, we complete their argument by excluding the possibility of linear polarization as a consistent interpretation. We go further and introduce the dual of the generalized QCC. The dual QCC exhibits an intriguing effect, where a horizontally polarized interferometer with just one arm can give rise to interference which is vertically polarized. The interference appears to arise as the result of the phase difference between the physical arm and a phantom arm. This peculiar effect arises from the interplay between the preselected and postselected states, which characterize WVs. The QCC has not yet been unambiguously experimentally demonstrated. The QCC dual offers an alternative pathway to experimental realization.
From its very beginning, quantum theory has been revealing extraordinary and counter-intuitive phenomena, such as wave-particle duality, Schrödinger cats and quantum non-locality. Another paradoxical phenomenon found within the framework of quantum mechanics is the 'quantum Cheshire Cat': if a quantum system is subject to a certain pre- and postselection, it can behave as if a particle and its property are spatially separated. It has been suggested to employ weak measurements in order to explore the Cheshire Cat's nature. Here we report an experiment in which we send neutrons through a perfect silicon crystal interferometer and perform weak measurements to probe the location of the particle and its magnetic moment. The experimental results suggest that the system behaves as if the neutrons go through one beam path, while their magnetic moment travels along the other.