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Journal of Physics B Atomic Molecular and Optical Physics

Atomic Cheshire Cat: untying energy levels from the de Broglie motion

Mehwish Nawaz, Rameez‐ul‐Islam, Tasawar Abbas, Manzoor Ikram

Published Feb 20, 2019• 8 citations
Publisher

Abstract

Abstract We suggest an idea to implement the Cheshire Cat in atomic media. The proposal is based on the time-tested tools of the Bragg regime atom interferometry and can be easily executed under the prevailing cavity quantum electrodynamics (QED) research environment. Our analysis, based on weak measurements, clearly demonstrates that under certain specific pre- and post-selected scenarios, the atom strangely behaves as being in one arm of the interferometer whereas the signatures of its energy levels exist in the other arm simultaneously.

DOI
10.1088/1361-6455/ab08c5

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Abstract We suggest an idea to implement the Cheshire Cat in atomic media. The proposal is based on the time-tested tools of the Bragg regime atom interferometry and can be easily executed under the prevailing cavity quantum electrodynamics (QED) research environment. Our analysis, based on weak measurements, clearly demonstrates that under certain specific pre- and post-selected scenarios, the atom strangely behaves as being in one arm of the interferometer whereas the signatures of its energy levels exist in the other arm simultaneously.
We analyze the quantum Cheshire cat dilemma under an entangled scenario based on atom-field interferometric setup that can be readily executed experimentally. Once the accuracy and the faithful workability of the two-state vector formalism along with the weak measurement theme is guaranteed, our analysis suggests that the interpretation, i.e., separation of a certain specific feature from the quantum entity, furnished in the pioneering Cheshire cat paper [Y. Aharonov, S. Popescu, D. Rohrlich, and P. Skrzypczyk, New J. Phys. 15, 113015 (2013)], cannot be taken for granted as the sole reason for the phenomenon. Rather, our analysis of the considered entangled interferometric scenario suggests an alternative point of view where the inaccessibility of the defining features of a quantum system appears to be a more plausible reason than the disembodying of the feature from the system itself. Our proposal is based on the time-tested tools of the atomic Bragg diffraction, operating in off-resonant regime, and utilizes the entangled correlations engineered between two identical, two-level atoms in the momentum space.
A recent neutron interferometry experiment claims to demonstrate a paradoxical phenomenon dubbed the "quantum Cheshire cat" [Nat. Commun.5, 4492 (2014)]. We have reproduced and extended these results with an equivalent optical interferometer. The results suggest that the photon travels through one arm of the interferometer, while its polarization travels through the other. However, we show that these experimental results belong to the domain where quantum and classical wave theories coincide; there is nothing uniquely quantum about the illusion of this Cheshire cat.
In Alice's Adventures in Wonderland by Lewis Carroll, mischievous grin of a Cheshire cat is sometimes dis-embodied from her body. Quantum theory allows such a counterintuitive phenomenon, known as the quantum Cheshire cat effect [1], to happen for a pre- and post-selected (PPS) system. While observing the quantum Cheshire cat, the physical property and the carrier's location should be weakly probed with a measuring device to leave the quantum state with negligible disturbance. To date, experimental observations of the quantum Cheshire cat effect have been realized with a single neutron as well as with a single photon [2, 3]. However, their observations relied on weakly disturbing elements instead of a true weak measuring device. In this work, we report the experimental observation of the quantum Cheshire cat effect with a weak measuring device. To investigate the quantum Cheshire cat effect, we employ a photon pair generated by spontaneous parametric down conversion (SPDC). The elements of quantum Cheshire cat, namely the body (photon) and the grin (polarization), are indirectly probed with a quantum pointer using another single photon, where both the body and the grin are weakly measured without destroying the photon itself.
Cavity quantum electrodynamics (QED) in the strong-coupling regime provides the opportunity to monitor and control the dynamics of a simple quantum system. A single Cesium atom interacts strongly with single-photon fields in the mode of a high-finesse optical cavity. When the resulting coherent coupling rate dominates dissipation in the system, strong coupling is realized and the system displays distinctively quantum behavior. The coupling between atomic internal states and the quantized cavity field allows for diverse protocols in quantum state preparation, quantum communication, and quantum logic. However, the atom's external or motional state must also be taken into account. My research develops cavity QED in the limit where the coherent atom-field coupling dominates the atomic kinetic energy and thus significantly affects the atomic center-of-mass motion. In this regime, the interaction of the atom with the cavity field provides both a means of controlling atomic motion and a signal for detecting that motion in real time with high signal-to-noise. The sensing capability of the ``atom-cavity microscope" is exploited to trap single atoms with single-photon fields and to monitor their orbits in real time as they are bound in the cavity. Such real-time position sensing is the basis for a detailed strategy and ongoing experiment to actively stabilize select aspects of an atom's motion within the cavity. As the cavity-enabled position measurement approaches the standard quantum limit, this work begins to realize a quantum servo for atomic position and to address questions of optimal state estimation and state preparation. In combination with other progress in cavity QED, it furthers the goal of controlled atom-field interactions for quantum information science.
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.