Niels Bohr's celebrated complementarity principle, lying at the heart of standard quantum interpretation, stipulates the mutual exclusivity between the wave and the particle-like properties of quantum entities.
This implies the impossibility of jointly observing particle-like properties and wave-like features in any quantum experiment typified by, for example, the Young's double-slit experiment and its variants. Going beyond this framework of Bohrian wave-particle complementarity, the experiment reported in this paper achieves, for the first time, the following:
Unambiguous joint detection of the spatially separated distinct properties of a single photon in the respective two arms of an interferometer, while at the same time preserving the wave-like coherence between the superposed states of a single photon, which quantum mechanically evolves by simultaneously taking both arms of the interferometer. The particular properties of a photon, that have been shown to be spatially separated within the interferometer are the spatial and polarization degrees of freedom.
This intriguing effect implying a seeming disembodiment of the properties of a particle was suggested in 2013, by Yakir Aharonov and his collaborators.
It was then that the term ‘Quantum Cheshire Cat’ (QCC) was coined, in order to describe this effect and its possible variants. It was after taking cue from the alluring image of "a grin without a cat" (Cheshire Cat) conjured up by Lewis Carrol in his masterpiece Alice's Adventures in Wonderland. The key to the experimental realisation of the QCC effect lies in invoking an ingenious measurement scheme which has been called "weak measurement" involving the suitable use of a non-destructive, minimally disturbing interaction. Subsequent to the theoretical prediction, several experiments had earlier attempted to observe this effect. However, none of these experiments could perform joint weak measurements of two different properties in the respective two arms by ensuring unambiguously the presence of only one particle within the setup during each run of the experiment. Hence, this crucial requirement for a decisive verification of the QCC effect remained unrealised. This is precisely what has now been accomplished in the present experiment by judiciously designing the setup and by implementing the measurements with the desired high precision.
Thus, the effect demonstrated opens up an earlier unexplored avenue towards probing deeper into the subtleties of the wave-particle duality of quantum entities. Importantly, the interpretation of our experiment would require surpassing the widely accepted Bohrian dictum that nothing can be said about the behaviour/properties of a photon within an interferometer. This, in turn, would call for a suitable refinement of the Bohrian principle of wave-particle complementarity that can have wider ramifications, including a deeper understanding of the way the particle-like photonic properties localised in each arm of the interferometer can exhibit wave-like superposition inside the interferometer while evidencing the QCC type effect. Revealing the full conceptual import of this experiment and exploring possible applications of the QCC effect in the areas of Quantum Communication and Quantum Sensing certainly seem to be promising directions for future studies with exciting possibilities.