Fig. 3: Experimental observation of the quantum Cheshire cat effect.
From: Observing the quantum Cheshire cat effect with noninvasive weak measurement
The observation is realized by measuring weak values. Measurements of pointer states (solid circle) are recorded as a function of g for observables: a\({\hat{{{\Pi }}}}_{\text{l}}\otimes \hat{{\mathbb{I}}}\) and b\({\hat{{{\Pi }}}}_{\text{l}}\otimes {\hat{\sigma }}_{{x}}\). Note that the imaginary parts have zero value, so the results for \({\langle {\hat{\sigma }}_{{y}}\rangle }_{{\rm{p}}}^{a}\) are not presented. One standard deviation due to Poissonian counting statistics are considered as error bars. The black solid lines are the exact theoretical predictions. For given observables, weak values are extracted by taking the first-order derivative with polynomial curve fit at g = 0. The measurement results of \({\langle {\hat{{{\Pi }}}}_{\text{l}}\otimes \hat{{\mathbb{I}}}\rangle }_{{\rm{w}}}=0.018\pm 0.206\) and \({\langle {\hat{{{\Pi }}}}_{\text{l}}\otimes {\hat{\sigma }}_{{z}}\rangle }_{{\rm{w}}}=1.085\pm 0.206\) indicate the quantum Cheshire cat effect that the physical property (polarization) can be found in the path where the physical carrier (photon) does not exist.
