Researchers at the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) have achieved a non-contacting exchange of the polarization of two photons to unravel the exclusive quantum properties of the “Quantum Cheshire Cat.”
The study was performed by Prof. LI Chuanfeng, XU Jinshi, and XU Xiaoye from USTC in collaboration with Prof. CHEN Jingling from Nankai University.
Published in the Nature Communications journal, the study offers in-depth insights into the most fundamental problem of physics—“what is physical reality.”
In the classical realm, an object is predicted to carry all of its physical characteristics. By contrast, in the quantum realm, a quantum object may not act so—it can temporarily leave a few of its physical characteristics where it does not appear.
First hypothesized by Yakir Aharonov in 2013, this phenomenon is called the quantum Cheshire cat effect. Cheshire Cat is a grinning cat that is mentioned in the “Alice in Wonderland” fairy tale. Though it disappears, its grin still hangs in the air.
In the following years, the separation of the properties of particles from the particles was observed by experimental physicists through photon and neutron interference experiments. Soon, scientists found that the outcomes of such experiments could be elucidated by using the classic interference theory.
However, more complex experiments were required to demonstrate the exclusive quantum effects of the “Quantum Cheshire Cat.”
For the first time, Prof. LI’s team employed the two-photon system to reveal the exclusive quantum effect of two “quantum Cheshire Cats” that exchange grins. Characterization of the position of the Cheshire cat and its grin in experiments requires weak values. But it is extremely challenging to extract weak values in multi-body quantum systems.
In this research, the researchers demonstrated that it is possible to bypass the conventional weak measurement method by applying a perturbation to the system. The inherent relationship between the strength of the perturbation and the system detection probability can be used to directly obtain the weak value.
The researchers developed a two-photon hyper-entangled state where the path degrees of freedom and polarization of both the photons are in the maximum entangled state but there exist no correlations between both the degrees of freedom. Imaginary time evolution was then used to introduce perturbation to acquire the weak value of polarization and path of the photon.
The researchers used the weak values to observe that each individual photon and its polarization are separated, and finally, the other photon’s polarization is obtained. Thus, they achieved the non-contacting grin exchange of the two “quantum Cheshire cats.”
Liu, Z.-H. (2020) Experimental exchange of grins between quantum Cheshire cats. Nature Communications. doi.org/10.1038/s41467-020-16761-0.