Researchers at the University of Science and Technology of China (USTC), together with researchers from United Kingdom and Italy, have observed recovery of quantum correlations in classical environment without system-environment back-action.

Revivals of quantum correlations in composite open quantum systems are a useful dynamical feature against detrimental effects of the environment. Their occurrence is attributed to flows of quantum information back and forth from systems to quantum environments. However, revivals also show up in models where the environment is classical, thus unable to store quantum correlations, and forbids system-environment back-action. This phenomenon opens basic issues about its interpretation involving the role of classical environments, memory effects, collective effects and system-environment correlations. Moreover, an experimental realization of back-action-free quantum revivals has applicative relevance as it leads to recover quantum resources without resorting to more demanding structured environments and correction procedures.

Following a simple two-qubit model suitable to address these issues, the experimental group leaded by Prof. Chuan-Feng Li at USTC prepared a special kind of entangled photon state, namely the Bell diagonal state, in an entirely optical setup. One of the photon is directly sent to the state tomography part whereas the other photon is directed to the environment part and successively to the state tomography part. The environment is given by two paths of the photon separated by a beam-splitter, the reflected path and the transmitted path, and by the measurement process that traces over the time difference between the two paths themselves, creating a statistical mixture of them with equal probabilities. The effect of the two paths is to make the photon basis states undergo, apart from an unimportant global phase factor, unitary transformations with different initial phases. The evolution process is simulated by quartz plates.

In this experiment, they recover and control, against decoherence, quantum correlations without back-action. What’s more they finally give an interpretation of the phenomenon by establishing the roles of the involved parties. 

References:

Jin-Shi Xu, Kai Sun, Chuan-Feng Li, Xiao-Ye Xu, Guang-Can Guo, Erika Andersson, Rosario Lo Franco & Giuseppe Compagno, Experimental recovery of quantum correlations in absence of system-environment back-action, Nature Communications 4, 2851 (2013). 

(School of Physical Sciences)