Zhang's bedroom as seen these days. The father has added a bed so that he can come in and lie down for a while wherever he misses his daughter. [Photo provided to China Daily]

In August 2018, before visiting Hou in Beijing, Shi went to Zhang's home in a small mountainous town in East China's Fujian province. Zhang's study-bedroom was on the top floor of the family's sparsely decorated and slightly rundown four-story brick house, a few steps away from the chicken coop.

The room was kept as it had always been: on a writing desk in front of the window, a lamp and a pile of award certificates, proof of Zhang's striving for excellence as a student, was collecting dust. A teenage Zhang once sat there studying into the wee hours. Once in a while she might have looked up and peered into the world map covering an entire wall on her right side.

The last farewell - Zhang Yingying (middle) with her parents at the train station in Jianyang, her hometown, before she left for Beijing, where she took the flight to US. [Photo provided to China Daily]

It's a world she had set out to explore, as she journey from her native town to the bustling coastal city of Guangzhou 850 kilometers away to attend one of China's top universities, where she met Hou, her classmate. After they graduated in 2016 both were enrolled by the prestigious Peking University, Zhang in its master's program and Hou in its doctoral program. By the time Zhang left for the US, she had completed her studies at Peking University, before working briefly at the Chinese Academy of Sciences.