Hong Kong(CNN) When Dennis Hu came home to China for the 2020 Lunar New Year, he thought it would be a brief visit. He planned to enjoy the festivities with his family, renew his United States visa, and then head back to Boston to continue the fourth year of his doctorate in computer science at Northeastern University.
But a year and a half later, he's still stuck -- and he has no idea when he'll be able to return to the US.
Hu is one of more than 1,000 Chinese students who spent years working toward studying at a US university, only to have their studies stalled, first by Covid and then by an ambiguously worded visa ban imposed under the Trump administration.
Faced with the perceived threat of Chinese students conducting espionage on US soil amid heightened tensions with China, then-US President Donald Trump introduced the ban that effectively prevents graduate science, technology, engineering and mathematics (STEM) students from several Chinese universities from gaining visas to the US, the world's biggest research hub.
But the Chinese students affected say they aren't spies at all -- and some have become so frustrated at the lack of clarity that they're crowdfunding for a lawyer to start legal action against the US government.
"The ban is based on a simple presumption: If you have been to a certain school, you will be targeted and labeled as a spy," Hu said. "I think it's a policy of discrimination based on nationality.
"The combination of the (ban) and the pandemic have led to a complete derailing of (students') studies, their careers and their lives."
Experts say the issue goes far beyond the individual students who are directly impacted, warning that shutting out international graduate students who contribute to important STEM studies could have implications for the quality of American research.
It could also impact the ability of US institutions to recruit Chinese students, and exacerbate the already-fraught relations between the two countries.
For the first few months of last year, a US travel ban to stop the rapid spread of Covid prevented Hu from returning to the US from his hometown in China.
Then, last May, Hu's problem turned political.
For years, US intelligence has warned that China is using student spies to steal American secrets. The Australian Strategic Policy Institute (ASPI), an independent think tank, says recruiting students is part of China's military-civil fusion strategy of using civilians to boost its military power, something that has been increasing in the past decade.
With more than 370,000 Chinese students in the US -- almost twice as many as any other source country -- US authorities are faced with a major dilemma: how to strike the balance between protecting America's open academic environment and mitigating the risk to national security.
"It is legitimate to be concerned about vulnerabilities within universities," said Robert Daly, the director of the Wilson Center's Kissinger Institute on China and the United States and a former US diplomat in Beijing. But, he adds, it has to be measured against the "enormous benefit we've had from the brain drain contribution of Chinese students and scholars over the past 40 years in the United States."
Beijing has called claims of student espionage "groundless," arguing that "unsubstantiated" accusations are a type of bigotry and discrimination that will ultimately damage US interests.
Undeterred, Trump signed a presidential proclamation in May last year clamping down on visas for students with perceived military ties.
The proclamation -- which Beijing slammed as "political persecution" -- claimed China is engaged in a "wide-ranging and heavily resourced campaign" to acquire sensitive US technologies and intellectual property, partly to bolster the Chinese military's modernization and capability.
The proclamation didn't specify who would be banned, but Hu was worried. He had studied for his undergrad at Northwestern Polytechnic University -- one of seven leading universities administered by the Ministry of Industry and Information Technology (MIIT) whose portfolio includes China's defense industry.
With the virus raging and travel bans in place, Hu opted to delay his studies by a year. Hu, like other students, hoped Trump's challenger Joe Biden would be elected and rescind the proclamation, widely known as the PP10043.
But while the Biden administration resumed issuing student visas to Chinese nationals in May, it has continued to defend the proclamation. This month, a US State Department spokesperson told CNN the policy is "narrowly targeted" as it affects less than 2% of Chinese student visa applicants and is needed to protect US research enterprise and national security interests.
That leaves Hu in limbo. If he applies for a new US visa, he fears he'll be asked about his university record and then rejected under PP10043.
It took Kerry Fang years of study before he was accepted to a PhD program at the University of California, Davis, on a full scholarship, and only 10 minutes for his dream to end at the US Consulate General in Shanghai last month.
Initially, Fang tried to play down that he had studied at a college linked to China's military. After all, it was seven years ago, and he had letters of support from his US university.
But as the staffer at the US consulate pressed him for answers, the 30-year-old admitted he'd studied at Harbin Engineering University, another leading university affiliated with China's MIIT. The staffer talked with her supervisor, then began typing.
A few minutes later, he said she handed him a refusal form and told him: "Because of the presidential ban, you cannot get a visa now."
"I graduated from university seven years ago, and the ban still won't let me go," Fang said. "If the US wants to target China, it should not target a specific group of people, and it should give the power to the visa officer, so that the officer can make his own judgment, rather than doing it in this broad brushstroke manner."
Wan -- who CNN is only referring to by his last name due to his fear of repercussions -- had a similar experience when he went to apply for his visa in May to pursue a Master's degree in electrical and computer engineering. The 24-year-old knew Trump had signed something related to Chinese students the year before, but he didn't think it would impact him.
Wan was only asked two questions at the consulate in Guangzhou: where had he gone to undergraduate school, and what was his major.
Once the visa officer heard the name of his school -- Nanjing University of Aeronautics and Astronautics, another MIIT-affiliated university -- her facial expression changed, he said. She stopped asking questions and kept typing. After a minute or two, she told him his visa was rejected and handed him a piece of paper stamped with the same code: "PP10043."
Since the policy was imposed in May last year, at least 1,000 students have been banned under the proclamation, the US State Department said last year. The State Department has not replied to CNN's request for updated figures on students affected.
Some were refused a visa, while others had their visas revoked and were expelled from the US. Many others may have decided not to apply for a visa due to the ban.
China's Ministry of Foreign Affairs has urged the US to withdraw its visa restrictions. Last month, Chinese Foreign Ministry spokesperson Zhao Lijian expressed "grave concern" that some Chinese visa applications had been denied.
"They are of great significance to enhancing mutual understanding between the two peoples and promoting the steady development of China-US ties," Zhao said, adding the US visa restrictions "carry on a poisonous legacy of the Trump administration and run counter to the US statement of 'welcoming Chinese students.'"
Chinese officials raised the US visa issue at recent high-level talks with US counterparts in Tianjin, according to Chinese state media.
China's denials are in part undermined by several cases of espionage prosecuted in the US. In recent years, for instance, student Zaosong Zheng was charged with attempting to smuggle biological research to China, and Chinese electrical engineering student Ji Chaoqun was indicted by a grand jury, accused of acting as an illegal agent.
Institutions with strong military and security links are "disproportionately implicated in theft and espionage," according to a 2019 Australian Strategic Policy Institute report.
But the US has tended to overstate the threat, said Daly from Wilson Center's Kissinger Institute on China and the United States. The Department of Justice said it does not track data on cases according to race, ethnicity or nationality. And there have also been a number of high-profile botched prosecutions of Chinese students on espionage-related charges -- last month, for instance, US prosecutors asked a judge to dismiss a case against a Chinese researcher accused of concealing her ties to the Chinese military on her visa application, Associated Press reported.
While Chinese espionage is undoubtedly taking place on US soil, the current policy is arbitrary and sweeping, said Eric Fish, author of "China's Millennials: The Want Generation." The universities being targeted aren't military universities per se -- they are civilian universities with thousands of students, most of whom have nothing to do with the military, he added.
"(US authorities are) really using a giant machete where they could be using a scalpel," Fish said.
While Fish doesn't think Biden would have initiated the policy, the tense US-China relationship means it might not be a politically savvy time to reverse it.
"If Biden were to rescind this, he would leave himself wide open to attacks from Republicans about being too soft on China, about throwing the door open to Chinese espionage, even though that's not really what the situation is," Fish said.
To Hu's friend Matthew Jagielski, a PhD graduate from Northeastern University, the thought of Hu being a spy is laughable.
"I definitely don't get the impression that his research is a military sensitive thing. I also don't get the impression that he is a person trying to sneak in or anything," he said. "He was very much a staple of the lab ... It's just really unfortunate."
It's not just Chinese students missing out. The Trump-era policy could impact the quality of US research as well, experts say.
Although the students affected make up only a tiny proportion of Chinese students overall, they're doing some of the most critical research. An estimated 16% of STEM graduate students in the US are Chinese nationals. About a third of top-tier AI researchers did their undergraduate degrees in China, but more than half of them went on to study, work and live in the US, according to Macro Polo, a Chicago-based think tank.
"What you're doing is you're keeping out thousands of very high-value Chinese students in very valuable fields that contribute a lot to American research, that contribute a lot to these labs," Fish said.
Hu's friend Jagielski puts it simply: "If you have fewer grad students, it means that less research is being done."
That worries US universities. Though Northeastern, Hu's university, said it couldn't comment on specific students, the university chancellor sent a letter to Chinese international students last year, saying it would "challenge shortsighted policies." In May, the vice provost for international affairs at prestigious Cornell University, Wendy Wolford, wrote to the US Secretary of State, criticizing the "highly charged and ambiguous atmosphere for students."
"Maintaining the flow of international students to the United States is necessary to both retaining our global preeminence and building stronger cultural and political bridges to the rest of the world," Wolford wrote.
Fang, for instance, was set to research the International Thermonuclear Fusion Reactor, with the aim of "developing clean energy for the future use of all mankind," he said.
Hu, ironically, was researching algorithm auditing in a bid to make Google search results more equal. "No matter what ethnicity you are, regardless of men and women, you get this kind of fair and friendly results," he said. "Even if I am studying at a US university, people all over the world can read my paper. So, from this point of view, even though I am in the US, in some sense I am also contributing to China and the world."
And the US's loss is China's gain. In fact, says Fish, the US policy is helping China do what it's been trying to accomplish for years -- draw its top talent back home.
In recent years, China's universities have been amping up their research capabilities in an attempt to lure top researchers. But while China's research output is improving, the US remains the world's research power house, meaning it's able to attract top global talent.
"One of the greatest sources of American power is that we still have the best universities in the world that attracts the top talent," Daly said. "And the excellence of our universities depends on openness and internationalization."
"China's attractiveness to talents is now gradually catching up with the United States, because China is a country that is becoming more and more attractive to talent in terms of economic strength," Fang said. "If the United States does not maintain the academic superiority, this part of the international talent will be lost, and the lost talent may go to China."
While concerns about espionage are legitimate, the scale of the threat is unclear -- and that makes it difficult to balance the potential loss of US intelligence against the benefit of Chinese students.
"It's not that there's no issue -- the concerns, given everything we know about China, are legitimate," said Daly, the former US diplomat in Beijing, adding it would be better to target students from sensitive sub-disciplines, rather than going after perceived military links.
"We're telling the world's biggest talent pool that it is an unwelcome or despised class in the United States, and that harms the American innovation system," Daly said. "There will always be vulnerabilities there, but our goal is to manage them and reduce them -- not eliminate them entirely. Eliminating all vulnerabilities is shutting ourselves off from the world's biggest talent pool."
Some politicians in the US, however, are pushing for even tougher rules.
Republican Senator Tom Cotton has introduced a bill restricting Chinese national graduate and postgraduate students in STEM fields, claiming it would "secure American research from Chinese Communist Party espionage and influence."
"The congressman's rhetoric is reinforcing the hatred of Asians and Chinese," Hu said.
Faced with uncertainty over the ban, some students are giving up on the US altogether, and heading to Canada, the United Kingdom, Australia, Singapore or Hong Kong. Wan is one of them -- he's now hoping to get to Australia in the spring. If he can, he says PP10043 will have only wasted six months of his life.
"I just can't afford wasting a year or two on this just because of the US visa restriction," he said, adding that he resigned from his job in Nanjing because he thought he would be able to go to the US. "If the time is wasted, it's a waste of youth."
Some say fewer Chinese students arriving in the US will reduce cross-cultural understanding at a time when it is needed most.
The move could also alienate prospective students disturbed by the discrimination Chinese students face in the US and the suspicion clouding the US-China relationship.
"I really think when you weigh the miniscule amount of espionage that this might be preventing against the massive competitive disadvantage the US is putting itself at with this, really it's a very counterproductive move," Fish said.
"A lot of these students kind of look at the US with wide eyes before coming over, thinking it's the land of the free -- it's where the top scientists in the world train. And then they get here and they encounter a very uncertain status. I think they go back home with very negative feelings towards the US that they didn't have before."
That's the feeling for Hu, who is still stuck waiting for something to change.
"I feel quite sad. I think we are a group of very serious researchers, but we are being unjustifiably accused and even framed," he said. "This ban won't guarantee national security for the US -- instead it will endanger America's academic prestige."
Correction: An earlier version of this story misstated where the consulate Wan applied for his visa was located. It was in Guangzhou.