LAUSANNE, Switzerland — At the Massachusetts Institute of Technology in the mid-1970s, Ernest J. Moniz was an up-and-coming nuclear scientist in search of tenure, and Ali Akbar Salehi, a brilliant Iranian graduate student, was finishing a dissertation on fast-neutron reactors.

The two did not know each other, but they followed similar paths once they left the campus: Moniz went on to become one of the nation’s most respected nuclear physicists and is now President Obama’s energy secretary.

Salehi, who was part of the last wave of Iranians to conduct nuclear studies at America’s elite universities, returned to an Iran in revolution and rose to oversee the country’s nuclear program.

Forty years later, they are facing off in intense one-on-one talks as the deadline approaches for a nuclear deal that could be one of the most important, and disputed, international accords in decades.

The two sides have set a deadline of March 31 for a framework agreement that can become the basis for a comprehensive deal to be reached by the end of June. The State Department said ‘‘difficult work’’ remained for negotiators.

Iran’s chief diplomat said Saturday that he had had productive discussions with his European counterparts and that negotiators were ready to begin drafting an initial agreement on an Iranian nuclear accord. But western diplomats cautioned that gaps remained and that it was still unclear if they could be bridged.

Moniz and Salehi have emerged as their countries’ No. 2 negotiators, a pair of atomic diplomats taking on the vast technical issues that lie beneath the political disagreements.

Their roles as deputies to the chief negotiators — Secretary of State John Kerry and Mohammad Javad Zarif, the Iranian foreign minister — signal that the two sides are down to the hardest issues about the kind of nuclear infrastructure Iran will be permitted to retain, and in recent days, those discussions have hit major road blocks.

The two had met only once before, in Vienna, more than a decade ago. But in the past five weeks, they have negotiated alone for more than 20 hours.

“We have a good rapport,” Moniz said as he poured himself a glass of well-aged Scotch and settled into the living room of his Lausanne suite, overlooking Lake Geneva.

The question is whether it is possible to dismantle enough of Iran’s nuclear infrastructure to assure the United States and its allies that they would have enough warning to stop Iran if it tried to build a nuclear bomb.

In a sign that the negotiators are getting closer to an initial agreement, foreign ministers from the other world powers have begun arriving in Lausanne.

“The endgame of the long negotiations has begun,” said Frank-Walter Steinmeier, the German foreign minister. “The final meters are the most difficult but also the decisive ones.”

Moniz, 70, understands his role well: He is providing not only technical expertise but also political cover for Kerry. If a framework agreement is reached in the next few days, it will be Moniz who will have to vouch to a suspicious Congress, to Israel, and to Arab allies that Iran would be incapable of assembling the raw material for a single nuclear weapon in less than a year.

Salehi, 66, will have his own problems selling an agreement to the generals and clerics in Tehran, many of whom are suspicious of Iran’s Western-educated negotiators and will have to be convinced that Iran has not backed down in the face of US demands.

Moniz, who was born in 1944 in Fall River, Mass., got hooked on science as a high school student in the post-Sputnik era of the late 1950s and early 1960s. After attending Boston College, he earned a doctorate in theoretical physics from Stanford and then he joined the faculty at MIT, where he fell in with a group of physicists who were active in the Union of Concerned Scientists and similar groups.

By Boston Globe