In a groundbreaking feat of computation, a team of international scientists—including Harvard researchers—have calculated the precise energy of a hydrogen molecule using a quantum computer.

While scientists have been able to determine the energy of hydrogen using paper and pencil since the 1930s, they now know they can use a quantum computer to perform this—and hopefully more complex calculations—with precision.  Using classical computers, calculating the energies of larger molecules was virtually unimaginable because “the numbers get literally astronomical,” according to University of Queensland Physics Professor Andrew G. White, one of the authors of the study published in “Nature Chemistry” on Jan. 10.

“A classical computer trying to do an exact simulation of a complex process would just blow up,” White said.

Unlike classical computers, quantum computers can account for an exponential number of possibilities when looking at a single piece of information.

Chemical calculations that would take massive amounts of time and rely heavily on approximation with classical computers can therefore be more feasibly—and precisely—performed with quantum computers, explained Alàn Aspuru-Guzik, an assistant professor of chemistry and chemical biology at Harvard and another author of the study.

“Approximate calculations are everywhere, and they take you very far, you can simulate very large systems, but they usually fail somewhere,” Aspuru-Guzik said.

He added that this recent quantum triumph is exciting because it opens the opens the door to bigger and better quantum computation.

“There might be roadblocks, but you don’t have to worry about errors,” Aspuru-Guzik said of the precision of quantum computing. “You can do calculations about things in the human body, without worrying about mistakes—that’s the dream.”

The researchers hope that their project will help generate interest in research that connects chemistry and quantum computing.

“Cool science happens at the interfaces of different fields,” Aspuru-Guzik said.

This experiment was designed in Cambridge but realized in Australia.  The software designed by Aspuru-Guzik and his team was implemented using quantum computers at the University of Queensland in Brisbane, Australia.

“We sat down with them—over e-mail—and talked back and forth on Skype about how best to lay out the quantum and chemical systems,” Harvard graduate student James D. Whitfield said of bringing the software and hardware together.

“It was almost as if they were down the hall,” added fellow graduate student Ivan Kassal.

The quantum computer in Brisbane takes up two square meters, and the researchers felt an “interesting parallel” between this moment and the early days of classical computers, Kassal said. Performing innovative calculations on such a large piece of machinery made them feel like the scientists of 60 years ago, who were using computers to crack codes or fire artillery in ground-breaking ways.

None of those World War II scientists could imagine their work someday leading to the wonders of Skype or iPhones, White added.

“The reason that computers are developed is never the reason they end up getting used,” he said.

Aspuru-Guzik and White agree that, as with the work done by the scientists of the past, the full implications of their research have not yet been revealed.

“In far less than 50 years, this will change the world,” White said.  “How, I have no idea.”

—Staff writer Julie R. Barzilay can be reached at jbarzilay13@college.harvard.edu.