Physicists Want to Build a Quantum Computer Out of Helium-3, the Lightest Atom a Laser Can Hold. It Tunnels Three Times Faster Than Lithium, and Its 'Stepstool' State Lasts Two Hours.

The atoms in today's neutral-atom quantum computers are chosen partly for convenience: rubidium, cesium and lithium are easy to cool and easy to hold in a focused laser beam. A team at the University of Chicago's Pritzker School of Molecular Engineering has proposed going lighter, all the way down to helium-3, the lightest atom that current technology can trap. The design, published in PRX Quantum, argues that the atom's small mass, unusual energy structure and fermionic character add up to a faster and more natural platform for quantum simulation than anything built so far.

The physics starts with weight. In an optical lattice, atoms hop between neighboring sites by quantum tunneling, and lighter atoms tunnel faster. "Helium is even lighter than lithium, so that provides quantum tunneling rates about three times faster, at least," said Zheyuan Li, a Ph.D. student in Jacob Covey's lab and co-first author of the paper. "Helium also has a much better-resolved energy structure, which means it could be laser-cooled much more easily than lithium atoms." Hydrogen, the only lighter option, is out of reach: the energy needed to lift it from its ground state to the excited state used for trapping is beyond what modern lasers deliver. Helium has the same problem in principle, but it has a second electron, and that gives it a metastable state partway up, a temporary landing between ground and excited levels. If reaching the excited state is like jumping onto a table, Covey's group says, the metastable state is a stepstool. In most elements such a state lasts seconds. Helium's lasts two hours.

The other ingredient is the missing neutron. Helium-4, the isotope in party balloons and MRI coolant, is a boson. Helium-3 has one fewer neutron and is a fermion, which means two atoms cannot occupy the same quantum state. That exclusion rule is what makes electrons in materials behave the way they do, and physicists have wanted a native fermionic quantum computer since the 1990s; the first working examples, based on lithium, appeared only early this year. "That would help us to implement fermionic quantum computing much more natively with this platform, rather than trying to use bosonic atoms like helium-4 and then trying to simulate fermionic structure," Li said. Princeton physicist Waseem Bakr, who was not involved in the work, called it "a compelling blueprint for the next generation of fermionic quantum simulators," one that "turns low mass into a real advantage: faster tunneling, faster transport and controllable motional qubits."

The optical tweezers themselves are single focused laser spots that hold one atom each; Covey compares them to a tractor beam. "You just have one beam, one focused spot, and the atom is attracted to that spot," he said. The plan is to build arrays of such traps, each holding a metastable helium-3 atom, and use the atoms' motion in the traps as an additional quantum resource alongside their internal states.

Building it is the next step, in collaboration with UChicago physicist Zoe Yan, a co-author who has worked with lithium-6. Cost dictates the order of operations. "Helium-3 is very expensive," said Rupsa De, the paper's other co-first author. "So we will start with helium-4 and do this first, and then proceed toward using helium-3." The experiment will not strain the world's helium supply: a single 5-liter tank, Covey said, "can last us for many years," since the goal is to trap tens of atoms, not the 3-followed-by-23-zeros in one balloon. "The foundation is there," he said, "and the progress is advancing now to the point where we would hope to have these atoms in tweezers for the first time probably within the next year or two."