Researchers have taken an important step toward the long-sought goal of a quantum computer, which in theory should be capable of vastly faster computations than conventional computers, for certain kinds of problems. The new work shows that collections of ultracold molecules can retain the information stored in them, for hundreds of times longer than researchers have previously achieved in these materials.
These two-atom molecules are made of sodium and potassium and were cooled to temperatures just a few ten-millionths of a degree above absolute zero (measured in hundreds of nanokelvins, or nK). The results are described in a report this week in Science, by Martin Zwierlein, an MIT professor of physics and a principal investigator in MIT’s Research Laboratory of Electronics; Jee Woo Park, a former MIT graduate student; Sebastian Will, a former research scientist at MIT and now an assistant professor at Columbia University, and two others, all at the MIT-Harvard Center for Ultracold Atoms.
Many different approaches are being studied as possible ways of creating qubits, the basic building blocks of long-theorized but not yet fully realized quantum computers. Researchers have tried using superconducting materials, ions held in ion traps, or individual neutral atoms, as well as molecules of varying complexity. The new approach uses a cluster of very simple molecules made of just two atoms.
Using this kind of two-atom molecules for quantum information processing “had been suggested some time ago,” says Park, “and this work demonstrates the first experimental step toward realizing this new platform, which is that quantum information can be stored in dipolar molecules for extended times.”
This vacuum chamber with apertures for several laser beams was used to cool molecules of sodium-potassium down to temperatures of a few hundred nanoKelvins, or billionths of a degree above absolute zero. Such molecules could be used as a new kind of qubit, a building block for eventual quantum computers. Courtesy of the researchers
“The most amazing thing is that [these] molecules are a system which may allow realizing both storage and processing of quantum information, using the very same physical system,” Will says. “That is actually a pretty rare feature that is not typical at all among the qubit systems that are mostly considered today.”
In the team’s initial proof-of-principle lab tests, a few thousand of the simple molecules were contained in a microscopic puff of gas, trapped at the intersection of two laser beams and cooled to ultracold temperatures of about 300 nanokelvins. “The more atoms you have in a molecule the harder it gets to cool them,” Zwierlein says, so they chose this simple two-atom structure.
The molecules have three key characteristics: rotation, vibration, and the spin direction of the nuclei of the two individual atoms. For these experiments, the researchers got the molecules under perfect control in terms of all three characteristics — that is, into the lowest state of vibration, rotation, and nuclear spin alignment.
“We have strong hopes that we can do one so-called gate — that’s an operation between two of these qubits, like addition, subtraction, or that sort of equivalent — in a fraction of a millisecond,” Zwierlein says. “If you look at the ratio, you could hope to do 10,000 to 100,000 gate operations in the time that we have the coherence in the sample. That has been stated as one of the requirements for a quantum computer, to have that sort of ratio of gate operations to coherence times.”
“The next great goal will be to ‘talk’ to individual molecules. Then we are really talking quantum information,” Will says. “If we can trap one molecule, we can trap two. And then we can think about implementing a ‘quantum gate operation’ — an elementary calculation — between two molecular qubits that sit next to each other,” he says.
Using an array of perhaps 1,000 such molecules, Zwierlein says, would make it possible to carry out calculations so complex that no existing computer could even begin to check the possibilities. Though he stresses that this is still an early step and that such computers could be a decade or more away, in principle such a device could quickly solve currently intractable problems such as factoring very large numbers — a process whose difficulty forms the basis of today’s best encryption systems for financial transactions.
Besides quantum computing, the new system also offers the potential for a new way of carrying out precision measurements and quantum chemistry, Zwierlein says.
Extending the coherence time of molecules
Quantum properties of atoms and molecules can be exploited for precision measurements or quantum information processing. The complex state structure of molecules can be exploited, but it is hard to preserve the coherence between pairs of those states in applications. Park et al. created fermionic molecules of NaK in the rovibrational ground state that maintained coherence between their nuclear spin states on a time scale of 1 second. This long coherence time makes dipolar ultracold molecules a valuable quantum resource.
Coherence, the stability of the relative phase between quantum states, is central to quantum mechanics and its applications. For ultracold dipolar molecules at sub-microkelvin temperatures, internal states with robust coherence are predicted to offer rich prospects for quantum many-body physics and quantum information processing. We report the observation of stable coherence between nuclear spin states of ultracold fermionic sodium-potassium (NaK) molecules in the singlet rovibrational ground state. Ramsey spectroscopy reveals coherence times on the scale of 1 second; this enables high-resolution spectroscopy of the molecular gas. Collisional shifts are shown to be absent down to the 100-millihertz level. This work opens the door to the use of molecules as a versatile quantum memory and for precision measurements on dipolar quantum matter.