Ultracold Molecules Hold
Promise
for Quantum Computing
New approach yields long-lasting configurations that could provide long-sought “qubit” material.
David L. Chandler | MIT News Office
for Quantum Computing
New approach yields long-lasting configurations that could provide long-sought “qubit” material.
David L. Chandler | MIT News Office
July 27, 2017 -- 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.
“Molecules have more ‘handles’ than
atoms,” Zwierlein says, meaning more ways to interact with each other and with
outside influences. “They can vibrate, they can rotate, and in fact they can
strongly interact with each other, which atoms have a hard time doing.
Typically, atoms have to really meet each other, be on top of each other
almost, before they see that there's another atom there to interact with,
whereas molecules can see each other” over relatively long ranges. “In order to
make these qubits talk to each other and perform calculations, using molecules
is a much better idea than using atoms,” he says.
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.”
“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 been able to trap
molecules for a long time, and also demonstrate that they can carry quantum
information and hold onto it for a long time,” Zwierlein says. And that, he
says, is “one of the key breakthroughs or milestones one has to have before
hoping to build a quantum computer, which is a much more complicated endeavor.”
The use of sodium-potassium
molecules provides a number of advantages, Zwierlein says. For one thing, “the
molecule is chemically stable, so if one of these molecules meets another one
they don't break apart.”
In the context of quantum
computing, the “long time” Zwierlein refers to is one second — which is “in
fact on the order of a thousand times longer than a comparable experiment that
has been done” using rotation to encode the qubit, he says. “Without additional
measures, that experiment gave a millisecond, but this was great already.” With
this team’s method, the system’s inherent stability means “you get a full
second for free.”
That suggests, though it remains to
be proven, that such a system would be able to carry out thousands of quantum
computations, known as gates, in sequence within that second of coherence. The
final results could then be “read” optically through a microscope, revealing
the final state of the molecules.
“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.
“These results are truly state of
the art,” says Simon Cornish, a professor of physics at Durham
University in the U.K.
The team also included MIT graduate
student Zoe Yan and postdoc Huanqian Loh. The work was supported by the National
Science Foundation, the U.S. Air Force Office of Scientific Research, the U.S.
Army Research Office, and the David and Lucile Packard Foundation.
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