Physicists find a way to control charged molecules—with quantum logic

National Institute of Standards and Technology (NIST) physicists have solved the seemingly intractablepuzzle of how to control the quantum properties of individual charged molecules, or molecular ions. Thesolution is to use the same kind of "quantum logic" that drives an experimental NIST atomic clock.

The new technique achieves an elusive goal, controlling molecules as effectively as laser cooling andother techniques can control atoms. Quantum control of atoms has revolutionized atomic physics,leading to applications such as atomic clocks. But laser cooling and control of molecules is extremelychallenging because they are much more complex than atoms.

The NIST technique still uses a laser, but only to gently probe the molecule; its quantum state is detectedindirectly. This type of control of molecular ions—several atoms bound together and carrying anelectrical charge—could lead to more sophisticated architectures for quantum information processing,amplify signals in basic physics research such as measuring the "roundness" of the electron's shape,and boost control of chemical reactions.

The research is described in the May 11 issue of Nature and was performed in the NIST Boulder group that demonstrated the first laser cooling of atomic ions in 1978.

"We developed methods that are applicable to many types of molecules," NIST physicist James ChinwenChou said. "Whatever trick you can play with atomic ions is now within reach with molecular ions.Now the molecule will 'listen' to you—asking, in effect, 'What do you want me to do?'"

"This is comparable to when scientists could first laser cool and trap atoms, opening the floodgatesto applications in precision metrology and information processing. It's our dream to achieve all thesethings with molecules," Chou added.

Compared to atoms, molecules are more difficult to control because they have more complex structuresinvolving many electronic energy levels, vibrations and rotations. Molecules can consist of many differentnumbers and combinations of atoms and be as large as DNA strands more than a meter long.

The NIST method finds the quantum state (electronic, vibrational, and rotational) of the molecular ionby transferring the information to a second ion, in this case an atomic ion, which can be laser cooledand controlled with previously known techniques. Borrowing ideas from NIST's quantum logic clock, theresearchers attempt to manipulate the molecular ion and, if successful, set off a synchronized motionin the pair of ions. The manipulation is chosen such that it can only trigger the motion if the molecule isin a certain state. The "yes" or "no" answer is signaled by the atomic ion. The technique is very gentle,indicating the molecule's quantum states without destroying them.

"The molecule only jiggles if it is in the right state. The atom feels that jiggle and can transfer the jiggleinto a light signal we can pick up," senior author Dietrich Leibfried said. "This is like Braille, which allowspeople to feel what is written instead of seeing it. We feel the state of the molecule instead of seeing itand the atomic ion is our microscopic finger that allows us to do that."

"Moreover, the method should be applicable to a large group of molecules without changing the setup.This is part of NIST's basic mission, to develop precision measurement tools that maybe other peoplecan use in their work," Leibfried added.

To perform the experiment, NIST researchers scavenged old but still functional equipment, including anion trap used in a 2004 quantum teleportation experiment. They also borrowed laser light from an ongoing quantum logic clock experiment in the same lab.

The researchers trapped two calcium ions just a few millionths of a meter apart in a high-vacuumchamber at room temperature. Hydrogen gas was leaked into the vacuum chamber until one calciumion reacted to form a calcium hydride (CaH+) molecular ion made of one calcium ion and one hydrogenatom bonded together.

Like a pair of pendulums that are coupled by a spring, the two ions can develop a shared motion becauseof their physical proximity and the repulsive interaction of their electrical charges. The researchersused a laser to cool the atomic ion, thereby also cooling the molecule to the lowest-energy state. Atroom temperature, the molecular ion is also in its lowest electronic and vibrational state but remains in amixture of rotational states.

The researchers then applied pulses of infrared laser light—tuned to prevent changes to the ions' electronicor vibrational states—to drive a unique transition between two of more than 100 possible rotationalstates of the molecule. If this transition occurred, one quantum of energy was added to the two ions'shared motion. Researchers then applied an additional laser pulse to convert the change in the sharedmotion into a change in the atomic ion's internal energy level. The atomic ion then started scatteringlight, signaling that the molecular ion's state had changed and it was in the desired target state.

Subsequently, researchers can then transfer angular momentum from the light emitted and absorbed during laser-induced transitions to, for example, orient the molecule's rotational state in a desired direction.

The new techniques have a wide range of possible applications. Other NIST scientists at JILA previouslyused lasers to manipulate clouds of specific charged molecules in certain ways, but the new NISTtechnique could be used to control many different types of larger molecular ions in more ways, Chousaid.

Molecular ions offer more options than atomic ions for storing and converting quantum information,Chou said. For example, they could offer more versatility for distributing quantum information to differenttypes of hardware such as superconducting components.

The method could also be used to answer deep physics questions such as whether fundamental"constants" of nature change over time. The calcium hydride molecular ion has been identified as onecandidate for answering such questions. In addition, for measurements of the electron's electric dipolemoment (a quantity indicating the roundness of the particles charge distribution), the ability to preciselycontrol all aspects of hundreds of ions at the same time would boost the strength of the signal thatscientists want to measure, Chou said.