Two 2023 experiments with ultracold molecules have revealed unusually precise ways to tune molecular interactions: a very narrow magnetic resonance in sodium–lithium (NaLi) collisions and microwave-controlled, long-range bound states in sodium–potassium (NaK). Together, they show how external fields can reshape collisions at the quantum-state level. They do not yet let scientists direct an ordinary chemical synthesis on demand, but they provide new physical mechanisms that could eventually make individual reaction pathways controllable.
The distinction matters. The experiments observed resonant interactions and unusual loss or binding behavior in carefully prepared molecular samples. They did not report a practical reaction yield, a laboratory process for making a useful chemical, or a demonstrated coherent reaction-control protocol. Their importance is that they expand the set of tools available for studying and manipulating molecular collisions.
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What the two experiments actually found
The results involve different molecules, different external fields and different kinds of resonant state. Treating them as one discovery would hide the essential physics.
NaLi molecules: one unexpectedly narrow magnetic feature
Juliana Park and colleagues in Martin Ketterle’s MIT group studied collisions between two sodium–lithium molecules. The researchers expected molecule–molecule resonances to be difficult to see. A collision between two molecules can access many more internal and rotational states than an atom–molecule collision, giving resonant signatures more opportunities to broaden or wash out.
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Instead, the team found a single, extremely narrow peak in sample loss as the magnetic field was varied. At the field associated with that peak, the loss rate rose by more than two orders of magnitude. In other words, a very small change in the field selected a collision condition in which molecules disappeared from the observed sample far more rapidly than away from the feature.
The group proposed that waves entering a Feshbach resonance and waves leaving it could interfere destructively. Such interference might preserve an intermediate state long enough to enhance loss and produce the narrow signal. This remains a proposed explanation, not an established mechanism. Park explicitly cautioned, “We’re not sure if this is the right model.”
NaK molecules: microwave-linked long-range states
A separate German experiment used polar sodium–potassium molecules. Instead of relying on magnetically tunable electron energy levels, the researchers applied an oscillating microwave field. Polar molecules have electric dipoles, so the field can couple their rotational states and alter how one molecule interacts with another.
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Changing the microwave polarization changed the observed resonance. The interaction can be pictured as attractive at relatively long range but repulsive at shorter range, creating a potential well that binds a pair without requiring the molecules to approach as they would in an ordinary chemical bond. The resulting long-range pairs were described as supermolecules.
These are not conventional molecules made by sharing electrons at close range. The paired NaK molecules remain comparatively far apart and are held together by field-controlled electric-dipole interactions. University of Chicago physicist David DeMille called the state, “really quite unlike any molecular state that’s ever been seen before.”
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How a resonance gives researchers leverage
Magnetic Feshbach resonance in NaLi
A Feshbach resonance occurs when a colliding pair in an open, or scattering, channel is coupled to a bound state in another channel. A magnetic field shifts the energies of states with different magnetic moments. When a bound level is tuned near the collision energy, the coupling can strongly alter the collision outcome.
For the NaLi experiment, the relevant result is not simply that a magnetic field changed a rate. It is that the field selected an exceptionally narrow condition in a molecule–molecule collision, despite the large number of states that might normally obscure such a feature. The sharpness provides a sensitive handle: a field setting can distinguish a small region of the collision-state landscape from neighboring settings.
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Microwave-induced field-linked resonance in NaK
The NaK work uses a different control knob. Because NaK is polar, an oscillating microwave field can couple to the molecules’ rotational structure and change the dipole–dipole interaction. The polarization of that field determines how the dipoles couple, allowing the researchers to manipulate the resonance rather than merely scan a static magnetic field.
In the reported picture, the field creates a long-range potential that attracts molecules at larger separation and repels them closer in. A minimum between those regions can support a bound pair. The molecules are therefore linked by an externally engineered interaction landscape, not by the short-range electronic bonding that defines an ordinary chemical species.
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NaLi and NaK compared
| Feature | NaLi experiment | NaK experiment |
|---|---|---|
| Molecular species | Sodium–lithium molecules | Sodium–potassium molecules |
| Collision or state studied | Collisions between two molecules, observed through sample loss | Pairs linked into long-range bound states, called supermolecules |
| Main control field | Magnetic field | Oscillating microwave field |
| Interaction mechanism | Magnetically tuned Feshbach resonance involving coupled collision and bound-state channels | Microwave-controlled electric-dipole interaction with a long-range potential well |
| Distinctive observation | A single extremely narrow loss peak; loss increased by more than two orders of magnitude at the feature | Field-linked resonances whose behavior changed with microwave polarization |
| What is established | A resonant loss feature was observed | Microwave-induced field-linked resonant states were experimentally confirmed |
| What remains uncertain | The proposed destructive-interference explanation is not confirmed | Practical chemical-reaction control remains a future possibility, not a demonstrated application |
Why ultracold temperatures make these effects visible
At ordinary temperatures, molecules move with a broad distribution of kinetic energies and collide through many partial waves and internal states. A narrow quantum feature can then be averaged over a wide range of conditions. Ultracold samples restrict the available collision energies and make external-field shifts comparable to the energy scale of the collision itself.
That does not mean the molecules stop moving or that every collision becomes simple. Molecules still carry rotational, vibrational and hyperfine structure, and those degrees of freedom create many possible channels. The advantage is experimental resolution: when the energy distribution is narrow, a field-tuned resonance can stand out instead of being buried in thermal averaging.
Ultracold conditions also make it possible to prepare and monitor a small, well-defined molecular ensemble. In the NaLi work, the signature was a change in the number of molecules remaining after collisions. In the NaK work, the field-linked state was identified through the response of the molecular pairs as the microwave conditions were changed.
What these observations do—and do not—demonstrate
Demonstrated in the reported studies
- Two molecules can exhibit a sharply resolved resonant feature even when molecule–molecule collisions offer many possible internal states.
- A magnetic field can tune NaLi collisions into a condition with a loss rate more than two orders of magnitude higher than away from the narrow feature.
- An oscillating microwave field can induce and manipulate resonances in polar NaK molecules.
- Electric-dipole interactions can bind NaK pairs at comparatively long range, producing states unlike conventional short-range chemical bonds.
Not demonstrated
- No experiment reported a useful chemical product, reaction yield or application outside the laboratory system.
- The NaLi destructive-interference model has not been independently established; it is the team’s proposed interpretation.
- Neither study showed that researchers can already direct an ordinary chemical reaction along a chosen pathway at will.
- A controllable p-wave superfluid was discussed as a possible future direction, not produced in these experiments.
The 2023 report does not give an exact temperature for the experiments, a reaction rate, a reaction yield or a field setting that can be transferred directly to another molecular species. Those omissions are important because resonance positions and widths depend on the species, internal state, field geometry and preparation method.
Why the findings matter for reaction control
Chemical reactions are governed by how reactants move through a landscape of interaction potentials. If an external field can shift a bound state, create a long-range well or change the coupling between collision channels, it can alter which outcomes are accessible. The NaLi and NaK results demonstrate two complementary ways to engineer that landscape.
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A magnetic Feshbach resonance can provide a narrow tuning point for coupling scattering states to a bound molecular channel. A microwave field can reshape dipolar interactions over a longer distance and can add a polarization-controlled directional element. These mechanisms give researchers independent questions to test: whether a collision can be made more likely, whether an unwanted loss channel can be suppressed, and whether a selected internal state can remain coherent long enough to influence the outcome.
DeMille summarized the broader promise as follows: “what’s really exciting is that you can use these states to influence how the molecules are interacting in a controlled way.” That is a statement about capability under development, not a claim that a practical synthesis has already been controlled.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Possible next steps in the field
Map resonance structure more precisely
For NaLi, a central task is to determine why the feature is so narrow and why it produces such a large increase in loss. Comparing line shapes under different molecular internal states and collision conditions could test whether interference is responsible or whether another multichannel mechanism explains the observation.
Control interactions without forcing close contact
The NaK field-linked state offers a way to modify interactions while keeping molecules at long range. Researchers could investigate how the microwave frequency, intensity and polarization alter the potential well, the lifetime of the pair and the conversion between free molecules and linked states. The reported work establishes the resonance and its field dependence; it does not establish a general recipe for arbitrary molecules.
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Build coherent many-body systems
Long-range, tunable interactions are also useful for quantum simulation. Xin-Yu Luo discussed a possible route toward a controllable p-wave superfluid, an exotic form of quantum matter. That idea is a proposed application. The reported NaK experiment did not create the superfluid, and the NaLi experiment did not demonstrate a many-body phase.
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Limits that will shape practical progress
- Complex molecular structure: Molecules have more internal states than atoms, creating additional loss pathways and making calculations and state preparation harder.
- Resonance specificity: A field value that works for NaLi or NaK cannot be assumed to work for another species. Each molecular combination has its own energy levels and couplings.
- Short-range chemistry: Long-range binding is not the same as forming a conventional chemical bond. A field-linked pair must still be understood as it approaches regions where ordinary chemical forces and reactive channels become important.
- Model uncertainty: A striking line shape can reveal new physics without immediately identifying its microscopic cause. The proposed NaLi interference picture remains to be tested.
- Laboratory conditions: The effects rely on ultracold molecular samples and carefully controlled external fields. The report provides no evidence that the same control survives under ordinary pressure or temperature.
Where the results fit in the field’s timeline
The work builds on the MIT group’s 2022 observation of sodium–lithium atom–molecule Feshbach resonances, including one with a more-than-hundredfold increase in loss. The 2023 NaLi result extended the search to molecule–molecule collisions and found a single narrow feature where a broader, less visible response had been expected.
In parallel, the NaK study supplied experimental confirmation of microwave-induced field-linked resonances in polar molecules. Together, the papers published in Nature in 2023—Park and colleagues, DOI 10.1038/s41586-022-05635-8, and Chen and colleagues, DOI 10.1038/s41586-022-05651-8—show that both magnetic and microwave fields can be used to access unusual molecular states.
The practical answer to “Can scientists control chemical reactions with ultracold molecules?”
Not in the everyday sense yet. Scientists can now tune specific molecular interactions, reveal narrow resonances and create long-range field-linked pairs under ultracold laboratory conditions. Those capabilities are foundational: they let researchers study collision pathways with a level of state and field control that is difficult to achieve in conventional chemistry.
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