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Two 2023 experiments show how external fields can create and detect highly unusual interactions between ultracold molecules. At MIT, a sodium–lithium (NaLi) molecule collision produced an unexpectedly narrow Feshbach-resonance feature, with sample loss rising by more than two orders of magnitude. In a separate experiment, researchers used an oscillating microwave field to create field-linked resonances between polar sodium–potassium (NaK) molecules, binding pairs at relatively long range.
These are not demonstrations of practical chemical synthesis controlled on demand. They are new ways to tune quantum collisions and molecular interactions, potentially enabling coherent reaction control and exotic forms of quantum matter.
What the two experiments actually established
“Ultracold” molecular physics studies molecules prepared at temperatures low enough that their motion is governed by quantum mechanics and by a small number of controllable collision channels. In that regime, changing a magnetic or microwave field can shift energy levels, alter the interaction potential, and reveal resonances that would be washed out in a warmer gas.
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| Experiment | Molecules and collision | Control field | Main observation | Evidence status |
|---|---|---|---|---|
| MIT NaLi study | Two sodium–lithium molecules colliding | Magnetic field | A single, extremely narrow peak in sample loss; the loss rate increased by more than two orders of magnitude at the feature | Observed resonance; the microscopic explanation remains provisional |
| NaK field-linked study | Polar sodium–potassium molecules | Oscillating microwave field, with polarization varied | Resonances associated with long-range bound pairs, called “supermolecules” | Observed field-linked states and controllable dipolar interaction; applications are prospective |
The MIT work was led by Juliana Park in Martin Ketterle’s group. The NaK experiment was carried out by a German team and is discussed in the same 15 February 2023 Chemistry World report. The report identifies the primary papers as Park et al., Nature (2023), DOI 10.1038/s41586-022-05635-8, and Chen et al., Nature (2023), DOI 10.1038/s41586-022-05651-8.
Why a NaLi molecule–molecule resonance was surprising
Feshbach resonances in plain language
A Feshbach resonance occurs when an incoming scattering state is coupled to a molecular bound state whose energy can be brought into alignment. A magnetic field shifts the relevant internal energy levels. Near the alignment, the effective interaction between colliding particles can change sharply, producing a strong increase or decrease in collision-induced loss or in the probability of forming a bound pair.
Atom–molecule experiments had already shown that this method could work. In 2022, Ketterle’s MIT group reported two sodium–lithium atom–molecule Feshbach resonances. One of those resonances produced a loss-rate increase of more than a hundredfold, according to the later report. That result involved an atom colliding with a molecule, not two molecules colliding with each other.
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Why two molecules make the signal harder to see
Two molecules carry many more internal degrees of freedom than an atom and a molecule. Their rotational, vibrational, hyperfine and relative-motion states create numerous possible channels. Coupling to so many channels can spread a resonance over a broad range of magnetic field or make individual features too weak to resolve. The MIT team therefore expected molecule–molecule collisions to show no clean resonance.
The narrow loss feature
Instead, Park found one extremely narrow peak in the loss of NaLi molecules from the ultracold sample. At the field corresponding to that peak, the loss rate rose by more than two orders of magnitude. The narrowness matters: it indicates that a sharply defined quantum pathway survived despite the dense set of possible molecular states.
The measurement is a resonance observation, not a direct measurement of a useful chemical yield. “Loss” means molecules disappeared from the detected sample, which can happen through inelastic collisions or formation of products and states that the experiment no longer detects. The report does not provide a reaction yield or an application outside the laboratory.
A proposed interference explanation, not a settled answer
The MIT researchers proposed that waves entering the Feshbach resonance and waves leaving it interfere destructively. In that picture, destructive interference suppresses escape from an intermediate state, allowing population to remain there long enough to produce the sharp loss feature.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsThat interpretation is explicitly tentative. Park said, “We’re not sure if this is the right model.” The experiment establishes the narrow resonance and its large loss enhancement; it does not independently confirm that interference mechanism. Any account of the NaLi result should keep that distinction clear.
How the NaK field-linked resonance works
Why use microwaves instead of a magnetic Feshbach resonance?
Magnetic Feshbach resonances rely on magnetically tunable electron energy levels. That approach is not broadly available to molecules whose electrons are paired and therefore do not provide the needed magnetic structure. Polar molecules offer another handle: their permanent electric dipoles can interact when an external microwave field drives appropriate rotational transitions.
Polarization changes the interaction
In the NaK experiment, an oscillating microwave field was applied to the molecules. Changing the field’s polarization changes how the molecular dipoles are oriented and coupled. That, in turn, changes the effective interaction as a function of intermolecular distance and allows the resonance condition to be manipulated without relying solely on a magnetic level crossing.
The reported interaction picture is attractive at long range but repulsive at shorter range. The combination can create a potential well outside the chemically short-range region. Two NaK molecules can then occupy a bound, long-range state while remaining relatively far apart. The report calls these pairs “supermolecules.”
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Why a supermolecule is not an ordinary chemical bond
An ordinary chemical bond forms when atoms or molecules approach closely enough for their electronic wavefunctions to rearrange into a new short-range molecular state. A field-linked NaK pair is different. Its binding is dominated by field-controlled electric-dipole interactions, and the partners can remain separated by a comparatively large distance. Turning the microwave field off or changing its polarization can alter or remove the well.
David DeMille of the University of Chicago described the state as “really quite unlike any molecular state that’s ever been seen before.” That description refers to its long-range, externally engineered character, not to a claim that the pair is a new stable chemical substance.
What “controlling chemical reactions” means here
The headline phrase should be understood as a direction for research rather than a claim that ordinary reactions can already be steered at will. In these experiments, control means changing quantum interaction pathways with an external field and observing how the collision response changes.
- Prepare the molecules. The experiment creates NaLi or NaK molecules in selected internal quantum states and confines them in an ultracold sample.
- Apply a tuning field. A magnetic field is scanned for the NaLi Feshbach feature. An oscillating microwave field is applied to NaK, with its polarization adjusted as a control parameter.
- Bring a resonance into play. The field shifts the relative energy of a scattering state and a bound or dressed state, or reshapes the dipolar potential so that a long-range well appears.
- Measure the response. Researchers monitor molecular survival, collision loss or the population of a bound state as the field is varied.
- Use the response as a control knob. If the field can be changed reproducibly and rapidly enough, it may allow selected collision channels to be enhanced, suppressed or coherently coupled in future experiments.
This procedure is quantum-state control of interactions. It is not yet a recipe for choosing the products of a room-temperature synthesis, setting a desired industrial yield or making a general chemical reaction proceed on command.
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| Question | NaLi molecule–molecule result | NaK field-linked result |
|---|---|---|
| What is being tuned? | A magnetic-field-dependent Feshbach resonance involving a bound state | A microwave-dressed interaction between polar molecules |
| How do the molecules interact? | Through coupling between collision channels near a resonance | Through electric dipoles producing an attractive long-range region and a repulsive inner region |
| What was measured? | A single narrow sample-loss peak with more-than-two-orders-of-magnitude enhancement | Field-linked resonant states of NaK pairs |
| What is unusual? | A visible molecule–molecule resonance despite many possible internal channels | Binding at long range rather than formation of a conventional short-range chemical bond |
| What remains uncertain? | The proposed destructive-interference model for the narrow feature | How far the states can be turned into practical coherent reaction-control protocols |
The numerical loss statements must not be merged. The “more than a hundredfold” increase refers to one of the 2022 NaLi atom–molecule resonances. The “more than two orders of magnitude” increase refers to the separate 2023 NaLi molecule–molecule feature.
Why these states could matter for quantum science
A route to coherent reaction control
Chemical reactions normally involve many uncontrolled collisions and rapidly changing phases. Ultracold samples reduce the kinetic-energy spread and make individual quantum channels experimentally accessible. A field-linked state or a narrow Feshbach resonance can serve as an intermediary whose coupling is adjusted by the field. If coherence can be maintained, researchers may eventually manipulate the timing and pathway of a reaction rather than merely change its average rate.
The current results are an enabling step: they show that molecular interactions can be made resonant and field-sensitive even in systems with complicated internal structure. They do not establish coherent control of a complete reaction sequence.
Exotic quantum matter
Dipolar molecules interact over longer distances and with direction-dependent forces. Those properties are useful for studying many-body phases that are difficult to realize with ordinary atoms. Xin-Yu Luo discussed a possible route toward a controllable p-wave superfluid using related field-linked interactions. That is a proposed future application, not a superfluid demonstrated by the reported NaK experiment.
Likewise, the NaLi resonance could provide a way to explore strongly interacting molecular gases or other collective states. The experiments show a useful interaction feature; they do not by themselves establish a new phase of matter.
What the reports do not tell us
- No exact temperature is given. The report identifies the work as ultracold but does not state a temperature for either 2023 experiment.
- No reaction yield or product distribution is reported. A large loss signal is not equivalent to a measured synthetic yield.
- No practical application is demonstrated. The findings remain laboratory advances in molecular and quantum physics.
- The NaLi mechanism is not confirmed. Destructive interference is a proposed model, and the lead author cautioned that it may not be correct.
- The NaK pair is not an ordinary molecule. “Supermolecule” describes a long-range, field-linked state, not a conventional chemical bond.
- The proposed p-wave superfluid is future work. It should not be described as an outcome of the reported experiment.
How this changes the experimental toolbox
Before these results, researchers already had several ways to cool molecules and prepare selected quantum states, but molecular complexity made resonant control difficult. The NaLi observation shows that complexity does not automatically erase every narrow molecule–molecule feature. The NaK work shows that microwave dressing can supply an electric alternative when magnetic tuning is unsuitable.
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Together, the experiments broaden the available choices:
- Use magnetic fields when a suitable Feshbach level can be tuned into resonance.
- Use microwave fields to engineer dipolar interactions in polar molecules.
- Compare short-range collision loss with long-range bound-state formation.
- Use the width, position and field dependence of a resonance to identify which quantum channels are participating.
That broader toolbox is valuable because no single resonance mechanism works for every molecular species. The right approach depends on the molecule’s internal structure, electric dipole, magnetic properties and the degree of control required.
Where the work goes next
The next scientific questions are more specific than simply “can reactions be controlled?” For NaLi, researchers need to determine whether the proposed interference picture reproduces the measured line shape and how the feature changes with molecular state, density and collision energy. Independent tests would help separate that explanation from alternatives involving other coupled channels.
For NaK, important questions include how long the field-linked pairs remain intact, how their lifetime depends on microwave polarization and intensity, and whether the interaction can be switched quickly without heating the gas. Demonstrating controlled association and dissociation, rather than only observing a resonance, would be a stronger step toward coherent manipulation.
For both systems, a practical reaction-control protocol would need reproducible state preparation, field stability, knowledge of loss channels and a way to detect specific products. None of those capabilities is established by the 2023 report alone.
Bottom line for readers
The NaLi and NaK experiments reveal two complementary ways to engineer molecular interactions at ultracold temperatures. NaLi produced an unexpectedly narrow magnetic Feshbach-resonance feature in molecule–molecule collisions, with loss increasing by more than two orders of magnitude. NaK demonstrated microwave-controlled, long-range field-linked states whose binding is unlike an ordinary chemical bond.
Those observations move the field closer to controllable quantum chemistry and novel many-body matter. They are evidence of new interaction mechanisms—not proof that scientists can already direct everyday chemical reactions or manufacture a p-wave superfluid on demand.
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