Introduction

The unique ability to fine-tune the interaction between ultracold atoms has led to the realization of a number of quantum phenomena1, among which the Efimov effect has become a quantum workhorse that allows for the exploration of some of the deepest issues of universal few-body physics2,3,4,5. Near a magnetic field dependent Feshbach resonance, the strength of the interatomic interaction is characterized by the s-wave scattering length a, which can assume arbitrarily large values compared to the characteristic range of the interactions, i.e., the van der Walls length \({r}_{{{{{{{{\rm{vdW}}}}}}}}}={(m{C}_{6}/{\hslash }^{2})}^{1/4}/2\), where m is the atomic mass, and C6 is the dispersion coefficient. However, not all Feshbach resonances are the same. The intricate nature of the hyperfine interactions in alkali-metal atoms allows for different couplings between the open channel and the corresponding closed channel carrying the Feshbach state. As such, a resonance is said to be broad (narrow) in case of a strong (weak) coupling and is characterized by the dimensionless strength parameter \({s}_{{{{{{{{\rm{res}}}}}}}}}\,\gg\, 1\) (\({s}_{{{{{{{{\rm{res}}}}}}}}}\,\ll \,1\))1.

Regardless of the strength of the Feshbach resonance, the Efimov effect occurs at ∣a∣→∞ due to the formation of an induced long-range three-body interaction of the form −1/R2, where R is the hyperradius5 providing the overall size of the system. This interaction gives rise to a log-periodic series of bound Efimov states whose absolute position is determined by the short-range three-body physics (Fig. 1)2,3,4,5. In the case of a broad resonance, the three-body potential supporting Efimov states features a universal repulsive wall near R ≈ 2rvdW thus preventing the atoms from probing small hyperradii. In fact, this repulsive wall is the hallmark characterizing the van der Waals (vdW) universality, according to which the ground Efimov state dissociates into the three-atom continuum at \({a}_{-}^{(0)}\approx -9.73{r}_{{{{{{{{\rm{vdW}}}}}}}}}\)6,7. This was observed across several different Feshbach resonances in 133Cs and 85Rb8,9. For narrow resonances, however, this result is expected to be modified as yet another length scale emerges, namely \({r}_{\star }=2\bar{a}/{s}_{{{{{{{{\rm{res}}}}}}}}}\,\approx\, 1.912{r}_{{{{{{{{\rm{vdW}}}}}}}}}/{s}_{{{{{{{{\rm{res}}}}}}}}}\). Since now r⋆ > rvdW, three-body observables are expected to depend on r⋆ (or equivalently, \({s}_{{{{{{{{\rm{res}}}}}}}}}\)) rather than rvdW alone10,11,12,13,14,15,16. Indeed, for intermediate resonances (\({s}_{{{{{{{{\rm{res}}}}}}}}}\,\gtrsim\, 1\)) deviations from the Efimov-van der Waals universality were already confirmed in recent precision measurements and calculations17,18,19,20,21,22. For 7Li atoms, although the Feshbach resonances are narrower than those above, experimental observations of a− are consistent with the vdW universality, thus challenging our understanding of universality.

Fig. 1: Efimov spectrum and energy scales.
figure 1

Schematic illustration of the Efimov scenario (universal theory) in the vicinity of a Feshbach resonance. The horizontal axis is the inverse scattering length a−1, and the dashed vertical line corresponds to the position of the Feshbach resonance (∣a∣ = ∞). The vertical axis indicates the wavenumber corresponding to the three-atom continuum (gray region) and the discrete spectrum of Efimov trimers (solid curved lines). The straight solid line originating at the Feshbach resonance position corresponds to the universal dimer state. The extreme points of the trimers' spectrum are labeled to indicate Efimov resonances and the first excited Efimov state is highlighted. The energy scales relevant to this work are indicated and are specific to the 894 G resonance in 7Li.

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Here, we show that as the resonance becomes narrower, the three-body interaction is reshaped with respect to that of a broad resonance (in any atomic species). While the universal repulsive wall near R ≈ 2rvdW disappears, the system also develops an additional potential barrier ranging from R ≈ 4rvdW to a distance proportional to r⋆, leading to a double-well structure absent for broad resonances. Specifically, we experimentally explore the energy spectrum using coherent spectroscopy in the vicinity of the atom-dimer threshold for 7Li atoms polarized in the \(\left\vert F=1,{m}_{F}=0\right\rangle\) state, which features a Feshbach resonance at 894 G with \({s}_{{{{{{{{\rm{res}}}}}}}}}\approx 0.493\), and observe an Efimov state above the atom-dimer threshold. This provides strong evidence of the reshaping for the three-body interactions for narrow resonances, and further elucidates some of the mechanisms leading to other puzzeling observations with 7Li atoms23,24,25,26.

Results and discussion

The DITRIS interferometer

In contrast to traditional cold-atom few-body experiments, which utilize inelastic losses to uncover Efimov features27,28,29,30, we perform high-resolution coherent spectroscopy of the Efimov state on the a > 0 side of the Feshbach resonance. Following the proof-of-principle demonstration of ref. 31, we generate a DImer-TRImer Superposition (DITRIS) state by rf association and let it evolve in time. The accumulated relative phase between its constituents is then measured in an interferometer-like sequence. The method works best in the region around \({a}_{\star }^{(1)}\) –the value of a at which the Efimov state merges with the atom-dimer threshold—where there is a clear separation of energy scales (Fig. 1). The difference between the trimer and dimer bound states must be smaller than their depth below the three-atom continuum on the one hand but larger than the temperature of the latter on the other. In this energy regime the straightforward measurement combining rf association and loss-spectroscopy fails due to rf power broadening30. Our procedure thus goes beyond the existing methods. As a second condition, the rf pulse must be short enough in time such that it is Fourier broadened beyond the trimer-dimer energy difference ∣ET − ED∣. When three atoms are subjected to such a broadband rf pulse they have the option to either form a dimer while one atom remains free or a trimer. This effectively creates a superposition of the two chemically different bound states.

The double pulse sequence is illustrated in Fig. 2. The first pulse generates DITRIS states from a fraction of a gas of free atoms. Then, following the accumulation of a relative phase according to their binding energies, the second pulse attempts to dissociate them. The dimer and trimer pathways interfere, and one observes oscillations in the number of free atoms as a function of the free evolution time. The frequency of the oscillations is proportional to ∣ET − ED∣. The DITRIS method is thus a measurement of the trimer binding energy with respect to the atom-dimer continuum. The two requirements (separation of energies and short, Fourier-broadened pulses) set the lower and upper limits of detectable ∣ET − ED∣. It lies between the temperature of the free-atom continuum and the pulse bandwidth, respectively (see Methods). In this regime the conversion efficiency is limited by the pulse duration, i.e. it is not saturated by the phase space density argument32, and therefore remains low, such that ∣CA∣ ≫ ∣CD∣, ∣CT∣ (see Fig. 2). As a result, the oscillations appear as a small signal on top of a large background. However, making the pulses longer would decrease the upper detection limit and is therefore not favorable. To faithfully extract the main frequency contribution, we use a Fourier transform-inspired three-parameter fit (for details, see Supplementary Note 2 and 3 as well as ref. 31). As is typical for frequency measurements, the accuracy increases for longer measurements. The free evolution time is thus varied over a wide range of values (up to ~100 μs) limited only by the coherence time of the superposition state (see Methods).

Fig. 2: Experimental double pulse sequence.
figure 2

(i) Initial state of the three-atom continuum. (ii) A first rf pulse transfers a fraction of the initial state to dimer and trimer bound states creating a superposition state. (iii) As the wave function evolves, each constituent gains a phase proportional to its binding energy. (iv) A second rf pulse mixes the states. For simplicity, only the free-atom part is depicted. (v) Using absorption imaging, the number of free atoms is measured. (vi) An example of a measured signal as a function of free evolution time and its three-parameter fit. The signal has a large background term (\({\left\vert {C}_{A}\right\vert }^{2}\)), fast oscillating terms (not shown) and a term that oscillates at ω = ∣ET − ED∣/h. The latter is extracted via a three-parameter fit, where A indicates the amplitude.

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Trimer spectroscopy

Having established a reliable tool for measuring ∣ET − ED∣ we apply the double pulse sequence for various values of the magnetic field (scattering length) with the goal of finding the point at which ET→ED. In Fig. 3a, measurements from the DITRIS interferometer (filled circles) are represented together with the data from the previous incoherent rf association spectroscopy (open circles)30. At large scattering lengths, the Efimov state is relatively deeply bound, (ET − ED)/h ≲ − 100 kHz, and our new measurements agree with those obtained from incoherent spectroscopy30. However, as the scattering length decreases and the Efimov state becomes more weakly bound, instead of the expected gradual approach towards the atom-dimer continuum2,3,4,5, a sharp turn in the energy is observed. Subsequently, the experimental signal disappears for energies below the lower detection limit [see arrows in the shaded region in Fig. 3a). The latter is set by the temperature via ∣ET − ED∣/h ≲ kBT/h ≈ 30 kHz, due to the loss of coherence amplitude33 (see also Supplementary Note 2). Most surprisingly, however, meaningful frequencies reemerge when the scattering length is further decreased (gray circles in Fig. 3a). The Efimov state binding energy quickly changes away from the threshold again and becomes undetectable above the higher frequency detection limit set by the pulse bandwidth [see the upper gray dashed line in Fig. 3a) leading to measurements with no dominant frequency contribution (gray arrow) (see Supplementary Fig. 3). Figure 3a also shows our theoretical results for the energies of the 7Li Efimov state (squares). These results, along with the physical interpretation of the phenomena controlling the observations, are discussed later in the text.

Fig. 3: Trimer energy from experiment and theory.
figure 3

a The values of (ET − ED)/h obtained from the double pulse sequence (filled circles) are shown together with data from rf association followed by loss (open circles)30 as a function of inverse scattering length multiplied by 1000. For the former, the errorbars (1σ fitting error) are smaller than the point size and so are all scattering length errorbars (see Methods). The horizontal shaded region and dashed lines show the respective lower and upper detection limits of the DITRIS interferometer. The numerical results (filled squares) for (ET − ED)/h < 0 were obtained from the methodology used in refs. 39,40 while the results for (ET − ED)/h > 0 are those extracted from Fig. 5. Being treated differently, the two regions are connected by a dashed line. b, c On the left panel—examples of the experimental signal (number of atoms as a function of the time between pulses). Each point is the average of 10–20 measurements and the errorbars show the standard deviation. On the right panel—results of the three-parameter fit applied to the corresponding signals which clearly indicate the presence of dominant frequencies. The horizontal (smaller than point size) and vertical errorbars are 1σ fitting errors. (For further details see Supplementary Note 2 and ref. 31). b For data below the threshold, at a = 265a0, and c for data above the threshold, at a = 156a0. The experimental signal and three-parameter fit for the remaining points are shown in Supplementary Fig. 3. Note that the experimental data predict the crossing of the threshold somewhere between 177 a0 and 160 a0.

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Experimentally we are only sensitive to the absolute value of the energy difference, which leads to two equally plausible scenarios: the trimer either crosses into or bounces off the atom-dimer continuum. Although the latter scenario has been indicated in the literature to occur for broad resonances34,35, our numerical simulations for 7Li instead show that the Efimov trimer crosses the atom-dimer continuum threshold due to a reshape of the three-body interaction potential associated with the narrow character of its Feshbach resonance (see discussion below).

We emphasize that the trimer remains in a metastable state well inside the atom-dimer continuum. This is demonstrated in Fig. 3b, c, where two-time sequences of the DITRIS interferometer are compared. In Fig. 3b, we show a signal obtained below the threshold and in Fig. 3c one from above it. Both signals are similar, and no observable decay is detected within the first 100 μs, covering up to 10 full oscillations. Although a thorough investigation of the trimer lifetime with the DITRIS interferometer is beyond the scope of this work, it is clear from these signals that the coherence time exceeds the expected lifetime of the Efimov trimer. (Our numerical simulations estimate the lifetime of the trimer state within the experimental range to be around 10–20 μs.) Interestingly, a recent theoretical study (performed for broad resonances) has provided a possible interpretation of such unusually large coherence times33, with coherence still being observed for times as long as twice the lifetime of the Efimov state. Although this result does not fully explain the experimentally observed coherence times, our analysis below demonstrates fundamental differences between the three-body physics for broad and narrow resonances as 7Li, which can potentially lead to substantial modifications of the coherence times.

Finally, we argue the implausibility of attributing the nonzero signal from the DITRIS interferometer above the threshold to any molecular state other than the Efimov state. Although one cannot completely rule out that a non-universal (non-Efimovian) trimer state exists by accident in the same energy region in which our observations are performed, this coincidence is very unlikely. In particular, the region in phase space (above the threshold) that we explore experimentally is extremely narrow, covering only a few a0 in scattering length and only a few tens of kHz in energy. Moreover, for DITRIS interferometry to provide a detectable signal, it is necessary for all states involved in the problem to be extraordinarily large. Near the region where we observe the crossing, the dimer state itself should be ~160a0, and the trimer should be comparable to or even greater than that. On the other hand, a non-Efimovian accidental state could only originate from short-range physics and would be ≲ rvdW = 32a0 for 7Li. For such small states, the coupling between them and the initial atomic state (with a size comparable to the average interatomic distance, i.e., ~104a0 for our case) would be extraordinarily small due to the poor Frank-Condon factor, and the DITRIS interferometer would be inefficient. As a result, since we know that the only weakly bound dimer state is the Feshbach dimer, it is reasonable to accept that the only trimer that can overlap well with both the dimer and the initial atomic state is an Efimov state.

Theory and numerical simulations

In order to better understand the nature of the 7Li Efimov trimer near the atom-dimer threshold, we have performed numerical calculations using the adiabatic hyperspherical representation (see Methods). In the following, we first present a two-channel interaction model with variable \({s}_{{{{{{{{\rm{res}}}}}}}}}\). This model gives insight into the crucial difference between broad and narrow resonances in the context of three-body Efimov interactions. Building upon the physical picture that emerges from the two-channel model, we then develop a multichannel theory using realistic 7Li two-body potentials. This latter model qualitatively reproduces the trimer’s crossing of the atom-dimer threshold, thus verifying the experimental observations. We note that while necessary approximations in our theoretical model hinder quantitative agreement with the experiment, our findings clearly identify the physical mechanism controlling the experimental observations.

Three-body interactions near narrow resonances

The two-channel model we use for the interatomic interaction contains the proper van der Waals physics and a set of parameters chosen to produce a Feshbach resonance with the 7Li background scattering length, abg ≈ − 25a036, but variable values for \({s}_{{{{{{{{\rm{res}}}}}}}}}\) (see Supplementary Note 4). In the adiabatic hyperspherical representation, a great deal of physical insight can be obtained from the hyperspherical effective potentials Wν(R), which are solutions of the adiabatic Hamiltonian at fixed values of the hyperradius R. In Fig. 4 (see also Supplementary Fig. 5), we show the effective potentials relevant for Efimov physics at a = ± ∞ and various values of \({s}_{{{{{{{{\rm{res}}}}}}}}}\) between 0.41 and 246, thus covering the broad, intermediate and narrow resonance regimes. Asymptotically, all potentials approach the universal form \(-({s}_{0}^{2}+1/4){\hslash }^{2}/2\mu {R}^{2}\) with s0 ≈ 1.00624, which supports infinitely many Efimov states. However, at shorter distances the potentials are drastically reshaped as the resonance strength enters the narrow resonance regime. (We note that similar results have been found in a recent publication16).

Fig. 4: Reshape of three-body interactions for narrow resonances.
figure 4

a Effective potentials, Wν(R), for the relevant channel supporting an infinity of Efimov states for different values of \({s}_{{{{{{{{\rm{res}}}}}}}}}\) in units of \({E}_{{{{{{{{\rm{vdW}}}}}}}}}={\hslash }^{2}/m{r}_{{{{{{{{\rm{vdW}}}}}}}}}^{2}\). As \({s}_{{{{{{{{\rm{res}}}}}}}}}\) evolves from the regime of broad (\({s}_{{{{{{{{\rm{res}}}}}}}}}\gg 1\)) to narrow (\({s}_{{{{{{{{\rm{res}}}}}}}}}\ll 1\)) resonances a repulsive interaction emerges for R ≳ 4rvdW and extends up to R ≈ 3r*, where \({r}_{*}\,\approx\, 1.912{r}_{{{{{{{{\rm{vdW}}}}}}}}}/{s}_{{{{{{{{\rm{res}}}}}}}}}\). The double-well structure of the three-body interaction for narrow resonances allows for trimer states to exist above the atom-dimer continuum for finite values of a > 0 as shape resonances. b 100-fold zoom of the dot-dashed box in a. Likewise, c shows a 10-fold zoom of the dot-dashed box in b. For more values of \({s}_{{{{{{{{\rm{res}}}}}}}}}\) see Supplementary Fig. 5.

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For our broadest resonance (\({s}_{{{{{{{{\rm{res}}}}}}}}}\,\approx\, 246\)), representing atomic species like 85Rb and 133Cs, the effective potential displays the expected universal repulsive wall near R ≈ 2rvdW (thick black curve in Fig. 4a), which prevents atoms from probing the small R region representing the hallmark of vdW universality6,7. As \({s}_{{{{{{{{\rm{res}}}}}}}}}\) is tuned towards the intermediate (\({s}_{{{{{{{{\rm{res}}}}}}}}}\,\approx\, 2.56\), similar to 39K) and narrow (\({s}_{{{{{{{{\rm{res}}}}}}}}}\approx 0.41\), similar to 7Li) resonance regime, the effective potentials are reshaped, and the universal repulsive wall eventually disappears. Remarkably, the three-body potentials develop a repulsive barrier for R ≳ 4rvdW (Fig. 4b), which extends up to R ≈ 3r⋆ (Fig. 4c), as a result of the strong mixing between the open and closed hyperspherical channels (see Supplementary Note 4). Therefore, in the \({s}_{{{{{{{{\rm{res}}}}}}}}} < 1\) regime, the effective potentials display a double-well structure, where interactions within the inner well (R ≲ 4rvdw) are dominated by vdW interactions while interactions in the outer well (R ≳ 3r⋆) are dominated by Efimov physics. For the \({s}_{{{{{{{{\rm{res}}}}}}}}}\,\approx\, 0.41\) case, the closest to 7Li (\({s}_{{{{{{{{\rm{res}}}}}}}}}\,\approx\, 0.493\)), the potential barrier height is found to be ~10 MHz (0.02EvdW) at a = ± ∞, i.e. much larger than the range of binding energies found experimentally. Importantly, and relevant to our present experiment, this barrier also persists for finite values of a > 0 (see Supplementary Note 4).

Multichannel calculations for 7Li

To provide a more quantitative analysis of the effect of the repulsive barrier in the parameter regime of the experiment, we perform additional numerical calculations that characterize the energy of the 7Li Efimov state using a more realistic interaction model based on the methodology developed in refs. 18,19. We note that modifications to this model were made (see Supplementary Note 4) in order to compensate for strong (short-ranged) electronic exchange interactions37,38. Yet, our model displays the correct physics at distances R ≳ rvdW, thus preserving the major features relevant to the central physical question we explore here, i.e., whether the repulsive barrier allows for Efimov states to exist above the atom-dimer threshold.

While our results for (ET − ED)/h < 0 in Fig. 3a were obtained using a methodology that provides a direct characterization of the energy of the Efimov state39,40, for (ET − ED)/h > 0, the analysis of the near-resonant energy regime in the atom-dimer continuum is much more subtle. However, a convenient way to characterize the existence of the 7Li Efimov state above the atom-dimer threshold is to compare the energy dependence of the 7Li atom-dimer elastic cross-section, σAD, with that of a system without the barrier, i.e., a system controlled by a broad (\({s}_{{{{{{{{\rm{res}}}}}}}}}=\infty\)) Feshbach resonance, \({\sigma }_{AD}^{\infty }\)35. It is crucial, however, that this comparison is performed when the two physical systems have the same value for the atom-dimer scattering lengths, \({a}_{AD}={a}_{AD}^{\infty }\), such that both cross-sections converge to the same value, 4π∣aAD∣2, as the collision energy vanishes. In this case, if aAD < 0 and an Efimov state exists above the threshold, the cross-section difference should display the enhancement whenever the collision energy coincides with that of the 7Li Efimov state. In practice, the above procedure is most meaningful in the case of weak inelastic transitions, which has led us to suppress the short-range decay mechanisms in our model. Also, since, in our case, the values of aAD we obtained are only approximately the same (differing by no more than 2%), we define the weighted cross-section difference as

$$\Delta {\sigma }_{AD}^{w}(E)={\sigma }_{AD}(E)-\frac{| {a}_{AD}{| }^{2}}{| {a}_{AD}^{\infty }{| }^{2}}{\sigma }_{AD}^{\infty }(E),$$
(1)

which ensures that the cross-section difference vanishes as the collision energy E→0. In the above expression, \({\sigma }_{AD}(E)=\pi | 1-{S}_{AD}(E){| }^{2}/{k}_{AD}^{2}\) where SAD is the diagonal S-matrix element associated with the atom-dimer channel and \({k}_{AD}^{2}=2m/3E/{\hslash }^{2}\), with m being the atomic mass. The results for \(\Delta {\sigma }_{AD}^{w}\) for various aAD in Fig. 5 clearly show the expected enhanced scattering of 7Li with respect to the broad resonance case thus demonstrating the existence of a 7Li Efimov state above the threshold as a direct consequence of the existence of the repulsive barrier. The energy of the Efimov state is associated with the maximum value of \(\Delta {\sigma }_{AD}^{w}\) occurring at smaller values of E as ∣aAD∣ increases, i.e., when the state approaches the atom-dimer threshold from above, and is displayed in Fig. 3a. We note that simplified, asymptotic models have failed to explain our experimental observations, indicating the importance of van der Waals interactions in order to properly describe the reshape of the three-body interactions41,42. Note also that, as shown in Fig. 3a, when (ET − ED)/h < 0, the theory results predict deeper energies close to the Feshbach resonance, which dive faster towards the atom-dimer continuum. We attribute such discrepancies to the simplifications adopted for otherwise nearly intractable, truly multichannel interactions in lithium. Even the most advanced attempt to model these interactions38 have not reached fully converged results, leading to a significant discrepancy between theory and experiment24,25 for the spin state considered here. Most importantly, however, is that both our experimental data and theoretical simulations refute the conventional expectation that an Efimov state simply merges with the atom-dimer continuum for the case of narrow resonances.

Fig. 5: Evidence of resonant scattering above atom-dimer threshold.
figure 5

Weighted elastic cross-section difference [Eq. (1)] between 7Li atom-dimer collisions and that of a system controlled by a broad Feshbach resonance for approximately the same value for ∣aAD∣ and Re[aAD] < 0. The existence of the repulsive barrier on the entrance atom-dimer channel for 7Li leads to the enhancement of elastic collisions just above the threshold as compared to that without the barrier, indicating the presence of the Efimov state above the atom-dimer threshold and with energy indicated by the closed circles. Note that at higher energies \(\Delta {\sigma }_{AD}^{w} < 0\), most likely due to the fact that ∣aAD∣ and \(| {a}_{AD}^{\infty }|\) are only approximately the same, but also due to other multichannel effects causing modifications on the scattering at such energies.

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In summary, our experimental and theoretical observations of the existence of an Efimov state above the atom-dimer continuum provides strong evidence of a fundamental reshaping of the three-body interactions for narrow resonances. Although our theoretical analysis allows us to point out that this phenomenon is universally valid for narrow Feshbach resonances, much is still needed to fully characterize the 7Li Efimov states, in particular, with respect to their lifetime. The coherence times observed with the DITRIS interferometer are clearly much longer than the estimations of the trimer lifetime obtained from our numerical simulations without coherence. Since we show here that for 7Li atom-dimer collisions are enhanced, this raises the intriguing question on whether the character of the DITRIS superposition state, along with the form of the three-body interaction, can conspire to form more long-lived superposition states in a way that coherence can still be observed at long times33. Although some open questions still remain, our current observations provide evidence that Efimov physics at a narrow Feshbach resonance deviates from the expectations from vdW universality, where the Efimov state simply disappears at the atom-dimer threshold.

Successful application of the DITRIS interferomter to coherent spectroscopy of the Efimov energy level, together with a notable demonstration of coherent manipulation of 4He halo dimers by ultrashort laser pulses43, reveals the great potential of the coherent approach to few-body physics phenomena. Future investigation into the superposition state lifetime, and the extension of the unique capabilities of the DITRIS interferometer to other atomic species and mixtures are expected to greatly advance our understanding of Efimov physics in ultracold atoms.

Methods

Experimental details

Standard laser cooling and evaporation techniques are used to produce a gas of 3 × 104 bosonic lithium atoms at 1.5 μK and an average density of 1.25 × 1012 cm−3 in a crossed optical dipole trap. The temperature corresponds to 30 kHz and is our lower detection limit. To image the atoms we use absorption imaging which is sensitive to free atoms only.

At the core of the experiment lies the 10 μs pulse, which is Fourier broadened to address both the dimer and the trimer simultaneously. The duration 10 μs refers to the full-width at half-maximum (FWHM). There is also a (measured) turn-on/turn-off time of τ0 = 4 μs, which means that the pulse is at its maximal value during τc = 6 μs. The experimental rf pulse envelope is modeled as

$$\chi (t)=\left\{\begin{array}{ll}{\sin }^{2}\left(\frac{\pi }{2}\frac{t+{\tau }_{c}/2+{\tau }_{0}}{{\tau }_{0}}\right) &-{\tau }_{0}-\frac{{\tau }_{c}}{2} \, < \, t \, < -\frac{{\tau }_{c}}{2}\\ 1 &-\frac{{\tau }_{c}}{2} \, < \, t \, < \, \frac{{\tau }_{c}}{2}\\ {\sin }^{2}\left(\frac{\pi }{2}\frac{t-{\tau }_{c}/2-{\tau }_{0}}{{\tau }_{0}}\right) &\frac{{\tau }_{c}}{2} \, < \, t \, < \, \frac{{\tau }_{c}}{2}+{\tau }_{0}\end{array}\right.$$
(2)

and plotted in Fig. 6. The Fourier transform of χ(t) is also shown in Fig. 6 in the low-frequency domain. It closely resembles a sinc, the transform of an ideal rectangular pulse, but is slightly broadened. It is FWHM is 117 kHz which is the value we use as our upper detection limit.

Fig. 6: Pulse shape and spectrum.
figure 6

The pulse shape of Eq. (2) is compared to a pure square pulse with the same FWHM. The FFT of the former is slightly broader.

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The method was refined with respect to the proof-of-principle in ref. 31 mainly by reducing the atom number fluctuations. This was achieved by stabilizing the magnetic field to a relative stability of 5 × 10−5 and by improved statistics (Supplementary Note 2).

Scattering length calibration

In Fig. 3a, the values of (ET − ED)/h are shown as a function of inverse scattering length. In the experiment we vary the magnetic field bias to tune the scattering length. The calibration is performed via the dimer binding energy, which is frequently measured during the DITRIS interferometer data accusation. The measurement protocol can be found elsewhere44. Finally, the dimer binding energy is related to the scattering length via coupled channel calculations45. Given the high accuracy of the dimer binding energy measurement and coupled channels calculations, the scattering length uncertainty is <a0 in the region explored in the experiment.

DITRIS coherence time

The longest feasible free evolution time is given by the decoherence of the superposition state31. The possible relevant parameters are the elastic collision rate and the trimer’s intrinsic lifetime. The low signal-to-noise ratio does not permit precise measurement of the decohrence time but empirically we do not observe signs of decay for <150 μs. In practice, the three-parameter fit allows neglecting the decay in the data analysis by keeping the range of the evolution time <150 μs.

Theory

The adiabatic hyperspherical representation provides a simple and conceptually clear description of the three-body system in terms of the hyperradius R, characterizing the overall size of the system, and the set of hyperangles, Ω46. Bound and scattering properties47 of the system are determined from solutions of the hyperradial Schrödinger equation:

$$\begin{array}{l}\left[-\frac{{\hslash }^{2}}{2\mu }\frac{{d}^{2}}{d{R}^{2}}+{W}_{\nu }(R)-E\right]{F}_{\nu }(R)\\+\mathop{\sum}\limits_{{\nu }^{{\prime} }\ne \nu }{W}_{\nu {\nu }^{{\prime} }}(R){F}_{{\nu }^{{\prime} }}(R)=0.\end{array}$$
(3)

where μ is the three-body reduced mass, E the total energy, Wν is the hyperspherical effective potentials governing the radial motion and \({W}_{\nu {\nu }^{{\prime} }}\) the nonadiabatic couplings driving transitions between different channels, characterized by the collective index ν. The hyperspherical effective potentials are defined as

$${W}_{\nu }(R)={U}_{\nu }(R)+\left\langle {\Phi }_{\nu }(R;\Omega )\left| \frac{{d}^{2}}{d{R}^{2}}\right| {\Phi }_{\nu }(R;\Omega )\right\rangle,$$
(4)

where Uν(R), the hyperspherical potentials, and Φν(R; Ω), the channel functions, are solutions of the hyperangular adiabatic equation

$${\hat{H}}_{{{{{{{{\rm{ad}}}}}}}}}{\Phi }_{\nu }(R;\Omega )={U}_{\nu }(R){\Phi }_{\nu }(R;\Omega ),$$
(5)

obtained at fixed values of R. The adiabatic Hamiltonian contains the hyperangular kinetic energy, as well as all the atomic and interatomic interactions in the system. We explicitly define the terms of the adiabatic Hamiltonian used in our studies in Supplementary Note 4.