Ultracold atoms in optical lattices are ideal to study fundamentally new quantum many-body systems1, 2 including frustrated or topological magnetic phases3, 4 and supersolids5, 6. However, the necessary control of strong long-range interactions between distant ground state atoms has remained a long-standing goal. Optical dressing of ground state atoms via off-resonant laser coupling to Rydberg states is one way to tailor such interactions5, 6, 7, 8. Here we report the realization of coherent Rydberg dressing to implement a two-dimensional synthetic spin lattice. Our single-atom-resolved interferometric measurements of the many-body dynamics enable the microscopic probing of the interactions and reveal their highly tunable range and anisotropy. Our work marks the first step towards the use of laser-controlled Rydberg interactions for the study of exotic quantum magnets3, 4, 9 in optical lattices.
At a glance
Neutral ultracold atoms in optical lattices are among the most promising platforms for the implementation of analog quantum simulators of condensed matter systems. However, the simulation of magnetic Hamiltonians, often emerging as an effective model in more complex many-body systems, is difficult with contact interactions due to the low energy scale of the associated superexchange process10. Long-range interactions offer an alternative way to directly achieve strong effective spin–spin interactions. Such interactions emerge between magnetic atoms and between ultracold polar molecules11, trapped ions12 or ground state atoms resonantly13 or off-resonantly coupled (‘dressed’) to Rydberg states5, 6, 7, 8. Rydberg dressing is especially appealing due to the simplicity of realizing atomic lattice systems with unity filling, combined with the great tunability of the interaction strength and shape, which might be exploited to explore exotic models of quantum magnetism3, 4. While effects of long-range spin interactions have been observed in many-body systems of polar molecules14, ions15, 16, 17 and resonantly excited Rydberg atoms18, 19, none of these approaches combines the advantages of Rydberg dressing, which permits the realization of strong spin interactions in lattices with near-unity filling. So far, Rydberg dressing in a many-body system remains an experimental challenge, for which up to now only dissipative effects have been measured20, 21, 22, 23, 24. For two atoms, first promising experimental results have been reported recently for near-resonant strong dressing25, where, however, the assumption of a weak Rydberg-state admixture required for the realization of various many-body models5, 6, 9, 26, 27 does not hold28.
Here we demonstrate Rydberg dressing in a two-dimensional (2D) near-unity-filled atomic lattice with tailored extended range interactions between approximately 200 effective spins. In contrast to our previous experiments18, 29 on resonantly coupled Rydberg gases, all atoms participate here in the spin dynamics. We exploit the temporal control over such interactions to perform interferometric measurements on the many-body system that directly reveal the induced correlations via single-site-sensitive local detection30. Our experiments illustrate the versatility of Rydberg dressing by tuning the range and isotropy31, 32 of the interaction potential induced by optical coupling to high-lying P-states. First-principles potential calculations together with an exact solution of the many-body dynamics accurately reproduce these measurements. From this we conclude that stimulated Rydberg–Rydberg state transitions, recently observed as anomalous broadening24, act collectively in a many-body system, implying that rare one-body decay events can globally affect the entire spin lattice.
Dressed atomic states emerge as new eigenstates of an atom driven by a laser field with a Rabi frequency Ω and frequency detuning Δ. Considering two levels |g and |e in the weak dressing regime, Ω Δ, the dressed ground state contains a small admixture β = Ω/(2Δ) of the state |e. As a result, it acquires a finite lifetime τr/β2, which, however, greatly exceeds the lifetime τr of the bare excited state. Choosing |e to be a Rydberg state, the laser coupling also induces effective interactions between two dressed atoms in state (Fig. 1a). At large interatomic distances R this interaction, U(R) ≈ β4V (R), corresponds to the Rydberg–Rydberg atom interaction potential V (R) reduced by the probability β4 to excite both atoms at once. At short distances, however, the strong interaction between Rydberg atoms blocks this simultaneous excitation within a critical distance, Rc, determined by V (Rc) = 2ℏΔ. As a result, the induced interaction acquires a soft-core shape and saturates to a value of U0 = ℏΩ4/(8 |Δ| 3) (Fig. 1c)5, 6, 8. Involving other atomic ground states in the dynamics, one naturally obtains various kinds of lattice models of interacting spins that have been proposed for metrology applications8, 33 or the exploration of exotic quantum magnetism3, 4. In the simplest case, a single additional ground state that is not coupled to the Rydberg state (Fig. 1a, b) results in a system described by a 2D Ising Hamiltonian
Here, is a spin-1/2 operator with eigenstates |↑ and |↓, corresponding to the Rydberg-dressed and uncoupled atomic ground state, respectively. The longitudinal field δ arises from the single-atom light shift, δ ≈ Ω2/(4Δ) and Ui, j = U0/(1 + (R/Rc)6) denotes the dressing-induced interaction between spins located on lattice sites i and j at a distance R/alat = |i − j|, where alat denotes the lattice constant. The collective contribution Δi(coll) = ∑ j≠iN(Ui, j/2) results from the transformation from the original atomic-state representation to effective spin operators18, 34. The interaction-induced energy shift resulting from this additional longitudinal field is measurable as a frequency shift in an interferometric Ramsey sequence (Supplementary Figs 1 and 2). Furthermore, due to the extended range of the interactions, this collective field depends on the spins nearby; hence, spins near the edge of a finite system evolve differently compared to spins in the centre. We directly observe this as an inhomogeneous spin distribution significantly contributing to dephasing of the Ramsey fringe (Fig. 1d).
The optically switchable spin interactions Ui, j ∝ Ω(t)4 enable such interferometric measurements of the many-body system by sequential application of the interaction Hamiltonian equation (1) and a transverse magnetic field induced by microwave coupling of the two ground states33, 35.
Our experiments started with a 2D degenerate gas of rubidium-87 in the |F, mF = |1, −1 hyperfine state, confined in a single antinode of a vertical (z -axis) optical lattice. In this single x–y plane, we then switched on a square optical lattice with alat = 532 nm spacing and prepared about 190 atoms in a unity-filling Mott insulator with a defect fraction of about 3%. The chosen atom number ensured a negligibly small number of doubly occupied sites. Transitions from the state |1, −1 (‘spin down’, |↓) to |2, −2 (‘spin up’, |↑) were driven globally via microwave pulses. To introduce long-range interactions, the state |↑ was laser-coupled to the 31PJ state (J = 1/2 or J = 3/2), which has a lifetime of approximately τr = 27 μs (see ref. 36). The excitation beam at a wavelength of 297 nm propagated in the plane of the atoms along the diagonal of the cubic lattice (Fig. 1b). A static magnetic field was used to set the quantization axis either along the z-direction or aligned with the laser beam wavevector k, which allowed selective coupling to different Rydberg states, depending on the polarization of the excitation beam (Supplementary Fig. 3 and Supplementary Information). The positions of all the atoms were then detected with single-lattice-site resolution and single-atom sensitivity37. By optically removing atoms in state |↑ before imaging, we could also perform spin-resolved detection and, in particular, measurements of spin–spin correlations.
In a first experiment, we aim to reveal the characteristic spin correlations in the many-body system that emerge over time as a result of the long-ranged spin interactions. To this end, we employ an interferometric spin-echo sequence (Supplementary Fig. 1), which is sensitive to interaction-induced phase shifts, whereas the influence of single-particle effects including the collective longitudinal field (Fig. 1d) is suppressed. Starting with all atoms in state |↓ we first applied a π/2 microwave pulse on the |↓ − |↑ transition to generate an equal superposition of the two spin states. This was followed by two identical Rydberg-dressing pulses of duration t/2 each, separated by a π microwave pulse. After closing the interferometer with another π/2 pulse, in the absence of interactions the system returns to its initial state with all the population in |↓, such that any deviations from this are expected to provide a precise probe of the interaction-induced dynamics described by the Hamiltonian equation (1). Optical dressing to the 31P1/2 Rydberg state was performed with a detuning Δ/2π = 6 MHz and a Rabi frequency of Ωs/2π = 1.33(7) MHz, determined independently by Ramsey interferometry (Supplementary Fig. 2 and Supplementary Information). These laser parameters ensure the system is in the weak dressing regime with a small Rydberg-state admixture of β = 0.11(1), corresponding to a population probability of β2 = 0.012(2). The resulting long-range spin interactions with U0/2π ≈ 1.8 kHz induce correlated phase rotations during the two dressing stages peaking at , and ultimately lead to measurable spin correlations at the end of the interferometric echo sequence. The time dependence of U0 arises from the finite rise time of Ω. Our spin-resolved detection scheme provides direct access to longitudinal correlations , where measures the |↓-population at site i. Measurements versus time or, equivalently, versus the interaction phase ϕ0 permit the dynamical growth of the spin–spin correlations to be traced in the regime of small interaction phase, whereas for large ϕ0 the correlation signal is expected to decrease. Figure 2a shows the measured spin correlation function g(2)(R) = ∑ i≠jδij, R gi, j(2)/∑ i≠jδij, R, which is obtained from a translational average constrained to a given distance R by the Kronecker symbol δij, R. The observed correlation functions resemble the soft-core-shaped interaction potential shown in Fig. 1c. This behaviour is readily understood in the short-time limit for Neffϕ02 1, where Neff denotes the number of spins within the range Rc of the interaction potential. In this short-time limit, one obtains a direct proportionality g(2)(R) = ϕ02/(4(1 + (R/Rc)6)2) between the induced correlations and the square of the soft-core potential. However, for all non-zero interaction times probed in the experiment, the correlations deviate from this simple expression due to the presence of surrounding spins (Fig. 2c), as reproduced by the analytic theory (see Supplementary Information). This directly highlights the importance of many-body effects in our long-range interacting system despite the seemingly small interaction phases.
Next to the optical switchability, Rydberg dressing also enables designing the extent and anisotropy of the interactions between the spins. In the following, we demonstrate this control capability by selecting different coupled Rydberg states. As shown in Fig. 3a, changing the Rydberg state from 31P1/2 to 31P3/2 leads to notable modifications of the measured correlation function and, hence, the underlying spin interactions. In contrast to 31P1/2, the 31P3/2 state features repulsive interactions with a tenfold larger magnitude (Supplementary Fig. 4), which implies a 50% larger interaction radius Rc/alat ≈ 3 that is reflected in the enhanced correlation range shown in Fig. 3a. In both cases, the angular symmetry of the induced interactions is dictated by the strong applied magnetic field, which causes isotropic interactions to emerge for the Bz-configuration (Fig. 1b) used in the measurements of Fig. 2. Rotating the magnetic field permits tailoring of the anisotropy of the dressing-induced interactions, which is maximized by aligning the magnetic field with the wavevector k of the circularly polarized dressing laser in the plane of the spin lattice (Supplementary Fig. 4). The correlation measurements confirm the expected anisotropy, and we observe an aspect ratio of ~3/2, in quantitative agreement with the theory (Fig. 3b).
In addition to probing the coherent spin interactions, the applied interferometric technique allows an in-depth characterization and understanding of decoherence in the present system, which is of great importance for future applications of Rydberg dressing. The measured spin correlations (Fig. 2c) reveal an unexpected homogeneous offset, in addition to the characteristic spatial dependence. This requires a process affecting the system globally, and can be explained in terms of an additional dissipation channel triggering the loss of all particles in the dressed |↑ state. Such a loss process is also consistent with the measured atom number distributions that steadily develop a bimodal structure with increasing dressing time (Fig. 4 and Supplementary Figs 5 and 6). Incoherent transitions to other Rydberg states due to black-body radiation present a plausible mechanism24, since such transitions project the |↑ state onto a nearby Rydberg state, and thereby produce a real Rydberg excitation (Supplementary Fig. 8). The production of such atoms with opposite parity with a small rate β2γBB can induce strong dipolar exchange interactions, and thereby trigger a fast avalanche-like atom loss due to strong resonance broadening24, 38, 39. We can incorporate this picture into our theoretical description by assuming a stochastically triggered instantaneous loss of all |↑-state atoms as a simplification, still permitting an analytical solution of the many-body dynamics (Supplementary Information). This loss process, being spatially uniform, does not modify the shape of the correlations, but leads to an overall scaling and can account for a uniform offset. Indeed, the measured correlations shown in Fig. 2 are well explained by this model and the offset is reproduced to within 20%. We extract a value of γBB/2π = 1.6 kHz, approximately half the literature value for 31P1/2-states36.
Furthermore, we observe a decreasing atom number N with increasing total dressing time t (Fig. 4), consistent with the predicted exponential loss N(t) = N(0)e−(N(0)/4)β2γbbt (Supplementary Information). Due to the dependence on system size via N(0), the extracted value of 130(20) μs is significantly lower than the anticipated value of τr/β2 = 2.2 ms in the absence of additional loss processes; however, it surpasses the bare Rydberg state lifetime by a factor of five. The spin-resolved measurement agrees equally well with the predicted dynamics, strongly supporting the developed understanding of the many-body dynamics.
The demonstrated spatio-temporal control over long-range spin interactions in a unity-filled lattice holds promise for a new generation of quantum simulation platforms. We anticipate that significant improvement in the current coherence time is possible in designed lattice systems with fewer spins or reduced dimensionality18, an appropriately adjusted Rydberg-state detuning to avoid coupling to broadened resonances, or via stroboscopic dressing that allows detrimental impurity Rydberg atoms to decay or be laser-quenched29 before triggering the avalanche loss. Our results pave the way towards experimental explorations of more complex quantum magnets3, 4, 9 and the study of novel phenomena in Rydberg-dressed atomic lattices26, 27.
The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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We thank A. Glätzle, P. Zoller, M. Cheneau and N. Henkel for discussions. We acknowledge funding by MPG, EU (UQUAM, SIQS, RYSQ, HAIRS, Marie Curie Fellowship to J.-y.C.) and the Körber Foundation.
- Supplementary information (10,131,966 KB)