Alec Douglas, Lin Su, Michal Szurek, Robin Groth, Sandra Brandstetter, Ognjen Marković, Oriol Rubies-Bigorda, Stefan Ostermann, Susanne F. Yelin, Markus Greiner
6 min
This paper by Alec Douglas and collaborators at Harvard introduces a breakthrough experimental platform in quantum optics: geometrically ordered 2D arrays of ultracold atoms with subwavelength spacing (atoms spaced at 266 nm, closer than the 841 nm emission wavelength). Unlike traditional 'point-like' ensembles where collective effects are limited to a single Dicke mode, these extended arrays create a rich network of long-range, photon-mediated interactions. This enables many-body super- and subradiance—collective emission where groups of atoms synchronize to emit light much faster (superradiance) or slower (subradiance) than independent atoms due to quantum interference of their dipoles.
The key innovation is achieving site-resolved imaging during collective emission using quantum gas microscopy. This allows direct observation of spatial correlations as the array evolves, transforming cooperative decay from a simple collective process into a strongly correlated many-body quantum system. They load ~500 erbium atoms into an 'accordion lattice' (tunable spacing from 266 nm to 3 μm via 488 nm light projection) and study dynamics across regimes.
In subwavelength arrays, atoms are so close that near-field dipole-dipole interactions dominate, forming an anisotropic, long-range network. Photons don't propagate freely like in dilute gases; instead, evanescent (non-radiating) fields mediate coherent spin exchange between neighbors. This creates dissipative many-body physics beyond Dicke superradiance:
Theory predicts spin waves (long-wavelength modes) cascade to reinforce emission, distinct from homogeneous Dicke behavior.
Quantum gas microscopy typically struggles with subwavelength arrays because photons from collective emission overwhelm single-atom detection. They innovate with blowout of ground-state atoms + lattice expansion, imaging excited-state population with single-site resolution. This reveals:
The arrays exhibit strongly correlated quantum dynamics:
This confirms the system explores a landscape of collective modes, with spatial patterns directly imaged—impossible in prior cavity or dense-cloud experiments.
This cavity-free system enables photon storage (subradiant modes as memory), capture (directional superradiant release), and atom-photon entanglement. The accordion lattice tunes spacing/wavelength ratio, accessing regimes from Dicke-like to fully many-body. Applications span quantum photonics (nanophotonic-free light-matter programming) to dissipative quantum simulation (e.g., spin models via emission). It's a new workbench for many-body quantum optics, bridging theory and experiment.
Why it matters: Prior work was confined to homogeneous ensembles; this unlocks extended, programmable arrays for next-gen quantum tech.
When quantum emitters couple indistinguishably to light, they can synchronize into a collective light matter system with radiative properties profoundly different from those of independent particles. To date, the resulting collective effects have largely been confined to point like or homogeneous ensembles. Here, we open access to a qualitatively new collective regime by realizing geometrically ordered, spatially extended atom arrays with subwavelength spacing. This establishes a fundamentally new platform in which collective emission is no longer confined to a single Dicke mode but instead emerges from an ordered network of photon mediated interactions. We find that 2D atom arrays undergo strong super and subradiant emission. Despite subwavelength spacing, we achieve site resolved imaging and directly observe the buildup of spatial correlations, demonstrating the transformation of cooperative decay into a strongly correlated many-body process. We observe extensive scaling of superradiance, uncover superradiant revivals, and reveal the ferromagnetic nature of superradiance and the antiferromagnetic nature of subradiance. Our results realize a novel programmable platform for exploring and utilizing dissipative many-body quantum physics, opening new possibilities for photon capture, storage, and atom photon entanglement.
Alex: So the speed per excitation isn't steady—it changes as these patterns form?
Sam: Correct. They track a normalized emission rate: total photons out per moment divided by excitations still left. For lone atoms, it's flat. Here, it starts average, spikes above for the burst as coherence builds, then drops below—ten times slower at the end—for the trap. This reveals the array acting as one big unit.
Alex: And does this get stronger with more atoms?
Sam: Yes—the burst peak grows roughly with atom number raised to about 1.1, meaning cooperativity spans the whole array despite distances. Late slowdown plateaus regardless of size, limited by atoms' fuzzy positions in the trap—about a twentieth of the light wavelength wide.
Alex: What happens if they don't excite every atom fully?
Sam: Shortening the excitation pulse leaves a fraction lit, imprinting a phase wave along the laser path—like seeding a directional ripple. Fewer excitations then emit faster per one, as they fit a tighter bright mode for aimed light out. Yet over 15% always lingers in dark states, dynamically created by interactions pushing them apart.
Alex: Interesting—tuning starting point changes paths through all possibilities.
Sam: Exactly. And varying grid spacing with the accordion confirms geometry filters modes: superradiance oscillates, peaking sharply when spacing hits half-wavelength fractions, as close packing channels light coherently.
Alex: So changing the spacing tunes which directions the light can go out... like opening specific doors for the photons?
Sam: Yes. Light photons carry a limited sideways push—their maximum in-plane momentum is fixed by the wavelength. In a tight atomic grid, that push is smaller than the grid's repeating pattern allows, splitting the grid into zones: one where atoms can easily send light into open space, and another where it's blocked. As spacing grows, the light's push matches the grid's pattern edges, folding new paths back in through Bragg scattering. This opens discrete channels coherently, boosting the fast emission sharply.
Alex: And they saw that directly by stretching the grid first?
Sam: They expand the accordion lattice to set spacings from 266 nanometers up, then excite and measure how much the emission deviates from what lone atoms would do—peaking at fractions like half-wavelength. Data matches theory, with those peaks showing new radiative paths light up exactly when geometry aligns.
Alex: Huh... so without a mirror enclosure, the grid itself controls light paths.
Sam: Precisely—this cavity-free setup lets structured interactions emerge from photon links between atoms. They visualize the shift from clustered bright bursts to spread-out dark traps.
Alex: So overall, this setup turns a grid of atoms into something like a switchable light trap... without needing those bulky cavities.
Sam: That's a fair summary. The arrays provide precise control over how light and atoms share energy, with patterns that emerge naturally from their spacing and order, as confirmed by both imaging and the model.
Alex: A grounded path to useful quantum devices, then... building on what they've demonstrated.
Sam: It is. This work establishes ordered atomic arrays as a versatile platform for studying and harnessing light-matter teamwork, with clear evidence from direct imaging and tuned geometry. The balance of experiment and theory sets a strong foundation for further exploration.
Alex: Thanks, Sam—that's a clear picture of where this fits in quantum optics. Listeners, that's our look at many-body effects in atomic arrays. Thanks for joining ResearchPod.