The DAALI consortium significantly advanced a number of “platforms for the future” for quantum optics. Key successes include:
- The ability to achieve long interaction times and even trap single atoms near whispering gallery mode (WGM) resonators. The high cooperativities of WGM resonators and the long interaction times enable more complex quantum processes to be performed, as evidenced by the implementation of a photonic quantum RAM protocol with ~80% efficiencies. Beyond this specific result demonstrated within DAALI, we envision many other prominent future results that take advantage of the advances of these systems.
- The realization of the first quantum gas microscope for quantum degenerate 84Sr atoms, which enables the manipulation and imaging of every individual atom in the array. This platform is expected to have significant impact for quantum optics, due to the ability to generate large, defect-free arrays that feature selective radiance, and the ability to access long-lived optical transitions. More broadly, this quantum gas microscope should open up unique opportunities in quantum simulation and the exploration of quantum many-body physics, e.g. SU(N) models.
- Significant progress toward a new generation of quantum interfaces between photonic crystal waveguides and cold atoms. The combination of new designs for photonic crystal waveguides and novel materials (GaInP), and the planned integration with tweezer arrays and structured light traps, should address the major challenges faced by previous attempts to implement this technology. In particular, our approach should allow to interface multiple atoms in a controlled and scalable fashion, while simultaneously yielding better figures of merit such as atom-photon coupling strengths.
- Development of a novel nanophotonics (nanofiber) interface to rare earth ion chelates, which protect the ions from non-radiative decay channels and ensure high optical quality of the ions.
- Key advances to push atom-nanofiber interfaces toward the “selective radiance” regime, in which unit-filled chains of atoms with sub-λ/2 lattice constants are coupled to the nanofiber. It is known theoretically that selective radiance can enable exponentially suppressed error probabilities (versus atom number) for important quantum applications like quantum memories, and it is widely believed that novel regimes of many-body physics with photons will become accessible.
- Vastly improved fiber integrated quantum memories, featuring a storage time up to 3 orders of magnitude longer for entangled photon pairs than previous demonstrations in integrated memories. This establishes the viability of photonic systems going forward.
- Advancement of Rydberg atom ensembles for quantum information applications. Novel demonstrated capabilities include the purification of single photons in a Rydberg ensemble, marking the first experimental demonstration of single-photon filtering with non-classical states and storage in Rydberg states, and networking of multiple ensembles.
Accompanying the development of physical platforms was the development of a number of novel protocols and conceptual breakthroughs:
- Theoretical proposal and experimental demonstration of a technique to generate optical spin waves in atomic media with wavevectors that are highly mismatched from light, k ≫ ω/c, using robust geometric phase imprinting. This is particularly interesting in the context of arrays, where such excitations are predicted to be highly subradiant.
- Proposal for a photon-photon gate using 2D sub-wavelength arrays with Rydberg interactions demonstrates a strong polynomial reduction in errors over previous protocols, and should enable a ~1% error even with small, 5x5 arrays. Compared to the best gates demonstrated so far, with 60% error, this provides a viable path toward high-efficiency quantum operations with atoms and light and further establishes atom arrays as a future platform for quantum optics.
- Materials with ultrahigh refractive index would be game changing for optics and photonics, as the index dictates the minimum footprint of optical devices and the minimum length scale that can be resolved (e.g. microscopy). The theories developed within DAALI elucidate why the refractive index of known materials is of order unity, and provide a path toward possibly synthesizing low-loss, ultrahigh index materials (n~30) in the future.
Overall, DAALI directly resulted in two commercial or IP opportunities, related to the turnkey-laser system developed for the atom-nanophotonics interface, and the quantum-information storage device patent filed and licensed. In the second case, on operating licence was granted to the company WelinQ, which is considering this technology for the development of a next generation quantum memory. Multiple opportunities also exist for longer-term exploitation potential, including potential routes to realize ultrahigh-index materials, and new platforms for photon quantum information processing using atom arrays.
