Ananya Sitaram

We demonstrate a continuous spectroscopy technique with frequency sensitivity well below the natural transition linewidth, while maintaining a locking range hundreds of times larger. The method exploits the position dependence of a continuous, broadband magneto-optical trap (MOT) operating on the 7.5-kHz-wide intercombination line of strontium. We show that the frequency sensitivity is fundamentally insensitive to the effective MOT laser linewidth. By applying active feedback on the MOT position to a dispersion-optimized frequency comb, which serves as the reference for stabilizing the MOT laser (Boughdachi , arXiv:2507.23617), we achieve a frequency instability below 4.4 × 10 − 13 after 400 s of averaging in both the optical and radio-frequency domains, surpassing the performance of conventional hot-vapor modulation transfer spectroscopy. Our method is a broadly applicable alternative route to frequency references in the high 10 − 14 range around 100 s.

We demonstrate a quasicontinuous sub- μ K strontium source achieved without the use of a high-finesse cavity-locked laser. Our frequency reference is based on a dispersion-optimized, fiber-based frequency comb that enables sub-kHz linewidths. The long-term stability of the comb is defined by an external rf reference: either a 10 MHz rf signal from the Dutch Metrology Institute (VSL) or a tunable rf source whose long-term stability is maintained by monitoring and stabilizing the position of a narrow-line magneto-optical trap (MOT) [Sillus , Rev. Sci. Instrum., 92, 033204 (2021); Heizenreder , Continuous cloud position spectroscopy using a magneto-optical trap, Phys. Rev. Appl. (2026)]. The comb-stabilized system is benchmarked against a conventional cavity-locked laser and achieves comparable performance in broadband and single-frequency MOTs using the narrow 1 S 0 → 3 P 1 laser-cooling transition. We generate high-flux, sub- μ K samples of all three bosonic strontium isotopes and demonstrate quasicontinuous outcoupling from the MOT. These results highlight the system’s suitability for compact, robust, and field-deployable continuous cold-atom devices.

Continuous superradiance using a narrow optical transition has the potential to improve the short-term stability of state-of-the-art optical clocks. Even though pulsed superradiant emission on a mHz linewidth clock transition has been shown, true continuous operation, without Fourier limitation, has turned out to be extremely challenging. The trade-off between maintaining a high atomic flux while minimizing decoherence effects presents a significant obstacle. Here, we discuss the design of a machine that could overcome this problem by combining a high-flux continuous beam of ultracold strontium atoms with a bowtie cavity for the generation of superradiant lasing. To evaluate the feasibility of our design, we present simulation results for continuous high-efficiency cooling, loading, and pumping to the upper lasing state inside the bowtie cavity. We then present two different models for simulating the generated superradiant field by taking into account position-dependent shifts, collisional decoherence, light shifts, and atom loss. Finally, we estimate a laser linewidth of less than 100 mHz , limited by atom number fluctuations, resulting in an output power of hundreds of fW .

Most atomic physics experiments are controlled by a digital pattern generator used to synchronize all equipment by providing triggers and clocks. Recently, the availability of well-documented open-source development tools has lifted the barriers to using programmable systems on chip (PSoCs), making them a convenient and versatile tool for synthesizing digital patterns. Here, we take advantage of these advancements in the design of a versatile clock and pattern generator using a PSoC. We present our design with the intent of highlighting the new possibilities that PSoCs have to offer in terms of flexibility. We provide a robust hardware carrier and basic firmware implementation that can be expanded and modified for other uses.

We demonstrate a compact magneto-optical trap (MOT) of alkaline-earth atoms using a nanofabricated diffraction grating chip. A single input laser beam, resonant with the broad ¹S₀ → ¹P₁ transition of strontium, forms the MOT in combination with three diffracted beams from the grating chip and a magnetic field produced by permanent magnets. A differential pumping tube limits the effect of the heated, effusive source on the background pressure in the trapping region. The system has a total volume of around 2.4 l. With our setup, we have trapped up to 5 × 10^{6 88}Sr atoms at a temperature of ∼6 mK, and with a trap lifetime of ∼1 s. Our results will aid the effort to miniaturize quantum technologies based on alkaline-earth atoms.

The mammalian immune system has the ability to discriminate between pathogens and innocuous microbes by detecting conserved molecular patterns. In addition to conserved microbial patterns, the mammalian immune system may recognize distinct pathogen-induced processes through a mechanism which is poorly understood. Previous studies have shown that a type III secretion system (T3SS) in Yersinia pseudotuberculosis leads to decreased survival of this bacterium in primary murine macrophages by unknown mechanisms. Here, we use colony forming unit assays and fluorescence microscopy to investigate how the T3SS triggers killing of Yersinia in macrophages. We present evidence that Yersinia outer protein E (YopE) delivered by the T3SS triggers intracellular killing response against Yersinia. YopE mimics eukaryotic GTPase activating proteins (GAPs) and inactivates Rho GTPases in host cells. Unlike wild-type YopE, catalytically dead YopER144A is impaired in restricting Yersinia intracellular survival, highlighting that the GAP activity of YopE is detected as a danger signal. Additionally, a second translocated effector, YopT, counteracts the YopE triggered killing effect by decreasing the translocation level of YopE and possibly by competing for the same pool of Rho GTPase targets. Moreover, inactivation of Rho GTPases by Clostridium difficile Toxin B mimics the effect of YopE and promotes increased killing of Yersinia in macrophages. Using a Rac inhibitor NSC23766 and a Rho inhibitor TAT-C3, we show that macrophages restrict Yersinia intracellular survival in response to Rac1 inhibition, but not Rho inhibition. In summary, our findings reveal that primary macrophages sense manipulation of Rho GTPases by Yersinia YopE and actively counteract pathogenic infection by restricting intracellular bacterial survival. Our results uncover a new mode of innate immune recognition in response to pathogenic infection. The type III secretion system (T3SS) is a macromolecular protein export pathway found in gram-negative bacteria. It delivers bacterial toxins into eukaryotic cells to promote pathogenic infection. T3SSs and the bacterial toxins delivered are critical arsenals for many bacterial pathogens of clinical significance, such as Yersinia, Salmonella and Shigella. On the other hand, the mammalian immune system may recognize the T3SS as a danger signal to signify pathogenic infection, and to stimulate appropriate defense against pathogens. Here, we show that the innate immune system recognizes the activity of YopE delivered by the Yersinia T3SS. Modulation of host Rho GTPases by YopE elicits a defensive response, which results in killing of bacteria in host cells. Inhibition of host Rho GTPases by Clostridium difficile Toxin B, another bacterial toxin, mimics the YopE-triggered killing effect. Our study demonstrates that host cells sense manipulation of Rho GTPases by bacterial toxins as a surveillance mechanism, revealing new insights into innate immune recognition of pathogenic infections.