Atomic Interactions Broaden Light Spectra, Revealing Insights into Sensing Technology

Researchers are increasingly exploring Rydberg-mediated photon-photon interactions for applications in nonlinear optics and quantum technologies. Xinghan Wang, Yupeng Wang, and Aishik Panja, all from the Department of Physics and Astronomy at Purdue University, alongside Qi-Yu Liang from both the Department of Physics and Astronomy and the Purdue Quantum Science and Engineering Institute at Purdue University, present compelling new evidence regarding these interactions within cold-atom Rydberg electromagnetically induced transparency. Their collaborative work demonstrates how increasing photon-photon interactions induce nonlinear spectral broadening and resonance shifts, revealing key insights into Rydberg physics. This research is significant because it not only clarifies the underlying mechanisms governing these interactions, but also establishes conditions for unbiased microwave field characterisation, potentially advancing the development of highly sensitive atomic sensors.

Scientists are unlocking new capabilities in atomic sensors by meticulously examining the subtle interplay between light and matter at the Rydberg level. Recent work demonstrates how interactions between Rydberg atoms, atoms with highly excited electrons, influence the precision of microwave and radio-frequency sensing techniques. This research, conducted with laser-cooled rubidium atoms, reveals a surprising divergence in behaviour between three- and four-level excitation schemes, offering crucial insights for building more sensitive and reliable devices. The study clarifies conditions under which microwave field characterisation can be performed without introducing systematic bias, a critical step towards achieving quantum-limited sensing performance. Researchers experimentally investigated Rydberg interaction-induced nonlinearity within cold-atom Rydberg electromagnetically induced transparency (EIT), a phenomenon where a medium becomes transparent to a probe laser due to the application of a control laser. In a standard three-level EIT system, increasing the intensity of the probe laser produces predictable nonlinear spectral broadening accompanied by shifts in the resonance frequency. However, when a microwave field is introduced to create a four-level system, pronounced nonlinear broadening occurs without any detectable spectral shifts, challenging existing theoretical models and necessitating a refined understanding of many-body interactions within Rydberg EIT spectroscopy. The three-level data aligns with a conditional superatom model, suggesting collective atomic behaviour dominates under certain conditions. Conversely, the four-level observations are well-described by a simpler dephasing model, indicating that the microwave dressing effectively suppresses the resonance shifts observed in the three-level system. By comparing experimental results with three representative theoretical frameworks, the study provides key insights into the role of these interactions and their impact on the resulting optical spectra. This work advances fundamental knowledge of many-body physics and paves the way for practical improvements in atomic sensor technology, potentially leading to more accurate and stable measurements across a wide range of frequencies. Increasing the probe photon rate in a three-level Rydberg EIT system induced spectral broadening alongside measurable resonance shifts. Specifically, observations revealed a resonance shift occurring concurrently with nonlinear spectral broadening, a phenomenon not previously reported in similar experiments. This shift signifies a change in the energy levels of the atoms due to their interactions, validating certain theoretical models. In contrast, a microwave-dressed four-level system, subjected to comparable nonlinear broadening, exhibited no detectable spectral shifts, demonstrating a fundamental difference in how interactions manifest in the four-level configuration. The observed nonlinear broadening in the three-level system increased with higher probe photon rates, while the magnitude of the resonance shift remained consistent with predictions based on the C6 coefficient characterising van der Waals interactions. In the four-level system, nonlinear broadening reached comparable levels, yet the absence of spectral shifts highlights a distinct interaction mechanism. These findings clarify the conditions under which microwave field characterisation can be performed in the nonlinear regime without introducing systematic bias, advancing both fundamental understanding of many-body physics and the practical development of atomic sensors. A spin-polarized cloud of 87Rb atoms serves as the foundation for Rydberg EIT investigations. Atoms are initially prepared in the ground state |1⟩= |5S1/2, F = 2, mF = −2⟩, defining the hyperfine and magnetic sublevels. A weak σ−-polarized probe laser then couples this ground state to an intermediate state |2⟩= |5P3/2, F = 3, mF = −3⟩, which is subsequently linked to a Rydberg state |3⟩= |61S1/2, mj = −1/2⟩ via a σ+-polarized control laser. To further refine the system, a microwave (MW) field addresses the |3⟩↔|4⟩= |61P3/2, mj⟩ transition, creating a four-level system for detailed analysis. The atomic cloud is first laser-cooled within a magneto-optical trap and then transferred to a far-detuned 1064nm optical dipole trap, employing a procedure detailed in a previous publication. A bias magnetic field of 3.7 Gauss is applied along the probe propagation direction to establish a defined quantization axis and facilitate optical pumping, ensuring precise control over the atomic states and their interactions. The resulting dipole-trapped atomic cloud achieves a temperature of 14 μK via gray molasses cooling, minimising Doppler broadening through the use of counter-propagating 780nm probe and 479nm control beams. To mitigate the effects of atomic motion and maximise spectral resolution, the dipole trap is modulated with a period of 180μs and a 50% duty cycle, with probing occurring during the off-time. Following 50 modulation cycles, the dipole trap is switched off, allowing the atoms to disperse for 10ms before a 2ms probe measurement. Normalization is achieved by averaging counts obtained without atoms, providing a consistent baseline for transmission data. The MW fields are delivered via a broadband horn antenna (FMWAN159-10SF, Fairview Microwave) driven by a signal generator (Anritsu 68369A/NV), and despite utilising a single antenna, all σ+, σ−, and π polarization components are present at the atoms due to reflections and scattering. Scientists have long recognised the potential of Rydberg atoms for exquisitely sensitive sensors, but realising that potential has proven surprisingly difficult. The challenge lies in untangling the complex interactions between these highly excited atoms and the electromagnetic fields used to probe them. This new work offers a crucial step forward by carefully dissecting how interactions between Rydberg atoms affect microwave and radio-frequency sensing, revealing subtle differences in behaviour depending on the experimental setup. It’s not simply about detecting a signal; it’s about ensuring that signal isn’t distorted by the very process of measurement. The significance extends beyond fundamental atomic physics. Precise, compact, and highly sensitive radio-frequency sensors have applications ranging from medical diagnostics to materials science and even national security. Existing technologies often struggle with size, power consumption, or sensitivity. Atomic sensors, leveraging the predictable quantum properties of atoms, offer a potential route to overcome these limitations, but only if the underlying physics is fully understood and controlled. The surprising finding that a simple dephasing model accurately captures behaviour in a specific four-level system suggests that some interactions are less complex than previously assumed, offering a pathway to streamlined sensor design. Crucially, this research clarifies conditions for unbiased measurements, a critical hurdle for practical applications. Future work will likely focus on extending these findings to more complex scenarios, exploring different atomic species, and ultimately, integrating these sensors into real-world devices. The path to a fully realised atomic sensor is still long, but this work illuminates a clearer route forward.