Rydberg atom-based electromagnetic field sensors are currently the focus of intense research efforts, promising high sensitivity, tunability, and compactness with SI-traceable measurements. However, the presence of a cell around the atoms alters the microwave field distribution, particularly due to reflection by materials like quartz. To characterize these sensors, measuring how the presence of the cell affects the microwave field distribution is therefore essential. Electro-optical probes offer high spatial resolution but require time-consuming scanning. Infrared electromagnetic thermography provides direct field mapping but is limited by the opacity of glass to infrared radiation. In contrast, fluorescence thermography allows for electric field mapping through optically transparent media. In this paper, we report the application of this technique to map the microwave electric field inside a quartz cell intended for atomic cooling and trapping. This provides a direct observation of the standing waves inside the cell, in good agreement with numerical simulations. This technique could be a valuable tool for the design and optimization of atomic cells for applications requiring good control of the microwave field distribution inside the cell, such as atomic clocks or electric field sensors based on hot or cold Rydberg atoms.
Imaging of electric and magnetic fields is limited by the fact that conventional field probes rely on conductive elements that perturb the fields they are meant to measure and require either spatial scanning or arrays to produce field maps. Atomic sensors based on Rydberg states offer a minimally invasive and broadband alternative, enabling absolute field measurements without external calibration. In this work, we present a field imaging technique that combines the spatial resolution of planar laser-induced fluorescence-dip spectroscopy with the field sensitivity and spectral resolution of electromagnetically induced transparency (EIT) using Rydberg states of atomic vapor. By imaging the change in fluorescence due to EIT, we spatially resolve the field-induced shifts in atomic energy levels, enabling the imaging of arbitrary electric field distributions from MHz to tens of GHz at the ∼V/cm level and static magnetic fields at the ∼mT level. We also image resonant microwave electric fields at the ∼5 mV/cm level. This technique achieves a spatial resolution of 160 µm, limited by our camera, with a fundamental resolution limit near 5 µm.
Quantum systems obeying well-defined laws of physics are very attractive metrological sensors. In principle, they are intrinsically calibrated and allow direct traceability to the SI system. Several fundamental and derived units are nowadays already based on the properties of quantum systems. Neutral atoms are e.g. used for the realization of the second. Leveraging the advantageous properties of such systems to other derived units and establishing traceability still represents a significant challenge, as real-life macroscopic scales must be linked with the microscopic quantum world. In this work, we investigate the use of a vapor of neutral atoms as a highly sensitive probe for electric fields in the microwave domain. By optically exciting the atoms to high-energy states—known as Rydberg states—the atoms become extremely sensitive to electric fields due to their large electric dipole moment. Through optical readout of the atomic state, this enhanced sensitivity can be exploited to turn the atoms into precise sensors for electric fields. In contrast to classical methods of measuring electric fields using metallic probes or antennas, a dilute atomic vapor has the distinct advantage of exerting only minimal influence on the fields being measured. In this study, we aim to leverage these benefits in an experiment designed to measure microwave electric field distributions above a coplanar waveguide structure with high precision and spatial resolution. In order to translate the high accuracy of the atomic sensors to accurate measurements of electrical parameters of the coplanar structure like power, voltage standing wave ratio or S-parameters, we plan to integrate the coplanar waveguide structure into a large atomic vapor cell where it is directly surrounded by the atomic gas. In this case the fields are only minimally disturbed by the vapor cell itself and allow for simple models to translate field measurements into electrical measurements. The methods we are developing in this project are not only aiming to provide traceable measurements for high frequency electrometry but also have the potential to be extended to higher frequency in the terahertz regime or to high resolution imaging of electric field distributions.
Research of hot atomic vapor systems has a long history. Recently, much-improved technologies, such as lasers, microwaves and digital/analog control systems, have led to exciting progress in this reviving field. Due to its cryogenic-free and vacuum-free deployment potential, room-temperature and quantum-enabled sensing devices using gaseous atoms have become a popular research topic, and engineering such devices has received considerable attention. Proof-of-principle experiments have been performed demonstrating new ideas such as quantum-enhanced motion sensors, next-generation magnetometers, Rydberg enabled microwave sensors, etc. This thesis focuses on my work related to electromagnetic-field sensing using Rydberg atoms in vapor cells, where Rydberg atoms are excited and detected by electromagnetically induced transparency (EIT) spectroscopy. I first investigate the detection of strong magnetic fields (
sim 1~T) in the Paschen-Back regime. The diamagnetic (quadratic) response of the Rydberg levels makes the atoms sensitive to small fluctuations in field strength on a large background. By implementing an isotope-mixed cell, we demonstrate that accurate measurement, within a relative uncertainty of
pm0.12%. Many Rydberg-enabled radio frequency and microwave sensing applications require a capability of zeroing or intentional tuning of small DC electric fields inside the vapor cell. For this purpose, I explore an in-situ DC electric-field sensing and tuning method without using any bulk or thin film electrodes. I find that the photoelectric effect on the vapor cell walls (with invisible, atomic Rb aggregate layers covering the surfaces) can generate tuning fields up to 0.8~V/cm, with an inhomogeneity of about 2%. The spatial distribution of these fields can be directly monitored by quadratic Stark shifts of the Rydberg atoms. This work may inspire new approaches for DC-field control in miniaturized, metal-free atomic vapor-cell devices. Like many other quantum measurements on hot gaseous samples, short interaction time and other technical decoherence processes are the major factors which limit the precision and, in certain cases, the accuracy of the metrological results. In order to study these effects in the context of EIT, I performed an EIT experiment using a
Lambda-type quantum system consisting of the hyperfine ground states and the first excited electronic state of the Rb atoms contained in a buffer-gas-free thermal vapor cell without anti-relaxation coating. Technical line-broadening effects due to inhomogeneity of stray magnetic fields, laser frequency jitter, transverse beam intensity distribution, interaction time distribution, and atom-wall/impurity gas collisions are studied. Experimental results and numerical simulations are compared. These results provide crucial information and quantitative insights for vapor cell sensor design. The development of compact and multi-functional spectroscopy vapor cells is widely considered to be a critical engineering step towards manufacturing field-deployable sensors for real life applications. My R&D efforts towards fabrication of glass spectroscopy cells with integrated, highly-conductive silicon components are presented in the last part of my thesis. I explored various technical challenges and successfully developed a proof-of-principle prototype from scratch. The device allows the application of electric fields inside the cell for particle trapping and for tuning Rydberg transitions. Further, the highly-conductive silicon components enable microwave polarization filtering for microwave sensing applications, plasma-physics studies, etc. Preliminary experimental/technical data and simulations are presented. In summary, in my thesis I report on a comprehensive study of several spectroscopic and field sensing applications of Rydberg and ground-state atoms in vapor cells, as well as forays into novel methods to engineer vapor cells with embedded electrodes.
Even in this day and age, precise absolute measurement of electric fields is not trivial. The authors extend quantum sensing via cold atoms and electromagnetically induced transparency to the strong-field regime. In this regime, existing techniques are insufficient due to the complex and highly nonlinear response of the atoms. This is an essential step toward calibration-free rf electric-field sensors for a wide range of applications that include antenna calibration, metamaterial characterization, and subwavelength imaging.
An atom-based approach is presented to detect absolute microwave (MW) electric fields (E-fields). The approach uses Rydberg atoms in vapor cells at room temperature. The MW E-field measurements utilize a bright resonance prepared within an electromagnetically induced transparency (EIT) window. The large transition dipole moments between energetically adjacent Rydberg states enable this method to make traceable E-field measurements with a sensitivity that is several orders of magnitude higher than the current standard for MW E-field measurements. The method can be used to image MW E-field in the near field regime with a subwavelength resolution of
mathrmlambdaMW/650, where
mathrmlambdaMW is the wavelength of the MW E-field. A high accuracy of $\\mathrm{1\\%}$ has been reached by minimizing the effects of the vapor cell geometry on the measured MW E-field. The dissertation also presents an alternative technique to perform the MW E-field measurement using dispersive properties of the EIT spectrum with a prism vapor cell. Recently, we applied a homodyne detection technique with a Mach Zehnder interferometer to achieve a new sensitivity limit for the MW E-field measurement,
mathrmsim3 muVcm−1Hz−1/2. The new sensitivity is one order of magnitude better than our prior best sensitivity presented in Ref. [Nat. Phys. 8, 819 (2012)]. The Rydberg atom-based method is promising to be a new standard for MW E-field measurements and to develop into portable devices in the field of MW technologies.
We have recently shown [Nat. Phys.8, 819 (2012)] that Alkali atoms contained in a vapor cell can serve as a highly accurate standard for microwave (MW) electric field strength as well as polarization. Here we show for the first time that Rydberg atom electromagnetically induced transparency can be used to image MW electric fields with unprecedented precision. The spatial resolution of the method is far into the subwavelength regime ∼λ/650 or 66 μm at 6.9 GHz. The electric field resolutions are similar to those we have already demonstrated ∼50 μV cm(-1). Our experimental results agree with finite element calculations of test electric-field patterns.