The Rydberg electromagnetically induced transparency (EIT)–Autler Townes (AT) splitting proportional to the target microwave electric field strength is an atom-based primary traceable standard in microwave electrometry. The minimum detected microwave electric field is limited when the EIT-AT splitting is indistinguishable. We design a method for auxiliary microwave-dressed Rydberg atoms that extends the electrometric span. We theoretically and experimentally show that the low bound of the direct SI-traceable microwave electric field is extended by two orders of magnitude, which is from several mV/cm to $\mu \mathrm{V}/\mathrm{cm}$ in a room-temperate Rb cell with a modest setup.

The microwave (MW) field can be measured by the Autler–Townes (AT) splitting of the electromagnetically induced transparency (EIT) spectrum in the Rydberg atomic system; however, the EIT-AT splitting method fails in weak MW fields. We used the amplitude modulation of the MW field to resolve the EIT-AT splitting in weak MW fields. The EIT-AT splitting interval can be directly obtained, and the minimum detectable MW strength is improved by six times compared with the traditional EIT-AT splitting method. The proposed method is more intuitive and convenient for measuring the strength of weak MW fields in practical applications.

Microwave phase detection is an essential technique in antenna metrology, radar, and communications. The Rydberg atom-based mixer has been successful in the measurement of the phase of a signal microwave field by creating a beat in the presence of a local microwave field, in which the phase information is transferred into the phase of the beat. In this study, we extend the function of the Rydberg atom-based mixer by adding alternating current Zeeman modulation, and find that the phase information is transferred into the amplitude of the beat, not only its phase. As the results, the phase information can be measured simply and real-time. The results show that another ‘dimension’ operation can be introduced into the Rydberg atom-based mixer to expand the functions and applications of the mixer, which is very useful for future real-time sensing applications. An experiment involving real-time measurement and phase-modulated digital communication based on the beat+Zeeman method is presented.

We theoretically and experimentally studied quantum microwave electrometry in a cold atomic system using Rydberg electromagnetic induction transparency (EIT) and Autler–Townes splitting (EIT-AT splitting). In cold atoms, a spectral linewidth of ∼500 kHz for EIT was achieved owing to a significant reduction in residual Doppler width, i.e. by at least an order of magnitude, compared to that in vapor cells at room temperature. Therefore, the minimum microwave electric field intensity (E _MW) that can be measured is 430 µV cm⁻¹, which is one order higher sensitivity in the EIT-AT regime than that in vapor cells at room temperature. Unlike microwave electrometry in atomic vapor cells, EIT-AT splitting cannot be observed if E _MW is so large that the EIT-AT splitting interval Δ f m exceeds the absorption peak width of the cold atom while scanning the frequency of the probe laser (ω _p ). Moreover, EIT-AT splitting can be observed if Δ f m exceeds the natural linewidth Γ eg of the intermediate states while scanning the coupling laser (ω _c ) and maintains a high spectral resolution with a high signal-to-noise ratio. While scanning ω _c , the upper microwave electric field intensity is limited by the scanning range of our setup. Using our system, we measure the maximum field to be 21.6 mV cm⁻¹, nearly three times higher than that of 6.8 mV cm⁻¹ while scanning ω _p . The results indicate that the linear range of E _MW measured using EIT-AT splitting considerably improves in cold Rydberg atoms.

Multistep laser resonance ionization spectroscopy of lutetium (Lu) has been performed at TRIUMF's off-line laser ion source test stand. The even-parity Rydberg series $6s^{2}nd{}^2{D}_{3/2}, 6s^{2}nd{}^2{D}_{5/2}$, and $6s^{2}ns{}^2{S}_{1/2}$ were observed converging to the $6s^2$ ionization potential. The experimental results have been compared to those of previous work. Fifty-one levels of Rydberg series $6s^{2}nd{}^2{D}_{5/2}$ and 52 levels of Rydberg series $6s^{2}ns{}^2{S}_{1/2}$ were reported. Additionally, six even-parity autoionization (AI) series converging to Lu ionic states $5d6s{}^3{D}_1$ and $5d6s{}^3{D}_2$ were observed. The level energies of these AI states were measured. The configurations of the AI states were assigned by relativistic multichannel theory within the framework of multichannel quantum defect theory.

We have systematically investigated the influence of the gas temperature (T), the Rabi frequencies of the probe laser (Ω_p), the coupling laser (Ω_c) and the radio-frequency (RF) (Ω_RF) on the Rydberg electromagnetically induced transparency (EIT) and Autler–Townes (AT) splitting (Δ_f) by defining a general Doppler mismatch factor D _g = Ω_RF/Δ_f in the Rydberg atom-based microwave electrometry. The effect of T on D _g is studied in detail from 0 to 1000 K, the results show that D _g is insensitive to T when T < 10 μK or T > 10 K, while D _g changes significantly with 10 K > T > 10 μ K . Then the effects of Ω_p, Ω_c and Ω_RF on factor D _g at T = 300 K (typical room temperature) and T = 10 μK (typical temperature of cold atom by laser cooling) are studied in detail, respectively. The results show that the linewidth of Rydberg EIT (Γ_EIT) can be used as a key parameter to characterize the dependence of D _g on Ω_p and Ω_c in both cases. D _g is insensitive to T, Ω_p and Ω_c when Ω_RF > 3Γ_EIT which means that Γ_EIT determines the lower limit of the linear region of the RF electric field strength measured by EIT–AT splitting. More interesting, the range where D _g is insensitive to Ω_p and Ω_c can be greatly expanded by lowering the gas temperature to 10 μK. The ranges of parameters where D _g is insensitive to T, Ω_p, Ω_c and Ω_RF are given, and such relationship can be easily scaled to other atomic systems. The results can help the selection of various parameters in the experiments and specific applications to ensure the accuracy of measuring the RF electric field.