Improving the spectral resolution and measurement range of quantum microwave electrometry by cold Rydberg atoms

Abstract

Abstract 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 µ V 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:msub> <mml:mi>f</mml:mi> <mml:mi>m</mml:mi> </mml:msub> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:msub> <mml:mi>f</mml:mi> <mml:mi>m</mml:mi> </mml:msub> </mml:math> exceeds the natural linewidth <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi mathvariant="normal">Γ</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>eg</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> 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.