Broad Instantaneous Bandwidth Microwave Spectrum Analyzer with a Microfabricated Atomic Vapor Cell

We report on broad instantaneous bandwidth microwave spectrum analysis with hot <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mmultiscripts><a:mrow><a:mi>Rb</a:mi></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>87</a:mn></a:mrow></a:mmultiscripts></a:mrow></a:math> atoms in a microfabricated vapor cell in a large magnetic field gradient. The sensor is a MEMS atomic vapor cell filled with isotopically pure <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mmultiscripts><c:mrow><c:mi>Rb</c:mi></c:mrow><c:mprescripts/><c:none/><c:mrow><c:mn>87</c:mn></c:mrow></c:mmultiscripts></c:mrow></c:math> and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi mathvariant="normal">N</e:mi><e:mn>2</e:mn></e:msub></e:math> buffer gas to localize the motion of the atoms. The microwave signals of interest are coupled through a coplanar waveguide to the cell, inducing spin-flip transitions between optically pumped ground states of the atoms. A static magnetic field with large gradient maps the frequency spectrum of the input microwave signals to a position-dependent spin-flip pattern on absorption images of the cell recorded with a laser beam onto a camera. In our proof-of-principle experiment, we demonstrate a microwave spectrum analyzer that has <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mo>≈</h:mo><h:mn>1</h:mn><h:mtext> </h:mtext><h:mtext> </h:mtext><h:mi>GHz</h:mi></h:math> instantaneous bandwidth centered around 13 GHz, 3 MHz frequency resolution, 2 kHz refresh rate, and a <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mo>−</j:mo><j:mn>23</j:mn><j:mtext> </j:mtext><j:mtext> </j:mtext><j:mi>dBm</j:mi></j:math> single-tone microwave power detection limit in 1 s measurement time. A theoretical model is constructed to simulate the image signals by considering the processes of optical pumping, microwave interaction, diffusion of <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mrow><l:mmultiscripts><l:mrow><l:mi>Rb</l:mi></l:mrow><l:mprescripts/><l:none/><l:mrow><l:mn>87</l:mn></l:mrow></l:mmultiscripts></l:mrow></l:math> atoms, and laser absorption. We expect to reach more than 25 GHz instantaneous bandwidth in an optimized setup, limited by the applied magnetic field gradient. Our demonstration offers a practical alternative to conventional microwave spectrum analyzers based on electronic heterodyne detection. Published by the American Physical Society 2024