Maximizing quantum enhancement in axion dark matter experiments

We provide a comprehensive comparison of linear amplifiers and microwave photon counters in axion dark matter experiments. The study is done assuming a range of realistic operating conditions and detector parameters, over the frequency range between 1 and 30 GHz. As expected, photon counters are found to be advantageous under low background, at high frequencies (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>ν</a:mi><a:mo>&gt;</a:mo><a:mn>5</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>GHz</a:mi></a:math>), they can be implemented with robust wide-frequency tuning or a very low dark count rate. Additional noteworthy observations emerging from this study include: (1) an expanded applicability of off-resonance photon background reduction, including the single-quadrature state squeezing, for scan rate enhancements; (2) a much broader appeal for operating the haloscope resonators in the overcoupling regime, up to <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>β</c:mi><c:mo>∼</c:mo><c:mn>10</c:mn></c:math>; (3) the need for a detailed investigation into the cryogenic and electromagnetic conditions inside haloscope cavities to lower the photon temperature for future experiments; (4) the necessity to develop a distributed network of coupling ports in high-volume axion haloscopes to utilize these potential gains in the scan rate.