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We report dark photon results from HAYSTAC phase II using data from previously reported axion searches. Additionally, we present an analysis of an unpublished dataset covering a region between $19.46–19.52\mathrm{\mu eV}$ . This region overlaps with a recently reported dark photon signal at $19.5\mathrm{\mu eV}$ with a kinetic coupling strength of $\left|{\chi }_{\mathrm{rand}}\right|\simeq 6.5\times {10}^{-15}$ resulting from a reanalysis of previously published data from the TASEH collaboration. Given HAYSTAC’s sensitivity, if such a signal were present, it would have appeared as a large $17.1\sigma$ excess above the noise. However, no such signal was observed. We thus exclude couplings $\left|{\chi }_{\mathrm{rand}}\right|\ge 4.90\times {10}^{-15}$ at the 90% confidence level over the newly reported region. In addition, using our previously reported axion data, we exclude couplings $\left|{\chi }_{\mathrm{rand}}\right|\ge 2.90\times {10}^{-15}$ between $16.96–19.46\mathrm{\mu eV}$ at the 90% confidence level. 

This Letter reports new results from the HAYSTAC experiment’s search for dark matter axions in our galactic halo. It represents the widest search to date that utilizes squeezing to realize subquantum limited noise. The new results cover $1.71\mathrm{\mu }\mathrm{eV}$ of newly scanned parameter space in the mass ranges $17.28–18.44\mathrm{\mu }\mathrm{eV}$ and $18.71–19.46\mathrm{\mu }\mathrm{eV}$ . No statistically significant evidence of an axion signal was observed, excluding couplings $\left|g_{\gamma }\right|\ge 2.75\times \left|g_{\gamma }^{\mathrm{KSVZ}}\right|$ and $\left|g_{\gamma }\right|\ge 2.96\times \left|g_{\gamma }^{\mathrm{KSVZ}}\right|$ at the 90% confidence level over the respective region. By combining this data with previously published results using HAYSTAC’s squeezed state receiver, a total of $2.27\mathrm{\mu }\mathrm{eV}$ of parameter space has now been scanned between $16.96–19.46\mathrm{\mu }\mathrm{eV}$ $\mu \mathrm{eV}$ , excluding $\left|g_{\gamma }\right|\ge 2.86\times \left|g_{\gamma }^{\mathrm{KSVZ}}\right|$ at the 90% confidence level. These results demonstrate the squeezed state receiver’s ability to probe axion models over a significant mass range while achieving a scan rate enhancement relative to a quantum-limited experiment. Published by the American Physical Society2025 

We design and test a low-loss interface between superconducting three-dimensional microwave cavities and two-dimensional circuits, where the coupling rate is highly tunable. This interface seamlessly integrates a loop antenna and a Josephson junction-based coupling element. We demonstrate that the loss added by connecting this interface to the cavity is 1.28 kHz, corresponding to an inverse quality factor of 1/(4.5×106). Furthermore, we show that the cavity's external coupling rate to a 50 Ω transmission line can be tuned from negligibly small to over 3 orders of magnitude larger than its internal loss rate in a characteristic time of 3.2 ns. This switching speed does not impose additional limits on the coupling rate because it is much faster than the coupling rate. Moreover, the coupler can be controlled by low frequency signals to avoid interference with microwave signals near the cavity or qubit frequencies. Finally, the coupling element introduces a 0.04 Hz/photon self-Kerr nonlinearity to the cavity, remaining linear in high photon number operations.