We demonstrate microwave kinetic inductance detectors (MKIDs) whose sensitivity is limited by photon noise at 50 mHz signal frequencies. The sub-Hz part of the detection spectrum is important for millimeter-wave instruments, yet photon noise below 1 Hz in MKIDs has not previously been demonstrated unambiguously. Devices are feedhorn-coupled, direct-absorber-type and consist of lumped-element superconducting resonators fabricated from a hybrid of stoichiometric TiN, Al, and an amorphous-Si passivation layer. Viewing a 7K thermal load and a passband centered on 2mm, the device noise spectrum is white down to 50 mHz and has an amplitude consistent with photon noise. We will present these measurements and discuss the multiple design choices that lead to the result.
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