Week-long-lifetime microwave spectral holes in an erbium-doped scheelite crystal at millikelvin temperature.

Rare-earth-ions (REI) doped crystals have remarkable optical and spin properties characterized by narrow homogeneous linewidths, which can be studied despite the large inhomogeneous broadening of the ensemble line through spectral hole burning (SHB). Here, we report SHB spectroscopic measurements in a scheelite crystal of CaWO₄ by pumping the spin transition of a paramagnetic REI (Er³⁺) at microwave frequency and millikelvin temperatures, with nuclear spin states of neighboring ¹⁸³W atoms serving as the auxiliary levels. The repeated application of pairs of microwave pulses generates a periodic modulation of the Er³⁺ density profile, which we observe spectrally and in the time-domain as an accumulated echo. The lifetime of the holes and accumulated echoes rises steeply as the sample temperature is decreased, exceeding a week at 10mK. Our results demonstrate that millikelvin temperatures can be beneficial for signal processing applications requiring long spectral hole lifetimes.