Persistent spectral hole burning and atomic frequency comb at microwave frequency in Er3+:CaWO4

The interaction of electron spins with neighboring nuclear spins in a host crystal leads to rich physics and dynamics, as observed in semiconducting quantum dots, color centers in diamond and donors in silicon. Here, we report a new phenomenon, using a crystal of CaWO4 containing Erbium ions, at millikelvin temperature. Erbium has a doublet ground state with a large magnetic moment behaving as an effective electron spin-1/2. In CaWO4, Er3+ couple to the magnetic moment of neighboring 183W nuclear spins (14% abundance). Under a field of 450mT, the Erbium spin is brought in resonance with a superconducting resonator at 7.8GHz used for detection. By applying a microwave tone, we observe spectral holes created in the absorption of Er and these holes exist over 20 hours, which is much longer than the Er3+:CaWO4 spin-lattice relaxation time (0.2s). We interpret the holes as being caused by dynamic nuclear polarization of the nearby W nuclear spin leading to an Overhauser field seen by the Er3+, while its persistent existence demonstrates the stability of polarization within W nuclear spins at low temperature. Furthermore, by applying repeated double-pulse sequence, we are able to generate an atomic frequency comb in the spin ensemble, which persists for at least 120 hours at 10mK.