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

TL;DR

This study demonstrates that spectral holes in erbium-doped scheelite crystals can last over a month at millikelvin temperatures, significantly enhancing potential applications in quantum and classical information processing.

Contribution

It reports the first observation of extremely long spectral hole lifetimes exceeding a month in Er$^{3+}$-doped CaWO$_4$ at millikelvin temperatures, using microwave spectral hole burning techniques.

Findings

Spectral hole lifetime exceeds one month at 10 mK.

Longer hole lifetimes are achieved at lower temperatures.

Millikelvin temperatures improve spectral hole stability for signal processing.

Abstract

Rare-earth-ion (REI) ensembles in crystals have remarkable optical and spin properties characterized by narrow homogeneous linewidths relative to the inhomogeneous ensemble broadening. This makes it possible to precisely tailor the ensemble spectral density and therefore the absorption profile by applying narrow-linewidth radiation to transfer population into auxiliary levels, a process broadly known as spectral hole burning (SHB). REI-doped crystals find applications in information processing, both classical (pattern recognition, filtering, spectral analysis) and quantum (photon storage), all protocols requiring suitable ensemble preparation by SHB as a first step. In Er$^{3+}$-doped materials, the longest reported hole lifetime is one minute, and longer lifetimes are desirable. Here, we report SHB and accumulated echo measurements in a scheelite crystal of CaWO$_4$ by pumping the electron spin transition of Er$^{3+}$ ions at microwave frequencies and millikelvin temperatures, with nuclear spin states of neighboring $^{183}$W atoms serving as the auxiliary levels. The lifetime of the holes and accumulated echoes rises steeply as the sample temperature is decreased, exceeding a month at 10 mK. Our results demonstrate that millikelvin temperatures can be beneficial for signal processing applications requiring long spectral hole lifetimes.