${\mathrm{Tm}}^{3+}:{\mathrm{Y}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ Materials for Spectrally Multiplexed Quantum Memories

We investigate the relevant spectroscopic properties of the 795 nm ${}^3{\mathrm{H}}_6\leftrightarrow {}^3{\mathrm{H}}_4$ transition in 1% ${\mathrm{Tm}}^{3+}:{\mathrm{Y}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ at temperatures as low as 1.2 K for optical quantum memories based on persistent spectral tailoring of narrow absorption features. Our measurements reveal that this transition has uniform coherence properties over a 56 GHz bandwidth, and a simple hyperfine structure split by $\pm 44\mathrm{MHz}/\mathrm{T}$ with lifetimes of up to hours. Furthermore, we find a ${{}^3\mathrm{F}}_4$ population lifetime of 64 ms—one of the longest lifetimes observed for an electronic level in a solid—and an exceptionally long coherence lifetime of $490\mu \mathrm{s}$—the longest ever observed for optical transitions of ${\mathrm{Tm}}^{3+}$ ions in a crystal. Our results suggest that this material allows realizing broadband quantum memories that enable spectrally multiplexed quantum repeaters.

Figure 1

Transient spectral hole decay. Hole burning measurements at 10 K reveal a ${{}^3\mathrm{H}}_4$ lifetime of 1.3 ms and an exceptionally long ${{}^3\mathrm{F}}_4$ lifetime of 64 ms. A relatively large probability $\beta =0.65$ of decay to the ${{}^3\mathrm{F}}_4$ bottleneck level is measured. Inset: Simplified energy level diagram showing measured excited-state lifetimes, branching ratio, and transition wavelengths.

Figure 2

Persistent spectral hole decay measured at 2.0 K for a field of 272 G. A double exponential decay is observed at low fields, possibly from two inequivalent subgroups of ${\mathrm{Tm}}^{3+}$ ions. Inset: Measured persistent hole spectrum with an applied field of 0.64 T. A single pair of antiholes appear with a splitting of $\pm 44\mathrm{MHz}/\mathrm{T}$.

Figure 3

Two-pulse photon echo decays. Measurements at 1.2 K and 454 G yield a coherence lifetime of $490\mu \mathrm{s}$ for 1% Tm:YGG (circles). For comparison, echo decays are shown for Tm:YAG (squares) [17] and Tm:LN (triangles) [25] at similar temperatures and fields.