Optical Spin-Wave Storage in a Solid-State Hybridized Electron-Nuclear Spin Ensemble

Solid-state impurity spins with optical control are currently investigated for quantum networks and repeaters. Among these, rare-earth-ion doped crystals are promising as quantum memories for light, with potentially long storage time, high multimode capacity, and high bandwidth. However, with spins there is often a tradeoff between bandwidth, which favors electronic spin, and memory time, which favors nuclear spins. Here, we present optical storage experiments using highly hybridized electron-nuclear hyperfine states in ${{}^{171}\mathrm{Yb}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$, where the hybridization can potentially offer both long storage time and high bandwidth. We reach a storage time of 1.2 ms and an optical storage bandwidth of 10 MHz that is currently only limited by the Rabi frequency of the optical control pulses. The memory efficiency in this proof-of-principle demonstration was about 3%. The experiment constitutes the first optical storage using spin states in any rare-earth ion with electronic spin. These results pave the way for rare-earth based quantum memories with high bandwidth, long storage time, and high multimode capacity, a key resource for quantum repeaters.

Figure 1

Experimental setup. (a) Energy level diagram of the optical ${{}^2\mathrm{F}}_{7/2}(0){\leftrightarrow }^2{\mathrm{F}}_{5/2}(0)$ transition for site II of ${{}^{171}\mathrm{Yb}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal at zero magnetic field. The $\mathrm{\Lambda }$ system used for optical storage uses transition ${\nu }_1$ for the input and output pulses and transition ${\nu }_2$ for the control pulses, and ${\nu }_{\mathrm{MW}}$ for the MW pulses. Transitions ${\nu }_3$ and ${\nu }_4$ are added for the different optical pumping steps. (b) The absorption spectrum after performing class cleaning and state initialization to the $|4{⟩}_g$ ground state over a 40 MHz bandwidth. Inset: enlargement of the ${\nu }_1$ and ${\nu }_2$ transitions of the $\mathrm{\Lambda }$ system. (c) Optical depth of the antihole at ${\nu }_1$ frequency and the hole at ${\nu }_2$ frequency, as a function of delay after the state initialization. (d) Experimental setup (see text for details). Fabry-Perot cavity (FP), Pound-Drever-Hall module (PDH), phase modulator (PM), phase locked loop (PLL), half-wave plate ($\lambda /2$), polarization beam splitter (PBS), Faraday rotator (FR), microwave frequency doubler ($\times 2$), microwave switch (SW).

Figure 2

Optical AFC echoes. (a) Example of an optical AFC echo for a delay of $1/\mathrm{\Delta }=5\mu \mathrm{s}$, with an efficiency of ${\eta }_{\mathrm{AFC}}=15\%$. The input pulse duration was 100 ns. (b) AFC echo efficiencies as a function of delay $1/\mathrm{\Delta }$ (see text for fit model). (c) Two examples of measured AFCs for $1/\mathrm{\Delta }=1$ and $1/\mathrm{\Delta }=5\mu \mathrm{s}$, in terms of optical depth $d$ as a function of frequency detuning on the transition ${\nu }_1$.

Figure 3

AFC spin-wave storage results. (a),(b) Measured Rabi oscillations on the optical and MW transitions (see Supplemental Material [51]). (c) Pulse and timing sequence of the AFC spin-wave storage experiment, including input and output pulses (transition ${\nu }_1$), control pulses (transition ${\nu }_2$), and MW pulses (transition ${\nu }_{\mathrm{MW}}$). Note that for a perfect control pulse the AFC echo at $1/\mathrm{\Delta }$ is completely suppressed. (d) The intensity of optical output pulse as a function of the total memory storage time $T_{\mathrm{M}}$, with $1/\mathrm{\Delta }=7\mu \mathrm{s}$. The data were fitted to the function $\exp (-2T_S/T_2^s)$, resulting in a spin coherence time of $T_2^s=1.2(2)\mathrm{ms}$. Inset: Example of output pulse trace for $T_M=107\mu \mathrm{s}$.