Coherent Spin Control at the Quantum Level in an Ensemble-Based Optical Memory

Long-lived quantum memories are essential components of a long-standing goal of remote distribution of entanglement in quantum networks. These can be realized by storing the quantum states of light as single-spin excitations in atomic ensembles. However, spin states are often subjected to different dephasing processes that limit the storage time, which in principle could be overcome using spin-echo techniques. Theoretical studies suggest this to be challenging due to unavoidable spontaneous emission noise in ensemble-based quantum memories. Here, we demonstrate spin-echo manipulation of a mean spin excitation of 1 in a large solid-state ensemble, generated through storage of a weak optical pulse. After a storage time of about 1 ms we optically read-out the spin excitation with a high signal-to-noise ratio. Our results pave the way for long-duration optical quantum storage using spin-echo techniques for any ensemble-based memory.

Figure 1

AFC spin-wave storage experiment. In (a) a simplified sketch of the experimental setup is shown. A highly coherent continuous-wave 580.04 nm laser is split into an input mode and a control mode, with about 560 mW of power in the control mode. The output mode crosses the input mode inside the 1 cm long crystal, which is cooled to around 4 K. The input mode is in a double-pass configuration to increase the optical depth. The output mode is sent through a Fabry-Perot (FP) cavity (2.5 MHz bandwidth) to filter out photon emission noise from the crystal at other frequencies (see Ref. [29]). The single-photon avalanche diode (SPAD) was gated with an acousto-optic modulator not shown. $\mathrm{PBS}=\mathrm{polarization}$ beam splitter. $\mathrm{FR}=\mathrm{Faraday}$ rotator. In (b) the timing of the storage sequence is shown. The total storage time is $1/\mathrm{\Delta }+T_S$, where $T_S$ is the spacing between the control pulses. The rf spin-echo sequence is inserted in between the optical control pulses. The rf pulses at 34.5 MHz are applied using a coil placed around the crystal [see (a)]. In (c) the hyperfine states of the ground and excited states and the two transitions of the chosen $\mathrm{\Lambda }$ system in ${\mathrm{Eu}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ are shown. In (d) we show the characterization of the population inversion precision of the $XX$ and $XY\text{−}4$ sequences. Initially, all ions are polarized into $|s⟩$. The relative population in $|g⟩{\rho }_g$ is measured after applying $N$ sequences. The error bars represent the error in the absorption measurement used to estimate ${\rho }_g$. More experimental details are given in the text.

Figure 2

Photon counting histograms. In (a) we show AFC spin-wave storage without spin-echo manipulation. In (b) and (c) we show spin-wave storage with spin-echo manipulation of a single mode (b) and of five input modes (c). Each histogram shows data recorded with (dark trace) and without (bright trace) the input pulse, which allows one to measure the SNR in the output mode. The SNR is $11\pm 2$ in (a), $10\pm 2$ in (b). and $7\pm 1$ in (c). The input mode(s) have mean photon numbers of $\mu =1.1\pm 0.1$ in (a) and $\mu =2.0\pm 0.1$ in (b) and (c). The storage efficiencies were here (a) $\eta =5.7\pm 0.4\%$, (b) $\eta =5.1\pm 0.4\%$, and (c) $\eta =3.1\pm 0.3\%$. In the case of five-mode storage all parameters are given as averages over the modes. Indicated errors are statistical.