Demonstration of Atomic Frequency Comb Memory for Light with Spin-Wave Storage

We present a light-storage experiment in a praseodymium-doped crystal where the light is mapped onto an inhomogeneously broadened optical transition shaped into an atomic frequency comb. After absorption of the light, the optical excitation is converted into a spin-wave excitation by a control pulse. A second control pulse reads the memory (on-demand) by reconverting the spin-wave excitation to an optical one, where the comb structure causes a photon-echo-type rephasing of the dipole moments and directional retrieval of the light. This combination of photon-echo and spin-wave storage allows us to store submicrosecond (450 ns) pulses for up to $20\mu \mathrm{s}$. The scheme has a high potential for storing multiple temporal modes in the single-photon regime, which is an important resource for future long-distance quantum communication based on quantum repeaters.

Figure 1

(a) The experiment was performed on the ${}^3\mathrm{H}_4\to {}^1\mathrm{D}_2$ transition in ${\mathrm{Pr}}^{3+}$. The ground and excited state manifolds both have three hyperfine levels denoted $M_I=\pm 1/2$, $\pm 3/2$, $\pm 5/2$. The three-level lambda system was formed by the levels labeled $|g⟩$, $|s⟩$ and $|e⟩$, following the notation in 19. (b) Experimental absorption spectrum showing the AFC on the $|g⟩\leftrightarrow |e⟩$ transition created within a 18 MHz wide transmission hole using the spectral holeburning sequence described in the text. Here the comb consists of 9 peaks with spacing $\Delta =250\mathrm{kHz}$. The holeburning sequence also empties the $|s⟩$ level, whereas $|\mathrm{aux}⟩$ is used for population storage. Note that the second AFC in the center of the spectrum is due to the weaker transition from the ground state $\pm 1/2$ to the excited state $\pm 5/2$. (c) The pulse sequence showing the input pulse to store, the two control fields for the back-and-forth transfer to the spin state, and the retrieved output pulse.

Figure 2

The experimental setup. The spectral hole burning and storage pulse sequences were created using a frequency-stabilized laser and acousto-optic modulators similar to the setup in 28 (not shown). A beam splitter (BS) split the light into a strong and a weak beam, whose two modes were overlapped in the crystal cooled to 2.1 K. The detected output pulse propagating to the right originated from the combination of a weak input pulse incident on the crystal from the left, and two strong control pulses incident from the right. Note that the signal disappeared when the control pulses were blocked directly after the BS. An AOM was used to direct only the transmitted input pulse and the output pulse onto the photodiode (PD), effectively working as a detector gate.

Figure 3

Storage in the spin state using two control pulses. Shown are (from left) the partially transmitted input pulse, control pulses (strongly attenuated by the closed optical gate), and the output pulse (magnified by a factor 10 for clarity). Here $1/\Delta =4\mu \mathrm{s}$ and $T_s=7.6\mu \mathrm{s}$, resulting in a total storage time $1/\Delta +T_s=11.6\mu \mathrm{s}$. Inset: The AFC echo observed at $1/\Delta =4\mu \mathrm{s}$ when the control pulses are not applied. Note that the vertical scales have been normalized to 100 with respect to the input pulse before the crystal; thus, these yield (in percent) the efficiency for the echo and transmission coefficient for the input pulse.

Figure 4

Experimental traces for spin-storage times $T_s=5.6$, 7.6, 10.6, and $15.6\mu \mathrm{s}$. All other parameters are the same as those in Fig. 3. The input pulses are superimposed to the left (truncated) and the output pulses for different $T_s$ are seen to the right. The leakage of the control pulses through the optical gate is not shown. For clarity there is also a break in the horizontal scale. The decay of the signal (see inset) as a function of $T_s$ is due to inhomogeneous spin dephasing. The solid curve is a fitted Gaussian function corresponding to 26 kHz (full-width at half maximum) spin broadening.

Figure 5

Storage of two temporal input modes. The normalized input modes shown here (dashed line) are recorded before the crystal using a reference detector. The AFC was prepared with a periodicity of $\Delta =200\mathrm{kHz}$, corresponding to $5\mu \mathrm{s}$ storage time on the optical transition. The spin-storage time was set to $T_s=7.6\mu \mathrm{s}$, resulting in a total storage time of $1/\Delta +T_s=12.6\mu \mathrm{s}$.