Demonstration of a Memory for Tightly Guided Light in an Optical Nanofiber

We report the experimental observation of slow-light and coherent storage in a setting where light is tightly confined in the transverse directions. By interfacing a tapered optical nanofiber with a cold atomic ensemble, electromagnetically induced transparency is observed and light pulses at the single-photon level are stored in and retrieved from the atomic medium. The decay of efficiency with storage time is also measured and related to concurrent decoherence mechanisms. Collapses and revivals can be additionally controlled by an applied magnetic field. Our results based on subdiffraction-limited optical mode interacting with atoms via the strong evanescent field demonstrate an alternative to free-space focusing and a novel capability for information storage in an all-fibered quantum network.

Figure 1

Experimental setup. (a) A 400 nm diameter nanofiber is overlapped with an ensemble of cold cesium atoms. The signal to be stored is guided inside the nanofiber while the control propagates at the outside, with an angle $\alpha \sim 13°$. (b) Transmission through the fiber of a pulse resonant on the $|g⟩=\{6S_{1/2},F=4\}\to \{6P_{3/2},F=5\}$ transition as a function of the input power. The low saturation power results from the confined geometry over the cloud length. (c) Transmission profile for a signal pulse at the single-photon level as a function of the detuning $\delta$ from the $|g⟩\to |e⟩=\{6P_{3/2},F=4\}$ transition.

Figure 2

Electromagnetically induced transparency for the guided light. The control is on resonance on the $|s⟩\to |e⟩$ transition while the signal is detuned by $\delta$ from the $|g⟩\to |e⟩$ resonance. Profiles are displayed as a function of $\delta$, for four values of the control power.

Figure 3

Slow light and storage at the single-photon level. (a) Transmitted pulses for different control powers. The reference is measured without atoms. (b) Storage and retrieval. In the absence of control, the blue and purple points give the transmitted pulse without and with atoms. The red data correspond to the memory sequence, showing leakage and retrieval. The black line indicates the control timing. After the end of the input pulse, the reference and absorption curves are superimposed and correspond to the background noise level. The mean photon number per pulse is 0.6 and the efficiency is $10\pm 0.5\%$. (c) Efficiency as a function of the control linear polarization angle. The zero angle corresponds to a vertical polarization.

Figure 4

Memory lifetime and revivals. (a) Normalized retrieval efficiency $\eta$ as a function of the storage time. The data are fitted according to the expression given in the text. (b) and (c) Retrieval efficiency in the presence of a dc magnetic field of 0.4 and 0.6 Gauss, respectively. Revivals are observed at multiples of the half Larmor period. The arrows indicate the theoretical positions calculated from the calibrated field.