Coherent Optical Memory Based on a Laser-Written On-Chip Waveguide

Tian-Xiang Zhu, Chao Liu, Liang Zheng, Zong-Quan Zhou, Chuan-Feng Li, and Guang-Can Guo

Phys. Rev. Applied 14, 054071 – Published 30 November 2020

Abstract

Quantum memory is the core device for the construction of large-scale quantum networks. For scalable and convenient practical applications, integrated optical memories, especially on-chip optical memories, are crucial requirements because they can be easily integrated with other on-chip devices. Here, we report the coherent optical memory based on a type-IV waveguide fabricated on the surface of a rare-earth ion-doped crystal (i.e., $E u^{3 +}$ : $Y_{2} Si O_{5}$ ). The properties of the optical transition ( $^{7} F_{0} \to^{5} D_{0}$ ) of the ${Eu}^{3 +}$ ions inside the surface waveguide are well preserved compared to those of the bulk crystal. Spin-wave atomic frequency comb storage is demonstrated inside the type-IV waveguide. The reliability of this device is confirmed by the high interference visibility of $97 \pm 1 %$ between the retrieval pulse and the reference pulse. The developed on-chip optical memory paves the way towards integrated quantum nodes.

Received 13 August 2020
Accepted 29 October 2020

DOI:https://doi.org/10.1103/PhysRevApplied.14.054071

Physics Subject Headings (PhySH)

Integrated optics Optical quantum information processing Quantum communication Quantum memories Quantum repeaters

Rare-earth doped crystals Waveguides

Quantum Information, Science & Technology

Authors & Affiliations

Tian-Xiang Zhu^1,2, Chao Liu^1,2, Liang Zheng^1,2, Zong-Quan Zhou ^1,2,*, Chuan-Feng Li ^1,2,†, and Guang-Can Guo^1,2

¹CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei 230026, China
²CAS Center For Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China

^*zq_zhou@ustc.edu.cn
^†cfli@ustc.edu.cn

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Vol. 14, Iss. 5 — November 2020

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Figure 1
(a) Experimental setup. The crystal is placed in a cryostat with a base temperature of 3.2 K. The incident laser beam is split into two parts, i.e., the input mode and the preparation and control mode. They are modulated by independent acousto-optic modulators (AOMs), and combined via a beam splitter (BS, with a reflectivity of 8%) and finally coupled into the single-mode fiber. The Gaussian beam with a FWHM of $7 μ m$ is coupled into the type-IV waveguide via a $3 \times beam$ expander (BE, GBE03-A, Thorlabs) and a lens with a focal length of 75 mm. The output beam is coupled into the single-mode fiber by the lens set for spatial filtering. Finally, the output beam is sent into a photoelectric detector after passing through a temporal gate based on AOM3. FC denotes fiber coupler and PBS denotes polarization beam splitter. (b) Top view of the type-IV waveguide under a microscope in the $b \times D 1$ plane of the crystal. (c) Front view of the type-IV waveguide under a microscope in the $D 2 \times D 1$ plane of the crystal. The scale bar of (b),(c) is $50.0 μ m$ . The ridges have a width of $4.8 μ m$ and a depth of $13.0 μ m$ . (d) The guided mode intensity distribution at the exit surface of the crystal. The FWMH of the guided mode is about $33 μ m \times 30 μ m$ (horizontal $\times$ vertical) at the exit surface of the crystal. The scale bar of (d) is $50.0 μ m$ .
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Figure 2
Two-pulse photon echo measurements implemented in three different regions inside the sample where $t$ is the spacing between the two incident pulses. The red solid lines are the results of the linear fitting. (a) In the type-IV waveguide, the coherence time $T_{2}$ is fitted as $124 \pm 4 μ s$ . (b) In the bulk-1 area, $T_{2}$ is fitted as $113 \pm 3 μ s$ . (c) In the bulk-2 area, $T_{2}$ is fitted as $119 \pm 4 μ s$ .
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Figure 3
(a) The hyperfine energy level diagram of the $^{7} F_{0} \to^{5} D_{0}$ transition of $^{151} {Eu}^{3 +}$ ions in the $Y_{2} {Si O}_{5}$ crystal at zero magnetic field. The input signal is resonant with the $| \pm 1 / 2 ⟩_{g} \to | \pm 5 / 2 ⟩_{e}$ transition with a frequency of $f_{0}$ . The control pulse is resonant with the $| \pm 3 / 2 ⟩_{g} \to | \pm 5 / 2 ⟩_{e}$ transition with a frequency of $f_{1}$ . The $| \pm 5 / 2 ⟩_{g} \to | \pm 3 / 2 ⟩_{e}$ transition with a frequency of $f_{2}$ is necessary for the AFC preparation process, which helps to isolate a single class of ions [32]. (b) The time sequence for the spin-wave AFC scheme. The duration of the control pulse ( $τ_{c}$ ) is $2.5 μ s$ .
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Figure 4
(a) The temporal traces of the input pulse (black line), the AFC echo (green line), and the spin-wave AFC echo (red line). The blue line indicates the positions of the two control pulses and the traces in the yellow region are magnified 10 times. (b) The prepared AFC at the $| \pm 1 / 2 ⟩_{g} \to | \pm 5 / 2 ⟩_{e}$ transition. The peak comb absorption depth ( $d$ ) is $0.79 \pm 0.02$ , the background absorption depth ( $d_{0}$ ) is $0.00 \pm 0.03$ , and the finesse ( $F$ ) of the combs is about $2.6 \pm 0.3$ . (c) Exponential decay of the spin-wave AFC echo amplitude with storage time in the spin state ( $t_{s}$ ). The red solid line is the result of the linear fitting, and the inhomogeneous linewidth of the spin transition ( $| \pm 1 / 2 ⟩_{g} \to | \pm 3 / 2 ⟩_{g}$ ) is $33 \pm 1$ kHz.
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Figure 5
(a) Interference between the readout echo and a reference pulse. Here $ϕ$ corresponds to the relative phase between the input pulse and the reference pulse. The data points are fitted using $I (φ) = (I_{\max} / 2) [1 + V \sin (φ + φ^{'})]$ [32, 51], where $I_{\max}$ is the maximum intensity, $V$ is the interference visibility, and $φ^{'}$ is the initial phase. The fitting curve is represented by the red line with $V = 0.97 \pm 0.01$ . (b) The traces of the readout echo in the cases of constructive (red line) and destructive (black line) interference.
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