On-Demand Integrated Quantum Memory for Polarization Qubits

Tian-Xiang Zhu, Chao Liu, Ming Jin, Ming-Xu Su, Yu-Ping Liu, Wen-Juan Li, Yang Ye, Zong-Quan Zhou, Chuan-Feng Li, and Guang-Can Guo

Phys. Rev. Lett. 128, 180501 – Published 2 May 2022

Abstract

Photonic polarization qubits are widely used in quantum computation and quantum communication due to the robustness in transmission and the easy qubit manipulation. An integrated quantum memory for polarization qubits is a useful building block for large-scale integrated quantum networks. However, on-demand storing polarization qubits in an integrated quantum memory is a long-standing challenge due to the anisotropic absorption of solids and the polarization-dependent features of microstructures. Here we demonstrate a reliable on-demand quantum memory for polarization qubits, using a depressed-cladding waveguide fabricated in a ${}^{151}{Eu}^{3 +} : Y_{2} {SiO}_{5}$ crystal. The site-2 ${}^{151}{Eu}^{3 +}$ ions in $Y_{2} {SiO}_{5}$ crystal provides a near-uniform absorption for arbitrary polarization states and a new pump sequence is developed to prepare a wideband and enhanced absorption profile. A fidelity of $99.4 \pm 0.6 %$ is obtained for the qubit storage process with an input of 0.32 photons per pulse, together with a storage bandwidth of 10 MHz. This reliable integrated quantum memory for polarization qubits reveals the potential for use in the construction of integrated quantum networks.

Received 12 February 2022
Accepted 28 March 2022

DOI:https://doi.org/10.1103/PhysRevLett.128.180501

Physics Subject Headings (PhySH)

Integrated optics Optical quantum information processing Quantum memories Quantum optics

Rare-earth doped crystals Waveguides

Quantum Information, Science & TechnologyAtomic, Molecular & Optical

Authors & Affiliations

Tian-Xiang Zhu^1,2,3, Chao Liu^1,2,3, Ming Jin^1,2,3, Ming-Xu Su^1,2,3, Yu-Ping Liu^1,2,3, Wen-Juan Li⁴, Yang Ye⁴, Zong-Quan Zhou ^1,2,3,*, Chuan-Feng Li^1,2,3,†, and Guang-Can Guo^1,2,3

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
³Hefei National Laboratory, Hefei 230088, China
⁴Center for Micro and Nanoscale Research and Fabrication, University of Science and Technology of China, Hefei 230026, China

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

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Vol. 128, Iss. 18 — 6 May 2022

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Images

Figure 1
Experimental setup. (a) Schematic of the setup. The signal and the preparation beams generated by the acousto-optic modulation in double-pass configurations are combined by a beam splitter (BS) and coupled into the type-III waveguide by lens group. The polarization of the signal qubits is prepared by a polarization beam splitter (PBS), a half wave plate (HWP), and a quarter-wave plate (QWP). The HWP after the BS is employed to match the axial direction of the crystal. Another lens group is used for collecting the emergent light from the waveguide. A phase plate (PP) is used to compensate the phase difference between the TE and TM modes because of waveguide birefringence and the phase difference accumulated in the setup. The polarization of signal photons is analyzed using a QWP, HWP, and BS. The signal is finally coupled into a fiber coupler (FC) and detected by a single photon detector (SPD). (b) Energy diagram of transition for site-2 ${}^{151}{Eu}^{3 +}$ ions in ${}^{151}{Eu}^{3 +} : Y_{2} {SiO}_{5}$ at zero magnetic field [58]. (c) The vertical view of type-III waveguides and the electrodes with a scale bar is $100 μ m$ . (d) The front view of the two type-III waveguides with a scale bar is $20 μ m$ . The left waveguide is the one used in the experiment, which has a lower insertion loss as compared to the right one. (e) The beam profile of the TM mode at the exit of the waveguide as measured by a charge coupled device with a scale bar is $20 μ m$ .
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Figure 2
(a) Time sequence for the measurement of the linear Stark splitting. After preparation of the antihole, the spectral feature is measured with sweeping optical pulse with applied electric fields. (b) Time sequence for SMAFC storage. After preparation of AFC, two electric pulse actively control the readout times of the echo. The standard AFC echo is suppressed by the first electric pulse and the SMAFC echo is retrieved after the second electric pulse.
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Figure 3
SMAFC memory for weak coherent pulses. Figures 1 and 1 are the measured comb structures with input states of $| H ⟩$ and $| V ⟩$ , respectively. The average peak absorption depth is 3.6 (4.0) for $| H ⟩$ ( $| V ⟩$ ) with negligible background absorption. Figs. 1 and 1 are photon counting histograms for SMAFC storage, with input states of $| H ⟩$ and $| V ⟩$ , respectively. Echoes retrieved after 2 s are magnified by 10 times. (e) The device efficiency of the SMAFC memory for input states of $| H ⟩$ (red) and $| V ⟩$ (black). The fitted $γ$ is $335 \pm 6 kHz$ ( $333 \pm 7 kHz$ ) for $| H ⟩$ ( $| V ⟩$ ), according to Eq. (1).
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Figure 4
The reconstructed process matrix of the storage process. (a) The real part of the process matrix as obtained by the quantum process tomography [61]. The horizontal axes are the basis operators in the two-dimensional Hilbert space. (b) The imaginary part of the process matrix with the largest imaginary element of 0.035.
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