High-fidelity entanglement between a trapped ion and a telecom photon via quantum frequency conversion

Abstract

Entanglement between a stationary quantum system and a flying qubit is an essential ingredient of a quantum-repeater network. It has been demonstrated for trapped ions, trapped atoms, color centers in diamond, or quantum dots. These systems have transition wavelengths in the blue, red or near-infrared spectral regions, whereas long-range fiber-communication requires wavelengths in the low-loss, low-dispersion telecom regime. A proven tool to interconnect flying qubits at visible/NIR wavelengths to the telecom bands is quantum frequency conversion. Here we use an efficient polarization-preserving frequency converter connecting 854 nm to the telecom O-band at 1310 nm to demonstrate entanglement between a trapped ⁴⁰Ca⁺ ion and the polarization state of a telecom photon with a high fidelity of 98.2 ± 0.2%. The unique combination of 99.75 ± 0.18% process fidelity in the polarization-state conversion, 26.5% external frequency conversion efficiency and only 11.4 photons/s conversion-induced unconditional background makes the converter a powerful ion-telecom quantum interface.

Fig. 1

Experimental setup. Atom–photon entanglement is generated between a single trapped ⁴⁰Ca⁺-ion, confined and laser-cooled in a linear Paul trap, and a single photon at 854 nm. The photons are collected with a HALO (“High-numerical-Aperture Laser Objective”) and coupled to a single-mode fiber. A combination of two quarter waveplates (QWP) and one half wave plate (HWP) is inserted behind the fiber to compensate all unitary rotations of the polarization state caused by the single-mode fiber and several dichroic mirrors between the fiber and the HALO. A flip mirror can be inserted behind the fiber to send the photons either to the converter lab via a 90 m long single-mode fiber or to the projection setup for 854 nm. The polarization-preserving frequency converter is realized with a nonlinear waveguide crystal in a single-crystal Mach–Zehnder configuration (for details see main text and Methods). The converted photons are detected with superconducting single-photon detectors. PBS polarizing beam splitter, DM dichroic mirror, BS beam splitter, BPF band pass filter, FBG fiber Bragg grating

Fig. 2

Ion–photon entanglement scheme and quantum-state tomography. a Atom–photon entanglement is generated via spontaneous decay from the P_3/2- to the D_5/2-state after excitation with π-polarized laser light at 393 nm. Emitted photons at 854 nm are collected along the quantization axis, thereby suppressing π-polarized photons. Atomic state analysis is realized via coherent pulses on the optical transition at 729 nm and the RF-transition in the ground state followed by fluorescence detection (details see Supplementary Note 1). b Real and imaginary part of the density matrix of the ion–photon entangled state, measured via quantum-state tomography. The different heights of the diagonal elements (red bars) results from the different Clebsch–Gordan coefficients of the σ⁺- and the σ⁻-transitions

Fig. 3

Characterization of the quantum frequency converter. a The external conversion efficiency of the two interferometer arms depending on the pump power of the mixing field at 2456 nm. The data points are fitted with ${\eta }_{\mathrm{ext}}\left(P_{\mathrm{P}}\right)$ = ${\eta }_{\max }{\sin }^2\left(\sqrt{{\eta }_{\mathrm{nor}}{P}_{\mathrm{P}}}L\right)$. b The absolute values of the process matrix of the coherent polarization-preserving down-conversion measured with a laser. The process fidelity is determined by the element corresponding to the identity operation (red bar). We achieve a value of 99.75 ± 0.18%. The error bars are deduced from a Poissonian distribution

Fig. 4

Quantum-state tomography of the ion–telecom-photon entangled states. Real and imaginary parts of the density matrices for: a the converted “bare” ion–photon entangled state still revealing the asymmetry in the diagonal elements (red bars) due to the asymmetric Clebsch–Gordan coefficients, b the converted ion–photon entangled state projected onto a Bell state by introducing polarization-dependent losses