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.
Highlights
Entanglement between a stationary quantum system and a flying qubit is an essential ingredient of a quantum-repeater network
There is a demand for interfaces connecting the telecom-wavelength regime and the visible/NIR range in a coherent way, i.e., preserving quantum information encoded in a degree of freedom of a single photon, such as its polarization
It has been shown that χ2-based quantum frequency converters (QFC) preserves nonclassical properties of single photons and photon pairs, such as second-order coherence[18,19,20], time-energy entanglement[21], time-bin entanglement[22], orbital angular momentum entanglement[23], polarization entanglement[24], and photon indistinguishability[25,26]; nonclassical correlations between telecom photons and spin waves in cold atomic ensembles[27,28] have been demonstrated
Summary
Entanglement between a stationary quantum system and a flying qubit is an essential ingredient of a quantum-repeater network. Using near-resonant QFC based on FWM in an atomic ensemble, entanglement of a spin–wave qubit with the polarization state of a telecom photon has been realized[16]. We generate entanglement between an atomic quantum bit in a single trapped 40Ca+ ion and the polarization state of a single photon at 854 nm.
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