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

TL;DR

This paper demonstrates high-fidelity entanglement between a trapped calcium ion and a telecom-wavelength photon using quantum frequency conversion, enabling efficient ion-photon quantum interfaces for long-distance quantum communication.

Contribution

It introduces a polarization-preserving quantum frequency converter connecting 854 nm to 1310 nm, achieving high fidelity entanglement with a trapped ion and telecom photons.

Findings

Achieved 98.2% entanglement fidelity between ion and photon.

Demonstrated 99.75% polarization-state conversion fidelity.

Achieved 26.5% external conversion efficiency.

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 $^{40}$Ca$^{+}$ ion and the polarization state of a telecom photon with a high fidelity of 98.2 $\pm$ 0.2$\%$. The unique combination of 99.75 $\pm$ 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.