High-fidelity entanglement between a trapped ion and a telecom photon via quantum frequency conversion (1710.04866v2)

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.

• The paper demonstrates high-fidelity entanglement (98.2% ± 0.2%) between a trapped ion and a telecom photon via quantum frequency conversion, crucial for quantum communication networks.
• A polarization-preserving quantum frequency converter based on a PPLN waveguide enables efficient (26.5%) and high-fidelity (99.75% ± 0.18%) conversion from 854 nm to 1310 nm with minimal noise.
• This high-fidelity linkage of stationary and flying qubits is a key step towards integrating trapped ions into long-distance telecom-wavelength quantum networks and quantum repeaters.

High-Fidelity Entanglement between a Trapped Ion and a Telecom Photon via Quantum Frequency Conversion

This paper presents a significant advancement in the field of quantum communication by demonstrating high-fidelity entanglement between a trapped ion and a telecom photon using quantum frequency conversion. The research addresses the critical challenge of connecting stationary quantum systems, such as trapped ions, with flying qubits suitable for long-range communication in the telecom wavelength regime.

The authors report successful entanglement between a $^{40}$Ca$^+$ ion and the polarization state of a telecom photon with an impressive fidelity of 98.2% ± 0.2%. This high level of fidelity is achieved by employing a polarization-preserving quantum frequency converter (QFC) that efficiently maps the ion's emission wavelength of 854 nm to the telecom O-band at 1310 nm. The QFC is characterized by a remarkable process fidelity of 99.75% ± 0.18%, along with a 26.5% conversion efficiency and minimal conversion-induced noise, registering only 11.4 photons/s.

The experimental setup integrates a linear Paul trap for confining and cooling a single $^{40}$Ca$^+$ ion, with a novel frequency conversion apparatus based on a periodically-poled lithium niobate (PPLN) ridge waveguide. This innovative design effectively mitigates the polarization dependence often associated with such conversion processes, making it a robust solution for integrating trapped-ion quantum nodes into larger telecom networks.

The paper highlights the potential for the QFC to serve as a building block for quantum repeater networks, which are essential for establishing long-distance quantum entanglement. The fidelity and purity metrics provided in the paper—97.7% ± 0.2% fidelity and 95.8% ± 1.3% purity after conversion—underscore the capability of the system to maintain the integrity of quantum information through the conversion process.

Furthermore, the research paves the way for the entanglement of ions over extended fiber-optic networks. The authors suggest future developments could include the transition to the telecom C-band at 1550 nm, which offers even lower transmission losses. Additionally, advancements in spectral filtering and detection technologies could further enhance signal-to-noise ratios, crucial for scalable quantum networks.

From a theoretical perspective, this work contributes to the understanding of entanglement dynamics across hybrid systems combining atomic and photonic elements. Practically, it sets a precedent for deploying trapped-ion systems in real-world quantum networks, offering a promising route to hybrid networks linking various quantum platforms through telecom-wavelength photons.

In conclusion, this research demonstrates a novel and effective linkage of stationary and flying qubits across different spectral regimes, achieved without compromising the fidelity of the entangled state. This progress not only brings quantum communication technology closer to practical quantum information networks but also opens avenues for multi-platform interoperability, essential in the development of a global quantum internet.