Deterministic spin-photon entanglement from a trapped ion in a fiber Fabry-Perot cavity

The development of efficient quantum network nodes is crucial for advancements in quantum computing and secure communication. These nodes require long coherence times, efficient coupling between stationary and traveling qubits (usually photons), and high-fidelity qubit manipulation. Several approaches exist using trapped ions, neutral atoms, NV-centers, or quantum dots, with varying photon collection methods. Fiber-based Fabry-Perot resonators offer advantages due to their small mode volume, enhanced light-matter coupling, and direct fiber coupling. While previous work coupled various emitters to these resonators, entanglement between light and matter in this setting hadn't been demonstrated. This work aims to address this gap by creating a quantum network node of a trapped ¹⁷¹Yb⁺ ion coupled to a fiber Fabry-Perot cavity, generating and verifying a maximally entangled atom-photon state.

The paper reviews existing work on quantum network nodes, highlighting various approaches using trapped ions, neutral atoms, NV-centers in diamond, and semiconductor quantum dots. Different photon collection techniques are discussed, including high numerical aperture objectives and optical cavities to enhance light-matter interaction. The advantages of fiber-based Fabry-Perot resonators are emphasized, particularly their small mode volume, strong light-matter coupling, and direct fiber integration. The authors point out that while these resonators have been coupled to various emitters, entanglement between light and matter within this setup remained unproven, forming the motivation for their research.

The experiment uses a single trapped ¹⁷¹Yb⁺ ion in a radiofrequency Paul trap embedded within a fiber Fabry-Perot cavity. The ion is Doppler-cooled, and entanglement is generated by initializing the ion in the |0⟩ state, exciting it to the |e⟩ state using a short laser pulse, and detecting the subsequent decay into |g⁺⟩ and |g⁻⟩ states, which emits a single photon into the cavity mode. A magnetic field suppresses ΔmF = 0 emission, resulting in circularly polarized photons. The photon's polarization is measured using a polarizing beam splitter and single-photon counters. The atomic state is read out via microwave pulses and fluorescence detection. Entanglement is verified by measuring correlations in different bases (σz, σxσx, σyσy) using waveplates to rotate the photon polarization and microwave pulses to manipulate the atomic state. Full quantum state tomography is performed using a maximum likelihood estimation to obtain the density matrix and calculate fidelity. The coherence time of the atomic qubit is measured using a Ramsey-like sequence.

The researchers successfully generated and verified a maximally entangled atom-photon state with a fidelity of 90.1(1.7)%. The single-shot success probability for entanglement generation and detection was 2.5 × 10⁻³, leading to an entanglement rate of 62 Hz. Correlations were measured in the σz basis (90.7 ± 3.9)% contrast and rotated bases σxσx (81 ± 16)% and σyσy (87.0 ± 2.6)%. Full quantum state tomography confirmed the high fidelity of the entangled state. The coherence time of the atomic qubit was measured as (496 ± 42) μs for the |g⟩ qubit and (1022 ± 121) μs for the |g⟩ + |f⟩ qubit. The photon duration was measured at 9.9(7) ns (FWHM), significantly shorter than previous cavity-based atom-photon entanglement experiments. The effective cooperativity was determined to be C0,eff = 0.056(13).

The high fidelity of the generated entanglement, combined with the short photon duration and relatively high generation rate, demonstrates the potential of this system as a node in a quantum network. The fiber-based Fabry-Perot cavity provides good optical access for precise qubit control. The limitations in fidelity are attributed to errors in atomic state detection, imperfect excitation of the |e⟩ state, dark counts, magnetic field noise, and timing jitter. The short photon duration is beneficial for impedance matching in quantum networks. While the entanglement rate is currently lower than some other systems, improvements in ion localization, cavity locking, and experimental sequence optimization are expected to increase the rate significantly.

This work demonstrates a high-fidelity deterministic atom-photon entanglement source using a trapped ion in a fiber Fabry-Perot cavity, suitable for quantum network applications. Future work could focus on enhancing the entanglement rate through improved cavity stabilization and ion localization, and integrating frequency conversion to telecom wavelengths for long-distance quantum communication. Exploring different atomic qubits with longer coherence times is also a potential avenue for improvement.

The main limitations are the imperfect localization of the ion and the stability of the fiber cavity length, reducing the effective cooperativity. Other factors influencing fidelity include errors in atomic state detection, imperfect state preparation and manipulation, dark counts, and magnetic field noise. The current entanglement generation rate, while appreciable, is not yet optimized.