High-fidelity remote entanglement of trapped atoms mediated by time-bin photons (2406.01761v1)

Abstract: Photonic interconnects between quantum processing nodes are likely the only way to achieve large-scale quantum computers and networks. The bottleneck in such an architecture is the interface between well-isolated quantum memories and flying photons. We establish high-fidelity entanglement between remotely separated trapped atomic qubit memories, mediated by photonic qubits stored in the timing of their pulses. Such time-bin encoding removes sensitivity to polarization errors, enables long-distance quantum communication, and is extensible to quantum memories with more than two states. Using a measurement-based error detection process and suppressing a fundamental source of error due to atomic recoil, we achieve an entanglement fidelity of 97% and show that fidelities beyond 99.9% are feasible.

• The paper demonstrates high-fidelity entanglement of trapped atomic ions using time-bin encoded photons, achieving a measured fidelity of 97.0%.
• It employs ultrafast laser pulses and a 50:50 beam splitter setup to overcome polarization errors and achieve an entanglement rate of 0.35 per second.
• The study paves the way for scalable quantum networks by integrating photonic links with robust error suppression techniques for long-distance quantum communication.

High-Fidelity Remote Entanglement of Trapped Atoms via Time-Bin Photons

The paper presented examines the advancement of quantum networks by demonstrating a remote entanglement protocol using time-bin encoded photons for trapped atomic ions. This research is crucial for achieving scalable quantum computation and long-distance quantum communication. The integration of photonic interconnects between quantum processing nodes is proposed as a method to surpass current limitations in scaling quantum networks.

Methodology and Results

The authors report on the high-fidelity entanglement of two remote atomic qubits, specifically $^{138}\mathrm{Ba}^{+}$ ions, using time-bin photonic qubits. The entanglement process involves the use of precise ultrafast laser pulses to excite ions and generate single photons, which are collected through optical fibers and used in an entanglement swapping procedure via a 50:50 beam splitter. This procedure results in a heralded entangled state with a measured fidelity of $97.0\%$, with possibilities for achieving fidelities exceeding $99.9\%$ through further optimizations.

Several key findings are highlighted:

• The successful entanglement is attributed to time-bin encoding, which enhances robustness against polarization errors and reduces susceptibility to birefringence in optical components.
• An entanglement rate of approximately $0.35$ entanglements per second was achieved. This, coupled with the development of a measurement-based error detection process, and suppression of atomic recoil errors, supports the high-fidelity results.

Error Analysis and Mitigations

Critical assessments of the system's limitations were undertaken, particularly considering common sources of error in quantum experiments. The principal sources of fidelity error identified include fluctuations in laser intensity, temporal wavefunction mismatch at the beam splitter, and potential entanglement with atomic motions due to recoil. The methodology for error suppression involves rigorous calibration and stabilizing the mode-locked laser system, effective photon collection techniques, and maintaining precise control over laser beam geometries and intensities.

Theoretical and Practical Implications

The transition from theoretical models to successful practical implementation of high-rate and high-fidelity entanglement protocols via time-bin encoding extends implications for both theoretical quantum network architectures and practical quantum technologies:

1. Theoretical implications: The breakthrough demonstrates the feasibility of modular quantum computers interconnected via photonic links, supporting the prospects of a networked approach to quantum computing.
2. Practical implications: The demonstration enhances the potential for reliable quantum communication over extended distances. The success in eliminating polarization dependencies opens avenues for integrating more complex quantum systems and extending entanglement capabilities beyond binary qubit systems to high-dimensional quantum memories or qudits.
3. Future directions: Further enhancements in laser stability, cooling methods, and error correction mechanisms are suggested to push boundaries toward even higher fidelities and operational rates. Additionally, extending this methodology to accommodate multiple time-bins could lead to more sophisticated quantum networking and high-throughput quantum communication systems.

In summary, this work advances the understanding of time-bin photonic qubits in long-distance quantum communication, aligning with the broader agenda of developing scalable and robust quantum networks. Future efforts should focus on extending these techniques to encompass more complex quantum states and enhancing the existing quantum communication infrastructure.