Hertz-rate metropolitan quantum teleportation

Quantum teleportation, transferring an unknown quantum state using entanglement, measurement, and classical communication, is fundamental to quantum information technologies like quantum networks and distributed quantum computation. Since its proposal in 1993, it's been demonstrated across various platforms (atomic ensembles, single atoms, trapped ions, solid-state systems, NMR, and quantum optics). Quantum optics-based systems, particularly discrete-variable (DV) systems, are promising for global-scale quantum networks requiring long-distance state distribution (thousands of kilometers). While impressive progress has been made, including teleportation over 1200 km using a satellite, a high-rate system for metropolitan networks remains a crucial goal. This paper presents a Hertz-rate quantum teleportation system over a metropolitan fiber network, using a high-performance time-bin entangled light source and weak coherent single-photon sources with decoy states. Photonic time-bin qubits are teleported at 7.1 ± 0.4 Hz over 64 km, achieving an average single-photon fidelity of ≥90.6 ± 2.6%, exceeding the classical limit.

The paper reviews the history and existing methods of quantum teleportation across different platforms, highlighting the limitations of continuous-variable systems in terms of distance and the need for high-rate teleportation in discrete-variable systems for building large-scale quantum networks. It mentions previous achievements in long-distance quantum teleportation, particularly the satellite-based experiment reaching over 1200 km, but emphasizes the lack of a high-rate system for metropolitan applications, which is the gap addressed by this research.

The experimental setup consists of three nodes: Alice (sender), Bob (receiver), and Charlie (mediator). Alice prepares photonic time-bin qubits using a weak coherent single-photon source and an unbalanced Mach-Zehnder interferometer (UMZI). Bob generates time-bin entangled photon pairs using a periodically poled lithium niobate (PPLN) waveguide. Alice sends her qubit to Charlie, and Bob sends his entangled idler photon to Charlie via separate 22km fiber channels. Charlie performs a Bell state measurement (BSM) on the photons received from Alice and Bob. The BSM result is classically communicated to Bob, who then performs a unitary transformation on his signal photon to recover Alice's original state. The system uses a fully running feedback system to stabilize the arrival times and polarization of photons, ensuring indistinguishability at Charlie. The experimental setup includes various components such as fiber Bragg gratings, optical circulators, variable optical attenuators, superconducting nanowire single-photon detectors, and active feedback systems for timing and polarization control. The synchronization between the three nodes is achieved through classical optical pulses. Prior entanglement distribution and indistinguishability of photons are measured using a Franson interferometer and Hong-Ou-Mandel (HOM) interference, respectively. Quantum state tomography (QST) is used to reconstruct the density matrices of the teleported states, allowing for fidelity calculations.

The experiment achieved a quantum teleportation rate of 7.1 ± 0.4 Hz over a 64-km fiber channel. An average single-photon fidelity of ≥90.6 ± 2.6% was achieved, significantly exceeding the classical limit of 2/3. The fidelity of equatorial states was 80.4 ± 2.0%, while the fidelity for the pole states (|e⟩ and |l⟩) were 92.2 ± 1.0% and 92.4 ± 1.1%, respectively. The average fidelity calculated using quantum state tomography (QST) was 86.4 ± 4.5%. The visibility of the Hong-Ou-Mandel (HOM) dip was 35.3 ± 1.0%, indicating a single-photon indistinguishability of 88.8 ± 2.4%. The decoy state method was used to account for the use of weak coherent states instead of genuine single photons.

The high teleportation rate and fidelity achieved in this experiment represent a significant advancement in quantum teleportation technology. The successful demonstration of Hertz-rate quantum teleportation over a metropolitan-scale fiber network opens the way for practical applications in quantum communication and networking. The results demonstrate the feasibility of building large-scale quantum networks using existing fiber optic infrastructure. The high fidelity, surpassing the classical limit, validates the quantum nature of the teleportation process. The use of the decoy state method ensures the robustness and reliability of the results. Future improvements could focus on further enhancing the indistinguishability of photons to increase the fidelity, and scaling up the system for even longer distances and higher rates.

This research successfully demonstrated high-rate quantum teleportation over a metropolitan fiber network, achieving a teleportation rate of 7.1 ± 0.4 Hz and an average single-photon fidelity exceeding the classical limit. This significant advancement paves the way for the development of practical quantum networks and the realization of a future quantum internet. Future work could involve improving the indistinguishability of photons to further enhance fidelity and exploring the use of advanced error correction techniques to mitigate noise.

The study uses weak coherent states instead of perfect single photons, requiring the decoy state method to estimate the fidelity with genuine single photons. Residual distinguishability of photons in different degrees of freedom limits the fidelity of teleportation, particularly for superposition states. Improvements in the stabilization and control of the quantum channel could further improve the overall performance.