Quantum storage of entangled telecom-wavelength photons in an erbium-doped optical fibre (1409.0831v2)

Abstract: The realization of a future quantum Internet requires processing and storing quantum information at local nodes, and interconnecting distant nodes using free-space and fibre-optic links. Quantum memories for light are key elements of such quantum networks. However, to date, neither an atomic quantum memory for non-classical states of light operating at a wavelength compatible with standard telecom fibre infrastructure, nor a fibre-based implementation of a quantum memory has been reported. Here we demonstrate the storage and faithful recall of the state of a 1532 nm wavelength photon, entangled with a 795 nm photon, in an ensemble of cryogenically cooled erbium ions doped into a 20 meter-long silicate fibre using a photon-echo quantum memory protocol. Despite its currently limited efficiency and storage time, our broadband light-matter interface brings fibre-based quantum networks one step closer to reality. Furthermore, it facilitates novel tests of light-matter interaction and collective atomic effects in unconventional materials.

• The paper demonstrates effective quantum storage using the AFC protocol in an erbium-doped fibre, preserving entanglement with minimal fidelity loss.
• The experimental setup achieved significant Bell inequality violation and maintained entanglement of formation above 49.9% post-storage.
• The study outlines avenues for enhancing recall efficiency and integrating quantum memories into current telecom infrastructure.

Quantum Storage of Entangled Telecom-Wavelength Photons in an Erbium-Doped Optical Fibre

In the endeavor to advance quantum information networks, the integration of quantum memories that can interface seamlessly with existing telecom infrastructure is paramount. This paper presents a significant step towards this integration through the demonstration of quantum storage of entangled telecom-wavelength photons within an erbium-doped optical fibre using the atomic frequency comb (AFC) protocol. The research successfully addresses the challenge of achieving atomic quantum memory compatible with standard telecom fibre infrastructure, a hurdle that had not been overcome until now.

Methodology and Experimental Setup

The paper employs a cryogenically cooled 20-meter-long silicate fibre doped with erbium ions. This setup is utilized to store and recall photons at 1532 nm, which are entangled with 795 nm photons. The AFC protocol is implemented to create a series of absorption lines in the inhomogeneously broadened atomic transition of erbium ions. These lines are equally spaced by frequency, and upon photon absorption, generate a collective atomic excitation. The subsequent re-emission of the photon occurs when these phases align after a pre-determined storage time.

Key Findings and Results

1. Fidelity and Entanglement Preservation: The fidelity of the entangled photon states before and after storage was recorded at 82.5% and 80.8%, respectively, indicating minimal degradation during storage. The experiment preserved significant entanglement, as evidenced by the entanglement of formation values remaining above 53% and 49.9% before and after storage.
2. Violation of Local Realistic Theories: The CHSH Bell-inequality test demonstrated significant violation with values of $S_\mathrm{in} = 2.38 \pm 0.05$ before storage and $S_\mathrm{out} = 2.33 \pm 0.22$ after storage, highlighting the maintenance of quantum correlations post-storage.
3. Efficiency and Practical Limitations: While the recall efficiency of the stored photons was noted as currently limited (~1%), the paper identifies avenues for enhancement through improved spectral burning and lower erbium doping concentrations.

Implications and Future Directions

The findings mark a critical development in quantum communication by achieving storage of non-classical states in a telecom-compatible medium. This has direct implications for the practicality of deploying quantum repeaters within existing fibre networks, leveraging the wide availability and operational robustness of erbium-doped fibres.

Looking forward, this paper opens the path for more advanced light-matter interactions in non-conventional materials, potentially leading to quantum internet components that integrate seamlessly with current technologies. Furthermore, the approach raises questions about the possible scale of separated atomic absorbers beyond the current constraints, inviting new experimental explorations in cavity QED-type experiments.

Ultimately, advancements in this area could lead to more efficient quantum repeaters and improved quantum networks, fundamentally impacting the development of global quantum communications and distributed quantum computing. The demonstrated integration of entangled photon storage within telecom infrastructure embodies a crucial stride towards realizing these ambitions.