Coherence time of over a second in a telecom-compatible quantum memory storage material (1611.04315v2)

Abstract: Quantum memories for light will be essential elements in future long-range quantum communication networks. These memories operate by reversibly mapping the quantum state of light onto the quantum transitions of a material system. For networks, the quantum coherence times of these transitions must be long compared to the network transmission times, approximately 100 ms for a global communication network. Due to a lack of a suitable storage material, a quantum memory that operates in the 1550 nm optical fiber communication band with a storage time greater than 1 us has not been demonstrated. Here we describe the spin dynamics of $^{167}$Er$^{3+}:$Y${2}$SiO${5}$ in a high magnetic field and demonstrate that this material has the characteristics for a practical quantum memory in the 1550 nm communication band. We observe a hyperfine coherence time of 1.3 seconds. Further, we demonstrate efficient optical pumping of the entire ensemble into a single hyperfine state, the first such demonstration in a rare-earth system and a requirement for broadband spin-wave storage. With an absorption of 70 dB/cm at 1538 nm and $\Lambda$-transitions enabling spin-wave storage, this material is the first candidate identified for an efficient, broadband quantum memory at telecommunication wavelengths.

• The paper demonstrates a quantum memory in 167Er:Y2SiO5 with a hyperfine coherence time of 1.3 seconds at 7 T, enabling long-distance quantum repeater networks.
• The research achieves 95% polarization efficiency through efficient optical pumping into a single hyperfine state, optimizing broadband spin-wave storage.
• The study reports 70 dB/cm absorption at 1538 nm and precise hyperfine control, simplifying integration with existing telecom infrastructures.

Coherence Time of Over a Second in a Telecom-Compatible Quantum Memory Storage Material

The paper discusses the advancements in quantum memory using the material $$^{167}\mathrm{Er}^{3+}:\mathrm{Y}_{2}\mathrm{SiO}_{5}$$, with a focus on applications in quantum communication networks. A quantum memory system capable of operating in the 1550 nm telecommunications band, with coherence times exceeding 1 second, is demonstrated. Such a development is crucial for enabling long-distance quantum communication, particularly for Quantum Repeater Networks (QRNs) that seamlessly integrate with the existing fiber optic infrastructure.

Key Findings and Numerical Results

1. Hyperfine Coherence Time: The authors report a significant improvement in hyperfine coherence time, achieving 1.3 seconds in a high magnetic field (7 T). This is unprecedented for Kramers ions, marking a major milestone compared to non-Kramers systems.
2. Efficient Optical Pumping: The research demonstrates the efficient optical pumping of the entire ensemble into a single hyperfine state, $|\pm 7/2\rangle$, achieving a polarization efficiency of 95%. Such capability is critical for realizing broadband spin-wave storage required in high-bandwidth quantum memory applications.
3. Optical Depth and Band Transitions: The paper observed an absorption of 70 dB/cm at 1538 nm, paired with $\Delta m_I = \pm1$ transitions showing appreciable oscillator strengths. The transition frequencies and hyperfine structures were meticulously characterized using AM spectroscopy, facilitating the precise control needed for high-fidelity quantum operations.

Implications and Theoretical Impact

The research has significant implications for both practical and theoretical advancements in quantum communication:

• Practical Applications: The demonstrated coherence time is sufficient for global networks or QRNs, allowing nodes spaced over distances exceeding 1000 km to effectively operate even without error correction. This sets a new standard for the integration of quantum memories within existing telecommunication infrastructure.
• Material Characteristics: The use of $^{167}\mathrm{Er}$ offers direct compatibility with the telecom wavelengths, potentially eliminating the need for complex interfacing techniques such as frequency conversion. This simplifies the system architecture and enhances the practicality of deploying quantum technologies at scale.
• Broadband Quantum Memory: The capability for efficient spin-pumping alludes to future developments in broadband memory techniques. These could rival or surpass current non-Kramers systems by leveraging the wide hyperfine bandwidth intrinsic to $^{167}\mathrm{Er}$.

Future Prospects in Quantum Communication

The experiment opens avenues for future exploration into ZEFOZ techniques in Kramers systems, aiming to reach population lifetime limits upwards of 10 minutes. This would further bridge the gap between theoretical potential and practical application of solid-state quantum memories. Additionally, reducing the cross-relaxation through non-adjacent hyperfine levels presents a pathway for increasing coherence times further.

In conclusion, $$^{167}\mathrm{Er}^{3+}:\mathrm{Y}_{2}\mathrm{SiO}_{5}$$ represents a compelling candidate for telecom-compatible quantum memories with integration prospects in future quantum internet frameworks. Continued research in this domain could vastly enhance the scalability and efficiency of quantum networks, pushing the frontier of secure global communication.