Researchers have developed a novel method for quantum communication that uses arrays of ytterbium-171 atoms, a development that could pave the way for more scalable and efficient quantum networks. The new approach, detailed in a recent publication in Nature Physics, directly generates entanglement between atoms and photons at telecom-band wavelengths, overcoming a significant hurdle in long-distance quantum communication. This breakthrough is expected to have profound implications for quantum information science, potentially leading to enhanced quantum computing and more secure communication networks in the near future. The work was led by a team at the University of Illinois at Urbana-Champaign.
The core of this new technique lies in its ability to avoid the inefficiencies and noise associated with converting quantum signals from the visible or ultraviolet spectrum to the telecom band, which is optimal for transmission over optical fibers. By using the unique properties of ytterbium-171, the researchers have demonstrated high-fidelity entanglement in a parallelized manner, suggesting that the system is inherently scalable. This method not only offers a more robust way to transmit quantum information but also leverages atomic properties that are beneficial for other quantum technologies, such as atomic clocks and quantum metrology.
A New Approach to Quantum Entanglement
The foundation of quantum networking is the phenomenon of entanglement, a quantum-mechanical connection between particles that allows them to influence each other instantaneously, regardless of the distance separating them. In this study, the researchers focused on creating entanglement between ytterbium-171 atoms and photons. The choice of ytterbium-171 was crucial due to its long-lived metastable state, which makes it an excellent candidate for storing quantum information. This property is also why ytterbium is used in some of the world’s most precise atomic clocks.
The team was able to achieve high-fidelity entanglement by utilizing a specific transition in the ytterbium atom that occurs at a wavelength of 1389 nm, which falls within the telecom O-band. This is a significant advantage over many existing quantum communication systems that rely on qubits operating at visible or ultraviolet wavelengths. Such systems require a conversion process to shift the photons to the telecom band for long-distance transmission, a step that often degrades the signal and introduces errors. By generating the entangled photons directly in the desired wavelength, the new method is more efficient and reliable. The researchers employed a technique known as time-bin encoding to establish the entanglement, a method that is well-suited for transmission over optical fibers.
Parallelization and Scalability
A key feature of the new system is its scalability, which is achieved through a parallelized architecture. The researchers arranged the ytterbium-171 atoms in a one-dimensional array and imaged this array onto a commercial fiber array. This setup allows for the simultaneous collection of photons and the generation of entanglement across multiple atoms in the array. In their experiments, the team demonstrated that this parallel approach resulted in consistently high entanglement fidelity with minimal crosstalk between the different network sites. This ability to perform parallel operations is a critical requirement for building large-scale quantum networks that can handle a high volume of information.
To further enhance the system’s robustness, the researchers developed a “mid-circuit networking protocol.” This protocol is designed to maintain the coherence of the qubits—the fundamental units of quantum information—during networking operations. Preserving coherence is one of the major challenges in quantum computing and communication, as qubits are extremely sensitive to their environment and can easily lose their quantum properties. By implementing this protocol, the team was able to ensure that the quantum information remained intact throughout the entanglement process. According to the researchers, with some technical upgrades, the fidelity of the atom-photon entanglement could potentially reach 99%.
Future Directions and Upgrades
The current experimental setup has already demonstrated the potential for building a scalable quantum network, but the research team is already working on next-generation improvements. One of the main goals is to further increase the efficiency of single-photon collection. The team has suggested that replacing the conventional objective lenses used in the current setup with a macroscopic confocal cavity could enhance collection efficiency by orders of magnitude. A confocal cavity is a structure made of two mirrors that can trap and amplify light, which would allow for a much higher rate of successful entanglement generation.
Building a Second-Generation System
The Covey Lab at the University of Illinois is in the process of designing a second-generation ytterbium experiment that will incorporate such a cavity. This new system will be specifically tailored for the 1389 nm transition and is expected to enable high-rate, long-distance quantum communication. The ultimate objective is to extend the demonstrated time-bin encoded entanglement to achieve remote atom-atom entanglement. This would allow for the creation of entangled links between atoms in different locations, either within the same experimental apparatus or across separate devices connected by optical fiber. Achieving this would be a major milestone towards the realization of a practical quantum internet.
Implications for Quantum Technologies
The advancements demonstrated in this research have far-reaching implications for the broader field of quantum information science. The ability to create robust, high-fidelity entanglement between atoms and telecom-band photons is a key building block for a variety of quantum technologies. For example, a network of entangled atomic clocks could lead to unprecedented levels of precision in timekeeping and navigation. Such a network could also be used for sensitive remote sensing applications, such as detecting subtle changes in gravitational fields.
In the realm of quantum computing, this technology could be used to connect multiple quantum processors together, creating a distributed quantum computer that is far more powerful than any single device. The protocols and design strategies developed in this study provide a clear roadmap for the future development of such systems. While there are still many challenges to overcome before a global quantum network becomes a reality, this work represents a significant and practical step in that direction.
