While long-distance relationships pose challenges for humans, they are even more complex for entangled states. The farther apart the quantum memories are, the harder it is to maintain their connection. Overcoming this barrier has been a key hurdle in quantum communication. However, recent advancements suggest that this distance issue might not be insurmountable.
A group of physicists, led by Xi-Yu Luo, Chao-Yang Wang, and Ming-Yang Zheng from the University of Science and Technology of China, has achieved a significant breakthrough by establishing the longest fiber-based matter-to-matter entanglement to date. They successfully entangled quantum memories over an optical fiber stretching 420 kilometers (261 miles), surpassing previous records by over four times and exceeding the fundamental limits of direct transmission.
“Our experiment provides a test bed for studying quantum network applications beyond metropolitan scale,” the researchers explain in a paper published in Physical Review Letters.
Quantum entanglement, although a well-established concept, remains a fascinating phenomenon. It involves linking two or more particles so that their properties are intertwined. Measuring one particle instantly reveals the properties of the other, regardless of the distance between them. This phenomenon is described by physicists as particles sharing a quantum state.
While many entanglement experiments focus on particles of light, known as photons, a future quantum internet will require something more durable. This is where quantum memories come into play—devices that can store quantum information. In this recent experiment, the quantum memories were made up of laser-cooled rubidium atom clouds. The setup involved two memory units, named Alice and Bob, connected by the 420-kilometer optical fiber. A central node, called Charlie, detects photons emitted by Alice and Bob. If Charlie observes the correct interference pattern, it indicates successful entanglement between Alice and Bob, despite their physical separation.
The researchers successfully demonstrated matter-to-matter entanglement over the entire 420 kilometers. Their success relied on three main techniques, none of which were novel individually, but their combination and enhancement enabled the system to function over these unprecedented distances.
The first challenge was the distance itself. Quantum memories emit photons at wavelengths that optical fibers quickly absorb. To prevent signal loss, the team converted the photons to wavelengths used in the telecommunications industry, allowing them to travel with minimal transmission losses.
The second challenge involved protecting quantum signals from external interference. Even minor temperature changes and vibrations can disrupt the quantum interference needed for entanglement. To address this, the researchers developed an advanced stabilization system that continuously corrected fluctuations, maintaining synchronization of quantum states over the entire link.
Lastly, they employed a single-photon entanglement scheme, which requires only one photon to complete the journey, instead of two. “A more efficient quantum frequency conversion protocol helps us to reduce the fiber loss and extend the transmission distance,” the researchers note in their paper. “Additionally, we employ a novel phase stabilization scheme to ensure the implementation of the experiment based on single-photon interference, which can be conveniently applied to other platforms.”
The experiment also uncovered another milestone. A 2017 paper, now a benchmark for quantum communication, suggests a limit, known as the PLOB bound, on how much quantum information can be sent through a lossy channel without repeaters or memories. It’s akin to trying to throw paper airplanes across a windy field—the longer the distance, the fewer airplanes reach the destination. Luo and his team demonstrated that beyond 320 kilometers, their system achieved entanglement more effectively than direct transmission through the same fiber, according to the PLOB bound.
While the result doesn’t defy the PLOB bound, it shows that memory-based quantum networks can surpass the capabilities of the best possible direct optical fiber link.
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The development of a practical quantum internet remains a distant goal. However, long-distance entanglement between quantum memories is considered a crucial building block. This experiment demonstrates that this component can operate over hundreds of kilometers of optical fiber. “Furthermore, our experiment also demonstrates the capability of high-rate entanglement generation at shorter distances (∼100 kilometers), which is of significant interest for the construction of quantum repeaters,” the researchers write. “Combining this feature with subsecond lifetime storage based on an optical lattice, entanglement swapping can be employed to enable the connection of many such remote entanglement segments.”
The findings are set to be published in Physical Review Letters.
This article was fact-checked by Michael Irving and edited by Michael Irving. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
