While today’s classical data signals can be amplified in cities or oceans, quantum signals cannot. Quantum signals must be stopped, copied, and passed on by specialized machines called quantum repeaters. Experts believe quantum repeaters will play a key role in future communication networks, enhancing security and enabling connections between remote quantum computers. The research has realized the optimization of quantum repeaters, thereby improving the connection performance between quantum computers.
Princeton Study and Quantum Repeater Advancements
The Princeton study, published Aug. 30 in Nature, details a new way to build quantum repeaters. Jeff Thompson, the study’s main author, said they have been working on this project for many years. They have combined advancements in photonic design and materials science. Most quantum repeaters emit light in the visible spectrum. Still, this light degrades quickly when traveling through fiber, causing signal attenuation. Therefore, it needs to be converted before being transmitted over long distances. The innovative device utilizes erbium-doped calcium tungstate crystals. These ions produce light at the right infrared wavelengths. This reduces the speed at which quantum signals decay and eliminates the need for signal conversion. As a result, networks can be made simpler and more reliable.
Enabling Quantum States Transmission
Ideally, that photon would be encoded with information from the ion, Jeff Thompson said. Or, more specifically, from a quantum property of ions called spin. Quantum repeaters use signals from distant nodes to create entanglement between their spins. This allows quantum states to be transmitted from end to end, even though some loss may occur.
Erbium Ions and Calcium Tungstate
Erbium ions are currently the most commonly used material in optical fiber communication systems. They provide high-quality, stable, long-distance, and low-loss laser output. The research team first started working with erbium ions several years ago. Still, the first versions used different crystals that contained too much noise. Noise in quantum systems causes the emitted photons’ frequency to randomly fluctuate, which disrupts the precise quantum interference required for quantum networks. To answer this question, the lab started working with Nathalie de Leon, an electrical and computer engineering associate professor, and Robert Cava, a renowned solid-state materials scientist and the Russell Wellman Moore Professor of Chemistry at Princeton University. They aim to find new materials that can accommodate single erbium ions with less noise. Material. They winnowed down the list of candidates from hundreds of thousands to hundreds, then dozens, then three. Each of the three finalists spent half a year testing. The results for the first material needed to be clearer, and the second material resulted in poorer quantum properties of erbium. But the third, calcium tungstate, is just right.
Promising Results and Future Improvements
The team ultimately demonstrated that the erbium ions in the new material emit indistinguishable photons, making them promising for use in quantum repeaters. According to graduate student Salim Ourari, who co-led the study, this puts the signal well above the high-fidelity threshold. While this work crosses an important threshold, additional work is needed to improve the storage time of quantum states in erbium ion spins. The team is improving calcium tungstate by reducing impurities affecting quantum spin states. This will minimize the negative impact of impurities on quantum communication signals.