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For some time, experts in quantum communication have understood that employing nonlinear optical methods could enhance the fidelity of quantum information transmission, safeguarding it against various errors. Nevertheless, earlier attempts to integrate these processes struggled to function under the notably low light conditions that quantum communication demands.
Recent advancements from a research group at the University of Illinois Urbana-Champaign have addressed these challenges by utilizing an indium-gallium-phosphide nanophotonic platform for nonlinear processes. This innovative approach significantly boosts efficiency, allowing operations to occur even at the level of single photons, marking a pivotal step towards the practical implementation of nonlinear optics in quantum communication systems.
“Our nonlinear system achieves a transmission fidelity of 94%, which is a marked improvement over the theoretical limit of 33% associated with linear optical systems,” explained Kejie Fang, an electrical and computer engineering professor leading the project. “This clearly showcases the potential of utilizing nonlinear optics in quantum communication. The key hurdle was efficiency, and our nanophotonic platform has demonstrated a substantial increase, paving the way for more effective technologies.”
The findings of this study were published in the journal Physical Review Letters.
Quantum information is transmitted through networks utilizing a protocol known as quantum teleportation. This method leverages quantum entanglement—where two quantum entities, usually single photons, exert influence over each other without a direct physical link—to relay quantum information from one party to another, avoiding traditional communication channels. This technique notably reduces the impact of external noise and imperfections inherent in communication systems.
Performance in quantum teleportation is limited by two primary factors. Firstly, standard linear optical components contribute to inherent inconsistencies during transmission. Secondly, the creation of entangled photons is prone to errors and excess noise, as entanglement sources frequently generate multiple photon pairs simultaneously. Consequently, it remains uncertain whether the specific photons used for teleportation are genuinely entangled.
“Multiphoton noise is a persistent issue in all practical entanglement sources and poses significant challenges for quantum networks,” noted Elizabeth Goldschmidt, a physics professor and co-author of the study. “The attractiveness of nonlinear optics lies in its ability to reduce the effects of this multiphoton noise due to the fundamental physics involved, facilitating the use of imperfect entanglement sources.”
Nonlinear optical components operate by merging photons of varying frequencies to generate new photons at different frequencies. In the context of quantum teleportation, the process employed is known as “sum frequency generation” (SFG), whereby the frequencies of two photons combine to create a new frequency. However, this process requires the initial photons to possess specific starting frequencies for it to succeed.
During quantum teleportation involving SFG, the procedure halts if two photons of identical frequency are detected. This characteristic effectively filters out predominant noise types found in most entangled photon sources, enabling higher telecommunication fidelities than would otherwise be achievable. However, the challenge lies in the low probability of successful SFG conversions, which has historically resulted in inefficiencies.
“While this issue has been acknowledged for a considerable time, it had not been thoroughly investigated due to the low success probability of SFG,” Fang continued. “Previously, the best success rates were as low as 1 in 100 million. Our work has achieved a ten-thousand-fold increase in conversion efficiency, bringing it to 1 in 10,000 using a nanophotonic platform.”
The research team expresses optimism that continued advancements may bolster the efficiency of quantum teleportation with nonlinear optical components even further. They anticipate that this technology could play a critical role in enhancing various quantum communication protocols, including entanglement swapping.
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