When particles of light called photons are paired together, or entangled, they can enable quantum communication across great distances. This entanglement means that changing the spin or other state of one photon also changes the paired photon, no matter how far apart they are. This link is what makes quantum communication possible. To test this ability, researchers built a quantum local area network (QLAN) that shared information among three systems in separate buildings using entangled photons. The team used a protocol called remote state preparation, where a successful measurement of one half of an entangled photon pair converts the other photon to the preferred state. The researchers performed this conversion across all the paired links in the QLAN—a feat not previously accomplished on a quantum network.
The QLAN connected a laboratory containing the photon source and the first node in the network to the second and third nodes using an existing fiber-optic network. Unlike conventional networks, the QLAN uses special switches that can make real-time adjustments without disconnecting users from the network. This allows network operators to respond to broken fibers or other problems by rerouting traffic—without disrupting the network’s speed or compromising security. The research demonstrates how quantum communications can scale from a lab to longer distances. It also helps lay the foundation for the quantum internet, which could eventually connect next-generation quantum computers. The findings could also improve quantum sensors, providing a new tool in the search for dark matter and other questions.
QLANs enhance the capabilities of local area networks that connect classical computing devices. Although entanglement can enable quantum key distribution (QKD), which has been the most common example of quantum communication, it also enables more general applications. The researchers who conducted this project have previously completed successful QKD experiments, but this method is limited because it only establishes security, not entanglement, between sites. For this new QLAN experiment, researchers from Oak Ridge National Laboratory (ORNL), Purdue University, and Stanford University located nodes named Alice, Bob, and Charlie in three different research laboratories in three separate buildings on the ORNL campus. While testing the QLAN, the team shared a signal from an antenna located in one of the laboratories to ensure that three GPS-based clocks assigned to each node were synchronized within a few nanoseconds and that they would not drift apart during the experiment. Having obtained precise timestamps for the arrival of entangled photons captured by photon detectors, the researchers sent these measurements from the QLAN to a classical network, where they compiled high-quality data from all three laboratory locations.
Oak Ridge National Laboratory
This work was funded by the Department of Energy Office of Science through the Early Career Research Program, the Transparent Optical Quantum Networks for Distributed Science Program, and the Office of Science Basic Energy Sciences program’s Materials Sciences and Engineering Division. Additional support was provided by the Intelligence Community Postdoctoral Research Fellowship Program at Oak Ridge National Laboratory and the Quantum Information Science and Engineering Network through the National Science Foundation.
Alshowkan, M., et al., Reconfigurable Quantum Local Area Network Over Deployed Fiber. PRX Quantum 2, 040304 (2021). [DOI: 10.1103/PRXQuantum.2.040304]