Entangled Photons Travel 38 Miles Through Suburban Skies, Paving Way for Quantum Networks

According to the National Institute of Standards and Technology (NIST), researchers have successfully transmitted entangled photons over a substantial distance, marking a significant advancement towards the development of quantum networks. In a study published in the Journal of Optical Communications and Networking, NIST researchers and their collaborators demonstrated that the delicate quantum states of photons, particles of light, can survive transit through real-world, non-laboratory conditions. This breakthrough opens possibilities for enhanced scientific research, powerful quantum computing, and highly secure communication systems.

The experiment involved sending entangled photons through approximately 62 kilometers (about 38.5 miles) of commercial fiber-optic cable. This cable ran from NIST’s campus in Gaithersburg, Maryland, to the University of Maryland in College Park, traversing suburban environments. Quantum networks rely on entanglement, a phenomenon where particles share a unified quantum state regardless of their separation. Measuring one entangled particle instantaneously influences the state of the other, a concept Albert Einstein famously described as “spooky action at a distance.” The potential applications of such networks are vast, ranging from improved astronomical imaging through synchronized telescopes to more precise earthquake detection with distributed sensors, and the simulation of complex molecules for drug discovery. Furthermore, entangled quantum networks could provide an unparalleled level of security for communications, as any attempt to intercept or tamper with the signals would be immediately detectable.

A primary challenge in building quantum networks is maintaining the integrity of entangled states, which are extremely fragile and susceptible to environmental interference. The researchers focused on using existing fiber-optic infrastructure, as constructing dedicated networks solely for quantum applications would be prohibitively expensive. However, the chosen fiber route presented significant challenges. Much of the cable was aerial, exposed to the elements, including temperature fluctuations that cause expansion and contraction, wind that can cause movement, and even birds landing on the lines. These physical disturbances can distort the polarization of photons, which is the property typically used to encode quantum information, thereby destroying the entanglement.

To overcome these environmental hurdles, the NIST team and their collaborators employed a sophisticated system. They used a commercial device to generate entangled pairs of photons, where the polarization of one photon was intrinsically linked to the other. One photon from each pair was measured in the lab, while its entangled partner traveled through the aerial fiber to the University of Maryland. To counteract the distortions introduced by the fiber, the researchers utilized specialized devices developed by Qunnect. These devices sent beams of reference light through the fiber and, at the receiving end, measured the exact polarization changes. This information was then used to apply the precise inverse of these transformations to the experimental photons, effectively stabilizing their quantum states and preserving entanglement throughout their journey.

The experiment successfully transmitted 1,500 entangled photons per second, a rate that, while needing further improvement for widespread practical application, demonstrates the feasibility of the technology. Over a 24-hour period, the entangled state was maintained 92.8% of the time, with only 7.2% of the time required for polarization corrections. Statistical analysis confirmed that the photons detected at both ends of the fiber remained entangled. While not a record for the longest distance achieved in a controlled underground environment, this study is notable for its demonstration of quantum entanglement distribution over a long stretch of exposed, real-world fiber, akin to a significant stress test for quantum networking systems.

Article by Mel Anara, based upon information from the National Institute of Standards and Technology.

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