Multidimensional Quantum Communications with Legacy Fibers

In a new “twist” on quantum communication using optical fiber, researchers from the University of Witwatersrand (Wits) and Huazhong University of Science and Technology (HUST) have demonstrated that multiple quantum patterns of twisted light can be transmitted across a conventional fiber link that supports only one light pattern. The research could open the way to secure transport of quantum data across fiber networks, using multiple dimensions of light.

The researchers made multiple patterns of light transmittable over a single fiber by engineering entanglement in two degrees of freedom of light, polarization, and pattern. The polarized photon was conveyed over the fiber, and the other photon was able to access the multiple light patterns.

Two photons are entangled, one in polarization and the other in orbital angular momentum (twisted light). By passing the polarization photon through the fiber and keeping the twisted light in air, multidimensional entanglement transport is possible even over single-mode fiber. Courtesy of Witwatersrand University.

“Our team showed that multiple patterns of light are accessible through conventional optical fiber that can only support a single pattern,” researcher Isaac Nape said. “We achieved this ‘quantum trick’ by engineering the entanglement of two photons. We sent the polarized photon down the fiber line and accessed many other patterns with the other photon.”

The researchers manipulated the qualities of the photon on the inside of the fiber line by changing the qualities of its entangled counterpart in free space.

“In essence, the research introduces the concept of communicating across legacy fiber networks with multidimensional entangled states, bringing together the benefits of existing quantum communication with polarized photons with that of high-dimension communication using patterns of light,” professor Andrew Forbes said.

Wits researcher Isaac Nape aligns a quantum entanglement experiment. Courtesy of Witwatersrand University.

Whereas spatial modes of light allow higher information capacity per photon, spatial mode entanglement transport requires custom multimode fiber and is limited by decoherence-induced mode coupling. “Our team found a new way to balance these two extremes, by combining polarization qubits with high-dimensional spatial modes to create multidimensional hybrid quantum states,” Nape said.

“The trick,” Forbes said, “was to twist the one photon in polarization and twist the other in pattern, forming ‘spirally light’ that is entangled in two degrees of freedom. Since the polarization entangled photon has only one pattern, it could be sent down the long-distance single-mode fiber, while the twisted light photon could be measured without the fiber, accessing multidimensional twisted patterns in the free space. These twists carry orbital angular momentum, a promising candidate for encoding information.”

The researchers demonstrated transfer of multidimensional entanglement states over 250 meters of single-mode fiber, showing that an infinite number of two-dimensional subspaces could be realized. This was confirmed by quantum state tomography, Bell violation measures, and a quantum eraser scheme. Each subspace could be used for sending information or for multiplexing information to multiple receivers. “Each transmission is still only a qubit (2D), but there are an infinite number of them because of the infinite number of twisted patterns we could entangle in the other photon,” Forbes said.

The new approach allows legacy networks to be used. “A consequence of this new approach is that the entire high-dimensional OAM Hilbert space can be accessed, but two dimensions at a time. In some sense it is a compromise between simple 2D approaches and true high-dimensional approaches,” Forbes said.

The research was published in Science Advances (www.doi.org/10.1126/sciadv.aay0837).