A special photon arrangement with a comb-like spectrum could bring efficient and secure quantum communications one step closer to reality. For the first time, an interdisciplinary team of researchers in the U.S. has achieved quantum entanglement of two photons in multiple dimensions, called high-dimensional entanglement, or hyperentanglement (Nat. Photon., DOI: 10.10.1038/NPHOT.2015.110).

Usually quantum encryption involves encoding two photons with the same polarization state, but photons also have properties like energy and spin. Associate professor of electrical engineering Chee Wei Wong at the University of California, Los Angeles (USA), and colleagues used these additional dimensions to create a phase-coherent bi-photon frequency comb to send photons entangled in multiple degrees of freedom. They then used a high-speed single photon detector at the National Institute for Standards and Technology (NIST) to receive and verify the high-dimensional entanglement of the photons.

The hyperentanglement of photons could do for quantum communications what wavelength division multiplexing does for data communications: weave more data into a single signal to enable higher data transmission rates. “Essentially, we’re leveraging wavelength division multiplexing concepts at the quantum level,” said Zhenda Xie, the paper’s first author and a researcher in Wong’s lab.

With each additional dimension of entanglement, a single photon pair can carry double the amount of information. This could enable secure, high-capacity data transfer with minimal error, which is useful in applications like military and government communications, banking and financial markets, quantum information processing and secure quantum key distribution.

“These observations demonstrate a new, fundamentally secure approach for dense information processing and communications,” said Wong. According to Wong, the observation of quantum entanglement in frequency-comb-like states was predicted decades ago by MIT professor Jeff Shapiro.