Change the color of quantum light on an embedded chip

Optical photons are ideal carriers of quantum information. But to work together in a quantum computer or network, they must have the same color (or frequency) and the same bandwidth. Changing the frequency of a photon requires changing its energy, which is particularly difficult on integrated photonic chips.

Recently, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed an integrated electro-optical modulator capable of efficiently altering the frequency and bandwidth of single photons. The device could be used for more advanced quantum computing and quantum networks.

The research is published in Light: science and applications.

Converting a photon from one color to another is usually done by sending the photon into a crystal with a powerful laser passing through it, a process that tends to be inefficient and noisy. Phase modulation, in which the oscillation of the photon wave is speeded up or slowed down to change the frequency of the photon, offers a more efficient method, but the device required for such a process, an electro-optical phase modulator, s proved difficult to integrate on a chip.

One material may be particularly suitable for such an application: thin-film lithium niobate.

“In our work, we adopted a new modulator design on thin-film lithium niobate which significantly improved the performance of the device,” said Marko Lončar, Tiantsai Lin Professor of Electrical Engineering at SEAS and lead author of the study. “With this integrated modulator, we have achieved record terahertz frequency shifts of single photons.”

The team also used the same modulator as a “time lens” – a magnifying glass that bends light in time instead of space – to change a photon’s spectral shape from fat to thin.

“Our device is much more compact and energy efficient than traditional bulk devices,” said Di Zhu, the first author of the paper. “It can be integrated with a wide range of classical and quantum devices on the same chip to achieve more sophisticated quantum light control.”

Di is a former postdoctoral fellow at SEAS and is currently a research scientist at the Agency for Science, Research and Technology (A*STAR) in Singapore.

Next, the team aims to use the device to control the frequency and bandwidth of quantum transmitters for applications in quantum networks.

The research was a collaboration between Harvard, MIT, HyperLight and A*STAR.

The article was co-authored by Changchen Chen, Mengjie Yu, Linbo Shao, Yaowen Hu, CJ Xin, Matthew Yeh, Soumya Ghosh, Lingyan He, Christian Reimer, Neil Sinclair, Franco NC Wong, and Mian Zhang.