Miniature optical frequency comb for integrated photonics can be used for quantum data encryption

Producing two frequencies within such a comb is the same as producing entangled photons.

Researchers at the University of Southern California (USC; Los Angeles, CA ) Viterbi School of Engineering have invented a new method to create an optical frequency comb, a tool that increases the potential applications of lasers by converting a single wavelength into multiple wavelengths, effectively creating tens to hundreds of lasers from a single laser.1 The new frequency-comb generator consists of optical microcavities covered with monomolecular layers of highly nonlinear small organic molecules. The new device requires 1000x less power to operate than a conventional frequency-comb generator, allowing for mobile applications.

The first applications of frequency combs focused on detecting trace amounts of chemicals and high-precision timekeeping; however, recently, a new application of great significance to society has emerged: quantum cryptography.

While many strategies are being pursued to enable quantum cryptography, one of the leading contenders is based on photon entanglement. Entangled pairs of photons must be created at exactly the same time with exactly the same properties, which is done naturally by a frequency comb. The first step in forming the frequency comb occurs when the primary laser generates a secondary pair of wavelengths; however, because of energy conservation, one wavelength must have higher energy and one wavelength must have lower energy. Additionally, the energies must sum to be exactly equal to the primary laser, and the two new wavelengths must appear at exactly the same time. Thus, frequency comb generators can be viewed as entangled photon generators.

While reducing the size and power requirements of the frequency comb were key technical hurdles, there are many integration and manufacturing challenges remaining before quantum cryptography on portable platforms will be commonplace.



1. Xiaoqin Shen et al., Science Advances (2018); doi: 10.1126/sciadv.aao4507.

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