Ultrafast frequency combs have found tremendous utility as precision instruments in domains ranging from frequency metrology, optical clocks, broadband spectroscopy, and absolute distance measurement. This sensitivity originates from the fact that a comb carries a huge number of co-propagating, coherently-locked frequency modes. Accordingly, it is the aggregate noise arising from these individual teeth that limits the achievable sensitivity for a given measurement. Correlations among various frequencies are the key factor in describing and using an optical frequency comb. We have developed methods, inspired from quantum optics, to extract amplitude and phase correlations among a multitude of spectral bands. From these, we can deduce the spectral/temporal eigenmodes of a given optical frequency comb (OFC), and use it to either study the dynamics or the laser, or to optimize metrology experiments such as, for instance, ranging in turbulent medium[1,2].
But beyond characterizing the classical covariance matrix of an OFC, one can, using non-linear effects, manipulate this noise and eventually reduce it even bellow quantum vacuum noise, producing squeezed optical frequency combs. We have demonstrated that by proper control of non-linear crystals, optical cavities and pulse shaping it was possible to embed within an optical frequency comb up to 10 spectral/temporal modes with non-classical noise properties. Furthermore, dividing the spectrum of this comb into 10 frequency bands, entanglement is certified for all of the 115974 possible nontrivial partitions of this 10 mode state. This is the first demonstration of full multipartite entanglement and this source is shown to be a very promising candidate for scalable measurement based quantum computing.
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