Kramers-Kronig detection in the quantum regime

We investigate the quantization of Kramers-Kronig detection technique initially developed for classical optical communications. It consists in mixing the unknown field with a strong monochromatic local oscillator on an unbalanced beam splitter. A single output of the beam splitter undergoes a direct detection of the optical intensity by means of a single photodiode. When the measured output verifies signal processing constraints, namely, the minimal phase and the single-sideband constraints, Kramers-Kronig detection reconstructs the phase of the signal from the intensity measurements via a digitally computed Hilbert transform. The local oscillator being known, Kramers-Kronig detection allows for reconstructing the quadratures of the unknown field. We show that this result holds in the quantum regime up to first order in the local oscillator amplitude and thus that Kramers-Kronig detection acts as a coherent detection able to measure both quadratures, making it a Gaussian measurement similar to double homodyne detection. We also study in detail the phase information measured by Kramers-Kronig detection for bosonic coherent states, monomode pure states, and mixed states. Finally, we propose and investigate a spectral tomography protocol for single-photon states that is inspired by Kramers-Kronig detection and relies on a spectral engineering of the single photon.