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Multi-photon nonclassical states of light based on high-gain parametric down-conversion

Subject Area Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2016 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 289382956
 
Final Report Year 2019

Final Report Abstract

The project ‘Multi‐photon nonclassical states of light based on high‐gain parametric down‐conversion’ was aimed at the study and application of the radiation resulting from high‐gain parametric down‐conversion, namely bright squeezed vacuum (BSV). This state of light is macroscopic, i.e., contains a large number of photons, and yet it is strongly nonclassical, because it manifests such quantum effects as squeezing and entanglement. In the course of the project we have developed several methods to study and control the mode structure and spectrum of BSV. The methods of the spectrum/mode structure control are based on the selective amplification of some modes in a nonlinear interferometer. In this device, one high‐gain PDC process is followed by another one, which can enhance or suppress it. Further development of nonlinear interferometry with high‐gain PDC led us to the first direct observation of phase sensitivity below the shot‐ noise limit, with tolerance to detection loss. The tolerance to detection loss was achieved by strong phase‐ sensitive amplification of the state before its detection. Another result of the project is the theoretical proposal to apply a similar strategy for protecting the nonclassical features of BSV against loss. Finally, according to the objectives of the proposal, BSV was used as a pump to generate optical harmonics of orders 2‐4, and up to 2 orders of magnitude efficiency increase has been demonstrated compared to the case of coherent light. All objectives of the project were therefore fulfilled. Meanwhile, three new unexpected results were obtained during the work on the project. The first one is again related to the use of a nonlinear interferometer, which became a hot topic during the last few years. Following a proposal of another group, we implemented the interferometer to measure the quadrature squeezing of highly multimode BSV. The second unexpected result is based on the idea of amplification before detection, and it is the theoretical proposal of sub‐shot‐noise imaging without the requirement of high detection efficiency. The third unexpected result was achieved when we used BSV for nonlinear optical experiments: the statistics of light at the output was so unusual that we observed a power‐law (Pareto) photon‐number distribution with undefined mean value. This last result was reported in a popular newsletter of Max‐Planck Society https://www.mpg.de/12627922/mpl_jb_20181. The results of the project have several potential applications. The sub‐shot‐noise phase sensitivity with tolerance to detection loss is likely to find applications in quantum‐enhanced metrology, spectroscopy and imaging. The high efficiency of BSV for multiphoton processes can be used in multiphoton spectroscopy, especially with fragile biological samples. As a result of the project, a method of absolute calibration of optical spectrometers has been developed and patented.

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