Femtosecond Quantum Information

The SPOPO (Synchronously Pumped Optical Parametric Oscillator) experiment uses optical frequency combs to explore the multimode aspect of the light. We push the boundaries of modern physics in our eternal quest for knowledge – and for fulfilling our supervisor’s darkest desires.

We pump a SPOPO to create multimode squeezed vacuum in frequency modes. The cavity contains a non-linear crystal (BiBo) to create the squeezing, and is resonant with the 76MHz pulsed light that comes in.

The modes then go through a pulsed gate. It consists in another non-linear crystal which performs SFG (sum-frequency generation) between the quantum light and another classical beam- the gate beam. This allows us to subtract a photon in the mode given by the frequency mode of the gate beam. This mode is selected using a SLM in a 4-f configuration that can shape each individual frequency in phase and amplitude.

The non-Gaussian light finally enters a double-homodyne detection (DHD), which measure orthogonal quadratures of the light q and p at the same time. However each quadrature is mixed with vacuum, thus Heisenberg’s inequality is still verified. This detection projects the quantum state on squeezed coherent state, and allows for a direct sampling of the Q Husimi function. We can alternate between the DHD, and the single homodyne detection (HD) with a half-waveplate on the signal beam. This half-waveplate also allows for the unbalancing of the DHD, with which we can measure on a squeezed basis.

Right now the project of the experiment is to use the experimental Q function of the non-Gaussian state to perform fidelity estimation on another known theoretical state. If the fidelity is high enough, it means that our experimental state shares the same properties as the theoretical state. It is a great tool for certifying certain properties such as Wigner negativity, or non-Gaussianity which are at the heart of quantum information.