Propagating Non-Gaussian States Generated by Degenerate 3-photon Downconversion

Nonclassical light sources have generated broad interest across a number of fields for their potential to enable novel quantum protocols. Spontaneous parametric downconversion (SPDC) processes have been widely adopted as an essential source of nonclassical light, generating various resource states. However, SPDC beyond second order has been an experimental challenge for decades. Here, with a superconducting parametric cavity designed for cubic interactions, we present experimental results for spontaneous three-photon downconversion when the cavity is flux pumped at three times the mode frequency. We measure propagating output states that are clearly non-Gaussian with a high degree of skewness in the quadrature amplitude distribution. Further, theory predicts that Wigner function negativity can persist for these states even after they escape from the cavity. With a linear detection scheme, we reconstruct the Wigner function of the propagating states from the directly measured moment matrix. We will present preliminary results on this reconstructed Wigner function.