Robust preparation of Wigner-negative states with optimized SNAP-displacement sequences

Hosting non-classical states of light in 3D microwave cavities has emerged as a promising paradigm for continuous-variable quantum information processing. Here we experimentally demonstrate high-fidelity generation of a range of Wigner-negative states useful for quantum computation, such as Schrödinger cat states, binomial states, Gottesman-Kitaev-Preskill (GKP) states and cubic phase states. To do so, we use a sequence of interleaved selective number-dependent arbitrary phase (SNAP) gates and displacements. We use two steps of optimization. In the first step we use a gradient descent algorithm to optimize the parameters of the SNAP and displacement gates. In the second step we optimize the envelope of the pulses implementing the SNAP gates. Our results show that this way of creating highly non-classical states in a harmonic oscillator is robust to fluctuations of system parameters such as the qubit frequency.