Linear Beamsplitter for Large Bosonic States in a Multi-Mode Superconducting Platform

Bosonic modes, thanks to their large Hilbert spaces, offer a hardware-efficient route toward scalable quantum information processing. To fully leverage the power of bosonic quantum computing, it is essential to engineer operations on large bosonic states with high fidelity. In superconducting circuits, the fundamental two-mode beamsplitter interaction has been realized, but unwanted nonlinear effects limit their linearity and thereby the size of the usable Hilbert space. Here, we implement a multi-mode, planar, bosonic superconducting processor, to demonstrate highly linear beamsplitter operations even for large excitations. Our implementation is based on an asymmetrically threaded superconducting quantum interference device (ATS) operated at a bias point that suppresses all static odd-order nonlinearities as well as the fourth-order Kerr term. At the same operating point, we also take advantage of microwave-activated third-order interactions to explore novel types of multi-mode gates, for example, enabling three-mode squeezing. By combining Gaussian and non-Gaussian gates across multiple modes in a single quantum chip, we demonstrate universal control in a multi-mode bosonic system.