Bosonic encoding is an approach for quantum information processing, promising reduced hardware overhead by encoding in the many levels of a harmonic oscillator mode. Recent implementations of such bosonic qubits in circuit QED have demonstrated quantum correction beyond break-even using only a single mode. However, scaling to multiple modes is challenging as it requires weak interaction for independent control, yet strong interaction for fast entanglement. Applying fast and efficient universal control on multiple modes remains an open problem.

In this seminar, we present the novel approach of the Conditional Not Displacement (CNOD) method, which enables fast generation of states and universal control over a harmonic oscillator with weak dispersive coupling to an ancillary qubit. Surprisingly, we find that extending such control to multiple harmonic oscillators does not require adding ancillary systems; it can be obtained with a single two-level system coupled to the various modes. The weak coupling, in turn, allows for excellent separability between the modes without sacrificing the speed of entanglement operations.

We demonstrate the control on a superconducting transmon qubit weakly coupled to a multi-mode superconducting cavity. We create both entangled and separable cat-states in different modes of the multi-mode cavity, showing entangling operations while maintaining independent control over each mode. Importantly, the operation time is not limited by the inverse of the coupling rate, which is the typical timescale, and we exceed it by almost 2 orders of magnitude. Our scheme allows for fast and efficient multi-mode bosonic encoding and measurement.

Based on arXiv:2301.09831