Fast entanglement of weakly interacting harmonic oscillators with superconducting qubits for bosonic encoded quantum computation

Bosonic encoding is a relatively nascent path to quantum computation in circuit QED. Dispersively coupling multiple Bosonic modes to a single superconducting ancilla qubit can be used to create entanglement between the bosonic modes. Typically these schemes use a large coupling of the bosonic modes to the ancilla for fast operation, however, this creates undesired anharmonicity in the form of Kerr nonlinearities, which degrade performance.

We use a symmetry-inspired approach for fast displacement gates conditioned on the state of the qubit, and utilize this to produce and measure both entangled and disentangled cat states in a multi-mode cavity weakly coupled to a single transmon qubit. Even though the typical coherence times of our setup are of the same order of magnitude as the coupling strength, through our control scheme we are able to measure a violation of CHSH inequality with an entanglement witness value of ~84%.