Propagating entangled qubits in quantum acoustics

The controlled interaction between superconducting qubits and acoustic devices provides a promising avenue for encoding and processing quantum information in phononic modes. The short wavelength and slow velocity of sound enable the emission and on-chip propagation of multiple flying phonon qubits in millimeter-scale acoustic channels. Entangled phonon states of itinerant qubits can be generated by a single superconducting qubit, with the size of the entangled state limited mostly by acoustic and superconducting qubit coherence times.

We explore the generation of multiqubit entangled graph states that can serve as a resource for quantum computation and sensing using a transmon circuit with a flip-chip-integrated lithium niobate surface acoustic wave channel. Phonon release from the second excited state interleaved with transmon qubit gates enables the generation of entangled phonon wavepackets. These phononic qubits can be made to interact with the emitter again, providing delayed quantum feedback. Using a second transmon qubit to catch phononic wavepackets, we characterize the performance of these operations and discuss progress towards the generation of graph states in this approach.