Researchers at the International Quantum Academy in Shenzhen have generated genuinely entangled states across as many as six phonon modes in two acoustic resonators on a single chip, and moved some of those states from one resonator to the other. The work, led by Tan Dian's group under academician Yu Dapeng with collaborators including Eric Lutz of the University of Stuttgart and Tobias Kippenberg of EPFL, was published online in Science on October 15 under the title Deterministic generation and distribution of genuine multipartite entanglement in macroscopic acoustic resonators.
Phonons, the quanta of vibration in a solid, can carry quantum information in the same way photons do. The team used high-overtone bulk acoustic wave resonators, or HBARs, which support many long-lived vibration modes and can be integrated on chips. Earlier quantum acoustics experiments had prepared, controlled and stored quantum states in mechanical modes. Creating large multipartite entanglement in such systems, and sending it between separate nodes, has remained difficult.
Instead of the three-dimensional microwave cavities used in many earlier setups, the team put two HBARs, superconducting qubits and a tunable coupler on one two-dimensional hybrid chip. By tuning the spacing of the acoustic mode frequencies and the qubit-phonon coupling, a single qubit could couple strongly to several mechanical modes.
The paper reports genuinely multipartite entangled states of two, four and six modes across the resonators, with fidelities of 0.912, 0.790 and 0.570. Two-mode and four-mode states were then transferred between the resonators with fidelities of 0.785 and 0.601. Tan explained that genuine entanglement means the correlations are shared by the whole multimode system and cannot be broken down into smaller, independent entangled pieces. The drop in fidelity as the number of modes grows shows how quickly the task gets harder.
To verify the states, the group developed a tomography method that reuses a limited number of qubits across modes, needing only two superconducting qubits. The team says this is the largest experimental characterization of a multimode phonon state on a quantum acoustics platform so far.
The researchers also propose an architecture that couples more qubits to more HBARs through controllable couplers, with the long-term aim of modular phonon-based quantum networks. For now the result remains basic research. Other contributors came from Southern University of Science and Technology, the Shenzhen branch of Hefei National Laboratory, Zhejiang University and Jimei University.
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