Entanglement and tomography of multi-mode superconducting bosonic systems
Quantum harmonic oscillators provide a hardware-efficient and intrinsically error-correctable platform for quantum information processing due to their large Hilbert space and rich phase-space structure. Realizing a quantum advantage, however, requires the generation, control, and characterization of entangled states distributed across many oscillators. Existing approaches typically rely on exciting nonlinear ancilla elements to mediate interactions and on extensive phase-space sampling for tomography, introducing dissipation, leakage, and significant measurement overhead.
In this talk, I will address both challenges within a bosonic circuit quantum electrodynamics (cQED) architecture, where quantum harmonic oscillators are realized as three-dimensional superconducting cavities coupled to transmon qubits. First, I will present a robust state-reconstruction method based on photon-number sampling that achieves high-fidelity reconstruction with the minimal number of measurements. Second, I will demonstrate an on-demand cross-Kerr interaction between two oscillators that enables a controlled-phase gate, allowing the generation of maximally entangled states without exciting the nonlinear coupler. Together, these results provide a path toward scalable quantum processors based on bosonic modes.
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