Star Code: Experimental Demonstration of Autonomous Error Correction with Two-Qutrits (Part II Experiment)

The implementation of the star code requires strong qutrit-qutrit (QQ) red (beam-splitting) and blue (two-photon squeezing) sideband interactions. We develop a coherence-preserving inductive coupler capable of providing individual sideband rates larger than 5 MHz while suppressing residual static cross-Kerr strength below 300 kHz. Utilizing this linear coupler for the star code scheme, we experimentally demonstrate lifetime improvements of the logical zero |L0>, logical one |L1>, and logical superposition states by factors of 3.2, 5.6, and 3.7 respectively. Among these, the lifetime of |L0> surpasses the longest coherence time in the system achieving partial-breakeven performance. To our knowledge, this is the first such realization in a transmon-based system with autonomous error correction.