Implementation of a Dual-Rail Qubit based on Fixed-Frequency Transmons and a Tunable Coupler

Conventional implementations of quantum error correction require substantial resource overheads, motivating schemes like the dual-rail encoding that exploits a hierarchy of error rates in combination with erasure conversion to increase the threshold and the effective code distance of an overarching error-correcting code [1]. For such schemes to be effective, error rates within the dual-rail subspace must be lower than the rate of relaxation errors. Previous implementations of dual-rail qubits have used superconducting 3D cavities [2] and frequency-tunable transmons [3]. Here, we experimentally demonstrate a dual-rail qubit realized using two fixed-frequency transmons coupled via a tunable coupler to perform coherent operations in the dual-rail subspace. We also explore the use of the coupler as an ancillary element for mid-circuit erasure checks. We tune-up and characterize logical gates acting on the dual-rail qubit and study the effectiveness of dynamical decoupling schemes on the dual-rail error rates. 

[1] A. Kubica et al., Phys. Rev. X 13, 041022 (2023).  

[2] K. S. Chou et al., Nat. Phys. 20, 1454 (2024).  

[3] H. Levine et al., Phys. Rev. X 14, 011051 (2024).