DUAL-RAIL ENCODING OF A FIXED FREQUENCY MULTIMODE TRANSMON QUBIT WITH ANCILLA-FREE ERASURE ERROR DETECTION

11:00 | 18 JUNE, 2024

Amplitude damping errors are a dominant source of error in high performance quantum processors. A promising approach in error-detection are “erasure qubits”, where amplitude damping errors are converted into detectable leakage outside of the computational subspace. Dual-rail encoding has been demonstrated in superconducting quantum devices to show extended coherence above that of the constituent elements, however, these architectures can require the use of an ancillary qubit to perform the erasure error detection.

Here we present a dual-rail encoding within a single fixed-frequency superconducting multimode transmon qubit. The three island, two junction device comprises two transmonlike modes with a detuning of 1.5 GHz, in a coaxial cQED architecture. A resonator on the opposing side of the substrate allows for the dispersive readout of the multimode state of the device, with an error-detected logical state assignment fidelity of above 99%. We demonstrate the utility of the logical encoded qubit as a quantum memory, showing the logical bit-flip and phase-flip error rates are more than one order of magnitude lower than the physical error rates. Finally, we discuss how the error-detected subspace can be used for investigations into the fundamentals of noise and decoherence in fixed-frequency transmon qubits.