Spin qubits encoded in gate-defined semiconductor quantum dots (QDs) are an advancing quantum computing platform, benefiting from small footprints, large charging energies, and compatibility with established semiconductor fabrication techniques. Among many spin-qubit modalities, the exchange-only (EO) qubit is particularly attractive for large-scale integration because it enables universal control using only baseband voltage modulation, eliminating the need for RF drives or local magnetic fields. However, because the EO qubit is encoded within a subsystem of a larger Hilbert space, it is susceptible to leakage, where information escapes the computational subspace. Leakage errors are particularly costly, as they are not directly compatible with standard quantum error-correction protocols.

Here, we demonstrate a leakage-protected idle (LPI) operating point for the EO qubit that suppresses leakage by energetically penalizing transitions out of the computational subspace. This protection is achieved by engineering a simultaneous, always-on exchange interaction among all three quantum dots comprising the EO qubit, opening an energy gap between the computational and leakage subspaces. We present methods for locating the LPI point and for measuring the induced gap. Using these, we characterize qubit coherence at the LPI point over a broad range of gap energies, identifying a regime in which leakage suppression and coherence are simultaneously enhanced.

More broadly, this work demonstrates precise coherent control of simultaneous exchange in an all-to-all-connected QD array. This gives rise to a virtual tunnel current with an associated direction, representing a distinct chiral quantum degree of freedom that could be used for alternative qubit designs or quantum simulations.