  In the endeavour to make quantum computers a reality, integrated
superconducting circuits have become a promising architecture. A major
challenge of this approach is decoherence originating from spurious atomic
tunneling defects at the interfaces of qubit electrodes, which may resonantly
absorb energy from the qubit's oscillating electric field and reduce the
qubit's energy relaxation time $T_1$. Here, we show that qubit coherence can be
improved by tuning dominating defects away from the qubit resonance using an
applied DC-electric field. We demonstrate a method that optimizes the applied
field bias and enhances the 30-minute averaged qubit $T_1$ time by 23\%. We
also discuss how local gate electrodes can be implemented in superconducting
quantum processors to enable simultaneous in-situ coherence optimization of
individual qubits.
