Mapping the positions of TLS defects in superconducting qubits

Material defects give rise to parasitic two-level-system defects (TLS) which are known to limit the coherence of superconducting qubits.

We present a method to determine the individual positions of TLS in a transmon qubit. For this, four independent gate electrodes are patterned on-chip which surround the qubit circuit. Application of a DC-voltage to such an electrode exposes the qubit to a DC-electric field whose strength varies rapidly with the distance to the electrode. Via this E-field, the resonance frequencies of TLS residing on the qubit electrodes can be tuned as observed by swap spectroscopy. By measuring the strengths by which a given TLS is tuned when different electrodes are biased, and comparing these to simulations of the location-dependent electric fields induced by the electrodes, we obtain the position of the TLS.

We find that about 50% of all observed TLS are residing on the leads connecting the Josephson junctions to the planar capacitor of the transmon qubit. Taking into account the enhanced electric fields of the qubit mode on the junction leads, this percentage is larger than expected, and points towards an approximately 8-times higher density of TLS on the eBeam-fabricated junction leads compared to the optically patterned qubit island and ground plane. This result also highlights the importance of the junction leads as a critical circuit part, and strongly advocates for techniques such as wire tapering to dilute the qubit mode’s E-field near the junctions.