Electric field spectroscopy of material defects in transmon qubits

Abstract

Superconducting integrated circuits have demonstrated a tremendous potential to realize integrated quantum computing processors. However, the downside of the solid-state approach is that superconducting qubits suffer strongly from energy dissipation and environmental fluctuations caused by atomic-scale defects in device materials. Further progress towards upscaled quantum processors will require improvements in device fabrication techniques which need to be guided by novel analysis methods to understand and prevent mechanisms of defect formation. Here, we present a technique to analyse individual defects in superconducting qubits by tuning them with applied electric fields. This provides a spectroscopy method to extract the defects' energy distribution, electric dipole moments, and coherence times. Moreover, it enables one to distinguish defects residing in Josephson junction tunnel barriers from those at circuit interfaces. We find that defects at circuit interfaces are responsible for about 60% of the dielectric loss in the investigated transmon qubit sample. About 40% of all detected defects are contained in the tunnel barriers of the large-area parasitic Josephson junctions that occur collaterally in shadow evaporation, and only ≈ 3% are identified as strongly coupled defects which presumably reside in the small-area qubit tunnel junctions. The demonstrated technique provides a valuable tool to assess the decoherence sources related to circuit interfaces and to tunnel junctions that is readily applicable to standard qubit samples.

Resolving the positions of defects in superconducting quantum bits

“…Figure 4b shows the resulting distribution of g, which is very similar to an independent and direct measurement of the coupling strengths reported in Ref. [22].…”

“…Further, detected defects are concentrated within 50 nm from the substrate-metal-vacuum edge since the qubit fields are focused in this region. b Distribution of extracted defect-qubit coupling strengths g, which is in good agreement to direct measurements using the same sample [22]. electric field.…”

“…2b. Horizontal traces indicate defects which most probably reside inside the qubit's stray Josephson junction where they do not experience any applied DC fields, as further detailed in the previously mentioned work [22]. Fig.…”

“…(3) are indicated by highlighted curves in Fig. 2b. Horizontal traces indicate defects which most probably reside inside the qubit's stray Josephson junction where they do not experience any applied DC fields, as further detailed in the previously mentioned work [22]. A distribution of measured γ t /γ b ratios is plotted in 3 bot top bot top 5V V b 5V V t text width DinA4, π meas.…”

“…This method can be further improved by performing independent measurements of the qubit-defect coupling strengths g and thus their effective dipole moment sizes [22], hereby reducing uncertainties concerning the interface at which a defect resides.…”