Abstract
Tunneling defects in disordered materials form spurious two-level systems which are a major source of decoherence for micro-fabricated quantum devices. For superconducting qubits, defects in tunnel barriers of submicrometer-sized Josephson junctions couple strongest to the qubit, which necessitates optimization of the junction fabrication to mitigate defect formation. Here, we investigate whether defects appear predominantly at the edges or deep within the amorphous tunnel barrier of a junction. For this, we compare defect densities in differently shaped Al/AlOx/Al Josephson junctions that are part of a Transmon qubit. We observe that the number of detectable junction-defects is proportional to the junction area, and does not significantly scale with the junction’s circumference, which proposes that defects are evenly distributed inside the tunnel barrier. Moreover, we find very similar defect densities in thermally grown tunnel barriers that were formed either directly after the base electrode was deposited, or in a separate deposition step after removal of native oxide by Argon ion milling.
Highlights
IntroductionMicroscopic tunneling defects forming parasitic two-level quantum systems[1,2] have attracted much attention in the superconducting quantum computing community due to their detrimental influence on qubit coherence[3–7]
Defects having an electric dipole moment may resonantly absorb energy from the oscillating electric field of the qubit mode, and efficiently dissipate it into the phonon[8] or BCS quasiparticle bath[9]. This gives rise to a pronounced frequency-dependence of qubit energy relaxation times T110,11, while strongly coupled defects which reside in the tunnel barrier of the Josephson junction may cause avoided level crossings in qubit spectroscopy[4,12,13]
We have studied densities of microscopic material defects in Josephson junctions of various shapes using superconducting Transmon qubits
Summary
Microscopic tunneling defects forming parasitic two-level quantum systems[1,2] have attracted much attention in the superconducting quantum computing community due to their detrimental influence on qubit coherence[3–7]. Defects having an electric dipole moment may resonantly absorb energy from the oscillating electric field of the qubit mode, and efficiently dissipate it into the phonon[8] or BCS quasiparticle bath[9]. This gives rise to a pronounced frequency-dependence of qubit energy relaxation times T110,11, while strongly coupled defects which reside in the tunnel barrier of the Josephson junction may cause avoided level crossings in qubit spectroscopy[4,12,13]. Josephson junctions remain a vulnerability to up-scaled quantum processors, where individual qubits may spontaneously be spoiled by strongly coupled junction-defects drifting into qubit resonance[30]. We investigate whether defects are predominantly formed at the edges of a tunnel junction or deep inside the tunnel barrier
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