On the nature of decoherence in quantum circuits: Revealing the structural motif of the surface radicals in α-Al₂O₃

Abstract

Quantum information technology puts stringent demands on the quality of materials and interfaces in the pursuit of increased device coherence. Yet, little is known about the chemical structure and origins of paramagnetic impurities that produce flux/charge noise that causes decoherence of fragile quantum states and impedes the progress toward large-scale quantum computing. Here, we perform high magnetic field electron paramagnetic resonance (HFEPR) and hyperfine multispin spectroscopy on α-Al 2 O 3 , a common substrate for quantum devices. In its amorphous form, α-Al 2 O 3 is also unavoidably present in aluminum-based superconducting circuits and qubits. The detected paramagnetic centers are immanent to the surface and have a well-defined but highly complex structure that extends over multiple hydrogen, aluminum, and oxygen atoms. Modeling reveals that the radicals likely originate from well-known reactive oxygen chemistry common to many metal oxides. We discuss how EPR spectroscopy might benefit the search for surface passivation and decoherence mitigation strategies.

Quantum bath suppression in a superconducting circuit by immersion cooling

“…2 that, unlike previous experiments on spins coupled to quantum circuits where the spin polarisation was saturated at about T = 50 mK 12 , surface spins are cooled to much lower temperatures in the presence of 3 He, with no other apparent change in the ESR spectra. The measured ESR spectrum is rather complex, consisting of many different species, and has been discussed in detail previously 12 , 33 . Here we focus on the species that are most suitable for intrinsic thermometry at these low temperatures, namely the two peaks labeled 1 and 3 that arise from atomic hydrogen 12 .…”

Chemical and structural identification of material defects in superconducting quantum circuits

“…Here, we use multifrequency as shorthand to mean microwave excitation/detection frequencies from conventional 10-30 GHz range to beyond 300 GHz and magnetic fields extending from 1 to well beyond 10 T. The use of multiple frequencies and fields allow increased resolution and separation of field-dependent (electron and nuclear Zeeman) and independent (exchange, zero-field and nuclear hyperfine) magnetic spin interactions, as sketched in figure 3 for a fictitious defect. For example, recent 3-10 T EPR measurements on α-Al 2 O 3 single crystals resolved three different radical centers (electron spin species with g-factor ∼2.0) [96]. High-field pulsed EPR-based nuclear hyperfine spectroscopy could identify one of the radicals as surface species through their couplings to multiple protons.…”

“…This approach has the benefit that computational modeling and interpretation of the measurements mutually constrain each other, leading to more reliable models of the electronic structure of the paramagnetic centers. For example, 27 Al and 1 H ENDOR and ELDOR-NMR combined with DFT modeling indicated that the unpaired electrons of α-Al 2 O 3 surface radicals were largely localized to two aluminum atoms and the surface oxygens to which they were bound [96]. Two large proton hyperfine interactions arose from protons bonded to the surface oxygen.…”

Low-loss α-tantalum coplanar waveguide resonators on silicon wafers: fabrication, characterization and surface modification

“…Historically, several approaches have been explored to increase their performance such as, participation ratio engineering [3]-typically resulting in bigger qubits-as well as optimal control [4,5], shielding [6] and signal filtering [7]. In contrast, the number of advancements based on understanding microscopic origin of decoherence and energy loss-including the judicious increase of the materials toolbox-is rather limited [8,9], with most published work focusing on only a few well documented materials such as Al [10][11][12][13][14][15], Nb [16][17][18][19][20], TiN [21,22], NbN [23,24], NbTiN [25][26][27], granular-Al [28,29], Re [30] and In [31]. Only recently, the suite of materials for superconducting quantum technology was further expanded, markedly resulting in qubit relaxation times as high as 0.5 ms for 2D transmon qubits by using α-tantalum (α-Ta) [32,33].…”