Origin of magnetic field dependence in the dielectric loss of superconducting devices

Dielectric loss is one of the main mechanisms of energy relaxation in superconducting qubits. This loss is believed to originate from electric-dipole active two-level systems (TLSs) present in the amorphous surfaces and interfaces surrounding the superconducting wires. However, the identity of the TLSs remains unknown. Recent experiments presented evidence that dielectric loss is affected by an external magnetic field, suggesting that the TLSs also have a magnetic dipole moment associated to them [1, 2].

Here we discuss two distinct theoretical models to explain the origin of this magnetic field dependence. The first, "p-orbital acceptor model", is based on spin-3/2 four-level systems such as boron acceptor impurities [1]. The second, called "p-orbital donor model" is based on the spin 1/2+3/2 six-level system formed by a spin-orbit-split donor impurity.

Our results show that measurements of the B-field dependence of dielectric loss may provide a method for identifying the microscopic species of TLS defects that cause loss. They also show that dielectric loss can be mitigated by magnetic fields even at low powers.

[1] Z. H. Zhang et al., arXiv:2402.17155 (2024).

[2] L. Athesian et al., Private Communication, MIT (2024).