Abstract: Quantum bits (qubits) are an excellent testbed for quantum many-body dynamics and for quantum computing. Especially superconducting qubits convince by the ease of scalability and a high coherence obeying Moore's law. However, one severe coherence-limiting factor is Two-Level-Systems (TLS)  that reside at substrate-qubit interfaces, dielectric layers or the microchip surface. TLS are microwave resonances arising from defects or impurities in solids at low temperatures (<1K), which is the typical operation temperature of superconducting qubits. Residing at surfaces or inside of amorphous bulk materials, TLS resonances are broadly distributed. Each TLS may couple to oscillating qubit fields by its electric dipole moment, which leads to decoherence and energy relaxation in the quantum circuit. We exploit this sensitivity of qubits to spectroscopically detect individual TLS in the qubit material in order to draw conclusions about the TLS density and their microscopic nature. Such information gives practical hints for the improvement of qubit fabrication and architecture to avoid TLS formation and mitigate their coupling to the qubit.
The here-presented work is two-fold. On the one hand, a TLS-sensor architecture has been developed that opens possibilities for basic research of TLS hosted in thin films, bulk dielectrics, and at circuit surfaces. Results obtained with a first tested sensor prototype indicate substantial differences between TLS residing in thick amorphous dielectrics and those hosted in the qubits’ Josephson junction. On the other hand, methods were developed to perform TLS experiments with ready-made qubit samples, which are based on resonant TLS detection under application of elastic strain and dc-electric fields. Hereby, locations of detectable TLS in the circuit of an Xmon qubit  were determined, revealing a rather puzzling species of TLS that remained so far unidentified.
Alexander Bilmes , G. Weiß, A.V. Ustinov, J. Lisenfeld