15 research outputs found
Identification of different types of high-frequency defects in superconducting qubits
Parasitic two-level-system (TLS) defects are one of the major factors
limiting the coherence times of superconducting qubits. Although there has been
significant progress in characterizing basic parameters of TLS defects, exact
mechanisms of interactions between a qubit and various types of TLS defects
remained largely unexplored due to the lack of experimental techniques able to
probe the form of qubit-defect couplings. Here we present an experimental
method of TLS defect spectroscopy using a strong qubit drive that allowed us to
distinguish between various types of qubit-defect interactions. By applying
this method to a capacitively shunted flux qubit, we detected a rare type of
TLS defects with a nonlinear qubit-defect coupling due to critical-current
fluctuations, as well as conventional TLS defects with a linear coupling to the
qubit caused by charge fluctuations. The presented approach could become the
routine method for high-frequency defect inspection and quality control in
superconducting qubit fabrication, providing essential feedback for fabrication
process optimization. The reported method is a powerful tool to uniquely
identify the type of noise fluctuations caused by TLS defects, enabling the
development of realistic noise models relevant to fault-tolerant quantum
control
Submicrometer-scale temperature sensing using quantum coherence of a superconducting qubit
Interest is growing in the development of quantum sensing based on the principles of quantum mechanics, such as discrete energy levels, quantum superposition, and quantum entanglement. Superconducting flux qubits are quantum two-level systems whose energy is sensitive to a magnetic field. Therefore, they can be used as high-sensitivity magnetic field sensors that detect the magnetization of a spin ensemble. Since the magnetization depends on temperature and the magnetic field, the temperature can be determined by measuring the magnetization using the flux qubit. In this study, we demonstrated highly sensitive temperature sensing with high spatial resolution as an application of a magnetic field sensor using the quantum coherence of a superconducting flux qubit. By using a superconducting flux qubit to detect the temperature dependence of the polarization ratio of electron spins in nano-diamond particles, we succeeded in measuring the temperature with a sensitivity of 1.3 µ Kµ at T  = 9.1 mK in the submicrometer range