The switching current distributions P(I) with different sweep rates are obtained in microwave-driven current-biased Josephson tunnel junctions. We observe the resonant peak caused by microwave-assisted tunneling in P(I). By measuring the magnitude of the microwave resonant peak as a function of the sweep rate, we develop a novel method of extracting the energy relaxation time T1 of the junction. With this simple method, it is determined that T1 of a Nb/AlOx/Nb Josephson junction is approximately 0.5 µs.
Quantum information processing using superconducting qubit based on Josephson junction has become one of the most promising candidates for possible realization of a quantum computer. In the heart of the qubit circuits, the superconducting microwave resonator plays a key role in quantum operations and measurements, which enables single-photon level microwave quantum optics. During last decade, the coherence time, or the lifetime of the quantum state, of the superconducting qubit has been dramatically improved. Among several technological innovations, the improvement of superconducting microwave resonator's quality has been the main driving force in getting the qubit performance almost ready for elementary quantum computing architecture. In this paper, I will briefly review very recent progresses of the superconducting microwave resonators especially aimed for quantum device applications during the last decade. The progresses have been driven by ingenious circuit design, material improvement, and new measurement techniques. Even a rather radical idea of three-dimensional large resonators have been successfully implemented in a qubit circuit. All those efforts contributed to our understanding of the qubit decoherence mechanism and as a result to the improvement of qubit performance.
We analyze the effect of dissipation and low-frequency current noise on quantum coherence in a current-biased Josephson junction that has low damping. Developing a stochastic Bloch equation for the reduced density matrix of the system, we determine the resonance response of the system to a weak external microwave current drive. We characterize the response by finding the spectroscopic coherence time
T2∗ as a function of the energy relaxation time
T1 and the current noise power spectral density. For current noise with a constant spectral density up to a frequency ${f}_{c}⪢{(2\ensuremath{\pi}{T}_{1})}^{\ensuremath{-}1}$, we find that the resonance broadening is proportional to the noise spectral density, while for ${f}_{c}⪡{(2\ensuremath{\pi}{T}_{1})}^{\ensuremath{-}1}$, the broadening is proportional to the rms current noise. We compare our results to microwave spectroscopy data on $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}$ Nb and Al tunnel junctions at milli-Kelvin temperatures and find good agreement between the measured response spectra and our model simulations.