In the design and investigation of superconducting qubits and related devices, a lumped element circuit model is the standard theoretical approach. However, many important physicalquestions lie beyond the scope of this approach, such as the consequences of very strong or otherwise unconventional Josephson junctions, the properties of small qubit devices, and the number of entangled electrons in superconducting Schrodinger cats. By performing gauge transformations on self-consistent solutions of the Bogoliubov-de Gennes equations, we develop here a formalism that is capable of addressing these questions. We then apply the formalism to a charge qubit and to an RF squid qubit. This theory provides a promising tool to accompany the remarkable experimental achievements driving superconducting qubits forward.
We analyze the charge-noise induced coherence time T2 of the fluxonium qubit as a function of the number of array junctions in the device, N. The pure dephasing rate decreases withN, but we find that the relaxation rate increases, so T2 achieves an optimum as a function of N. This optimum can be much smaller than the number typically chosen in experiments, yielding a route to improved fluxonium coherence and simplified device fabrication at the same time.
The Leggett-Garg inequality holds for any macrorealistic system that is being measured noninvasively. A violation of the inequality can signal that a system does not conform to ourprimal intuition about the physical world. Alternatively, a violation can simply indicate that „clumsy“ experimental technique led to invasive measurements. Here, we consider a recent Leggett-Garg test designed to try to rule out the mundane second possibility. We tailor this Leggett-Garg test to the IBM 5Q Quantum Experience system and find compelling evidence that qubit Q2 of the system cannot be described by noninvasive macrorealism.