What is Josephson junction?

Answer: C. A Josephson junction

A Josephson junction consists of two superconductors separated by a thin insulating film about 1–2 nm thick; a superconducting current flows through it without producing a voltage, and it acts as a lossless nonlinear inductor. A transmon is a resonant circuit that combines this junction with a capacitor. In an ordinary LC resonator made of a coil and a capacitor the energy levels are equally spaced, so the transition between |0⟩ and |1⟩ cannot be addressed on its own; the nonlinearity of the junction makes the spacing unequal, so the two lowest levels can be used as a qubit. The junction is usually made of aluminum and aluminum oxide formed by oxidizing its surface.

Answer: A. 4.8 GHz

The |1⟩→|2⟩ frequency is 5.0 GHz + (−0.2 GHz) = 4.8 GHz. Because of the nonlinearity of the Josephson junction, the level spacing gets slightly smaller for higher levels, so the |1⟩→|2⟩ frequency is lower. A 5.0 GHz microwave drive is detuned from the 4.8 GHz transition, so the qubit is less likely to leak into |2⟩, outside the qubit subspace. However, if the pulses are made too short, their frequency spread approaches 200 MHz and leakage to |2⟩ increases, so the pulse shape is carefully designed.