RESUMEN
A scheme for measuring small intrinsic critical currents I(c) in nanoscale devices is described. Changes in Josephson inductance L(J) are converted to frequency variations that are recorded via microwave reflection measurements at 700-800 MHz. The critical current is determined from the frequency shift of the reflection magnitude at zero phase bias assuming a sinusoidal current-phase relation. The method is used to study a multiwalled carbon nanotube transistor with Pd/Nb contacts inside a resistive on-chip environment. We observe gate-tunable critical currents up to I(c) â¼ 8 nA corresponding to L(J) > 40 nH. The method presented is also applicable to devices shunted by closed superconducting loops.
RESUMEN
We have performed microwave reflection experiments on a charge-phase qubit coupled to an LC oscillator. We find that the system behaves like an artificial molecule showing vibronic sideband transitions. The reflected signal is determined by a combination of the Franck-Condon principle and resolved-sideband cooling or heating of the oscillator.
RESUMEN
Landau-Zener (LZ) tunneling can occur with a certain probability when crossing energy levels of a quantum two-level system are swept across the minimum energy separation. Here we present experimental evidence of quantum interference effects in solid-state LZ tunneling. We used a Cooper-pair box qubit where the LZ tunneling occurs at the charge degeneracy. By employing a weak nondemolition monitoring, we observe interference between consecutive LZ-tunneling events; we find that the average level occupancies depend on the dynamical phase. The system's unusually strong linear response is explained by interband relaxation. Our interferometer can be used as a high-resolution Mach-Zehnder-type detector for phase and charge.
