Radio-frequency based monitoring of small supercurrents

Abstract

We consider the applicability of the established rf readout technique, which allows to obtain the supercurrent-phase relation of a Josephson element from impedance measurements in the phase-biased regime. In experiments on Nb-based single and double tunnel junctions, we demonstrate that this method holds even if the Josephson coupling energy is smaller than the thermal energy. Compared with conventional current–voltage measurements, we evaluate the rf technique to be favorable in particular for investigations of small supercurrents in low-capacitance Josephson elements.

Multiphoton transitions between energy levels in a phase-biased Cooper-pair box

“…The current in a qubit ring is probed by the tank circuit, which is weakly coupled through a mutual inductance M to the PBCQ 23,24 . The PBCQ is characterized by the inductance L P BCQ = L + L J , where L is the ring's inductance and L J is an inductance defined by L −1 J = (2e/ ) ∂I/∂δ.…”

“…The qubit was coupled to a high quality tank circuit 15 or nonresonantly to a single mode of the electromagnetic field of a superconducting on-chip resonator 16,17 . Multiphoton transitions between energy levels in superconducting devices were studied in several articles 18,19,20,21,22 ; in this work we present both the experimental observation and the theoretical description of the multiphoton transitions between the ground and the first excited state in the phase-biased charge qubit 6,8,9,10 making use of the impedance measurement technique 23,24 . We begin in Sec.…”

Observation of the second harmonic in superconducting current-phase relation of Nb/Au/(001)YBa 2 Cu 3 O x heterojunctions

“…Moreover, I s (ϕ) is measurable even if the thermal energy exceeds the Josephson coupling energy. In fact, critical currents down to 50 nA were recently detected at T = 4.2 K [17]. For the junction SQ10, β varied between 0.4 and 0.27 for temperatures in the range T = 1.7 ÷ 6.0 K. Since β < 1, we can extract the CPR from the α(ϕ e ) dependence for the complete phase range (see fig.…”

Dynamic behavior of a superconducting flux qubit excited by a series of electromagnetic pulses

“…Next we compare the theoretically (dashed) and numerically (marked by points) calculated curves for the phase shift a calculated under the assumption (12) from the (14) and (15). For the numerical calculations we use the following parameters of the pulse: T = 0 5 .…”

“…The impedance measurement technique [13,14] consists in that the tank circuit probes the effective inductance of the system via measuring the phase shift a between the voltage and current in the tank circuit. The phase shift a is related to the Josephson inductance L of the qubit as follows:…”