ac Josephson Effect and Resonant Cooper Pair Tunneling Emission of a Single Cooper Pair Transistor

We measure the high-frequency emission of a single Cooper pair transistor (SCPT) in the regime where transport is only due to tunneling of Cooper pairs. This is achieved by coupling on chip the SCPT to a superconductor-insulator-superconductor junction and by measuring the photon assisted tunneling current of quasiparticles across the junction. This technique allows a direct detection of the ac Josephson effect of the SCPT and provides evidence of Landau-Zener transitions for proper gate voltage. The emission in the regime of resonant Cooper pair tunneling is also investigated. It is interpreted in terms of transitions between charge states coupled by the Josephson effect.

Figure 1

(a) Schematic picture of the SCPT coupled to a Josephson junction and the on chip circuit ($R=750\Omega$, $C_C\approx 750\mathrm{fF}$). (b) Josephson branch of the SCPT at $C_G{V}_G/e=1$ and 0 (multiplied by 10). Upper inset: $I(V)$ characteristic of the SCPT at high bias showing the Josephson-quasiparticle peak (JQP) and the quasiparticle (QP) tunneling. Lower inset: SEM picture of the SCPT.

Figure 2

(a) Derivative of the PAT current of the detector vs $V_B$ of the SCPT ($d{I}_{\mathrm{PAT}}/d{V}_B$) (or a Josephson junction for the upper panel, noted TJ) for $C_G{V}_G/e=0$ and 1 (lower panel). The dashed line $J$ (LZ) indicates the slope of the Josephson (Landau-Zener) effect. The solid curves are $d{I}_{\mathrm{PAT}}/d{V}_B$ at $V_B=45\mu \mathrm{V}$, with the scale in $\mathrm{pA}/\mu \mathrm{V}$ indicated by the arrow on the curve and the zero line shown by the dotted line. (b) Voltage power spectrum of the ac Josephson effect extracted from the curves shown on A for a Josephson junction (TJ) and for the SCPT at $C_G{V}_G/e=0$ and 1. The curves are shifted for clarity. (c) Sketch of the phase dependence of the two lowest energy levels of the SCPT at $C_G{V}_G/e=1$ and illustration of the Landau-Zener (LZ) and Josephson ($J$) effect.

Figure 3

(a) Differential conductance of the SCPT. The resonances 1 and 3 (see text) are indicated on the plot. (b) Energy levels of the SCPT at different bias voltage. The arrows indicate transitions between states resulting from the coupling by the Josephson effect of charge states. The green rectangles represent degenerate states coupled by the Josephson term.

Figure 4

Derivative of the PAT current of the detector vs bias voltage $V_B$ of the SCPT for different gate voltage, indicated on each plot. The dashed lines indicate the transitions predicted by the theory of RCPT (see text). The dashed line noted $J$ correspond to ac Josephson effect. The color scale is not the same for all plots.