Quantitative study of quasiparticle traps using the single-Cooper-pair transistor

We use radio-frequency reflectometry to measure quasiparticle tunneling rates in the single-Cooper-pair transistor. Devices with and without quasiparticle traps in proximity to the island are studied. A ${10}^2$- to ${10}^3$-fold reduction in the quasiparticle tunneling rate onto the island is observed in the case of quasiparticle traps. In the quasiparticle trap samples we also measure a commensurate decrease in quasiparticle tunneling rate off the island.

Figure 1

(Color online) (a) Micrograph of a quasiparticle trap device with an island volume ${\mathrm{vol}}_i=750\mathrm{nm}\times 125\mathrm{nm}\times 7\mathrm{nm}$. Bright features are the normal metal traps $200–300\mathrm{nm}$ from SCPT junctions. (b) The rf circuit has a resonance frequency of $\sim 320\mathrm{MHz}$ and includes a chip inductor $(L=470\mathrm{nH})$, a parasitic capacitance $(C_p=0.53\mathrm{pF})$, and the SCPT. (c) Energy gap profile showing isolation of the leads from the normal metal bond pads.

Figure 2

(Color online) (a) Histogram of times spent in the even and odd states for a QT device, solid lines are fitted exponentials. Here $t_{odd}=3.1\mathrm{ms}$ and $t_{even}=964\mathrm{\mu }\mathrm{s}$. Inset: switching between even and odd states at $n_g=1$. (b) Upper: Averaged supercurrent oscillations. Lower: Energy band diagram of the SCPT at zero current bias. The quasiparticle bands (dashed line) are lifted by an energy $\delta \Delta$ creating a potential well of energy $\delta E$ relative to the lower Cooper pair band (solid line). (c) Allowed transitions of a quasiparticle including the relaxation rate to the bottom of the potential well on the island.

Figure 3

(Color online) Temperature dependence of (a) $t_{even}$ and (b) $t_{odd}$ for a QT device (triangles) and a NT device (squares). Solid lines are fits to the quasiparticle density in the leads (see text).

Figure 4

(Color online) (a) Values of $t_{odd}$ and $t_{even}$ plotted for QT (triangles) and NT devices (squares) at fridge base temperature. (b) The probability of having a quasiparticle on the island $\left(p_{odd}\right)$, determined from the time constants at this temperature. QT (NT) devices are indicated as filled (unfilled). The principal QT (NT) device is noted with an asterisk symbol (*).