Very High Frequency Spectroscopy and Tuning of a Single-Cooper-Pair Transistor with an On-Chip Generator

A single-Cooper-pair transistor (SCPT) is coupled capacitively to a voltage biased Josephson junction, used as a high-frequency generator. Thanks to the high energy of photons generated by the Josephson junction, transitions between energy levels, not limited to the first two levels, were induced and the effect of this irradiation on the dc Josephson current of the SCPT was measured. The phase and gate bias dependence of energy levels of the SCPT at high energy is probed. Because the energies of photons can be higher than the superconducting gap we can induce not only transfer of Cooper pairs but also transfer of quasiparticles through the island of the SCPT, thus controlling the poisoning of the SCPT. This can both decrease and increase the average Josephson energy of the SCPT: its supercurrent is then controlled by high-frequency irradiation.

Figure 1

(a) Sketch 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 $C_G{V}_G/e=0$ (multiplied by 10). Upper inset: $I(V)$ characteristic of the SCPT at high bias showing the JQP peak and the QP tunneling. Lower inset: SEM picture of the SCPT.

Figure 2

(a) Upper panels: Josephson branch of the SCPT at $C_G{V}_G/e=1$ (left panel) and $C_G{V}_G/e=0$ (right panel) and different voltage $V_S$ of the source, i.e., different Josephson frequencies. Lower panel: derivative of the voltage across the SCPT when current biased at $I=80\mathrm{pA}$ vs $V_S$ (see text) under HF irradiation for $C_G{V}_G/e=1$ and $C_G{V}_G/e=0$. The arrows correspond to voltage $V_S$ shown in the upper panels. The curves are shifted for clarity, the dashed line indicates no signal. (b) Variation of $V_B$ in logarithmic scale at small current (see text) under HF irradiation and for different values of gate voltage. The two vertical dashed lines correspond to the curves shown on (a), lower panel. The colored line on the left part is a guide to the eye.

Figure 3

(a) Calculated spectrum of the SCPT with parameters $E_C=65\mu \mathrm{V}$ and $E_J=28\mu \mathrm{V}$. The colored arrows represent transitions induced by HF irradiation, which involve the transfer of a CP to the island (red arrow) or from the island (red dashed arrow), or the transfer of a QP to the island (green arrow) or from the island (green dashed arrow). (b) Predicted gate dependence of the frequencies at which a change in the averaged phase dependence (see text) of the SCPT is expected, involving CP (black), QP (gray) tunneling or presence of an unpaired electron on the island (red). Poisoning is indicated on the figure. (c) Sketch of the QP density of states in the contacts and the energy of charge states in the SCPT at two gate values. The arrows illustrate the different photo-assisted tunneling processes possible.