Double-Dot Charge Qubit and Transport via Dissipative Cotunneling

We investigate transport through an exotic charge qubit composed of two strongly capacitively coupled quantum dots, each being independently connected to a side gate which in general exhibits a fluctuating electrostatic field (i.e., Johnson-Nyquist noise). Two quantum phases are found: the “Kondo” phase where an orbital-Kondo entanglement emerges and a “local moment” phase in which the noise destroys the Kondo effect leaving the orbital spin unscreened and resulting in a clear suppression of the conductance. In the Kondo realm, the transfer of charge across the setting is accompanied by zero-point charge fluctuations in the two dissipative environments and then the $I\mathrm{\text{−}}V$ characteristics are governed by what we call “dissipative cotunneling.”

Figure 1

Left: Schematic setup of a double-QD charge qubit being connected in series to reservoir leads. The two QDs are capacitively coupled to side gates and fluctuations of the gate voltages are included as a result of the finite resistances $R_{1,2}=Z_{1,2}(\omega =0)$. Right: Honeycomb pattern of the charging energy for $C_m\gg C_{gi}$, where $i=1,2$. Our concern is close to the degeneracy point $n_{g1}=n_{g2}=1/2$.

Figure 2

Normalized currents $I_L/c_L$ and $I_S/c_S$ as a function of the dimensionless bias voltage $eV/{\omega }_c$ ($\hslash =1$) in the Kondo regime. Curves from top to bottom correspond to $g/2={\alpha }_1+{\alpha }_2=0,0.1,0.2,0.5$. The inset is a log-log plot of the conductance for large dots (solid line) and for small dots (dashed line) versus $|t^{\prime }|$ in the Kondo regime in the specific situation where $g=0$ and $T_{KL}\gg T_K^{(\mathrm{s}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{l})}(g)$ $(|t|/D_0=0.0001)$.