Adiabatic Quantum Pumping at the Josephson Frequency

We analyze theoretically adiabatic quantum pumping through a normal conductor that couples the normal regions of two superconductor–normal-metal–superconductor Josephson junctions. By using the phases of the superconducting order parameter in the superconducting contacts as pumping parameters, we demonstrate that a nonzero pumped charge can flow through the device. The device exploits the evolution of the superconducting phases due to the ac Josephson effect, and can therefore be operated at very high frequency, resulting in a pumped current as large as a few nanoamperes. The experimental relevance of our calculations is discussed.

Figure 1

(a) Layout of the proposed quantum pump, consisting of two SNS Josephson junctions in a SQUID geometry with a common normal region (The dark gray areas represent the superconductor electrodes and light gray the normal conductor). (b) Diagramatic representation of the scattering matrix of the device.

Figure 2

Pumped charge per cycle $Q_P$ as a function of phase difference $\phi$ between the pumping parameters, for the case $a=b=\epsilon =1/2$ and different values of $k_{F}L$.

Figure 3

Color scale plots of $Q_P$ as a function of $k_{F}L$ and $\phi$ for three distinct cases of amplitude scattering on the beam splitter: $\epsilon =1/8$ for $a>b$ (top), $a=b=\epsilon =1/2$ (center), and $\epsilon =1/8$ for $b>a$ (bottom). The dashed lines in the center panel correspond to the curves shown in Fig. 2.