Figure 1

Schematic (a) and optical image (b) of the three-junction SQUID consisting of a superconducting ring of inductance $L$ interrupted by two Josephson junctions in its left arm and one in its right arm. The SQUID is fed by a current $I$, an applied flux ${\Phi }_a$ tunes the operation point. The crosses symbolize the junctions; their maximum supercurrents are denoted $I_{0k}$, with $k=1,2.3$. Junctions 1 and 2 are shunted by a common resistor $R$; junction 3 is shunted separately by a resistor which, by design, has the same magnitude $R$. $J$ denotes the screening current around the ring.Reuse & Permissions

Figure 2

Characterization of the three-junction SQUID ratchet. (a) Measured IVC at ${\Phi }_a=0$ (open circles) together with the simulated curve (solid line). Dashed line: calculated IVC for ${\Phi }_a=0.25{\Phi }_0$. (b) $I_c$ vs ${\Phi }_a$. Open symbols: measurement; solid line: simulated curve. Relevant model parameters: $2I_0=29.9\mu \mathrm{A}$, $I_{0}R=16\mu \mathrm{V}$ ($f_c\approx 7.7\mathrm{GHz}$), $s=0.4$, and $\Gamma =0.09$. (c) $V_{dc}$ vs $I_{ac}$ for a $f=50\mathrm{kHz}$ monochromatic drive (open symbols, $f/f_c=6.5\times {10}^{-6}$) together with simulated curves. Solid markers: full numerical simulation, with $f/f_c=0.005$; solid line: approximate solution using the calculated IVC at ${\Phi }_a=0.25{\Phi }_0$. For comparison, the dashed line shows the asymmetry in dc voltages [$V(+I)+V(-I)$], as determined from the calculated IVC. This curve, plotted using the ordinate for $I$ and the abscissa for $V$, has been divided by 4 to become comparable in magnitude to the other curves.Reuse & Permissions

Figure 3

The response $V_{dc}$ of the three-junction SQUID ratchet to stochastic drives. (a) $V_{dc}$ vs ${\Phi }_a$ at fixed $I_{\mathrm{noise}}=1.1I_0$. (b) $V_{dc}$ vs $I_{\mathrm{noise}}/I_0$ at ${\Phi }_a=0.25{\Phi }_0$. Open black circles in both graphs: measured data, obtained with $f_{\mathrm{cut}}=50\mathrm{MHz}$. Solid black circles in both graphs: full numerical simulation, with $f_{\mathrm{cut}}=0.0065f_c$ (corresponding to the experimental value). Solid line in (b) approximated theoretical curve valid for $f_{\mathrm{cut}}/f_0\to 0$. Other curves in (b) series of full numeric simulations for 7 larger values of $f_{\mathrm{cut}}/f_0$ between 0.05 and 10.Reuse & Permissions