  A macroscopic fluid pump works according to the law of Newtonian mechanics
and transfers a large number of molecules per cycle (of the order of 10^23). By
contrast, a nano-scale charge pump can be thought as the ultimate
miniaturization of a pump, with its operation being subject to quantum
mechanics and with only few electrons or even fractions of electrons transfered
per cycle. It generates a direct current in the absence of an applied voltage
exploiting the time-dependence of some properties of a nano-scale conductor.
The idea of pumping in nanostructures was discussed theoretically a few decades
ago [1-4]. So far, nano-scale pumps have been realised only in system
exhibiting strong Coulombic effects [5-12], whereas evidence for pumping in the
absence of Coulomb-blockade has been elusive. A pioneering experiment by
Switkes et al. [13] evidenced the difficulty of modulating in time the
properties of an open mesoscopic conductor at cryogenic temperatures without
generating undesired bias voltages due to stray capacitances [14,15]. One
possible solution to this problem is to use the ac Josephson effect to induce
periodically time-dependent Andreev-reflection amplitudes in a hybrid
normal-superconducting system [16]. Here we report the experimental detection
of charge flow in an unbiased InAs nanowire (NW) embedded in a superconducting
quantum interference device (SQUID). In this system, pumping may occur via the
cyclic modulation of the phase of the order parameter of different
superconducting electrodes. The symmetry of the current with respect to the
enclosed magnetic flux [17,18] and bias SQUID current is a discriminating
signature of pumping. Currents exceeding 20 pA are measured at 250 mK, and
exhibit symmetries compatible with a pumping mechanism in this setup which
realizes a Josephson quantum electron pump (JQEP).
