A particle under the action of a single applied force accelerates ballistically in accordance with Newton's second law. In the presence of a frictional force, an applied force will ultimately maintain a constant velocity rather than produce acceleration. Analogously, an externally applied voltage can ballistically accelerate the superfluid in a superconductor, leading to a supercurrent that grows with time; whereas a constant applied voltage in a resistive conductor merely maintains a constant current. This acceleration phase of the supercurrent lasts for a very brief period before resistive processes set in, making it difficult to observe in real time. The present work employed a measurement system that could simultaneously track and correlate current and voltage with subnanosecond timing accuracy, resulting in the first clear time-domain measurement of this transient phase where the quantum system displays a Newtonian like response. The technique opens doors for the controlled investigation of other time-dependent transport phenomena in condensed-matter systems. [This work was funded by the U. S. Department of Energy under Grant no. De-FG02-99ER45763 and the principal result was published in Phys. Rev. Lett. 102, 077001 (2009).]

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