Phase retrapping in a [phi] Josephson junction: Onset of the butterfly effect

We investigate experimentally the retrapping of the phase in a [phi] Josephson junction upon return of the junction to the zero-voltage state. Since the Josephson energy profile U sub(0)([psi]) in [phi] JJ is a 2[pi] periodic double-well potential with minima at [psi]=+ or -[phi]mod2[pi], the question is at which of the two minima ?[phi] or +[phi] the phase will be trapped upon return from a finite voltage state during quasistatic decrease of the bias current (tilt of the potential). By measuring the relative population of two peaks in escape histograms, we determine the probability of phase trapping in the + or -[phi] wells for different temperatures. Our experimental results agree qualitatively with theoretical predictions. In particular, we observe an onset of the butterfly effect with an oscillating probability of trapping. Unexpectedly, this probability saturates at a value different from 50% at low temperatures.