Punchthrough in a three-junction SQUID

Josephson latching logic devices can be powered by a bipolar trapezoidal waveform. Ideally, switched devices will always reset as the power supply reverses polarity. Under certain conditions, however, punchthrough can occur: a switched logic device fails to reset and proceeds into the next cycle in the nonzero voltage state. Original theory and experiment held that the punchthrough probability P should be identically zero provided the power supply transition time τ is greater than some critical value. The present experiment on a three-junction superconducting quantum interference device (SQUID) demonstrates, as more recent theories and simulations predict, that there is an exponential tail of P(τ) rather than an abrupt cutoff. We also find P can be strongly modulated by a small dc in the control line over the SQUID.