GB1502690A - Josephson junction storage cells - Google Patents

Abstract

1502690 Superconductive devices INTERNATIONAL BUSINESS MACHINES CORP 12 March 1976 [13 March 1975] 10081/76 Heading H3X In a data storage cell comprising at least one Josephson junction in which binary data is selectively represented by 0 and 1 flux quanta modes, the cell is arranged to exhibit an undercritically damped switching characteristic so that non-destructive read out of stored information is obtained by the ability to switch from an 0 quantum mode to a 2 quantum mode, followed by reversion to the 0 quantum mode, and the inability of a switching operation to affect a 1 quantum mode. Storage cells using vortex modes in long Josephson junctions. An interferometer type storage cell is shown in Fig. 2 which comprises a lower electrode metallization M2 deposited on a substrate, a thin oxide layer in the shaded area, and upper electrode metallization M3. In a two co-ordinate storage arrangement, Fig. 5 (not shown), writing is effected during the period when a control current I B aux is applied to an auxiliary line coupled to the junctions to decrease the maximum Josephson current. To switch a storage cell from an 0 to a 1 mode, a bit current is also applied to a control line coupled to inductive region of the cell, followed by an overlapping write current I w . For non-destructive readout producing a mode change only when an 0 mode is present, the overlapping sequence of bit and word currents I b and I w alone are used, the sequentially applied currents being of such magnitude as to circumvent a point x, Fig. 3, defining the minimum boundary of the 0 mode and falling within the shaded area of the mode characteristic shown. A switch to the 2 mode then occurs and this vortex mode is sustained until the currents are terminated. Reversion to the 0 mode then occurs. An alternative form of cell using vortex modes is described, Fig. 6 (not shown). Storage cells comprising a single Josephson junction shunted by an inductance. A suitable cell is shown in Fig. 10 which comprises metallized deposits M2, M3 with an intervening oxide layer forming a Josephson junction 38, the deposits also being connected at 40 to provide an inductive loop L. Lines for currents I B and I B aux are coupled to the inductive loop and the junction, respectively. No flux quantum is trapped within the inductance for an 0 state of the storage cell, while a trapped flux quantum is present for a 1 state. Writing and reading non-destructively is effected by the same programme of I B , I w and I B aux currents as previously described, the underdamped characteristic of the cell enabling a transition to an unstable 2 state for non-destructive readout. Sense circuits, Figs. 7, 8 and 9 (not shown). These are as described in Specification 1,450,343.