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Josephson Soliton Combinatorial Logic

IP.com Disclosure Number: IPCOM000041965D
Original Publication Date: 1984-Mar-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 3 page(s) / 54K

Publishing Venue

IBM

Related People

Kaplan, SB: AUTHOR [+2]

Abstract

Recording the passage of solitons in combinatorial logic as changes in the circulating currents of Superconducting Quantum Interference Devices (SQUIDs) in the logic devices solves the timing difficulty in performing combinatorial logical functions with soliton pulses. It is difficult to perform logical operations with pulsed logic signals. As an example, consider a 2-input AND gate: two pulses must be present in order to insure the correct AND operation, but it is very difficult to insure the simultaneous presence of two extremely short pulses. For this reason proposed logical operations with Josephson solitons have been limited to Programmable Logic Arrays (PLAs) in which soliton pulses are steered with logic levels. However, such simple arrays offer only two-stage logic.

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Josephson Soliton Combinatorial Logic

Recording the passage of solitons in combinatorial logic as changes in the circulating currents of Superconducting Quantum Interference Devices (SQUIDs) in the logic devices solves the timing difficulty in performing combinatorial logical functions with soliton pulses. It is difficult to perform logical operations with pulsed logic signals. As an example, consider a 2-input AND gate: two pulses must be present in order to insure the correct AND operation, but it is very difficult to insure the simultaneous presence of two extremely short pulses. For this reason proposed logical operations with Josephson solitons have been limited to Programmable Logic Arrays (PLAs) in which soliton pulses are steered with logic levels. However, such simple arrays offer only two-stage logic. In order to obtain a more powerful array, feedback from the outputs back to the inputs must be done with latches and combinatorial logic. The passage of a soliton through a Josephson transmission line can be recorded as a current level in a SQUID. Consider the device illustrated in Fig. 1: the device consists of two Josephson transmission lines (JTLs) connected by an inductor L. The shaded regions are isolation resistors (NN is in the top electrodes of the JTLs. The groundplane forms the bottom electrode of the junctions, and the inductor connects the top electrodes of sections 2 and 5. Sections 2 and 5 along with inductor L form a two junction SQUID; control line Ic is coupled to the SQUID. The LI0 product of the SQUID is less than I0/2 to avoid flux trapping. The SQUID and control line operate as the functional elements in a soliton bias gate. If Ic is turned on, a positive bias will appear in section 2 and a negative bias in section
5. If sections 1, 3, 4 and 5 are positively biased and a soliton enters section 1, the soliton will propagate through section 2 to section 3. As it passes through section 2 the SQUID will switch so that the circulating current will be reversed. Now there will be a negative bias in section 2 and a positive bias in section 5 -- the passage of the soliton has been recorded in this manner. If a second soliton enters section 1, it will be stopped and dissipated since section 2 will have a negative bias. The SQUID will not be switched by this soliton. When the control current is turned off, solitons are released into each of the lines and the SQUID resets. AND Gate The basic layout of the AND Gate is sketched in Fig. 2. The drawing is a top view of Josephson transmission lines: the shaded areas are isolation resistors in the top electrodes of the JTLs; the groundplane is the bottom electrode of the junctions; and inductors connect the top electrodes of sections 2 and 9 and of sections 4 and 7. These have Li0<I0/2. Control current Ic is coupled to L1 and L2, as shown. Sections 1, 3, 5, 6, 8 and 10 are positively biased (NN .4I0 At the beginning of a logic cycle control current Ic is turned on. This...