Tri-stable storage loops

A method of altering a series of pulses from alternating between a positive-current state and a null-current state to alternating between a negative-current state and the null-current state, the method comprising:

• providing alternate positive and negative control inputs to a storage loop in a reciprocal quantum logic (RQL) system to alternate the storage loop between
• a positive current storage state in which current circulates in the loop in a positive direction and
• a null current storage state in which essentially no current circulates in the loop;
• and
• inputting a positive single flux quantum (SFQ) signal pulse to the storage loop during the positive state to
• return the storage loop to the null state and subsequently to
• cause the storage loop to transition, on the next negative control input, into a negative current storage state in which current circulates in the loop in a negative direction, whereupon the control inputs alternate the storage loop between the negative state and the null state.

A circuit comprising:

• a control input line connected to an input node;
• a control Josephson junction (JJ) connected between a circuit ground and the input node;
• a storage inductor connected between the input node and an output node;
• a logic JJ connected between the circuit ground and the output node, the control JJ, storage inductor, and logic JJ forming a storage loop;
• a signal input line connected to the output node;
• and
• an output line connected to the output node;
• wherein the control JJ and storage inductor are sized to provide unidirectional flow of control inputs provided via the control input line;
• wherein the circuit is configured such that, in the absence of input on the signal input line, the storage loop has two stable states including a first stable state wherein essentially no current flows in the storage loop and a second stable state wherein one Φ0 worth of current flows in the storage loop, and
• wherein the circuit is configured such that, upon provision of a positive SFQ pulse to the signal input line, the storage loop obtains a third stable state wherein −Φ0 worth of current flows in the storage loop.