Patent Description:
The present invention relates generally to quantum and classical digital superconducting circuits, and specifically to tri-stable storage loops for use in RQL circuits, that is, loops capable of stably holding currents representative of positive, negative, and zero states until a held state is affirmatively altered by one or more input signals.

In the field of digital logic, extensive use is made of well known and highly developed complimentary metal-oxide semiconductor (CMOS) technology. As CMOS has begun to approach maturity as a technology, there is an interest in alternatives that may lead to higher performance in terms of speed, power dissipation computational density, interconnect bandwidth, and the like. An alternative to CMOS technology comprises superconductor based single flux quantum circuitry, utilizing superconducting Josephson junctions (JJs), with typical signal power of around <NUM> nanowatts (nW), at a typical data rate of <NUM> gigabits per second (Gb/s) or greater, and operating temperatures of around <NUM> kelvins.

In the context of systems and circuits in the reciprocal quantum logic (RQL) family, a storage loop is a loop capable of holding a superconducting current representative of a state, stably, until the current in such loop, and thereby the represented state, is affirmatively altered by an input signal, as opposed to by, for example, ambient AC conditions present in a larger circuit of which the storage loop may be a constituent. <CIT> describes a single flux quantum circuit apparently based on RSFQ (Rapid Single Flux Quantum) technology rather than RQL.

Aspects of the invention are set out in the accompanying claims. One example includes a reciprocal quantum logic (RQL) tri-stable storage loop circuit. A control input line provides a control input to an input end of a storage loop in the circuit. A signal input line provides a signal input to an output end of the storage loop. An output line propagates an output single flux quantum (SFQ) pulse from the output end of the storage loop. The storage loop is made up of a control Josephson junction (JJ) at the input end, a logic JJ at the output end, and a storage inductor connecting the input end to the output end, and wherein the storage loop is configured to selectively apply any of positive, negative, or no bias to the logic JJ based on the storage loop storing positive current, negative current, or no current, respectively.

Another example includes 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: with a control input line, providing alternate positive and negative control inputs to an input end of a storage loop, the storage loops comprising a control Josephson junction JJ at the input end, a logic JJ at an output end, and a storage inductor connecting the input end to the output end, 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; with an output line propagating an output single flux quantum SFQ pulse from the output end of the storage loop; and with a signal input line, inputting a positive single flux quantum SFQ signal pulse to the output end of 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, the storage loop being configured to selectively apply any of a positive, negative, or no bias to the logic JJ based on the storage loop storing positive current, negative current, or no current, respectively.

This disclosure relates generally to logical circuits for use in reciprocal quantum logic (RQL) systems and related methods. This disclosure more specifically relates to an inductive storage loop that can be driven into any of three stable states via the interaction of signals at Josephson junctions (JJs) at both ends of the loop. The inductive storage loop described herein enables single flux quantum (SFQ) logic to selectively apply positive, negative, or no bias at one of the junctions.

<FIG> shows an example tri-stable loop <NUM>. Tri-stable loop <NUM> includes control input line <NUM> provided to an input node connecting control JJ <NUM> to storage inductor <NUM>. At the opposite end of storage inductor <NUM> is an output node to which logic JJ <NUM>, signal input line <NUM>, and output line <NUM> are connected. Thus, storage loop <NUM> is formed between a circuit ground, control JJ <NUM>, storage inductor <NUM>, logic JJ <NUM>, and the circuit ground. Input lines <NUM>, <NUM> and output line <NUM> can be connected to, for example, Josephson transmission lines (JTLs) (not shown) to propagate SFQ pulses into or out of storage loop <NUM>, respectively.

Loop <NUM> applies additional bias to logic JJ <NUM>, such that an SFQ signal applied along a signal input line <NUM> produces an output that is propagated on output line <NUM>. To accomplish this, control junction <NUM> is triggered to put an SFQ of current into storage loop <NUM>. This may be done via RQL-encoded SFQ pulses supplied along control input line <NUM>, or direct application of AC power supplied along control input line <NUM>.

The selections of component sizes in storage loop <NUM> provide a unidirectional data flow. For example, control JJ <NUM> can be sized large relative to logic JJ <NUM> and storage inductor <NUM> can be sized large relative to propagation-path inductances in input line JTLs (not shown) to make loop <NUM> stable regardless of surrounding AC bias conditions. Signal direction is thereby enforced in circuit <NUM>. As an example, an SFQ pulse provided on control input line <NUM> can place one Φ<NUM> of current into storage loop <NUM>. The magnitude of current through such a storage loop is determined by the size of storage inductor <NUM> in storage loop <NUM>. Thus, the inductance value of an input inductor (not shown) on control input line <NUM> can be small (e.g., between about <NUM> pH and <NUM> pH, e.g., <NUM> pH) in comparison to the inductance value of storage inductor <NUM>. On the other hand, storage inductor <NUM> can sized to be relatively large (e.g., between about <NUM> pH and <NUM> pH, e.g., <NUM> pH) (e.g., about four times larger than the aforementioned input inductor) to reduce the magnitude of the stored current induced by a control input SFQ pulse provided on control input line <NUM>. In some examples, the magnitude of a current introduced at control input line <NUM> is about four times larger than the current stored in storage loop <NUM>. Control JJ <NUM> is sized such that any driving JTL (not shown) connected to the control input line <NUM> is capable of flipping control JJ <NUM> to put current into storage loop <NUM>, but the current in the storage loop <NUM> is never sufficient to unflip control JJ <NUM> and allow the stored pulse to back out of control input line <NUM>.

In RQL circuits, any Josephson junction, the superconducting phase of which is representative of a logical state, triggers in an alternating fashion: positive, negative, positive, negative, etc. <FIG> shows, as a function of time, the current in storage inductor <NUM> as control junction <NUM> is triggered in this alternating fashion by currents provided along control input line <NUM> with no signal inputs applied along signal input line <NUM>. Each positive triggering <NUM>, <NUM> of control junction <NUM> puts one Φ<NUM> (about <NUM> mA-pH) worth of current into storage loop <NUM>, positively biasing logic junction <NUM>. Each negative triggering <NUM>, <NUM> removes this biasing current (i.e., setting it back to zero). In some examples, the signal to control input line <NUM> can be configured to cause one triggering pair (e.g., <NUM>, <NUM>) every AC clock cycle. In other examples, the applied current provided to control input line <NUM> could be present across multiple AC clock cycles.

<FIG> shows a plot similar to that of <FIG> but with the addition of the effect of signal inputs applied on signal input line <NUM>. As the result of control signals provided along control input line <NUM>, control junction <NUM> still alternately triggers positively <NUM>, <NUM>, <NUM>, <NUM> and negatively <NUM>, <NUM>, <NUM>, <NUM>. Any SFQ pulses input to circuit <NUM> via signal input line <NUM> during times when there is zero current in loop <NUM> are insufficient to trigger logic junction <NUM> on their own. However, such SFQ pulses are capable of triggering logic junction <NUM> with the additional bias provided by current in storage inductor <NUM>.

Initially, control junction <NUM> is only capable of applying positive bias or no bias to logic junction <NUM>, because, as shown in <FIG>. in absence of signal input, the current in loop <NUM> only varies between () and one Φ<NUM> worth of current. However, the positive triggering of logic junction <NUM> annihilates the current stored in storage loop <NUM> and removes this positive bias, as shown at point <NUM>. After this point <NUM>, the next triggering <NUM> of control junction <NUM> is negative and control junction <NUM> is now only capable of applying negative bias.

Subsequent triggerings <NUM>, <NUM>, <NUM> of control junction <NUM> switch the applied bias between zero and -Φ<NUM> until logic junction <NUM> is triggered negatively <NUM> by the combination of this bias and an applied negative SFQ pulse at signal input line <NUM>. This again annihilates the current in storage loop <NUM>, which then returns to the original state wherein control junction <NUM> once again can apply only positive bias or no bias.

In view of the above description, tri-stable storage loops of the type illustrated in <FIG> provide the ability to interrupt an alternating series of pulses coming from one RQL signal such that it can selectively alternate not just between a positive-current state and a no-current state, but can also reach a negative-current state as well. Although in the above-described examples of <FIG> a first triggering (e.g., <NUM> or <NUM>) of control junction <NUM> is assumed to be in the positive direction, circuit <NUM> functions equivalently when the first triggering of controller junction <NUM> is negative, with the signs of all described currents being reversed. Construction of gates providing some logic functions benefit in part count, efficiency, etc. from the ability of an RQL signal to apply positive, negative, or no bias current to a decision-making Josephson junction. Tri-stable loops of the type described herein accordingly provide the benefit over earlier designs in construction of such RQL gates. As examples, storage loop <NUM> can be used to create component-efficient D flip-flops, majority gates, AND gates, OR gates, AND-OR gates, NAND gates, and NOR gates, among others, compatible with RQL systems. In some examples, multiple storage loops can be combined such that the storage loops share a common logic junction that is triggered only upon appropriate biasing created by current stored in a plurality of, a majority of, or certain of the storage loops.

<FIG> shows method <NUM> 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. Alternate positive and negative control inputs are provided <NUM> 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. A positive single flux quantum (SFQ) signal pulse is input <NUM> to the storage loop during the positive state. A logic JJ in the storage loop triggers <NUM> in the positive direction, annihilating the current in the storage loop and returning <NUM> the storage loop to the null state. On the next negative control input, the storage loop is caused <NUM> to transition into a negative current storage state in which current circulates in the loop in a negative direction, whereupon subsequent control inputs alternate <NUM> the storage loop between the negative state and the null state.

<FIG> shows method <NUM> of altering a series of pulses from alternating between a negative-current state and a null-current state to alternating between a positive-current state and the null-current state, which can continue from method <NUM> shown in <FIG>. A negative SFQ signal pulse is input <NUM> to the storage loop during the negative state. The logic JJ in the storage loop negatively triggers <NUM> to annihilate the current in the storage loop and thereby return <NUM> the storage loop to the null state. On the next positive control input, the storage loop is caused <NUM> to transition into the positive state, whereupon subsequent control inputs alternate <NUM> the storage loop between the null state and the positive state.

Claim 1:
A reciprocal quantum logic RQL tri-stable storage loop circuit comprising:
a control input line (<NUM>) configured to provide a control input to an input end of a storage loop (<NUM>);
a signal input line (<NUM>) configured to provide a signal input to an output end of the storage loop (<NUM>); and
an output line (<NUM>) configured to propagate an output single flux quantum SFQ pulse from the output end of the storage loop (<NUM>);
wherein the storage loop (<NUM>) comprises:
a control Josephson junction JJ (<NUM>) at the input end;
a logic JJ (<NUM>) at the output end; and
a storage inductor (<NUM>) connecting the input end to the output end, and
wherein the storage loop (<NUM>) is configured to selectively apply any of positive, negative, or no bias to the logic JJ (<NUM>) based on the storage loop (<NUM>) storing positive current, negative current, or no current, respectively.