Flux switch system

One example includes a flux switch system. The system includes an input stage configured to provide an interrogation pulse. The system also includes a plurality of flux loops configured to receive an input current. Each of the flux loops includes a Josephson junction configured to trigger to generate an output pulse in response to a first polarity of the input current and to not trigger to generate no output pulse in response to a second polarity of the input current opposite the first polarity. The system further includes an output stage configured to propagate the output pulse to an output of the flux switch system.

TECHNICAL FIELD

This disclosure relates generally to classical and superconducting computing systems, and more specifically to a flux switch system.

BACKGROUND

In a variety of different types of superconducting circuits, control currents are typically implemented to control data states. For example, currents can be inductively provided to superconducting circuits via inductive couplings to induce a flux in control loops to control data. A variety of different types of superconducting circuits implement such a control current, such as a superconducting quantum interference device (SQUID), a quantum flux parametron (QFP), or a variety of other devices. The control current that is provided to a superconducting circuit or a portion of a superconducting circuit may be unknown as to the current direction. Therefore, additional circuits can be implemented to query the sign of a control current to indicate the direction of current flow of the control current.

SUMMARY

One example includes a flux switch system. The system includes an input stage configured to provide an interrogation pulse. The system also includes a plurality of flux loops configured to receive an input current. Each of the flux loops includes a Josephson junction configured to trigger to generate an output pulse in response to a first polarity of the input current and to not trigger to generate no output pulse in response to a second polarity of the input current opposite the first polarity. The system further includes an output stage configured to propagate the output pulse to an output of the flux switch system.

Another example includes a method for determining a polarity of an input current. The method includes providing a bias current to each of a plurality of flux loops to provide a current in each of the flux loops. Each of the flux loops includes a Josephson junction. The method also includes inductively providing the input current to each of the flux loops and providing an interrogation pulse to the flux loops via an input stage. The method also includes determining that the input current has a first polarity in response to an output pulse being provided from an output stage resulting from a triggering of the Josephson junction of each of the flux loops in response to the interrogation pulse and based on the currents in each of the flux loops. The method further includes determining that the input current has a second polarity opposite the first polarity in response to no output pulse being provided from the output stage.

Another example includes a flux switch system. The system includes an input stage configured to provide a reciprocal quantum logic (RQL) interrogation pulse. The system also includes a first flux loop comprising a first inductor that is inductively coupled to an input current and a first Josephson junction. Each of the first inductor and the first Josephson junction are coupled to the input stage and the first flux loop is configured to conduct a first current in response to a bias current. The system also includes a second flux loop comprising a second inductor that is inductively coupled to the input current and a second Josephson junction. The second flux loop is configured to conduct a second current in response to the bias current. The first and second Josephson junctions can be configured to trigger to generate an RQL output pulse in response to a first polarity of the input current and the RQL interrogation pulse based on a relative of the first and second currents, and can be configured to not trigger in response to a second polarity of the input current and the RQL interrogation pulse. The system further includes an output stage coupled to the second inductor and the second Josephson junction. The output stage can be configured to propagate the RQL output pulse to an output of the flux switch system.

DETAILED DESCRIPTION

This disclosure relates generally to classical and superconducting computing systems, and more specifically to a flux switch system. The flux switch system can be configured as an analog-to-digital converter (ADC) to provide an indication of a sign of a current, such as in a superconducting circuit system. For example, the ADC can be a single-bit ADC that is configured to provide an output pulse (e.g., reciprocal quantum logic (RQL) pulse) in response to the current having a first current direction and to not provide an output pulse in response to the current having a second current direction. For example, the flux switch system can be provided an interrogation pulse (e.g., an RQL pulse) to interrogate the current direction of the current.

The flux switch system includes an input stage, an output stage, and a plurality of flux loops. The input stage is configured to receive an interrogation pulse (e.g., an RQL pulse) that can be provided to interrogate the sign of the input current to be monitored. The input current can be provided to the plurality of flux loops, such that the input current is inductively coupled to the flux loops to induce a current in the flux loops. As an example, the flux loops can include a first flux loop and a second flux loop that each receive a bias current that provides first and second currents that circulate in opposite orientations in the flux loops. Therefore, the first and second currents provide a steady-state equal and opposite amplitude with respect to a flux in the respective flux loops. As an example, the flux loops can each include a Josephson junction and an inductor that receives the induced current from the input current.

In response to the input current having a first direction, the input current can induce currents in a first direction to increase the amplitude of the first current and decrease the amplitude of the second current. The increase in amplitude of the first current can increase the bias of the Josephson junction associated with the first flux loop, such that the Josephson junction can trigger in response to the interrogation pulse. The triggering of the Josephson junction of the first flux loop can thus result in triggering of the Josephson junction of the second flux loop, and thus to provide an output pulse (e.g., an RQL pulse) from the output stage that is coupled to the second flux loop to indicate that the input current has the first current direction. However, in response to the input current having a second direction opposite the first direction, the input current can induce currents in a second direction opposite the first direction to decrease the amplitude of the first current and to increase the amplitude of the second current. The decrease in amplitude of the first current can decrease the bias of the Josephson junction associated with the first flux loop. As a result, the Josephson junction of the first flux loop does not trigger in response to the interrogation pulse. Therefore, no pulse is provided from the output stage to indicate that the input current has the second current direction.

FIG. 1illustrates an example of a flux switch system10. The flux switch system10can be implemented in any of a variety of superconducting computer systems to determine a sign (e.g., direction) of an input current IINin response to an interrogation pulse INTIN(e.g., a reciprocal quantum logic (RQL) pulse). For example, the flux switch system10can be configured as an analog-to-digital converter (ADC) to provide an indication of a sign of the input current IIN. For example, the flux switch system10can be a single-bit ADC that is configured to provide an output pulse INTOUT(e.g., an RQL pulse) in response to the input current IINhaving a first current direction and to not provide the output pulse INTOUTin response to the input current IINhaving a second current direction.

The flux switch system10includes an input stage12, a plurality of flux loops14, and an output stage16. The input stage12can be configured as a Josephson transmission line (JTL) that can be configured to propagate the interrogation pulse INTIN. For example, the interrogation pulse INTINcan be an RQL pulse, such that the input stage12can be biased via an RQL clock signal to receive the interrogation pulse INTINand to propagate the interrogation pulse INTINto the flux loops14. As an example, the interrogation pulse INTINcan be provided to the flux switch system10to interrogate the state of the input current IIN, such that the interrogation pulse INTINcan be provided at a time in which the input current IINhas a non-zero amplitude.

The flux loops14are demonstrated in the example ofFIG. 1as receiving both the input current IINand a bias signal BIAS. For example, the flux loops14can include a first flux loop and a second flux loop that each include an inductor and a Josephson junction. As an example, the input current IINcan be inductively coupled to each of the flux loops14, such as via the inductor associated with each of the first and second flux loops14. The bias signal BIAS can be provided to each of the flux loops14to provide first and second currents that circulate in opposite orientations in the respective flux loops14. Therefore, the first and second currents provide a steady-state equal and opposite amplitude with respect to a flux in the respective flux loops14. As described herein, the first and second currents can have amplitudes that are modified by the inductive coupling of the input current IINto adjust a bias of the respective Josephson junctions of the flux loops14.

The output stage16is coupled to a last of the flux loops14(e.g., the second flux loop14) and is configured to propagate an output pulse INTOUTin response to the interrogation pulse INTINand a first current direction of the input current IIN, and to not propagate the output pulse INTOUTin response to the interrogation pulse and a second current direction of the input current IIN. For example, in response to the input current IINhaving the first current direction, the input current IINcan induce currents in a first direction in each of the flux loops14to increase the amplitude of the first current of the first flux loop14and decrease the amplitude of the second current of the second flux loop14. The increase in amplitude of the first current can increase the bias of the Josephson junction associated with the first flux loop14, such that the Josephson junction can trigger in response to the interrogation pulse INTIN. The triggering of the Josephson junction of the first flux loop14can thus result in triggering of the Josephson junction of the second flux loop14, and thus to provide the output pulse INTOUT(e.g., an RQL pulse) from the output stage16to indicate that the input current IINhas the first current direction.

In response to the input current IINhaving a second direction opposite the first direction, the input current IINcan induce currents in a second direction in each of the flux loops14opposite the first direction to decrease the amplitude of the first current of the first flux loop14and to increase the amplitude of the second current of the second flux loop14. The decrease in amplitude of the first current can decrease the bias of the Josephson junction associated with the first flux loop14. As a result, the Josephson junction of the first flux loop14does not trigger in response to the interrogation pulse INTIN. Therefore, no pulse is provided from the output stage16to indicate that the input current IINhas the second current direction.

Therefore, the flux switch system10described herein provides a robust manner of interrogating a current direction of the input current IINcorresponding to a superconducting persistent current, as opposed to typical ADC implementations for interrogating current direction in superconducting circuits. For example, typical ADC designs can exhibit low operating margins and clock-induced noise in the measured circuitry. In order to function, such typical ADCs can be DC-biased to an operating point where the additional current induced in the ADC from the input current to be interrogated (e.g., with the correct polarity) provides a final threshold necessary to excite the Josephson junctions of the ADC, which can lead to a stream of digital “logic ones” at the output. Since the ADC is biased to a point where zero input current produces no data at the output, the scheme of a typical ADC only utilizes half of the input current range available from the superconducting circuitry, which is a major source of the low operating margins. However, the flux switch system10is interrogated when the input current IINis present to provide a first state in the first direction of the input current IINand a second state in the second direction of the input current IIN. In addition, typical ADCs for RQL interrogation rely on Josephson junctions that are biased by the ROL clock lines. The RQL clocked Josephson junctions in the typical ADC can induce noise in the circuitry being measured, which can lead to a distortion of the input signal and can reduce the fidelity of the measurement depending on the sensitivity of the superconducting circuitry being measured. However, the flux switch system10implements Josephson junctions that are not biased from the RQL clock signal, as described in greater detail herein, and thus mitigates deleterious effects of noise on the operation of the flux switch system10.

FIG. 2illustrates an example of a flux switch circuit50. The flux switch circuit50can be implemented in any of a variety of superconducting computer systems to determine a sign (e.g., direction) of an input current IIN. The flux switch circuit50can correspond to the flux switch system10in the example ofFIG. 1. Therefore, reference is to be made to the example ofFIG. 1in the following description of the example ofFIG. 2.

The flux switch circuit50includes an input stage52and an output stage54. In the example ofFIG. 2, the input stage52includes a JTL56and an input inductor LIN. The JTL56can be configured to receive and propagate an interrogation pulse INTIN, such as provided from a previous JTL stage. For example, the interrogation pulse INTINcan be an RQL pulse, such that the JTL56can be biased via an RQL clock signal (not shown) to propagate the interrogation pulse INTINto the input inductor LIN. As an example, the interrogation pulse INTINcan be provided to the flux switch circuit50to interrogate the state of the input current IIN, such that the interrogation pulse INTINcan be provided at a time in which the input current IINhas a non-zero amplitude. Similarly, the output stage54includes an output inductor LOUT, a first JTL58, and a second JTL60. As an example, the first and second JTLs58and60can likewise be biased via an RQL clock signal (e.g., via the same phase as the JTL56for at least the first JTL58).

The flux switch circuit50also includes a first flux loop62and a second flux loop64. The first flux loop62is coupled to the input inductor LINand includes a Josephson junction J1and an inductor L1. The second flux loop64is coupled to an output inductor LOUTand includes a Josephson junction J2and an inductor L2, such that the inductors L1and L2and the Josephson junctions J1and J2are configured in a balanced arrangement. As an example, the inductors L1and L2can have an approximately equal inductance that is less than an inductance of the inductance of the input inductor LINand the output inductor LOUT. As another example, the input inductor LINand the output inductor LOUTcan have an approximately equal inductance to balance the first and second flux loops62and64, as described in greater detail herein. The flux loops62and64further include a shared first bias inductor LBIAS1that is inductively coupled to a second bias inductor LBIAS2through which a bias current IBIASflows. Therefore, the bias current IBIASis inductively provided to each of the first and second flux loops62and64in opposite orientations to provide a steady-state bias to each of the Josephson junctions J1and J2via approximately equal and opposite current flow through the respective inductors L1and L2.

FIG. 3illustrates another example of a flux switch circuit100. The flux switch circuit100corresponds to a portion of the flux switch circuit50in the example ofFIG. 2. Therefore, reference is to be made to the example ofFIG. 2, and like reference numbers are used as in the example ofFIG. 2, in the following description of the example ofFIG. 3. The flux switch circuit100includes the first flux loop62and the second flux loop64. In the example ofFIG. 3, a current IBthat can correspond to the induced current provided by the bias current IBIAS(e.g., via the bias inductors LBIAS1and LBIAS2) is provided to each of the flux loops62and64. Therefore, the current IBis split as a first current I1that flows in a counter-clockwise orientation through the first inductor L1and the first Josephson junction J1about the first flux loop62and a second current I2through the second inductor L2that flows in a clockwise orientation through the second inductor L2and the second Josephson junction J2about the second flux loop64.

The example ofFIG. 3also demonstrates the input inductor LINcoupled to the first flux loop62and the output inductor LOUTcoupled to the second flux loop64. As described previously, the inductors L1and L2can have an approximately equal inductance that is less than an inductance of the inductance of the input inductor LINand the output inductor LOUT, and the input inductor LINand the output inductor LOUTcan have an approximately equal inductance to balance the first and second flux loops62and64. Therefore, the current I1flows through the Josephson junction J1(as opposed to the input inductor LINhaving a high impedance) and the current I2flows through the Josephson junction J2(as opposed to the output inductor LOUThaving a high impedance). Therefore, the currents I1and I2can be approximately equal in amplitude at steady-state corresponding to an absence of the input current IIN, as described in greater detail herein, to provide a steady-state bias of the Josephson junctions J1and J2, respectively.

Referring back to the example ofFIG. 2, the inductor L1is inductive coupled to an inductor L3and the inductor L2is inductively coupled to an inductor L4. The inductors L3and L4are arranged in series on a conductor66through which the input current IINis provided in either a first direction or a second direction opposite the first direction. Therefore, when the input current IINis provided on the conductor66, the input current IINinduces a current in the flux loops62and64that results in a net amplitude change of the currents I1and I2. Because the currents I1and I2flow in opposite directions through the inductors L1and L2, the input current IINthus changes the amplitude of the currents I1and I2relative to each other by increasing the amplitude of one of the currents I1and I2and equally decreasing the amplitude of the other one of the currents I1and I2based on the direction of the input current IIN. As a result, the bias of the Josephson junctions J1and J2can change relative to each other based on the current direction, and thus sign, of the input current IIN.

FIG. 4illustrates another example of a flux switch circuit150. The flux switch circuit150corresponds to a portion of the flux switch circuit50in the example ofFIG. 2. Therefore, reference is to be made to the examples ofFIGS. 2 and 3, and like reference numbers are used as in the example ofFIG. 2, in the following description of the example ofFIG. 4. The flux switch circuit150includes the first flux loop62and the second flux loop64. In the example ofFIG. 4, a current IBthat can correspond to the induced current provided by the bias current IBIAS(e.g., via the bias inductors LBIAS1and LBIAS2) is provided to each of the flux loops62and64. Therefore, the current IBis split as the first current I1that flows in a counter-clockwise orientation through the first inductor L1and the first Josephson junction J1about the first flux loop62and the second current I2through the second inductor L2that flows in a clockwise orientation through the second inductor L2and the second Josephson junction J2about the second flux loop64.

In the example ofFIG. 4, the input current IINis demonstrated as being provided on the conductor66in a first direction (e.g., first sign). As a result of the input current IINbeing provided on the conductor66, and thus through the inductors L3and L4, the inductor L3induces a current via the inductor L1on the first flux loop62that is additive to the first current I1. Therefore, the first current I1is demonstrated in the example ofFIG. 4as a thicker line to correspond to an increase in amplitude of the first current I1as a result of the additive amplitude (e.g., same direction as the current IB) provided from the induced coupling to the input current IINin the first direction. Similarly, the inductor L4induces a current via the inductor L2on the second flux loop64that is subtractive from the second current I2. Therefore, the second current I2is demonstrated in the example ofFIG. 4as a thinner line to correspond to a decrease in amplitude of the second current I2as a result of the subtractive amplitude (e.g., opposite direction as the current IB) provided from the induced coupling to the input current IINin the first direction.

Based on the increase in amplitude of the first current I1, the first current I1increases the bias of the first Josephson junction J1. As described previously, the interrogation pulse INTIN, demonstrated in the example ofFIG. 4at152, can be provided to, and propagate through, the input stage52to interrogate the sign of the input current IIN. In response to the interrogation pulse INTINbeing provided from the input stage52through the input inductor LIN(not shown in the example ofFIG. 4), the interrogation pulse INTINcan add to the increased bias of the Josephson junction J1to trigger the Josephson junction J1. Therefore, the Josephson junction J1can likewise generate a voltage pulse that propagates through the inductor L1and through the inductor L2to also trigger the Josephson junction J2. Accordingly, the Josephson junction J2can emit a voltage pulse corresponding to the output pulse INTOUT, demonstrated in the example ofFIG. 4at154, that can propagate to and through the output stage54to indicate that the input current IINhas the first sign, and thus flows in the first direction.

FIG. 5illustrates another example of a flux switch circuit200. The flux switch circuit200corresponds to a portion of the flux switch circuit50in the example ofFIG. 2. Therefore, reference is to be made to the examples ofFIGS. 2 through 4, and like reference numbers are used as in the example ofFIG. 2, in the following description of the example ofFIG. 5. The flux switch circuit200includes the first flux loop62and the second flux loop64. In the example ofFIG. 5, a current IBthat can correspond to the induced current provided by the bias current IBIAS(e.g., via the bias inductors LBIAS1and LBIAS2) is provided to each of the flux loops62and64. Therefore, the current IBis split as the first current I1that flows in a counter-clockwise orientation through the first inductor L1and the first Josephson junction J1about the first flux loop62and the second current I2through the second inductor L2that flows in a clockwise orientation through the second inductor L2and the second Josephson junction J2about the second flux loop64.

In the example ofFIG. 5, the input current IINis demonstrated as being provided on the conductor66in a second direction (e.g., second sign) opposite the first direction of the example ofFIG. 4. As a result of the input current IINbeing provided on the conductor66, and thus through the inductors L3and L4, the inductor L3induces a current via the inductor L1on the first flux loop62that is subtractive from the first current I1. Therefore, the first current I1is demonstrated in the example ofFIG. 5as a thinner line to correspond to a decrease in amplitude of the first current I1as a result of the subtractive amplitude (e.g., opposite direction as the current IB) provided from the induced coupling to the input current IINin the second direction. Similarly, the inductor L4induces a current via the inductor L2on the second flux loop64that is additive to the second current I2. Therefore, the second current I2is demonstrated in the example ofFIG. 5as a thicker line to correspond to an increase in amplitude of the second current I2as a result of the additive amplitude (e.g., same direction as the current IB) provided from the induced coupling to the input current IINin the second direction.

Based on the decrease in amplitude of the first current I1, the first current I1decreases the bias of the first Josephson junction J1. As described previously, the interrogation pulse INTIN, demonstrated in the example ofFIG. 5at202, can be provided to, and propagate through, the input stage52to interrogate the sign of the input current IIN. In response to the interrogation pulse INTINbeing provided from the input stage52through the input inductor LIN(not shown in the example ofFIG. 5), and based on the decrease in bias of the Josephson junction J1, the interrogation pulse INTINcan be insufficient to trigger the Josephson junction J1. Therefore, no voltage pulses propagate through the flux switch circuit50to and through the output stage54, and thus the output stage54does not provide the output pulse INTOUTto indicate that the input current IINhas the second sign, and thus flows in the second direction.

Referring back to the example ofFIG. 2, as described previously, the interrogation pulse INTINcan be provided as an RQL pulse that includes a positive fluxon followed in time by a negative fluxon (e.g., as a reciprocal pair). Therefore, the flux switch circuit50can be implemented in an RQL circuit. For example, based on the high inductance value of the input inductor LIN, the negative fluxon of the RQL pulse can be rejected (e.g., reflected), such that only the positive fluxon passes through the input inductor LINto the first Josephson junction J1. In addition, the output stage54includes a reset stage68coupled to a node70that interconnects the first and second JTLs58and60. The reset stage68is configured to provide a negative fluxon in response to the output pulse INTOUTto reset the Josephson junctions J1and J2(e.g., as well as the Josephson junction(s) of the first JTL58).

The reset stage68includes a JTL72that interconnects a low-voltage rail (e.g., ground) at an input and the node70(via an inductor LRST) at an output. As described previously, the triggering of the Josephson junction J1(e.g., in response to the input current IINbeing provided in the first direction as demonstrated in the example ofFIG. 4) results in a triggering of the Josephson junction J2to propagate a voltage pulse to the output stage54. The first JTL58can thus likewise propagate a voltage pulse that is provided to both the JTL72of the reset stage68and to the second JTL60. The second JTL60can thus propagate the voltage pulse as the positive fluxon of the RQL output pulse INTOUT. However, because the JTL72of the reset stage68is arranged opposite with respect to the input and output relative to the JTLs58and60, the voltage pulse provided to the JTL72is provided to the low-voltage rail, and is thus reflected as a negative fluxon. The negative fluxon therefore propagates back through the JTL58and to the Josephson junctions J1and J2, thus resetting the Josephson junctions J1and J2, as well as the Josephson junction(s) associated with the JTL58. The negative fluxon also propagates through the second JTL60and is output as the negative fluxon of the reciprocal pair of the RQL output pulse INTOUT.

In view of the foregoing structural and functional features described above, a methodology in accordance with various aspects of the disclosure will be better appreciated with reference toFIG. 6.FIG. 6illustrates an example of a method250for determining a polarity of an input current. It is to be understood and appreciated that the method forFIG. 6is not limited by the illustrated order, as some aspects could, in accordance with the present disclosure, occur in different orders and/or concurrently with other aspects from that shown and described herein. Moreover, not all illustrated features may be required to implement a methodology in accordance with an aspect of the present examples.

At252, a bias current (e.g., the bias current bras) is provided to each of a plurality of flux loops (e.g., the flux loops60and62) to provide a current (e.g., the currents I1and I2) in each of the flux loops, each of the flux loops comprising a Josephson junction (e.g., the Josephson junctions J1and J2). At254, the input current (e.g., the input current IIN) is inductively provided to each of the flux loops. At256, an interrogation pulse (e.g., the interrogation pulse INTIN) is provided to the flux loops via an input stage (e.g., the input stage52). At258, it is determined that the input current has a first polarity in response to an output pulse (e.g., the output pulse INTOUT) being provided from an output stage (e.g., the output stage54) resulting from a triggering of the Josephson junction of each of the flux loops in response to the interrogation pulse and based on the current in each of the flux loops. At260, it is determined that the input current has a second polarity opposite the first polarity in response to no output pulse being provided from the output stage.