Reducing losses in superconducting cables

Abstract

Methods and apparatus for cooling superconducting signal lines disposed on an interconnect, such as a flexible cable or a rigid substrate, are disclosed. The superconducting signal lines are cooled to a cryogenic temperature that is below the temperature at which at least some superconducting logic devices coupled to the interconnect operate. In some examples, a hermetically sealed conduit, heat pipe, or heat conductive tape is provided to cool the superconducting interconnect. In one example of the disclosed technology, a system includes at least two sets of superconducting logic devices, a cooling device adapted to cool the logic devices to a first operating temperature, an interconnect coupled to the superconducting logic devices, and a cooling device in thermal communication with the interconnect. The device is adapted to cool the superconducting signal lines on the interconnect to an operating temperature that is below the first operating temperature at which the superconducting logic devices operate.

Description

Background Art

[0001] Conventional semiconductor-based integrated circuit technology (such as technology based on silicon or gallium arsenide device technology) is reaching its physical limits in terms of device size, switching frequency, and power consumption. In addition, integrated circuits deployed in data centers are consuming increasing amounts of power. This includes power consumed by leakage current when semiconductor devices are not switching. There is great interest in building quantum computers that can perform certain types of useful calculations (e.g., quantum chemistry) faster and more economically than any existing digital computer technology.

[0002] One approach to addressing the limitations of conventional semiconductor technology is to use devices based on superconducting logic. Such devices are typically cooled to cryogenic temperatures in order to function in a superconducting state. Refrigeration equipment used to provide such cooling for superconducting logic devices is expensive and consumes a lot of energy. Therefore, there is still ample opportunity to improve computers built from superconducting logic-based devices, including interconnects for transmitting signals between multiple sets of superconducting logic-based devices. Summary of the invention

[0003] Apparatus and methods for cooling superconducting cables and interconnects to cryogenic temperatures are disclosed. In some examples, a flexible multi-conductor microstrip or stripline ribbon cable is cooled using a cryogenic gas or liquid (e.g., helium) enclosed in a conduit or other containment device. In some examples, a heat pipe or heat conducting tape is provided in thermal communication with the cable or interconnect to provide cooling. A superconducting interconnect coupling two or more groups of logic devices can be cooled to a temperature below that of the logic devices, thereby reducing attenuation of signals carried by the interconnect without using additional energy to provide more extensive cooling to refrigerate the logic devices.

[0004] Superconducting signal lines disposed on an interconnect, such as a flexible cable or a rigid substrate, may be cooled to a cryogenic temperature below the operating temperature of at least some superconducting logic devices coupled to the interconnect. In some examples, a hermetically sealed conduit, heat pipe, or heat conducting tape is provided to cool the superconducting interconnect. In one example of the disclosed technology, a system includes at least two sets of superconducting logic devices, a cooling device adapted to cool the logic devices to a first operating temperature, and an interconnect coupled to the superconducting logic devices. A cooling device is provided in thermal communication with the interconnect. The device is adapted to cool the superconducting signal lines on the interconnect to an operating temperature below the first operating temperature at which the superconducting logic devices operate. By cooling only the superconducting signal lines to a temperature below the coupled logic devices, a significant amount of energy may be saved for the entire system. The temperature used to cool the signal lines may be selected based on the materials used, distance constraints, operating constraints (such as frequency), and energy consumption.

[0005] This "Summary" is provided to introduce some concepts in a simplified form, which will be further described in the "Detailed Description" below. This "Summary" is neither intended to identify key features or essential features of the claimed subject matter nor to be used to limit the scope of the claimed subject matter. The foregoing and other objects, features, and advantages of the disclosed technology will become more apparent from the following detailed description made with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 is a block diagram depicting an example system that may be used to cool a superconducting interconnect that may be implemented in certain examples of the disclosed technology.

[0007] Figure 2 is a block diagram illustrating an example system including a catheter and an interconnect having a plurality of connections that may be implemented in certain examples of the disclosed technology.

[0008] Figures 3A to 3C is a diagram showing an example interconnect that may be cooled using certain disclosed conduits or heat pipes.

[0009] Figure 4A and 4B is a diagram illustrating alternative example interconnects that may be used in certain examples of the disclosed technology.

[0010] Figure 5A and 5B Another example of a superconducting interconnect that may be implemented in certain examples of the disclosed technology is shown.

[0011] Fig. 6A and 6B An example of a multi-drop interconnect cooled with thermally conductive tape that may be implemented in certain examples of the disclosed technology is shown.

[0012] Figure 7 is a graph showing the variation of losses in a superconducting transmission line with frequency and temperature that may be observed in certain examples of the disclosed technology.

[0013] Figure 8 is a flow chart outlining an example method of providing interconnections in a superconducting computer system that may be performed in certain examples of the disclosed technology.

[0014] Fig. 9 is a flow chart outlining an example method of cooling a conduit coupled to a superconducting logic device that may be performed in certain examples of the disclosed technology. DETAILED DESCRIPTION

[0015] I. General Considerations

[0016] Representative embodiments of methods, apparatus, and systems for designing, manufacturing, and assembling substrates that carry conductor and / or superconductor materials using connectors that form mechanical and / or electrical connections between such substrates are disclosed herein. The present disclosure is set forth in the context of representative embodiments, which are not intended to be limiting in any way.

[0017] As used in this application, the singular forms "a", "an" and "the" include the plural forms unless the context clearly indicates otherwise. In addition, the term "includes" means "comprises". Furthermore, the term "coupled" includes mechanical, electrical, magnetic, optical and other practical ways of coupling or linking items together, and does not exclude the presence of intermediate elements between the coupled items. In addition, as used herein, the term "and / or" refers to any one or more items in the phrase.

[0018] The systems, methods, and devices described herein should not be interpreted as limiting in any way. Rather, the present disclosure relates to all novel and non-obvious features and aspects of the various disclosed embodiments, individually and in various combinations and sub-combinations with each other. The disclosed systems, methods, and devices are not limited to any particular aspect or feature or combination thereof, nor do the disclosed things and methods require the presence of any one or more specific advantages or solutions to problems. In addition, any features or aspects of the disclosed embodiments may be used in various combinations and sub-combinations with each other.

[0019] Although the operations of some disclosed methods are described in a particular sequential order for ease of presentation, it should be understood that this description includes rearrangement unless the specific language set forth below requires a specific order. For example, in some cases, the operations described in sequence may be rearranged or performed simultaneously. Moreover, for simplicity, the accompanying drawings may not show the various ways in which the disclosed things and methods can be used in combination with other things and methods. In addition, the description sometimes uses terms such as "produce", "generate", "manufacture", "receive", "cool", "provide" and "suitable for" to describe the disclosed methods. These terms are high-level descriptions of the actual operations performed. The actual operations corresponding to these terms will vary depending on the specific implementation, and are easily discernible by those of ordinary skill in the art who benefit from the present disclosure.

[0020] As used in this application and the claims, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. Additionally, the term "includes" means "comprises." Furthermore, unless the context dictates otherwise, the term "coupled" means connected or linked mechanically, electrically, or electromagnetically, and includes both directly connected or linked and indirectly connected or linked through one or more intermediate elements that do not affect the intended operation of the described system.

[0021] In addition, certain terms such as "above", "below", "upper", "lower", etc. may be used. When applicable, these terms are used to provide certain descriptive clarity when dealing with relative relationships. However, these terms are not intended to imply absolute relationships, positions and / or orientations.

[0022] References to "superconducting" components such as superconducting interconnects, striplines, microstrips, signal lines, or logic devices refer to components made of superconductor materials that can be set to operate in a superconducting mode below the material's respective critical temperature. However, the referenced superconducting components may not necessarily be in a superconducting state when performing certain actions described herein. For example, coupling a superconducting cable to a set of superconducting logic devices refers to forming such a coupling, which can then, at a later point in time, be placed in a superconducting state by cooling the logic devices and interconnects below their respective critical temperatures.

[0023] The theories of operation, scientific principles or other theoretical descriptions presented herein with reference to the devices or methods of the present disclosure are provided for better understanding and are not intended to limit the scope. The devices and methods in the appended claims are not limited to devices and methods that function in the manner described by such theories of operation.

[0024] II. Introduction to the Disclosed Technology

[0025] Although high-frequency signals are transmitted over superconducting cables configured as stripline, microstrip or coaxial transmission lines, they are still slightly attenuated. The reason is that the small but significant number of normal (unpaired) electrons in the superconductor are accelerated in the usual way by the time-varying electromagnetic field generated by the supercurrent. However, unlike supercurrents, normal currents dissipate power in the surface resistance of the wire. This may distort the transmitted signal due to the reduction of its high-frequency components. Therefore, although superconductor wires can act as perfect superconductors at DC, at higher frequencies (e.g., GHz range), significant losses can be observed due to the presence of normal electrons.

[0026] The higher the frequency, the more pronounced this effect is. At a given frequency, the attenuation factor α describes how much the signal is attenuated as it propagates along the line. For example, α = 10-3 An attenuation factor of nepers / cm means that the signal amplitude decreases by 0.1% per centimeter, or about 10% per meter traveled. The attenuation factor is a combination of dielectric loss and superconductor loss. In general, the attenuation due to dielectric loss increases linearly with frequency, while the attenuation due to superconductor loss increases with the square of the frequency. Therefore, superconductor loss becomes the largest attenuation source at high frequencies.

[0027] Lowering the temperature of the cable can reduce this attenuation. In general, both dielectric and superconductor losses decrease as the temperature decreases toward absolute zero. Superconductor losses are particularly sensitive to temperature. At higher temperatures, there are more free (non-Cooper pairs) electrons causing losses.

[0028] The Bardeen-Cooper-Schrieffer (BCS) theory tells us that the density ratio of normal electrons to Cooper pairs is exponentially related to temperature. For niobium (Nb, element 41) below about 4.65K (1 / 2 of its critical temperature, typically 9.3K), the resulting surface resistance can be approximated by f 2 ×exp((-17.67 / T) / T), where T is the absolute temperature of the cable and f is the signal frequency. For example, the surface resistance of Nb can be reduced by about 49 times simply by lowering the temperature of Nb from 4.2K (the temperature of saturated liquid helium (He) at atmospheric pressure) to 2K. Therefore, when the cable is cooled from 4.2K to 2.0K, the portion of the attenuation coefficient due to superconductor losses will be reduced by about 49 times.

[0029] The contribution of dielectric losses to the attenuation coefficient is not that sensitive to temperature. As the cable cools, eventually the superconductor losses will be negligible, and the attenuation coefficient will consist primarily of dielectric losses ("loss tangent"). The loss tangent of interconnects with polyimide cables is very low at frequencies between 1 and 20 GHz at deep freeze temperatures (typically 100 times less than the corresponding room temperature value). This allows for fairly long distance (meter-scale) transmission of microwave (about 10 GHz) range signals without excessive attenuation, and also allows for the transmission of useful high RF power levels without excessive dielectric heating. Superconducting transmission lines work well with trace widths of a few microns. For typical nominal impedances, the dielectric thickness will also be a few microns.

[0030] Superconducting flexible controlled impedance cables such as microstrip, stripline or coplanar waveguide allow high-speed electrical signals to be transmitted over long distances with minimal loss compared to conventional cables using conductors. In addition, their physical cross-section can be very small, so a large number of signals can be transmitted in a single compact ribbon cable. However, if the cable temperature approaches or exceeds T c, performance advantages may be lost, such as would occur when connecting from a cryogenic environment to high temperature electronics.

[0031] Apparatus and methods for cooling superconductor cables and interconnects to cryogenic temperatures are disclosed. In some examples, cryogenic helium gas enclosed in a tube is used to cool a flexible multiconductor microstrip or stripline ribbon cable. For example, a Joule-Thomson valve can be used to cool a superconductor cable at 4.2K and an atmospheric pressure (14.70 pounds per square inch (psi) or 1.01325×10 5 Pascal (Pa)) received helium expands 32 times to reach a pressure of about 0.034 atmospheres (0.5 psi / 3.447×10 3 Pa), thereby reducing the gas temperature to 2 K. Larger pressure drops will produce lower temperatures, allowing longer cables or higher signal rates.

[0032] In some examples, helium at 2K from a single source can be passed through multiple ribbon cables in the same pipeline. Low losses mean that the helium is hardly heated by the multiple cables. A network of cables connecting multiple endpoints can be cooled by enclosing the network in a network of pipes spanning the endpoints. Each endpoint can have its own Joule-Thomson expansion valve to supply helium to the pipe network. Multiple exhaust points of sufficient diameter to generate a significant pressure gradient can be located at the center of the pipe network and connected to a helium liquefier for recompression and recycling.

[0033] Methods, devices and systems for designing and manufacturing substrates of various configurations are disclosed, including conductive or superconducting flexible cables using microstrip geometry. In some examples, the superconducting flexible cable has a plurality of signal conductors disposed on one side, and a power or ground plane disposed on the other side (e.g., the opposite side or the front side). In some examples, a typical signal line disposed on the surface of the flexible cable is 50 microns wide. In some examples, the width of the signal line can vary between 10 and 100 microns. In some examples, the signal line containing niobium (including selected niobium alloys) is intended to operate at deep freezing temperatures (e.g., about below 4K). High-density connections can be made between a pair of such flexible cables. In some examples, the two cables to be connected are connected end to end, with a small gap between the two ends. In other examples, the cables can be slightly adjacent or overlapped. A "bridging connector" containing a series of short electrical connections is clamped above the joint area, thereby creating overlapping electrical contacts, thereby providing electrical continuity between the corresponding signal lines of the two flexible cables. Similarly, another bridging connector is also clamped below the connection area, thereby providing electrical contact between the power or ground planes of the two flexible cables. In such examples, precision microfabricated pillars engage microfabricated holes to provide mechanical self-alignment of the bridge connector and the cable. In some examples, simple thin film conductors are used on the bridge connector. In some examples, gold contacts can be used. For example, the gold plating on the superconductor can be made thin enough (e.g., less than 20nm) so that the gold layer may become superconducting due to the proximity effect. In other examples, thicker conductor layers (e.g., copper, aluminum, silver, gold, or titanium) are used, but some resistance is created in the line.

[0034] III. Example Interconnects and Conduits

[0035] Figure 1 1 is a block diagram 100 depicting an example system that may be used to cool a superconducting interconnect in certain examples of the disclosed technology. In the example system, a sealable conduit is adapted to enclose at least a portion of a superconducting interconnect. The conduit may also be adapted to couple to one or more sets of superconducting logic devices. The conduit may also be adapted to receive a liquid or gas coolant to cool the superconducting interconnect to a temperature below that of the superconducting logic devices coupled to the interconnect.

[0036] like Figure 1As shown, the conduit 110 encloses a portion of the superconducting interconnect cable 120. The superconducting interconnect cable is configured to transmit signals between a first group of logic devices 130 and a second group of logic devices 135. The first group of logic devices and the second group of logic devices include superconducting devices that can operate in a frozen thermal environment. For example, the device may include a conventional CMOS-based or other type of device that can operate at a temperature below about 70K. The logic device may also include a superconducting logic-based device that operates at even lower temperatures (e.g., below 4K, or below 20mK). In some examples, the superconducting logic device may include a circuit based on a Josephson junction-based device. The superconducting logic device may include a circuit having one or more Josephson junction devices. A typical Josephson junction includes a thin insulator or conductor sandwiched between two superconductor materials. The thin layer provided is thin enough to allow current to flow from one part of the superconductor material to another part of the superconductor material on the other side of the thin layer. For example, the superconductor material may include niobium and an insulator provided by a thin layer of aluminum oxide (e.g., 5 to 20 angstroms). As an exemplary superconductor, the critical temperature (T c ) is about 9.2 degrees Kelvin. c Niobium is superconducting at temperatures below 100 °C; however, above T c At temperatures below 1000 °C, it behaves like a normal metal with electrical resistance. In a superconductor-insulator-superconductor (SIS) type of Josephson junction, the superconductor layer may be made of niobium and the insulator layer may be formed of aluminum oxide. A supercurrent flows through the Josephson junction depending on the sign of the phase difference between the two junction electrodes.

[0037] The dimensions, including layer thickness, aspect ratio, spacing, length, and width, can be selected based on the desired circuit performance. In some examples, interconnects up to one meter long or more can be constructed. Ideally, the interconnect temperature is selected so that the signal decays less than 10% over the length of the cable. In addition, because the logic devices can be maintained at a higher temperature than the interconnect, a significant amount of energy can be saved by limiting the lower cooling to the interconnect portion of the system.

[0038] The superconductor logic device is maintained at an operating temperature by enclosing the superconductor logic device in one or more cryostats 140 and 145. The cryostats provide cooling for the superconductor logic device and have at least one terminal or opening to allow a superconducting interconnect cable 120 to be received by the conduit 110. The superconducting interconnect cable 120 includes at least one signal line including a superconducting material disposed on an insulator. For example, a plurality of strip lines or microstrips formed of a superconductor or material may be disposed on a rigid or flexible insulator. Other examples detailing the construction and use of suitable superconducting interconnect cables, such as cable 120, are described with reference to the following examples.

[0039] The conduit 110 is configured to enclose at least a portion of the interconnect and has a hermetic seal. Suitable connections are provided to interconnect cables from the logic device to the terminals. The conduit is gas-tight and can be hermetic. The seal is provided in part by hermetic clamps 130 and 135, which allow the interconnect 120 to extend to the outside of the conduit while maintaining a hermetic seal. The conduit includes at least one Joule-Thomson valve 150. The Joule-Thomson valve is coupled to the conduit 110 and is adapted to receive a gas provided at a first temperature and a first pressure. The Joule-Thomson valve is also adapted to provide a gas coolant to the interior of the conduit 110 at a pressure lower than the first pressure and a second temperature lower than the first temperature. The Joule-Thomson valve 150 coupled to the conduit is adapted to provide a gas coolant to the interior of the conduit. The Joule-Thomson valve 150 is configured to receive high pressure gaseous helium and provide helium to the conduit at a lower temperature and pressure. For example, the Joule-Thomson valve can be configured to accept helium at 4.2K at one atmosphere and discharge the helium into conduit 110 at a low pressure of 2K at 0.5 psi. The cooled gaseous helium is provided to the Joule-Thomson valve by refrigerator 160. For example, refrigerator 160 can be a dilution refrigerator, such as a Leiden Cryogenics CF450 3He / 4He dilution refrigerator. However, any suitable cooling device can be used as will be readily appreciated by one of ordinary skill in the art having the benefit of this disclosure.

[0040] The conduit 110 also includes at least one exhaust port 170 adapted to receive a gaseous coolant from the interior of the conduit. The exhaust port 170 is coupled to receive helium from the conduit 110 and return it to the refrigerator 160 to be recompressed, cooled, and recycled back to the conduit. In some examples, the same refrigerator is also used to provide coolant to a cryostat containing a superconductor logic device. In other examples, a different unit provides cooling to the superconductor logic device.

[0041] IV. Example Conduits and Interconnections with Multiple Connections

[0042] Figure 2 2 is a block diagram 200 illustrating an example system that may be employed in certain examples of the disclosed technology, the example system including an example conduit and an interconnect having a plurality of connections. The system may include one, two, or more groups of superconducting logic devices, a cooling device adapted to cool the logic devices to below a first operating temperature, and an interconnect at least partially enclosed within the conduit and including a superconducting signal line. The cooling device is disposed in thermal communication with the interconnect and adapted to cool the superconducting signal line on the interconnect to a second operating temperature for the first operating temperature of the group of superconducting logic devices.

[0043] like Figure 2As shown, conduit 210 is gas-tight and encloses multiple branches of interconnects 220, 221, 222, etc. Interconnect 220 can be hermetically sealed and include connections such as hermetically sealed clamps 225 and 227 that connect interconnect 220 enclosed within the conduit to other interconnects coupled to one or more sets of cryogenic logic devices (e.g., 230 or 235). Each branch of conduit 210 is coupled to a Joule-Thomson valve (e.g., Joule-Thomson valve 240) that provides coolant gas at one end of conduit branch 210. Conduit 210 also includes multiple exhaust ports, such as exhaust ports 250 and 255, that allow coolant gas to be exhausted from the interior of conduit 210 so that it can be recovered and sent to a helium refrigerator. By providing helium gas to cool interconnect 220, the Q factor of the interconnect is reduced when the temperature of interconnect 220 within the conduit is substantially reduced to below the temperature at which the coupled cryogenic logic devices operate.

[0044] V. Example Superconducting Interconnects

[0045] Figures 3A-3C An example interconnect 300 is shown that may be cooled using an enclosed conduit or heat pipe as discussed herein. Figure 3A 3 is a plan view of an interconnect that includes a substrate 310 (which may be made of a polyimide-based film or other suitable material) on which a number of superconducting strip lines, such as strip lines 320, 321, and 322, have been disposed. The strip lines may be made of, for example, a 250 nanometer (nm) thick layer of niobium deposited on an optional 10 nm adhesion layer formed of titanium. In some examples, a glass substrate may be used to make a flexible superconducting cable. An initial polyimide film (e.g., Pyralin PI-2611 or HD-4100) having a thickness of about 20 micrometers (μm) may be spin-coated onto a glass substrate. Using a plasma process followed by a physical vapor deposition (PVD) process, a thin film of titanium may be applied to the cured polyimide film. Next, a signal line formed of a superconducting metal layer (e.g., a niobium layer) may be deposited. Next, the optional titanium / niobium stack may be covered with a copper layer or other suitable conductor (such as gold or silver). For example, a 500 nm copper layer may be used. Finally, photolithography and thin film processing techniques can be used to pattern the conductor traces to form signal lines (e.g., microstrip, stripline, or other paths on the substrate). The polyimide layer can then be applied and cured over the conductors. These processes can be repeated to form a structure with multiple conductor layers. As will be readily appreciated by those of ordinary skill in the related art having the benefit of the present disclosure, layer thickness, width, length, line spacing, and other aspects of the selection can be selected based on the desired performance characteristics of the interconnect.

[0046] Any suitable superconducting material may be used for the superconducting wire, including, for example, niobium, molybdenum nitride, titanium niobium nitride, niobium nitride, or alloys of these materials. Niobium thin films are susceptible to cracking and delamination from the underlying substrate, particularly when subjected to flexing, such as in the case of flexible cables. The superconducting stripline may be disposed on an optional bonding layer disposed in the middle on the substrate, which bonding layer may be made of, for example, titanium. Titanium may serve as an adhesion and transition layer for subsequently deposited metal layers. Interconnect 300 also includes a contact metallization layer disposed on the ends of the superconducting stripline. In some examples, the contact material may be made of a superconductor, while in other examples, a thin layer of a material such as copper, aluminum, silver, gold, or titanium may be used. Figure 3B 340 is a cross-section of interconnect 300 as shown by dashed line 340. As shown, there are a number of strip lines, including strip lines 320, 321, and 322 disposed on substrate 310. The front side of interconnect 300 includes a planar layer 350 disposed on the back side of interconnect 300. Planar layer 350 can be made of a superconductor material, such as niobium or other materials discussed above, or can be made of a conductor layer, such as one of the conductors listed above. In some examples, planar layer 350 can be configured to serve as a ground plane for interconnect 300. In some examples, planar layer 350 can be placed in a thermally conductive state using a heat pipe or a heat conductive tape. For example, a heat conductive tape formed of copper can be used to conduct heat from the planar layer to a cold plate, as discussed further below. Figure 3C 3 is a cross section of interconnect 300 taken at the section indicated by dashed line 360. As shown, each stripline has a portion of contact metallization 330, 331, and 332 disposed over the ends of the stripline. Examples of suitable materials include lead, indium, or copper.

[0047] In some examples, high temperature superconductor materials such as yttrium barium copper oxide (YBCO) or bismuth strontium calcium copper oxide (BSCCO) can be used to provide superconducting interconnects at higher relative temperatures (eg, up to about 70 K for these materials).

[0048] VI. Additional Example Interconnects

[0049] Figure 4A-4B Depicted are cross-sections of alternative interconnect structures that may be used in certain examples of the disclosed technology. Figure 4A An interconnect 400 is shown having a plurality of superconducting strip lines 410 disposed on a substrate such as a polyimide-based film 420. The interconnect may include a plurality of planar layers (e.g., planar layers 430 and 435) and a substrate 425. In some examples, one of the planar layers may be made of a superconducting material and another may be made of a conductor material. In other examples, both planar layers may be made of a superconductor material or a conductor material. Figure 4BThe interconnect 400 is shown after an additional substrate 427 having a planar layer made of a superconductor or conductor material 437 on the front side of the insulator. It should be noted that one or more of the planar layers 430, 435, and / or 437 can be arranged to be in a thermally conductive state through a heat pipe or other cooling device. In some examples, one or more of the planar layers can be connected to a power or ground signal.

[0050] Figure 5A and 5B A cross-section and plan view, respectively, of a superconducting "coaxial" interconnect 500 that may be employed in certain examples of the disclosed technology are shown. For example, the cable shown is an interconnect that may be enclosed within a conduit and cooled by a coolant such as liquid helium, or may be placed in thermal communication with a heat pipe or heat conducting tape.

[0051] As in Figure 5A As shown in the cross-section of , a single superconductor signal line 520 is enclosed in an insulator such as polyimide, Kapton, yttrium stabilized zirconia (YSZ), Ceraflex, fluorinated ethylene propylene, Teflon, polyester film or ceramic material. The example cable also includes a pair of signal lines 525, 526 enclosed in another portion of the interconnect 500. The pair of signal lines can be configured to transmit differential signals on the pair of signal lines. The interconnect 500 includes planar layers 530 and 535 located above and below the signal lines 520, 525 and 526. These planar layers are connected by a series of shielding vias (e.g., shielding vias 540, 541 and 542), which can provide electrical connections and mechanical rigidity to the interconnect 500 components.

[0052] Figure 5A The cross section of is indicated by dashed line 550. Figure 5B As shown in the plan view of FIG, a plurality of additional shielding vias are provided, which can be used to shield electromagnetic and interference between signal lines (e.g., between signal line 520 and differential pairs 525 and 526) located below the plane layer 530 (as shown by the dotted lines). The vias can be made of any suitable conductor, for example, including one or more of copper, aluminum, gold, silver or tungsten.

[0053] VII. Example Multi-Branch Interconnect and Thermal Conductive Belt

[0054] Fig. 6A is a diagram 600 illustrating an example system for using thermally conductive tape to cool a multi-drop interconnect 610 that may be employed in alternative examples of the disclosed technology. In other examples, heat pipes may be used in place of or in addition to thermally conductive tape.

[0055] like Fig. 6AAs shown, one or more superconducting signal lines 620, 621 and 622 are arranged on an insulating substrate 630. The insulating substrate is in turn arranged on a planar superconductor or conductor material layer on the opposite side of the interconnect 610 from the signal line 620. A plurality of conductive belts (e.g., belts 640, 641, 642 and 643) are placed in thermal communication with the planar layer, for example, by being arranged in contact with the planar layer. The distal end of the belt is coupled to a cooling plate maintained at a low temperature. The heat from the interconnect 610 is transferred to the cooling plate 650 via the belt. In some examples, the conductive belt includes a conductor layer arranged on the insulator portion of its interconnect coupling. In some examples, the conductor layer includes at least one of copper and aluminum, silver, gold or titanium. In some examples, the insulator includes at least one of polyimide, Kapton, yttrium-stabilized zirconium oxide (YSZ), Ceraflex, fluorinated ethylene propylene, polytetrafluoroethylene, polyester film or ceramic material. In other examples, in addition to or instead of the conductive belt, a heat pipe can be used.

[0056] Figure 6B is a multi-drop interconnect 610 in Fig. 6A 680 at the dashed line 670 of FIG. As shown, the planar layer on the side opposite to the signal line 620 is placed in contact with the heat conducting tape 640. The distal end of the heat pipe is clamped in the cooling plate 650. The cooling plate 650 is configured to cool the heat conducting tape to a temperature below the operating temperature of the logic device 660 and / or 665.

[0057] VIII. Example Properties of Superconducting Materials

[0058] Figure 8 800 is a graph showing the variation of losses in a superconducting transmission line as a function of frequency and temperature that may be observed in certain examples of the disclosed technology. The losses are measured by configuring the transmission line as a resonator, with both ends of the line open and weakly coupled to a network analyzer. For example, a first data sequence 710 shows the losses of a superconducting transmission line at 4.2K over the entire frequency range. A second sequence 720 and a third sequence 730 show the losses at 3.6K and 3K. A fourth sequence 740 and a fifth sequence 750 show the losses at 1.2K and 20mK. The quality factor ("Q") of the resonator is measured at each resonant frequency. The losses in the resonator are proportional to the inverse of the Q (1 / Q) and are plotted for several different temperatures. As shown in graph 800, it is clear that cooling below 4.2K results in a significant reduction in losses, and these reductions are more pronounced at higher frequencies.

[0059] IX. Example Methods for Cooling Superconducting Interconnects

[0060] Figure 8800 is a flowchart outlining an example method for providing interconnects in a superconducting computer system that may be performed in certain examples of the disclosed technology. For example, the apparatus described above (including an apparatus having conduits, heat conducting tapes, or heat pipes configured to receive a coolant) may be used to implement the illustrated method.

[0061] At process block 810, a superconducting interconnect, such as a cable, is coupled to the first set of superconducting logic devices and the second set of superconducting logic devices. For example, a flexible or rigid superconducting cable, such as discussed above, may be coupled. In some examples, a polyamide cable including at least one superconductor stripline disposed on a surface is coupled to the superconducting logic devices using a clamp or other suitable connection technique. In some examples, a conductor layer may also be disposed on at least a portion of the superconducting cable.

[0062] At process block 820, the first group of superconducting logic devices and / or the second group of superconducting logic devices are cooled to a first temperature. For example, a cryostat such as a dilution refrigerator or other refrigeration technology may be used to cool the superconducting logic devices to a selected temperature.

[0063] At process block 830, the superconducting interconnect or cable coupled at process block 810 is cooled to a second temperature that is lower than the first temperature at which the superconducting logic device is cooled. For example, the superconducting logic device can be cooled to about 4K, and the superconducting cable or interconnect can be cooled to less than 4K. In some examples, the second temperature is at least 25% lower than the first temperature, for example measured in degrees Kelvin. In some examples, the superconducting cable is cooled by providing a substantially sealed conduit, heat conducting tape, or heat pipe and thermal communication with the superconducting cable.

[0064] X. Example Methods of Cooling Interconnects via Conduit

[0065] Fig. 9 9 is a flowchart outlining an example method of cooling a conduit coupled to a superconducting logic device that may be performed in certain examples of the disclosed technology. For example, the sealed conduit discussed above may be used to perform the illustrated method.

[0066] At process block 910, a conduit is provided that encloses at least a portion of a superconducting cable coupled to one or more groups of superconducting logic devices. The conduit can be airtight or hermetically sealed to enclose a cooling liquid or gas provided to the interior of the conduit. The conduit can have one or more inlet valves (e.g., an inlet valve coupled to a Joule-Thomson valve) and one or more outlets. Thus, a coolant can be circulated to a refrigerator to provide cooling of the interconnects enclosed within the conduit.

[0067] At process block 920, a coolant is provided to the conduit at a temperature lower than the temperature of the superconducting logic device to which the interconnect is coupled. For example, a helium vapor flow can be provided to the conduit via a Joule-Thomson valve. In addition, a helium vapor flow can be provided out of the conduit via an exhaust valve for further cooling. In some examples, liquid helium is provided to the conduit enclosing at least a portion of the superconducting interconnect. The temperature at which the coolant is provided is lower than at least one of the groups of superconducting logic devices. For example, the coolant can be provided at 2K, 1K, 100mK, 20mK, or other suitable temperatures. The selection of temperature and coolant can be selected based on the electrical characteristics of the enclosed interconnect. In some examples, it is desirable to select the coolant and properties so that the attenuation of the signal transmitted via the interconnect is kept below 20% or below 10%.

[0068] At process block 930 , the exhaust coolant received via the conduit exhaust is chilled to a temperature such that it can be reintroduced into the conduit.

[0069] XI. Other Examples of the Disclosed Technology

[0070] Based on the above examples, other examples of the disclosed technology are disclosed.

[0071] In some examples of the disclosed technology, a method of providing interconnection in a superconducting computing system includes coupling a superconducting cable to a first group of superconducting logic-based devices and a second group of superconducting logic-based devices, cooling the first group of superconducting logic-based devices, the second group of superconducting logic-based devices, or the first group of superconducting logic-based devices and the second group of superconducting logic-based devices to a first temperature, and cooling the superconducting cable to a second temperature below the first temperature. In some examples, the first temperature is about 4 degrees Kelvin (K) and the second temperature is below 2 degrees Kelvin. In some examples, the first temperature is about 4.2 degrees Kelvin (K) and the second temperature is below 3 degrees Kelvin. In some examples, the first temperature is about 3.6 degrees Kelvin (K) and the second temperature is below 1.2 degrees Kelvin. In some examples, the first temperature is about 4 degrees Kelvin (K) and the second temperature is below 20 degrees Kelvin. In some examples, the first temperature is below a temperature selected from 4.2, 4.0, 3.6, 3.0, 2.4, 2.0, or 1.2 degrees Kelvin, and the second temperature is a temperature below the first temperature, the second temperature selected from 4.0, 3.6, 3.0, 2.4, 2.0, or 1.2 degrees Kelvin. In some examples, the second temperature is at least 25% less than the first temperature in degrees Kelvin. In some examples, the second temperature is at least 50% less than the first temperature in degrees Kelvin. In some examples, the second temperature is at least 75% less than the first temperature in degrees Kelvin.

[0072] In some examples, the method further includes providing a hermetically sealed conduit enclosing at least a portion of the superconducting cable, and providing a helium vapor flow into the conduit via a Joule-Thomson valve. In some examples, the method further includes providing the helium vapor flow out of the conduit via an exhaust valve. In some examples, liquid helium is provided to cool the superconducting cable. In some examples, a gas with a higher boiling point is provided into the conduit, depending on the characteristics of the superconductor used in the interconnect. In some examples, the method further includes refrigerating at least a portion of the helium vapor flow coming out of the conduit, and providing the refrigerated vapor into the conduit via a Joule-Thomson valve. In some examples, the method further includes providing a heat pipe or a heat-conducting belt that is thermally connected to the superconducting cable in addition to or in lieu of providing cooling vapor or liquid to the superconducting interconnect.

[0073] In some examples, the superconducting cable includes at least one superconducting signal line and at least one conductor plane, and cooling the superconducting cable includes providing at least one cooling plate in thermal communication with the at least one conductor plane. In some examples, cooling the superconducting cable includes providing liquid or gas phase helium to a hermetically sealed manifold enclosing at least a portion of the superconducting cable. In some examples, the two sets of superconducting logic devices are located on two different substrates. In some examples, the two sets of superconducting logic devices are located in two separate housings. In some examples, the two sets of superconducting logic devices are cooled by the same cryocooler, while in other examples, separate coolers are used.

[0074] In some examples of the method, in addition to or in lieu of a valve providing cooled liquid helium or gaseous helium, a heat pipe or heat conducting tape is provided. The heat pipe or heat conducting tape provides thermal communication between the cooling device and at least one interconnect. In some examples, the heat pipe or heat conducting tape includes a conductor layer disposed on an insulator portion of the interconnect coupling. In some examples, the conductor layer is a metal. In some examples, the conductor layer includes at least one of copper, aluminum, silver, gold, or titanium. In some examples, the insulator includes at least one of polyimide, Kapton, yttrium stabilized zirconium oxide (YSZ), Ceraflex, fluorinated ethylene propylene, polytetrafluoroethylene, polyester film, or a ceramic material. In some examples, the superconducting interconnect coupling includes at least one signal line including at least one of niobium, molybdenum nitride, or niobium titanium nitride.

[0075] In some examples of the disclosed technology, a device includes a sealable conduit adapted to seal at least a portion of a superconducting interconnect, the conduit further adapted to receive a liquid or gaseous coolant to freeze the superconducting interconnect, the conduit further adapted to couple the superconducting interconnect to one or more superconducting logic devices. In some examples, the device includes at least one Joule-Thomson valve coupled to the conduit and adapted to provide a gaseous coolant to the interior of the conduit. In some examples, the device includes an exhaust port coupled to the conduit and adapted to receive a gaseous coolant from the interior of the conduit. In some examples, the device is adapted to receive helium or a liquid as a coolant. In some examples, the first temperature is about 4 degrees Kelvin (K) and the second temperature is below 2 degrees Kelvin. In some examples, the first temperature is about 4.2 degrees Kelvin (K) and the second temperature is below 3 degrees Kelvin. In some examples, the first temperature is about 3.6 degrees Kelvin (K) and the second temperature is below 1.2 degrees Kelvin. In some examples, the first temperature is about 4 degrees Kelvin (K) and the second temperature is below 20 degrees Kelvin. In some examples, the first temperature is below a temperature selected from 4.2, 4.0, 3.6, 3.0, 2.4, 2.0, or 1.2 degrees Kelvin, and the second temperature is a temperature below the first temperature, the second temperature selected from 4.0, 3.6, 3.0, 2.4, 2.0, or 1.2 degrees Kelvin. In some examples, the second temperature is at least 25% less than the first temperature in degrees Kelvin. In some examples, the second temperature is at least 50% less than the first temperature in degrees Kelvin. In some examples, the second temperature is at least 75% less than the first temperature in degrees Kelvin. In some examples, depending on the characteristics of the superconductor used in the interconnect, a gas having a boiling point higher than helium is provided to the conduit.

[0076] In some examples, the device includes a superconducting interconnect, the superconducting interconnect including at least one signal line, the signal line including a superconducting material disposed on an insulator. In some examples, the device also includes at least one valve, the at least one valve coupled to the conduit and adapted to provide a liquid coolant to the interior of the conduit, and the device also includes an exhaust port, the exhaust port coupled to the conduit and adapted to receive the liquid coolant in gaseous or liquid form. In some examples, the exhaust port is coupled to a cryocooler, which can re-refrigerate the coolant and recirculate it back to the device valve. In some examples, the superconducting interconnect includes a polyimide cable, the polyimide cable including at least one superconductor strip line disposed on a surface of the cable, the polyimide cable forming a portion of the superconducting interconnect. In some examples, the device also includes a valve, the valve coupled to the conduit and adapted to receive a gaseous coolant provided at a first temperature and a first pressure, the valve further adapted to provide the gaseous coolant to the interior of the conduit at a pressure lower than the first pressure and a second temperature lower than the first temperature. In some examples, the device is adapted to receive helium or a liquid as a coolant. In some examples, the first temperature is about 4 degrees Kelvin (K) and the second temperature is below 2 degrees Kelvin. In some examples, the first temperature is about 4.2 degrees Kelvin (K) and the second temperature is below 3 degrees Kelvin. In some examples, the first temperature is about 3.6 degrees Kelvin (K) and the second temperature is below 1.2 degrees Kelvin. In some examples, the first temperature is about 4 degrees Kelvin (K) and the second temperature is below 20 degrees Kelvin. In some examples, the first temperature is lower than a temperature selected from 4.2, 4.0, 3.6, 3.0, 2.4, 2.0 or 1.2 degrees Kelvin, and the second temperature is a temperature lower than the first temperature, the second temperature is selected from 4.0, 3.6, 3.0, 2.4, 2.0 or 1.2 degrees Kelvin. In some examples, the second temperature is at least 25% less than the first temperature in degrees Kelvin. In some examples, the second temperature is at least 50% less than the first temperature in degrees Kelvin. In some examples, the second temperature is at least 75% less than the first temperature in degrees Kelvin. In some examples, a gas having a boiling point higher than helium is provided to the conduit, depending on the properties of the superconductor used in the interconnect. In some examples, in addition to or in place of a valve providing cooled liquid helium or gaseous helium, the device further comprises a heat pipe or a heat conducting belt. The heat pipe or heat conducting belt provides thermal communication between the cooling device and at least one interconnect. In some examples, the heat pipe or heat conducting belt comprises a conductor layer disposed on an insulator portion of the interconnect coupling. In some examples, the conductor layer is a metal. In some examples, the conductor layer comprises at least one of copper, aluminum, silver, gold, or titanium. In some examples, the insulator comprises at least one of polyimide, Kapton, yttrium-stabilized zirconium oxide (YSZ), Ceraflex, fluorinated ethylene propylene, polytetrafluoroethylene, polyester film, or a ceramic material.In some examples, the interconnect coupling includes at least one signal line including at least one of niobium, molybdenum nitride, or niobium titanium nitride.

[0077] In some examples of the disclosed technology, a system includes at least two sets of superconducting logic devices, a cooling device adapted to cool the at least two sets of superconducting logic devices to below a first operating temperature, at least one interconnect coupling the at least two sets of superconducting logic devices and including at least one superconducting signal line, and a cooling device in thermal communication with the at least one interconnect, the cooling device adapted to cool the at least one superconducting signal line to a second operating temperature below the first operating temperature of the at least two sets of superconducting logic devices. In some examples, in addition to or in lieu of a valve providing cooled liquid helium or gaseous helium, the system further includes a heat pipe or a heat conducting tape. The heat pipe or heat conducting tape provides thermal communication between the cooling device and the at least one interconnect. In some examples, the heat pipe or heat conducting tape includes a conductor layer disposed on an insulator portion of the interconnect coupling. In some examples, the conductor layer is a metal. In some examples, the conductor layer includes at least one of copper, aluminum, silver, gold, or titanium. In some examples, the insulator includes at least one of polyimide, Kapton, yttrium stabilized zirconium oxide (YSZ), Ceraflex, fluorinated ethylene propylene, polytetrafluoroethylene, polyester film, or a ceramic material. In some examples, the interconnect coupling includes at least one signal line including at least one of niobium, molybdenum nitride, or niobium titanium nitride. In some examples, the system also includes a hermetically sealed conduit enclosing at least a portion of the at least one interconnect, the conduit further configured to receive a coolant provided at a temperature below the first operating temperature.

[0078] In view of the many possible embodiments to which the principles of the disclosed subject matter may be applied, it should be recognized that the illustrated embodiments are merely preferred examples of the disclosed technology and should not be considered to limit the scope of the claimed subject matter. Instead, the scope of the invention is defined by the appended claims. We therefore claim as our invention all that falls within the scope of these claims.

Claims

1. A method for providing interconnection in a superconducting computing system, the method comprising: coupling a superconducting cable to the first set of superconducting logic-based devices and the second set of superconducting logic-based devices; cooling the first group of superconducting logic-based devices, the second group of superconducting logic-based devices, or the first group of superconducting logic-based devices and the second group of superconducting logic-based devices to a first temperature; as well as The superconducting cable is cooled to a second temperature lower than the first temperature. 2 . The method of claim 1 , wherein the first temperature is about 4 degrees Kelvin (K) and the second temperature is at or below 2 degrees Kelvin. 3 . The method of claim 1 , wherein the second temperature is at least 25% less than the first temperature in degrees Kelvin.

4. The method of claim 1, wherein cooling the superconducting cable to a second temperature lower than the first temperature comprises: providing a hermetically sealed conduit enclosing at least a portion of the superconducting cable; as well as A flow of helium vapor was provided into the conduit via a Joule-Thomson valve.

5. The method according to claim 4, further comprising: A flow of helium vapor is provided from the conduit via an exhaust valve.

6. The method according to claim 5, further comprising: At least a portion of the helium vapor flow from the conduit is refrigerated, and the refrigerated helium vapor flow is provided to the conduit via the Joule-Thomson valve.

7. The method of claim 1, wherein cooling the superconducting cable to a second temperature lower than the first temperature comprises: A heat pipe or heat conducting tape is provided in thermal communication with the superconducting cable.

8. The method according to claim 1, wherein: The superconducting cable comprises at least one superconducting signal line and at least one conductor plane; and Cooling the superconducting cable to a second temperature lower than the first temperature includes providing a cooling plate in thermal communication with the at least one conductor plane.

9. The method of claim 1, wherein cooling the superconducting cable to a second temperature lower than the first temperature comprises: Helium in liquid or gas phase is provided to a hermetically sealed manifold enclosing at least a portion of the superconducting cable.

10. A superconducting computing system, comprising: at least two sets of superconducting logic devices; a first cooling device adapted to cool the at least two groups of superconducting logic devices to below a first operating temperature; at least one interconnect coupling the at least two sets of superconducting logic devices, the at least one interconnect comprising at least one superconducting signal line; and A second cooling device is in thermal communication with the at least one interconnect, the second cooling device being adapted to cool the at least one superconducting signal line to a second operating temperature lower than the first operating temperature of the at least two groups of superconducting logic devices.

11. The superconducting computing system according to claim 10, further comprising: A heat pipe or heat conductive tape provides thermal communication between the second cooling device and the at least one interconnect.

12. The superconducting computing system of claim 11, wherein the heat pipe or heat conducting tape comprises a conductor layer disposed on an insulator portion of the interconnect.

13. The superconducting computing system according to claim 12, wherein: The conductor layer includes at least one of copper, aluminum, silver, gold, and titanium; The insulator comprises at least one of polyimide, fluorinated ethylene propylene, polytetrafluoroethylene, polyester film, and ceramic material; and The interconnect includes at least one superconducting signal line comprising at least one of niobium, molybdenum nitride, and niobium titanium nitride.

14. The superconducting computing system according to claim 10, further comprising: A hermetically sealed conduit encloses at least a portion of the at least one interconnect, the conduit further configured to receive a coolant provided at a temperature below the first operating temperature.

15. The superconducting computing system of claim 13, wherein the polyimide comprises Kapton.

16. The superconducting computing system of claim 13, wherein the ceramic material comprises at least one of yttria-stabilized zirconia (YSZ) and Ceraflex.

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