Flexible superconductor circuit

Abstract

The invention relates to a flexible superconductor circuit. The flexible superconductor circuit (1) comprises in this order a first substrate (2), a first superconductor layer (3) at a first side of the first substrate, a first normal conducting layer (4) and a second superconductor layer (5) wherein the first superconductor layer and the second superconductor layer, respectively, comprise a superconductor with a critical temperature larger than 4 K.

Description

[0001] Flexible superconductor circuit

[0002] Field of the invention

[0003] The invention relates to a flexible superconductor circuit for connecting electronic devices.

[0004] Background

[0005] A flexible superconducting circuit is known from US 11557709 B2. The known bilayer transmission line comprises a substrate, a copper layer and a superconducting layer of Niobium. The bi-layer transmission line is applied for connection between an external electronic control device to a cryogenic electronic system. The cryogenic electronic system may comprise, for example, qubit devices, quantum processors, sensing and detector systems, quantum internet apparatuses, medical devices, cryptographic devices, classical computing processors, and any other electronic devices. However, there are many other applications using cryogenic electronic circuits, such as multi-pixel superconducting photon detectors used in astronomy and quantum communication applications.

[0006] Cryogenic cooling equipment is provided for maintaining the cryogenic electronic circuits at the required operating temperature of near zero Kelvin. This cryogenic cooling equipment is often built up from a stack of separate temperature stages, wherein each lower stage is cooled down to a lower temperature respectively. Due to the fundamentals of thermodynamics, the power required to progressively cool down to lower temperatures increases exponentially. For example, a typical cryogenic cooling equipment consumes 20-30 kW for managing a thermal load of 12-18 pW at 100 mK.

[0007] The electronic control device is typically placed outside the cryogenic equipment to prevent power dissipation from heating up the cryogenic equipment as a whole and thus the cryogenic circuits as well. Therefore, a communication path is required for exchanging signals between cryogenic circuits at the final stage of the cryogenic equipment through the top of the cryogenic equipment to the external control electronics. Such a path is typically constructed from a cascade of semi rigid transmission lines, usually coax cables, to bridge the distance and to intercept mechanical tension and vibrations during the cooling down procedure and operation.

[0008] Recent developments show a possible shift from a part of the control electronic to cryogenics control stages, for example, at 4 K, however this still requires control lines to control devices between cryogenic stages.Cryogenic circuits, such as qubit devices, require communication with the external control device for controlling the qubits and signaling back an actual state of each qubit to be to the control device. This requires also high frequency, HF, and analogue signals. Typically, this signal can be in the range from low frequencies or DC to ultrahigh frequencies up to the infrared or visible wavelength ranges.

[0009] Recent cryogenic qubit devices have an increasing number of qubits. Each qubit requires individual communication to the control device outside the cryogenic device. This individual communication requires an increasing number of transmission lines for the qubits. For example, the qubit device can comprise 96 qubits and requires at least 288 individual transmission lines that should be guided through subsequent thermal stages to the outside. The transmission lines may comprise several coax connectors, for example, for bridging the consecutive stages of the cryogenic equipment. So, when the number of transmission lines increases, the total number of coax connectors in the transmission lines to bridge each stage is also increasing and relatively more space in the subsequent stages is required to accommodate for this increased number of coax connectors and may become a limiting condition for a further increase in numbers of qubits. The coax cables can be partly replaced by flexible superconducting transmission lines to reduce the volume of the connector. For example, eight coax cables can be replaced by a flexible superconducting transmission line comprising a dielectric layer provided with eight channels or signal lines at one side and a superconducting layer or ground layer provided at another side of the flexible superconducting transmission line. Each signal line comprises a bilayer of the superconductor layer and a copper layer. The bi-layer transmission line can be connected to the cryogenic device via known connectors. The known bi-layer transmission line has a close to zero resistance while allowing for minimal heat conductance.

[0010] |The known bi-layer transmission line transmission exhibits losses in transfer of high frequency signals corresponding to microwave lengths below the critical temperature of the superconductor.

[0011] Summary of the invention

[0012] It is therefore an object of the invention to mitigate the above indicated problem.

[0013] According to a first aspect of the invention this and other objects are achieved by a flexible superconductor circuit comprising in this order a first substrate, a first superconductor layer at a first side of the substrate, a first normal conductor layer and a second superconductor layer, wherein the first superconductor layer and the second superconductor layer, respectively,comprise a superconductor with a critical temperature larger than 4 K.

[0014] In this disclosure a superconducting circuit and a superconductor layer, and a superconducting material, respectively, are defined as a circuit, layer and a superconductor that has superconducting characteristics when exposed to temperature below the critical temperature of the superconductor that it comprises. In this disclose a superconductor that has a critical temperature larger than 4K is defined as a large-gap superconductor, for example, a niobium or niobium alloy. Furthermore, the normal conducting layer is a non-superconducting metal layer. The first substrate comprises polyimide. Layers of polyimide are suitable for forming flexible substrates with a desired thickness. A signal line can be formed by the first superconducting layer, the first normal conducting layer and the second superconducting layer. An advantage of this arrangement is that transmission of HF signals corresponding to microwave wavelengths is improved at temperatures below the critical temperature. In this disclosure a signal can be a high frequency signal in the range between 100kHz to 100 GHz. The invention is based on the insight that in the known bilayer transmission line surface currents are present in both the superconducting layer and the metal layer resulting in a non-zero resistance and increased signal loss in the transmission of HF signals. In the arrangement according to this disclosure the surface currents are predominantly existing in the superconducting layers resulting in a reduced resistance and reduced signal loss for the said high frequency signals. Furthermore, this arrangement provides a higher critical current. Furthermore, in case of any defects in the superconducting layer, the normal conducting layer provides a current by-pass to the superconducting layers. The normal conducting layer can also be used for testing at a temperature higher than the critical temperature Tc of the first and second superconducting layers. A further advantage is that it enables a communication line as a single component in contrast to a cascaded communication line existing of several pieces located in different temperature stages respectively and connected by respective connectors. The single flexible superconducting circuit can be used for communication between a control device and a cryogenic circuit with an increased length of about 1 meter.

[0015] US2024 / 396194 Al discloses a stack of a smaller gap superconductor layer, a second type layer, a larger gap superconductor layer, and again a second type layer and a smaller gap superconductor layer. Furthermore, that document discloses a communication line including a filter for reducing high-frequency signal components, wherein the larger gap superconductor layer may comprise Niobium layers and the smaller gap superconductors layers can be titanium layers, the second type layers can be copper, or the larger gap superconductor layermay comprise Niobium layers and the smaller gap superconductors layers can be tin metal layers, and the second type layers can be titanium layers. That document does not disclose that both the first type superconductor layer and the second type superconductor layer are large gap superconductors. Furthermore, the superconducting circuit according to US xx comprises a HF filter. The flexible circuit according to this disclosure is a communication line for transmitting broadband signals including HF frequency components and stabilizing DC characteristics by providing a current by-pass and thermal heatsink at locations where micro-cracks are present.

[0016] In an advantageous embodiment of the flexible superconducting circuit the first superconducting layer and the second superconducting layer comprises one of Niobium, Niobium Titanium, Niobium Titanium Nitride and Niobium Nitride. These superconductors have desired characteristics with respect to flexibility and to the critical temperature Tc. The different superconducting materials have different London penetration depths, and the thickness of the superconducting layers should be adjusted accordingly as is known by the skilled person in the field. The thickness of the subsequent layers is chosen such that the London penetration depth of the superconductor ensures that the superconductor layers carry the microwave signal. In this arrangement the screening currents in the superconducting layers are sufficient to minimize the currents in the normal conducting layer. Furthermore, in a case wherein the thickness of the first superconducting layer is not sufficient to screen the high frequency EM fields, the normal conducting layer is influenced and becomes partly superconducting, induced superconductivity, as is well-known to the skilled person. The arrangement according to this disclosure constitutes effectively a macroscopic superconductor-normal conductor-superconductor, SNS, junction wherein Andijeev scattering defines macroscopic material characteristics. The effective resistance of the normal conducting layer is influenced by the proximity effect with the superconductor layers surrounding the normal conducting layer and can be tuned to zero. The first normal conducting layer comprises one of copper Cu, copper nickel CuNi, silver Ag, and Phosphorus Bronze. An advantage of an alloy is that it has an approximately Residual Resistive Ratio, RRR of one. That is the ratio between room temperature conductivity and cryogenic conductivity as is well-known to the person skilled in the art.

[0017] In a further advantageous embodiment, the flexible superconducting circuit comprises a third superconducting layer at a second side of the first substrate facing away from the first side. This third superconducting layer acts as a ground layer. The third superconducting layercomprises one of Niobium, Niobium Titanium, Niobium Titanium Nitride and Niobium Nitride.

[0018] In a further advantageous embodiment, the flexible superconducting circuit further comprises a second normal conducting layer at a side of the third superconducting layer facing away from the first substrate. The second normal conducting layer can be a metal that is electrically conductive above the critical temperature and also has a thermalizing structure. In this disclosure thermalizing means cooling down the flexible communication circuit to a steady state. The second normal conducting layer can comprise silver (Ag), copper (Cu), gold (Au), copper nickel or phosphorus bronze. The second normal conducting layer provides a current bypass for the third superconducting layer and also a thermal path for reducing a cool down time in a cryogenic system. Further advantages are that the second normal conducting layer allows for testing at room temperature and improves solderability.

[0019] In a further advantageous embodiment, the flexible superconducting circuit comprises a first capping layer comprising Chrome or Nickel Chromium between the first superconducting layer and the first substrate and a third capping layer comprising Chrome or Nickel Chromium between the third superconducting layer and the first substrate. The first and third capping layers protect the superconducting layer against impurities dissolved in the first substrate and improves adhesion between the superconducting layers and the first substrate.

[0020] In a further advantageous embodiment, the first substrate, the first superconducting layer, the first normal conducting layer and the second superconducting layer are arranged as a microstrip.

[0021] In a further advantageous embodiment, the flexible superconducting circuit further comprises a second substrate at a side of the second superconducting layer facing away from the first normal conducting layer and a fourth superconducting layer at a second side of the second substrate facing away from the second superconducting layer. The second substrate can be like the first substrate. The fourth superconducting layer comprises one of Niobium, Niobium Titanium, Niobium Titanium Nitride and Niobium Nitride.

[0022] In a further advantageous embodiment, the flexible superconducting circuit comprises a third normal conducting layer at a side of the fourth superconducting layer facing away from the first substrate. The third normal conducting layer comprises one of copper Cu, or copper nitride Cu N, silver Ag or copper nickel Cu Ni or Phosphorus Bronze.

[0023] In a further advantageous embodiment, the flexible superconducting circuit comprises a second capping layer of Chrome or Nickel Chromium between the second superconductinglayer and the second substrate and a fourth capping layer between the fourth superconducting layer and second substrate. An advantage of the capping layers is that it improves adhesion between the substrate and the superconductor layers. A further advantage is that the capping layers protect the superconducting layers against impurities dissolved in the substrate or present in the environment.

[0024] In a further advantageous embodiment, the first substrate, the first superconducting layer, the first normal conducting layer, the second superconducting layer, the second substrate, and the fourth superconducting layer are arranged as a stripline.

[0025] The invention further relates to an electronic system comprising a flexible superconducting circuit.

[0026] These and other features and effects of the present invention will be explained in more detail below with reference to drawings in which preferred and illustrative embodiments of the invention are shown. The person skilled in the art will realize that other alternatives and equivalent embodiments of the invention can be conceived and reduced to practice without departing from the scope of the present invention.

[0027] Brief description of the drawings

[0028] Fig. 1 shows diagrammatically a cross-section in a lateral direction of flexible superconducting circuit according to an embodiment of this disclosure;

[0029] Fig. 2 shows diagrammatically a cross-section in a lateral direction of flexible superconducting circuit according to an embodiment of this disclosure;

[0030] Fig. 3 shows diagrammatically a cross-section in a lateral direction of flexible superconducting circuit according to an embodiment of this disclosure; and

[0031] Fig. 4 shows diagrammatically a cross-section in a lateral direction of the superconducting circuit according to an embodiment of this disclosure;

[0032] Fig. 5 shows diagrammatically a cross-section in a longitudinal direction of the superconducting circuit of the first normal conducting layer patterned in eight lines according to an embodiment of this disclosure; and

[0033] Fig. 6 shows diagrammatically an electronic system according to an embodiment of this disclosure.

[0034] Detailed description of embodiments

[0035] In the figures like numerals refer to similar components. The invention is explained with reference to Figs. 1-6.Although illustrative embodiments of the present invention have been described with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments. Various changes or modifications may be affected by one skilled in art without departing from the scope of the invention as defined in the claims.

[0036] Accordingly, reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, it is noted that the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0037] The flexible superconducting circuit, for example, a flexible transmission line can be used to connect a cryogenic device, for example, a cryogenic electronic circuit at a temperature of about 1 mK to an electronic device at room temperature or another electronic device at 4 K. The cryogenic electronic circuit can be a qubit device or an astronomic electronic circuit or other scientific instruments.

[0038] Fig. 1 shows diagrammatically a cross-section in a lateral direction of the superconducting circuit according to an embodiment in this disclosure. In this embodiment the flexible superconducting circuit is a flexible transmission line, The flexible transmission line comprises a first substrate 2, for example, a first foil of dielectric material. The dielectric material can be polyimide, for example, Kapton as delivered by DuPont. In an embodiment, the first foil 2 of polyimide may comprise a stack of layers of polyimide and adhesive, wherein the thickness of the stack is 50 pm. The adhesive can be epoxy-based, acrylic based, or polymer based. The stack can be adapted to obtain a predetermined thickness of the flexible superconducting circuit. The thickness of the flexible superconducting transmission line is about 0,25 mm. The first foil 2 has a rectangular shape. The length of and width of the flexible superconducting transmission line can be 1 m and 10 mm respectively. Furthermore, the flexible superconducting transmission line 1 comprises, a first superconducting layer 3 at a first side of the first foil. The first superconducting layer 3 is a thin film of niobium (Nb). niobium titanium (NbTi), niobium titanium nitride (NbTiN), titanium nitride (TiN) or other Niobium alloys. These superconducting materials have a critical temperature layer than 4K and are known as large cap superconductors. For example, the critical temperature of Niobium is 9.2 K. In embodiments the thickness of the first superconducting layer 3 can be in a range 50 nm- 2 pm, preferably between 150 - 400 nm and more preferably 250 nm. Theskilled person can select the thickness of the first superconducting layer 3 depending on process time of deposition of the superconductor, flexibility and maximum current through the superconducting circuit layer. A thickness of 250 nm for the first superconducting layer 3 appears well suitable with respect to flexibility of the superconducting layer and a desired maximum current through the superconducting circuit. The width of the first superconducting layer is in the range between 5 and 500 pm, preferably 150 pm. In embodiments the maximum current through the first superconducting layer is in a range between 2 and 5 mA.

[0039] Furthermore, the flexible superconducting transmission line 1 comprises a first normal conducting layer 4 at a side of the first superconducting layer 3. The first normal conducting layer 4 can be silver, copper, copper nickel, phosphorus bronze or another alloy. The thickness of the first normal conducting layer 4 can be in the range between 50 nm and 6 pm, preferably between 500 nm. This thickness allows for real time testing at room temperature. The width of the first normal conducting layer 4 is equal to that of the first superconducting layer 3.

[0040] Advantages of the first normal conducting layer 4 are that it provides current stabilization in the first superconducting layer 3, that the first normal conducting layer 4 is in electrical contact with the first superconducting layer 3, and that the first normal conducting layer 4 improves solderability, and that provides the first normal conducting layer provides a current bypass for the first superconducting layer and improves mechanical stability.

[0041] Furthermore, the first normal conducting layer 4 provides electrical conductivity above the critical temperature of the first superconducting layer 3 and can be used for connection between the first superconducting layer 3 and an external electronic device. Furthermore, the flexible transmission line 1 comprises a second superconducting layer 5 at the other side of the first normal conducting layer 4 facing away from the first superconducting layer 3. The second superconducting layer can be like the first superconducting layer 3.

[0042] Furthermore, the first normal conducting layer 4, the first superconducting layer 3 and the second superconducting layer 5 can be patterned in a plurality of lines or traces. For example, eight lines. The first and second superconducting layers 3,5 on the outside of the first normal conducting layer 4 transmit a high frequency signal at temperatures below the critical temperature. In this disclosure a high frequency signal is a signal with a frequency in the range between 100kHz to 100 GHz. The arrangement of the first superconducting layer 3, the normal conducting layer 4 and the second superconducting layer 5 according to this disclosure reduces the resistance experienced by high frequency signals at temperatures belowthe critical temperature compared to the known bi-layer circuit, while still allowing for testing at temperatures above the critical temperature of the first and second superconducting layers.

[0043] For example, this superconducting transmission line improves communication for a small signal red out from the qubits by a control circuit at room temperature because of the reduced resistance in a portion of the superconducting transmission line in a stage at a temperature below the critical temperature where the qubits are located.

[0044] Furthermore, the superconducting transmission line 1 comprises a third superconducting layer 6 arranged at a second side of the first foil 2 facing away from the first superconducting layer 3. The third superconducting layer 6 can be made of Niobium, Niobium Titanium, Niobium Titanium Nitride, Niobium Nitride or another Niobium alloy. The length and width of the third superconducting layer are equal to the first foil 2. The third superconducting layer 6 can be applied as a ground plane. The thickness of the third superconducting layer 6 is in the range between 50 nm and 2 pm, preferably 200nm. This thickness covers approximately the London penetration depth of the superconducting material of the layer. London penetration depth is well known to the person skilled in the art.

[0045] Furthermore, this thickness of the superconducting layer 6 allows for good connectivity. This third superconducting layer 6 acts as a ground plane.

[0046] Optionally, the superconducting transmission line comprises a second normal conducting layer 7 at the side of the third superconducting layer 6 facing away from the first foil 2. The second normal conducting layer can comprise one of silver, copper, copper nickel, phosphorus bronze or another alloy. The length and width of the second normal conducting layer 7 are equal to the third superconducting layer 6. The second normal conducting layer 7 improves solderability, provides a current bypass to the third superconducting layer and improves mechanical stability. Furthermore, the third non-superconducting metal layer 7 provides electrical conductivity above the critical temperature of the second superconducting layer 6 and can be used for connection between the second superconducting layer 6 and an external electronic device.

[0047] In this embodiment the first substrate 2, the first superconducting layer 3, the first normal conducting layer 4, the second superconducting layer 5, the third superconducting layer 6, and the second normal conducting layer 7 are arranged as a microstrip. For example, for communication between a cryogenic device such as a qubit device at a temperature of about 4K and an electronic device at room temperature.

[0048] Furthermore, the first normal conducting layer 4, the first superconducting layer 3, and the second superconducting layer 5, can be patterned in a plurality of lines on the firstsubstrate 2. For example, eight lines. In embodiments the flexible superconducting circuit also comprises at least one of a resistor, a capacitor and a coil.

[0049] Fig. 2 shows diagrammatically a cross-section in a lateral direction of a superconducting transmission line 20 according to an embodiment of this disclosure. The first substrate 2, the first superconducting layer 3, the first normal conducting layer 4, the second superconducting layer 5, the third superconducting layer 6, and the second normal conducting layer 7 of the flexible superconducting transmission 20 are like those as described with respect to Fig. 1. Furthermore, in this embodiment the flexible superconducting transmission line 20 comprises a first capping layer 8 of chromium between the first foil 2 and the first superconducting circuit 3 and a second capping layer 9 of chromium at a side of the second superconducting layer 5 facing away from the first normal conducting layer. The second capping layer 9 provides protection against impurities in the environment. Alternatively, the first and second capping layers 8,9 are made of nickel chromium. The thickness of the respective first and second capping layers is in the range between 5 and 50 nm, preferably 20 nm.

[0050] Furthermore, a third capping layer of chromium 10 can be applied between the first foil 2 and the third superconducting layer 6. The thickness of the third capping layer is in the range between 5 and 50 nm, preferably 20 nm. An advantage of the capping layers is that it improves adhesion between the superconducting layers 3,5 6 and the first foil 2. The capping layers of chromium protects the superconducting layers against impurities dissolved in the polyimide and act as an adhesion layer between the superconducting layers and polyimide due to chemical bonding. Alternatively, the third capping layer 8 is made of nickel chromium.

[0051] In this embodiment the first substrate 2, the first superconducting layer 3, the first normal conducting layer 4, the second superconducting layer 5, the third superconducting layer 6, the first capping layer 8, the second capping layer 9, the second normal conducting layer 7, and the third capping layer 10 are arranged as a microstrip. For example, for communication between a cryogenic device such as a qubit device at a temperature of about 4K and an electronic device at room temperature. Furthermore, the first normal conducting layer 4, the first superconducting layer 3, the second superconducting layer 5, the first capping layer 8 and the second capping layer 9 can be patterned in a plurality of lines. For example, eight lines.

[0052] Fig. 3 shows diagrammatically a cross-section in a lateral direction of the flexible superconducting transmission line 30 according to an embodiment in this disclosure. The first substrate or foil 2, the first superconducting layer 3, the normal conducting layer 4, the secondconducting layer 5, the third superconducting layer 6 and the third conducting layer 7 of the flexible superconducting layer 30 are like those as described with respect to fig. 2. In this embodiment the flexible superconducting transmission line further comprises a second substrate 11 or second foil of dielectric material, at a side of the second superconducting layer 5 facing away from the first normal conducting layer 4. The dielectric material of the second foil 11 is similar to that of the first foil 2. In embodiments the first and second foil can be formed in a single foil.

[0053] Furthermore, the superconducting transmission line 30 comprises a fourth superconducting layer 12 at the second side of the second foil 11 facing away from the second superconducting layer 5. The fourth superconducting layer 12 comprises niobium, niobium titanium nitride, niobium nitride or titanium nitride. The fourth superconducting layer 12 can be like the third superconducting layer 6. The fourth superconducting layer 12 also acts as a ground plane.

[0054] In an embodiment the superconducting transmission line 30 comprises a third normal conducting layer 13 at the side of the fourth superconducting layer 12 facing away from the second superconducting layer 5. The third normal conducting layer 13 is like the second normal conducting layer 7. The third normal conducting layer 13 improves solderability, provides a current bypass to the fourth superconducting layer 12 and improves mechanical stability. Furthermore, the third normal conducting layer 13 can be used for connection between the fourth superconducting layer 12 and an external electronic device.

[0055] In this embodiment the first foil 2, the first superconducting layer 3, the first normal conducting layer 4, the second superconducting layer 5, the third superconducting layer 6, the second normal conducting layer 7, the second foil 11, the fourth superconducting layer 12 and the third normal conducting layer 13 are arranged as a stripline. For example, for communication between a cryogenic device such as a qubit device at a temperature of about 20 mK and an electronic device at room temperature. Furthermore, the first normal conducting layer 4, the first superconducting layer 3 and the second superconducting layer 5 can be patterned in a plurality of lines. For example, eight lines.

[0056] Fig. 4 shows diagrammatically a cross-section in a lateral direction of the flexible superconducting transmission line according to an embodiment in this disclosure. The foil 2, the first superconducting layer 3, the first normal conducting layer 4, the second conducting layer 5, the third superconducting layer 6, the third conducting layer 7, the second foil 11, the fourth superconducting layer 12 and the third normal conducting layer 13 of the flexible superconducting transmission line 40 are like that described with respect to Fig. 3. In thisembodiment the flexible superconducting transmission line 40 comprises a first capping layer 8 of chromium between the first foil 2 and the first superconducting layer 3 and a second capping layer 9 of chromium between the second foil 11 and the second superconducting layer 5. Alternatively, the first and second capping layers 8,9 are made of nickel chromium. The thickness of the first and second capping layers 8,9 is in the range between 5 and 50 nm, preferably 20 nm. An advantage of the capping layers is that it improves adhesion between the superconducting layers 8,9 and the first foil 2 and the second foil 11 respectively.

[0057] Furthermore, the capping layers provide protection against impurities in the polyamide of the first and second foil.

[0058] Furthermore, a third capping layer 10 of chromium is applied between the first foil 2 and the third superconducting layer 6 and a fourth capping layer 14 is applied between the fourth superconducting layer 12 and the second foil 11. An advantage of the third and fourth capping layers is that these improve adhesion between the third and fourth superconducting layers 6, 12 and the first foil 2 and second foil 11, respectively and provide protection against impurities in the polyamide of the first and second foil. Alternatively, the third and fourth capping layers 10,14 are made of nickel chromium. In this embodiment the first foil 2, the first superconducting layer 3, the first normal conducting layer 4, the second superconducting layer 5, the third superconducting layer 6, the second normal conducting layer 7, the second foil 11, the fourth superconducting layer 12., the third normal conducting layer 13, the first capping layer 8, the second capping layer 9, the third capping layer 10 and the fourth capping layer 14 are arranged as a stripline. For example, for communication between a cryogenic device such as a qubit device at a temperature of about 20 mK and an electronic device at room temperature. Furthermore, the first normal conducting layer 4, the first superconducting layer 3, the second superconducting layer 5, the first capping layer 8 and the second capping layer 9 can be patterned in a plurality of lines. For example, eight lines.

[0059] Fig. 5 shows diagrammatically a cross-section of the first normal conducting layer 4 in a longitudinal direction of the flexible superconducting communication 20 according to an embodiment of this disclosure. In this embodiment the first normal conducting layer 4 is patterned as a plurality of parallel lines or traces 21, for example, eight parallel lines.

[0060] Furthermore, the first normal conducting layer 4 is provided with first and second contact pads 22,23 wherein the first contact pad 22 is arranged at one end of the respective lines 21 and the second contact pad 24 is arranged at another end of the respective lines 21. The lines 21 and contact pads 22,23 can be obtained by lift-off processes, laser ablation, chemicaletching or reactive ion etching of the first normal conducting layer 4. These processes are well-known to the person skilled in the art.

[0061] Fig. 6 shows diagrammatically an electronic system 50 comprising an electronic device 51, a cryogenic circuit 53 and a flexible planar superconducting transmission line 1 according to an embodiment of this disclosure described with respect to Figs.1-5. In this arrangement the electronic device 51 is an electronic control circuit. The cryogenic circuit 53 is a cryogenic electronic circuit provided in a cryogenic cooling device 52. Cryogenic electronic devices can be a qubit device or an astronomic electronic circuit or other scientific instruments. The cryogenic cooling device 52 can be a dilution refrigerator that has several stages 54 each kept at a predetermined temperature to bridge the environment temperature of 290 K to the cryogenic temperature of, for example, 10 mK. The stages 54 are separated by plates 55 provided with apertures large enough to pass the flexible planar superconducting transmission line 1. The plates 55 can be provided with clamping devices 56 made of a thermal conductive material to keep the portion of the superconducting transmission line passing the apertures at the temperatures of the respective plates.

[0062] The flexible superconducting transmission line 1 enters dilution refrigerator 52 through a dedicated port 57. The flexible superconducting transmission line 1 provides communication between the electronic control circuit and the cryogenic electronic circuit.

[0063] A process to manufacture the flexible superconducting transmission line like the one described in fig 1 comprises providing a first substrate or foil 2 of a dielectric material, and deposition of subsequently the first superconducting layer 3, the first normal conducting layer 4 and the second superconducting layer 5. The dielectric material is a polyimide, for example Kapton. The thickness of the polyimide layer is 50 pm. The deposition of the subsequent layers can be in-situ performed in a vacuum deposition apparatus. The first and superconducting layers 3,5 can be Niobium, Niobium Titanium, Niobium Titanium Nitride or Niobium Nitride. The thickness of the first superconducting layer 3 and second superconducting layer 5 are 250 nm. The first normal conducting layer 4 can be silver, copper, copper nickel, phosphorus bronze or another alloy. The thickness of the first normal conducting layer 4 is 500nm. Optionally, a second normal conducting layer 7 is deposited on the other side of the first foil 2 facing a from the first superconducting layer 3. The thickness of the second normal conducting layer is 200 nm. After deposition of the first normal conducting layer 4 the contact pads 24,25 can be obtained by lift-off processes, laser ablation, chemical etching or reactive ion etching the first normal conducting layer 4.The invention relates to a flexible superconducting circuit. The flexible superconducting circuit comprises in this order a first substrate, a first superconducting layer at a first side of the first substrate, a first normal conducting layer and a second superconducting layer, wherein the first superconductor layer and the second superconductor layer, respectively, comprise a superconductor with a critical temperature larger than 4 K.

[0064] This arrangement provides a reduced resistance and reduced signal loss for high frequency signals through the superconducting circuit at temperatures below the critical temperature, while still allowing for testing at temperatures above the critical temperature of the first and second superconducting layers.

[0065] The invention can be summarized by the following clauses:

[0066] 1. A flexible superconducting circuit (1) comprising in this order

[0067] a first substrate (2);

[0068] a first superconducting layer (3) at a first side of the first substrate;

[0069] a first normal conducting layer (4); and

[0070] a second superconducting layer (5).

[0071] 2. The flexible superconducting circuit according to clause 1 wherein the first superconducting layer (3) and the second superconducting layer (5) comprises one of Niobium, Niobium Titanium, Niobium Titanium Nitride and Niobium Nitride.

[0072] 3. The superconducting circuit according to clause 1 or 2 wherein the first normal conducting layer (4) comprises silver, copper, copper nickel, phosphorus bronze. 4. The flexible superconducting circuit according to any of the clauses 1-3 comprising a third superconducting layer (6) at a second side of the first substrate (2) facing away from the first side.

[0073] 5. The flexible superconducting circuit according to clause 4, wherein the third superconducting layer (6) comprises one of Niobium, Niobium Titanium, Niobium Titanium Nitride, and Titanium Nitride.

[0074] 6. The flexible superconducting circuit according to clause 4 or 5, comprising a second normal conducting layer (7) at a side of the third superconducting layer (6) facing away from the first substrate (2).

[0075] 7. The superconducting circuit according to clause 6 wherein the second normal conducting layer (7) comprises silver, copper, copper nickel, phosphorus bronze. 8. The flexible superconducting circuit according to any of the clauses 4 - 7 comprising a first capping layer (8) comprising Chrome or Nickel Chromium between the firstsuperconducting layer (3) and the first substrate (2) and a third capping layer (10) comprising Chrome or Nickel Chromium between the third superconducting layer (6) and the first substrate (2) respectively.

[0076] The flexible superconducting circuit according to clause 1-5, wherein the first substrate (2), the first superconducting layer (3), the first normal conducting layer (4) and the second superconducting layer (5) are arranged as a microstrip.

[0077] The flexible superconducting circuit according to any of the clauses 1- 9 further comprising a second substrate (11) at a side of the second superconducting layer (5) facing away from the first normal conducting layer (4) and a fourth superconducting layer (12) at a second side of the second substrate (11).

[0078] The flexible superconducting circuit according to clause 10, wherein the fourth superconducting layer (12) comprises one of Niobium, Niobium Titanium and Niobium Titanium Nitride.

[0079] The flexible superconducting circuit according to clause 10 or 11, comprising a third normal conducting layer (13) at a side of the fourth superconducting layer (12) facing away from the second substrate.

[0080] The flexible superconducting circuit according to any of the clauses 10-12 comprising a second capping layer (9) comprising Chrome or Nickel Chromium between the second superconducting layer (5) and the second substrate (11) and fourth capping layer (14) comprising Chrome or Nickel Chromium between the fourth superconducting layer (12) and second substrate (11) respectively.

[0081] The flexible superconducting circuit according to any of the clauses 10-13, wherein the first substrate (2), the first superconducting layer (4), the first normal conducting layer (5), the second superconducting layer (6), the second substrate (12), and the fourth superconducting layer (12) are arranged as a stripline.

[0082] Electronic system comprising a flexible superconducting circuit as described in any of the clauses 1-14.Although illustrative embodiments of the present invention have been described with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments. Various changes or modifications may be affected by one skilled in the art without departing from the scope of the invention as defined in the claims. Accordingly, reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, it is noted that the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

Claims

Claims1. A flexible superconductor circuit (1) comprising in this ordera first substrate (2),a first superconductor layer (3) at a first side of the first substrate,a first normal conductor layer (4); and.a second superconductor layer (5), wherein the first superconductor layer and the second superconductor layer, respectively, comprise a superconductor with a critical temperature larger than 4 K.

2. The flexible superconductor circuit according to claim 1 wherein the first superconductor layer (3) and the second superconductor layer (5) comprises one of Niobium, Niobium Titanium, Niobium Titanium Nitride, Niobium Nitride or Niobium-alloy.

3. The flexible superconductor circuit according to claim 1 or 2 wherein the first normal conducting layer (4) comprises silver, copper, copper nickel, or phosphorus bronze.

4. The flexible superconductor circuit according to any of the claims 1-3 comprising a third superconductor layer (6) at a second side of the first substrate (2) facing away from the first side.

5. The flexible superconducting circuit according to claim 4, wherein the third superconductor layer (6) comprises one of Niobium, Niobium Titanium, Niobium Titanium Nitride, Titanium Nitride.

6. The flexible superconductor circuit according to claim 4 or 5, comprising a second normal conducting layer (7) at a side of the third superconducting layer (6) facing away from the first substrate (2).

7. The flexible superconductor circuit according to claim 6 wherein the second normal conducting layer (7) comprises silver, copper, copper nickel, or phosphorus bronze.

8. The flexible superconductor circuit according to any of the claims 4 - 7 comprising a first capping layer (8) comprising Chrome or Nickel Chromium between the first superconductor layer (3) and the first substrate (2), and a third capping layer (10) comprising Chrome or Nickel Chromium between the third superconductor layer (6) and the first substrate (2), respectively.

9. The flexible superconductor circuit according claim 1-5, wherein the first substrate (2), the first superconductor layer (3), the first normal conducting layer (4) and the second superconductor layer (5) are arranged as a microstrip.

10. The flexible superconductor circuit according to any of the claims 1- 9 further comprising a second substrate (11) at a side of the second superconductor layer (5) facing away from the first normal conducting layer (4), and a fourth superconductor layer (12) at a second side of the second substrate (11).

11. The flexible superconductor circuit according to claim 10, wherein the fourth superconductor layer (12) comprises one of Niobium, Niobium Titanium and Niobium Titanium Nitride.

12. The flexible superconductor circuit according to claim 10 or 11, comprising a third normal conducting layer (13) at a side of the fourth superconductor layer (12) facing away from the second substrate.

13. The flexible superconductor circuit according to any of the claims 10-12 comprising a second capping layer (9) comprising Chrome or Nickel Chromium between the second superconductor layer (5) and the second substrate (11) and fourth capping layer (14) comprising Chrome or Nickel Chromium between the fourth superconductor layer (12) and second substrate (11) respectively.

14. The flexible superconductor circuit according to any of the claims 10-13, wherein the first substrate (2), the first superconductor layer (4), the first normal conducting layer (5), the second superconductor layer (6), the second substrate (12), and the fourth superconductor layer (12) are arranged as a stripline.

15. Electronic system comprising a flexible superconductor circuit as claimed in any of the claims 1-14.19

Images