Electrical assembly for a cryogenic cooling system

Abstract

An electrical assembly (1) is provided for a cryogenic cooling system, the electrical assembly comprising: a printed circuit board, (PCB 2); and a plurality of thermal connectors (6, 7, 8, 9) arranged along the PCB 2. Each said thermal connector (6, 7, 8, 9) is distally separated from an adjacent thermal connector by a respective folded portion (3, 4, 5) of the PCB 2.

Description

[0001] ELECTRICAL ASSEMBLY FOR A CRYOGENIC COOLING SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of cryogenic cooling systems and to an electrical assembly for such a system.

[0004] BACKGROUND TO THE INVENTION

[0005] Cryogenic cooling systems are commonly used to cool apparatus to temperatures below 100 Kelvin. These systems typically comprise a vacuum chamber containing a plurality of spatially dispersed thermal stages (also referred to as “flanges”), and a refrigerator that is arranged to cool one or more of the stages. Heat radiation shields are typically also provided to shield the lower temperature components from thermal radiation from higher temperature components of the system. An example of such a system is a dilution refrigerator. Such systems are commonly used for experimental purposes in which a sample (also referred to herein as a “device for measurement”) is mounted at a low temperature stage (typically the base temperature stage) and communicatively coupled to a processing device at a higher temperature location by an electrical assembly. This electrical assembly is also referred to herein as an “interconnect”.

[0006] An interconnect generally comprises an assembly of electrical connections, such as cables, extending from a relatively high temperature location to a relatively low temperature location inside the system. The interconnect is typically connected to one or more of the intermediate temperature stages for reducing the heat load onto the lower temperature stages. The components and structure of the interconnect may vary depending on the application requirements but may include coaxial cables, triaxial cables, twisted pair cables and microwave waveguides. Relevant considerations when designing an interconnect include impedance matching and reducing noise, losses and reducing heat conduction along the interconnect.

[0007] Quantum Information Processing, QIP, is one example of an experimental field in which there is an increasing demand to scale up the number of electrical connections that are provided to the device (which in the context of QIP may comprise a qubit array). This increasingly difficult to achieve within the limited space available in a cryogenic cooling system using existing interconnect designs.

[0008] SUMMARY OF THE INVENTION

[0009] A first aspect of the invention is an electrical assembly for a cryogenic cooling system, comprising: a printed circuit board, PCB; and a plurality of thermal connectors arranged along the PCB, wherein each thermal connector is arranged on the outside of the PCB and comprises a region of high thermal conductivity material; wherein each said thermal connector is distally separated from an adjacent thermal connector by a respective folded portion of the PCB; wherein the electrical assembly is elongate in a first axis along which the PCB substantially extends, the thermal connectors being separated from each other along the first axis, wherein each said folded portion extends along two or more planes and comprises two overlapping regions of the PCB having a normal in the first axis, the two overlapping regions being mutually connected by a connection region of the PCB having a normal orthogonal to the first axis, wherein the overlapping regions are connected to the adjacent thermal connectors by respective upper and lower regions of the PCB having a normal orthogonal to the first axis.

[0010] A PCB is therefore used to facilitate electrical connections across the electrical assembly. This provides a scalable and customisable solution for the end-user which can accommodate a higher density of electrical connections than conventional interconnects. The thermal connectors may be arranged in use to connect to respective thermal stages of a cryogenic cooling system for thermally grounding the PCB and reducing thermal conduction across the interconnect. The electrical assembly is configured so that the PCB is folded between each pair of adjacent thermal connectors to form a respective folded portion. This may provide several benefits. For example, it increases the creepage distance (i.e. the length of the conductive path along the surface of the PCB between adjacent thermal connectors), which reduces the effective thermal conductivity across the electrical assembly. This ensures that the electrical assembly does not place an undesirable heat load onto the low temperature components of a system to which it may be connected in use.

[0011] A flex PCB is typically used to form the PCB. This may be folded to form the folded portion(s) of the interconnect, and rigid portions of the PCB may be used to form the thermal connectors. Each said folded portion is preferably deformable to accommodate a change in the separation between adjacent thermal connectors. This provides the electrical assembly (or “interconnect”) with some mechanical tolerance so that an effective connection to components of the cryogenic cooling system can be maintained during thermal cycling (when the separation between the different temperature components of the system may change, creating strains on the electrical assembly). The electrical assembly may be substantially elongate along a first axis. In use, a thermal gradient is typically applied to the electrical assembly by the cryogenic cooling system, the thermal gradient being parallel to the first axis. The folded portion(s) of the PCB increase the elastic deformation of the electrical assembly, for example to allow a change in separation between adjacent thermal connectors of ±2 mm along the first axis. Each said folded portion of the PCB extends along two or more planes, which are preferably orthogonal. This contributes towards reducing the effective thermal conductivity across the electrical assembly and increasing the elastic deformation of the electrical assembly between adjacent thermal conductors.

[0012] As noted above, the electrical assembly is elongate in a first axis along which the PCB substantially extends. The thermal connectors are separated from each other along the first axis. Each said folded portion comprises two overlapping regions of the PCB having a normal in the first axis, the two overlapping regions being mutually connected by a connection region of the PCB having a normal orthogonal to the first axis. The overlapping regions are connected to the adjacent thermal connectors by respective upper and lower regions of the PCB having a normal orthogonal to the first axis (and optionally orthogonal to the normal of the connecting region also). This arrangement increases the creepage and elastic deformation of the PCB along the first axis. Additionally, it makes effective use of a notional sized area of PCB panel that may be cut and folded to form the PCB of the first aspect.

[0013] The electrical assembly typically takes the form of an insert for a cryogenic cooling system. In this way, the assembly may be fitted to or removed from a cryogenic cooling system as a unitary body. This allows for convenient installation to existing cryogenic cooling systems and for adjustment of the assembly on a benchtop, for example to change the components of the assembly based on the experimental need.

[0014] It is desirable that the electrical assembly does not form a heat leak that may prevent the low temperature components of the system from cooling to their intended operational base temperatures. Low thermal conductivity materials are therefore preferably used along the PCB for forming any electrical transmission lines or “traces”, these being separated by one or more layers of an electrically insulating material also having a low thermal conductivity. A low but finite thermal conductivity may be for example between 1 and 50 W / (m*K) at 100 K, between 0.1 and 5 W / (m*K) at 10 K, and between 0.01 and 0.75 W / (m*K) at 1 K. Typically, the effective thermal conductivity across each said folded portion is below 50 W / m K at a temperature of 20 K. An example of a suitable material used for forming the electrical connections in the PCB is a copper-nickel alloy, such as Constantan (comprising around 55% copper and 45% nickel) or Manganin (comprising around 84% copper, 12% manganese and 4% nickel). The PCB therefore preferably comprises electrical transmission lines formed from a copper-nickel alloy, optionally further comprising manganese and / or nickel. The electrically conductive material therefore forms a routing material and may be provided as a foil.

[0015] The electrical assembly has a plurality of regions of high thermal conductivity material, referred to as thermal connectors. A high thermal conductivity may be for example at least 100 W / (m*K) at or above 10 K, and at least 10 W / (m*K) at 1 K. An example of a suitable material for the thermal connector is copper. The plurality of thermal connectors typically comprises a first thermal connector and a second thermal connector, wherein the first thermal connector is thermally coupled to a first end of the PCB and the second thermal connector is thermally coupled to a second end of the PCB, wherein the PCB is configured to transmit electrical signals between the first end and the second end. The thermal connectors may take the form of a layer or block of high thermal conductivity material. Each thermal connector is arranged on the outside of the PCB for mechanically connecting to components of a cryogenic cooling system. For example, each said thermal connector is preferably configured to connect to a respective flange or “thermal stage” of the cryogenic cooling system. One or each thermal connector may be integrally formed within the PCB or it may be a separate component, such as an anchor or clamp, that is applied onto the PCB. The electrical assembly preferably comprises four said thermal connectors, each said thermal connector for connecting to a respective thermal stage of a cryogenic cooling system.

[0016] Typically, the plurality of thermal connectors comprises a first thermal connector and a second thermal connector; the electrical assembly further comprising: a first set of electrical components provided at the first thermal connector; and a second set of electrical components provided at the second thermal connector; wherein the first set of electrical components is electrically connected to the second set of electrical components by the PCB. In this scenario, the second thermal connector is typically on an opposite end of the electrical assembly from the first thermal connector. The second set of electrical components may comprise one or more electrical connectors, preferably for communicatively coupling with a device for measurement, which may be positioned on a thermal stage of the cryogenic cooling system in use (typically the base temperature stage). Alternatively, a device for measurement may be connected directly to the electrical assembly. For example, the second set of electrical components may comprise the device for measurement, which may be physically connected to the PCB. This provides the advantage of reducing the number of electrical connections / joints to be formed, which reduces signal integrity losses that can otherwise occur.

[0017] The device for measurement is preferably cooled below 1 K, preferably below 0.5 K, preferably still below 50 mK or 20 mK in use by components of the cryogenic cooling system. The device for measurement preferably forms part of a quantum

[0018] RECTIFIED SHEET (RULE 91) ISA / EP processing system and is preferably a qubit array comprising one or more qubits. The electrical assembly is preferably configured to supply electrical power to the device for measurement and may be communicatively coupled to perform read / write operations on the device.

[0019] Within a PCB stack, the PCB preferably comprises a first layer of electrically conductive material separated from a second layer of electrically conductive material by an electrically insulating layer. These layers of electrically conductive material may be used to form the traces. For example, the first layer may form an array of RF transmission lines and the second layer may form a backplane. Additionally (e.g. in an adjacent segment of the PCB) or alternatively, the first layer may form a first array of DC transmission lines and the second layer may form a second layer of DC transmission lines. This layered arrangement typically continues unbroken and throughout the PCB, across the folded portion(s) and the thermal connectors. The backplane may be a hatched (or “meshed”) ground plane. This reduces the effective width of the conduction along the backplane and between adjacent thermal connectors.

[0020] If DC lines are provided, these may be formed by the first and / or second layers of electrically conductive material in the PCB stack. The interconnect preferably comprises a first array of DC lines arranged on a first layer of the PCB and a second array of DC lines arranged on a second layer of the PCB, the first layer overlapping the second layer. A high density of electrical connections can then be provided. Optionally, the first array may provide an input channel to a device for measurement and the second array may provide a return channel. The array of RF lines and arrays of DC lines preferably electrically and communicatively couple the first set of electrical components to the second set of electrical components. Typically, the array of RF lines is arranged within a first segment of the PCB, and the first array of DC lines and the second array of DC lines is arranged within a second segment of the PCB, the second segment preferably being adjacent to the first segment. The two segments may correspond to distinct and non-overlapping widths of the PCB, through the thickness of the PCB. This further contributes towards making efficient use out of the space available on the PCB so that a high density of connections can be included.

[0021] Further aspects of the invention will now be discussed. Any of the features discussed in connection with one aspect may be equally applied in respect of the remaining aspects and share similar benefits.

[0022] A second aspect of the invention is a cryogenic cooling system comprising: a vacuum chamber; a plurality of thermal stages arranged inside the vacuum chamber; a cryogenic cooling device configured to apply a thermal gradient across the plurality of thermal stages; and an electrical assembly according to the first aspect, wherein each said thermal connector is connected to a respective said thermal stage.

[0023] The second aspect shares similar advantages as discussed above for the first aspect. The cryogenic cooling system of the second aspect may take a number of different forms. For example, it may be a “wet” system reliant of cryogenic exchange gases or liquid cryogens (typically helium or nitrogen contained within a dewar) for providing the majority of the cooling power. Alternatively it may be a “dry” or “cryogen-free” system reliant on one or more mechanical refrigerators (also referred to as cryocoolers), such as a pulse tube refrigerator, Stirling cooler or Gifford McMahon cooler for cooling the system. The cryogenic cooling device preferably comprises a pulse tube refrigerator. The cryogenic cooling device may comprise a plurality of refrigerators, some of which may be configured to obtain base temperatures of 1 K of less. For example, the cryogenic cooling device preferably further comprises a dilution refrigerator.

[0024] In use, the interface is typically arranged within an evacuated space formed by the vacuum chamber. The plurality of thermal stages preferably comprises a base temperature stage configured to be cooled to the lowest temperature of the plurality of thermal stages by the cryogenic cooling device, the base temperature stage being thermally coupled to a device for measurement, wherein the electrical assembly is communicatively coupled to the device for measurement. In particular, the device for measurement is preferably communicatively coupled or electrically connected to the PCB of the electrical assembly. The device for measurement may be arranged on the base temperature stage or on the electrical assembly itself, as earlier discussed. In use, the base temperature stage is preferably configured to be cooled to below 5 K, preferably below 20 mK by the cryogenic cooling device.

[0025] The invention finds particular benefit in the field of QIP where there is an increasing demand to scale up the number of electrical connections within a cryogenic cooling system. A third aspect of the invention is therefore a quantum processing system, comprising: a cryogenic cooling system according to the second aspect; and a qubit array comprising one or more qubits communicatively coupled to the electrical assembly. The qubit array may therefore form an example of the device for measurement earlier discussed. The QIP system typically comprises one or more electronic components configured to perform read / write operations on the qubit array. These components may be inside or outside the vacuum chamber and may comprise a processing device, such as a computer.

[0026] A fourth aspect of the invention is an electrical assembly for a cryogenic cooling system, comprising: a PCB having a plurality of rigid portions arranged along the PCB, each said rigid portion comprising a material having a high thermal conductivity, and one or more flexible portions, each said flexible portion connecting adjacent rigid portions of the PCB. The rigid portions may correspond to the thermal connectors discussed with reference to the first aspect. The flexible portion(s) may be folded to form the folded portion(s) from the first aspect.

[0027] BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Embodiments of the invention will now be discussed with reference to the following drawings, in which:

[0029] Figure 1 is a first perspective view of an electrical assembly according to a first embodiment of the invention;

[0030] Figure 2 is a second perspective view of the electrical assembly of the first embodiment;

[0031] Figure 3 is a schematic illustration of the electrical assembly of the first embodiment installed within a cryogenic cooling system;

[0032] Figure 4 is a PCB panel for forming the electrical assembly of the first embodiment;

[0033] Figure 5 is an enlarged view of a portion of the PCB panel shown in Figure 4; Figure 6 is a perspective view of a first surface of the PCB panel of the first embodiment; and

[0034] Figure 7 is a perspective view of a second surface of the PCB panel of the first embodiment.

[0035] DETAILED DESCRIPTION

[0036] An electrical assembly or “interconnect” 1 in accordance with a first embodiment of the invention is shown in Figures 1 to 3. Figure 1 shows a front-upper perspective view of the interconnect 1 whereas Figure 2 shows a rear-lower perspective view of the interconnect 1. The interconnect is substantially elongate along a first direction, which is in the vertical Z axis shown in Figure 1. The interconnect 1 is primarily formed by a PCB 2 which extends substantially along the first direction between a first set of electrical components and a second set of electrical components. The first set of electrical components takes the form of a first connector assembly 17 that is arranged at a first end of the interconnect 1. The second set of electrical components takes the form of a second connector assembly 19 that is arranged at a second end of the interconnect 1 . In use, the interconnect 1 is installed within a cryogenic cooling system and a thermal gradient is applied to the interconnect 1 by one or more cryogenic cooling devices 40, 41 , 42 of the system (Figure 3). The thermal gradient extends along the first direction with the first end being the warm end of the interconnect 1 and the second end being the cold end.

[0037] The PCB 2 is connected to a first plate 10, a second plate 11 , a third plate 12 and a fourth plate 13 which are separated from each other and spatially dispersed along the Z axis. Each said plate 10-13 forms part of a respective thermal stage 30-33 of a cryogenic cooling system and is cooled to a different base temperature in use. The plates 10-13 are formed of a high thermal conductivity material, typically copper. The plate assembly 10-13 may comprise a series of aligned

[0038] RECTIFIED SHEET (RULE 91) ISA / EP apertures which form line-of-sight ports 16 in the plates 10-13. The line-of-sight ports enable compatibility with existing cabling of the system and form a conduit for the cabling. However, a particular advantage provided by the interconnect 1 is that the PCB 2 may replace the need for such cabling, and so the line-of-sight ports may optionally be blanked off (as is shown for the first plate 10 in Figure 1) to reduce any heat radiation along the Z axis.

[0039] The PCB 2 is typically a “flex PCB”, comprising a flexible material, such as polyimide. It is particularly desirable that the PCB 2 is adapted to carry electrical signals between the first connector assembly 17 and the second connector assembly 19 by one or more transmission lines. It is also desirable that the PCB 2 does not place an undesirable thermal load on any low temperature components of the system. The electrically conductive material used in the PCB 2 therefore exhibits good electrical conductance and relatively poor thermal conductivity. Copper-nickel, such as Constantan, is particularly well suited for this purpose. The PCB 2 therefore typically comprises one or more layers of copper-nickel which form the transmission lines and which are electrically insulated by one or more polymer layers.

[0040] An assembly of thermal connectors 6, 7, 8, 9 is connected to the PCB 2 (see Figures 2-4). Each said thermal connector forms a rigid, high thermal conductivity portion of the interconnect 1 for thermally coupling part of the PCB 2 with a component of the cryogenic cooling system (in this case a corresponding connected plate 10-13). The thermal connectors 6-9 thermally ground the interconnect 1 to the different thermal stages of the cryogenic cooling system. In use, the connected parts of the interconnect 1 may therefore be cooled by components of the cryogenic cooling system, such as a cryogenic cooling device. This reduces the thermal conductance between the warm temperature end and the cold temperature end of the interconnect 1 . The thermal connectors 6-9 are spatially dispersed along the PCB 2 in the first direction and separated from each other by a respective folded portion 3, 4, 5 of the PCB 2. In the first embodiment the thermal connectors 6-9 are integrally formed within portions of the PCB 2. For example, distinct portions of the PCB 2 forming the thermal connectors may comprise one or more layers of copper plating that overlays the transmission lines, separated by an intermediate coverlay. The PCB stack may therefore have rigid portions which have a high thermal conductivity due to the copper plating applied, the rigid portions being separated by flexible folded portions 3-5 of the PCB 2 that have a low thermal conductivity. Standard flexi-rigid PCB manufacturing techniques can be used, such as I PC 2223, with the thermal conductors 6-9 forming the rigid portions of the stack, separated by intermediate flexible portions of the PCB 2. A low thermal conductivity material is however used to form the transmission lines or ‘traces’, as earlier discussed, instead of copper (as would ordinarily be used).

[0041] In the embodiment of Figure 3, the interconnect 1 is fitted to components of a dilution refrigerator. Various components of the dilution refrigerator have been omitted from Figure 3 for clarity, including the vacuum chamber, first cooling stage and the heat radiation shields. Each of the first, second, third and fourth plates 10-13 form removable sections of a corresponding thermal stage 30-33, also referred to in the art as a “flange”. A high thermal conductivity connection is formed between each of the plates 10-13 and the respective thermal stage 30-33. Several such removable plates may be provided along a thermal stage, and several interconnects can therefore be accommodated within a given cryogenic cooling system. With reference to Figure 3, the first thermal stage 30 is cooled by a second stage of a two-stage pulse tube refrigerator 40 to a base temperature of around 4 K in use. The second thermal stage 31 is cooled to a base temperature of around 0.5-1 K in use by a still 41 of the dilution refrigerator. The fourth thermal stage 33 is cooled to a base temperature below 20 m K by the mixing chamber 42 of a dilution refrigerator, and the third thermal stage 32 typically reaches a base temperature between that of the second thermal stage 31 and the fourth thermal stage 33 in use. Although not shown, in practice it is envisaged that multiple interconnects may be stacked in a row in close proximity and connected to the different thermal stages of the system. The interconnects may have slightly different dimensions or electrical components based on the experimental need.

[0042] RECTIFIED SHEET (RULE 91) ISA / EP Each thermal connector 6-9 may be directly connected to a corresponding plate 10-13 by a mechanical clamp or anchor. In the first embodiment, the interconnect 1 is mechanically connected to each of the plates 10-13 by a corresponding edge piece 10’-13’ (Figure 1). During an installation process and with the edge pieces 1O’-13’ removed, the interconnect 1 is arranged within the cryogenic cooling system so that the first thermal connector 6, second thermal connector 7, third thermal connector 8, and fourth thermal connector 9, are abutting against the first plate 10, second plate 11 , third plate 12 and fourth plate 13 respectively. The first edge piece 10’, second edge piece 11’, third edge piece 12’ and fourth edge piece 13’ are then connected to first plate 10, second plate 11 , third plate 12 and fourth plate 13 respectively so as to hold the interconnect 1 in place between the plates and the connected edge pieces. The thermal connectors 6-9 may comprise one or more folds to assist with forming this mechanical connection. The interconnect 1 therefore forms an insert for the cryogenic cooling system which may be inserted into the cryogenic cooling system or removed as a unitary member (typically either with or without the plates 10-13 being connected to the interconnect 1). This enables convenient servicing or adjustment of the interconnect 1 on a benchtop environment, outside of the cryogenic cooling system.

[0043] The folded portions 3, 4 and 5 of the PCB 2 advantageously reduce the effective thermal conductivity between the plates and across the interconnect 1. This is done by increasing the “creepage” of the PCB 2 (which is the length of the PCB 2 between the thermal connectors). The length of the folded portions 3, 4, 5 is therefore made larger than the separation between the adjacent thermal stages in the first direction. The folded portions 3-5 also enable a certain amount of elastic deformation of the interconnect 1 along the Z axis. This provides a relief from strain that can otherwise result from manufacturing tolerances or thermal contraction and expansion changing the separation between adjacent plates. The folded portions 3-5 therefore contribute towards reducing the heat conduction onto the low temperature components of the system and achieving a reliable mechanical and thermal connection between the system and the interconnect 1.

[0044] RECTIFIED SHEET (RULE 91) ISA / EP In order to preserve signal fidelity, it is desirable that the PCB 2 is formed from a single PCB structure, without any joints being required between PCB parts. To achieve this, a PCB panel 2’ is cut from a flat PCB blank. PCB blanks are typically available in relatively restricted, short dimensions. The PCB blank may have dimensions of around 60cm x 45 cm and be cut into a serpentine shape to form the planar PCB panel 2’ shown in Figure 4. The PCB panel 2’ is then folded along a plurality of fold lines to form the PCB 2 of the interconnect 1. This may be achieved by clamping parts of a flat PCB panel 2’ and bending the remaining panel. The shape of the PCB panel 2’ and the position of the fold lines is chosen to make effective use of available dimensions for the PCB blank (reducing waste), whilst enabling interconnect 1 to be retrofitted to an existing cryogenic cooling system having preset plate separations. A particular advantage achieved by the folds is that it increases the length of the PCB 2 in the Z axis beyond that of the PCB panel 2’. This allows the PCB 2 of the interconnect 1 to connect further than its maximum manufacturing area such that a single contiguous PCB can reach from the top (highest temperature) stage of a cryogenic cooling system all the way to the lowest temperature stage.

[0045] The PCB 2 extends substantially in the first direction between each of the thermal connectors 6-9 in an elongate manner. A first folded portion 3 of the PCB 2 connects the first thermal connector 6 to the second thermal connector 7. In this embodiment, the separation between the first plate 10 and the second plate 11 along the Z axis is larger than the separation between any of the other adjacent plates. The PCB panel 2’ is therefore constructed such that the first folded portion 3 is the longest of folded portions 3-5 in the Z axis. A second folded portion 4 of the PCB 2 connects the second thermal connector 7 to the third thermal connector 8 (this is the shortest of the folded portions 3-5 due to the smaller separation between the adjacent second and third plates 11 , 12). A third folded portion 5 of the PCB 2 connects the third thermal connector 8 to the fourth thermal connector 9. Each said folded portion 3-5 has a central folded region 18 comprising two overlapping portions of the PCB 2 having a plane normal along the Z axis. The two overlapping portions are connected by a connection portion of the PCB 2 having a normal orthogonal to the Z axis. Each central folded region 18 is

[0046] RECTIFIED SHEET (RULE 91) ISA / EP connected to the respective adjacent thermal connectors by upper and lower portions of the PCB 2. In this embodiment, the upper and lower portions have a plane normal to that of the overlapping portions and the connecting portions. One or more additional folds is typically provided at the point of connection between the PCB 2 and each of the plates 10-13, as shown in Figure 2.

[0047] As shown in Figure 3, a device for measurement 45 is connected to the fourth thermal stage 33 and cooled in use by the mixing chamber 42. In the context of a quantum information processing (QIP) system, the device 45 typically comprises a qubit array of one or more qubits. The device 45 is communicatively coupled to the interconnect 1 by electrical connectors 47, such as cables, which extend between the second connector assembly 19 and the device 45. The presence of any connections between different electrical components introduces a potential point of failure and may introduce signal losses. Therefore, in an alternative embodiment the device 45 may be formed on the interconnect 1 , directly connected to the transmission lines of the PCB 2 without any intermediate electrical connectors or cabling 47.

[0048] The interconnect 1 communicatively couples the device 45 to an electronic processing device (not shown), provided at a higher temperature location. This is typically a computer that is arranged outside of the vacuum chamber of the cryogenic cooling device. In the first embodiment, it is envisaged that the interconnect 1 terminates (on the warm temperature end) at the first thermal stage 30 where the first connector assembly 17 is provided. The first connector assembly 17 is therefore arranged inside the vacuum chamber and so additional electrical connections, typically in the form of cables, are needed to connect the first connector assembly 17 to the processing device outside the vacuum chamber. However, in alternative embodiments the first connector assembly may be arranged outside of the vacuum chamber (for example within an airtight gasket), thereby removing the need for any intermediate cabling extending inside the vacuum chamber.

[0049] Returning to the embodiment shown in Figure 1 and 2, electrical connectors forming part of the first connector assembly 17 are arranged on the outside of the first thermal connector 6, and electrical connectors forming part of the second connector assembly 19 are arranged on the outside of the fourth thermal connector 9. These form breakout sections for the transmission lines (traces) in the PCB 2. The electrical connections may be used to transmit power to a device 45 and / or communicate with the device 45, for example to perform a read / write operation on the device 45.

[0050] Each of the first and second connector assemblies 17, 19 includes an array of coaxial RF (radio frequency) connectors for RF transmission lines extending through the PCB 2 and an array of 25 way micro-D connectors for direct current (DC) transmission lines that also extend through the PCB 2. The specific form of the electrical connectors provided on the outside of the interconnect 1 may vary depending on the application. The direct DC connections may be used to apply a DC voltage or a low frequency pattern below 100Mhz to a device for measurement. The RF connections may be used for signal frequencies above 100Mhz.

[0051] The PCB stack is configured to accommodate two layers of electrically conductive material (typically Constantan) which are separated by an intermediate electrically insulating base layer (typically polymer) to form the RF and DC transmission lines. The RF and DC transmission lines are therefore integrally formed within the PCB 2 and extend unbroken across the PCB 2, from the first connector assembly 17 to the second connector assembly 19.

[0052] Figure 5 shows the arrangement of RF and DC transmission lines across a portion ‘A’ of the PCB 2 indicated in Figure 2. Across a first segment or “width” 56 of the PCB 2, there is an array of RF lines 50 formed from a first Constantan layer. The conductive traces formed by the first Constantan layer are also visible in Figure 6, which depicts a first surface of an area of the PCB 2. The RF lines 50 are linearly arranged along the PCB 2 (and in the first direction, for the portion A shown). The RF lines 50 can be included at a high spatial density, for example up to 5 lines per mm, with the higher densities being more susceptible to the effects of cross-talk. There may be a requirement for the RF connections to maintain a characteristic impedance, for example 50 Q. This is done on the PCB by running a route with a backplane having a certain width, so the impedance (which is the square root of the inductance divided by the capacitance) equals 50. Within the structure of the PCB stack, the RF lines 50 overlay a mesh backplane 52, which is formed by the second Constantan layer and electrically connected to the ground potential to create the 50 ohm routing. Both layers of traces are shown in Figure 5 for illustrative purposes, although only one layer is shown in each of Figures 6 and 7. Figures 7 depicts a second surface of an area of the PCB 2, which is opposite to the surface shown in Figure 6. As shown in Figure 7, the electrically conductive material 52 in the second layer is arranged in a lattice comprising a grid of orthogonal traces and which is offset from the electrically conductive material 50 in the first layer. The RF lines 50 therefore overlap the conductive traces for the backplane 52 at 45 degrees. This meshed arrangement ensures that the backplane spans the width of the first segment 56 but reduces the effective width of electrically conductive material for conducting heat between adjacent thermal connectors along the backplane. In contrast, if the backplane were provided as a solid layer of electrically conductive material (that is uninterrupted across the width of the first segment 56), the effective width of electrically conductive material in the second layer may be around three times higher, which could increase the effective thermal conductivity across the folded portions of the PCB 2.

[0053] DC lines 54, 55 are linearly arranged along the PCB 2 (and in the first direction for the portion A shown). The DC lines 54, 55 are arranged across a second segment or “width” 58 of the PCB 2, which is adjacent to the first segment 56 and does not overlap the first segment 56. Unlike the RF lines 50, there is no need to include a backplane for the DC lines, and so DC lines 54, 55 are formed by both the first and second Constantan layers. In other words, the DC lines run on both sides of the central polymer base layer within the PCB stack, which is why they are visible in the two surfaces of the PCB 2 shown by Figures 6 and 7. The DC lines may be provided at a higher density than the RF lines, for example up to 14 DC lines per mm (including the routing of DC on both sides). The width of the DC lines may be customised depending on the signals and the power of the electrical signal to be transmitted based on the application. In the present embodiment, the width of the portion A is around 30 cm, which contains 30 RF lines across the first segment 56 and 100 DC lines across the second segment 58 (although this is schematically illustrated). This is a much higher density of electrical connections than can be achieved using conventional interconnects formed of cable assemblies.

[0054] The interconnect 1 may comprise active or passive electrical components. A particular benefit provided by the interconnect 1 is that if a user wanted to include a new component, such as an active component requiring electrical power, the PCB 2 already provides the connections that would be needed for this component. In contrast, under the conventional approach a new electrical cable would need to be routed across the inside of the cryogenic cooling system, to and from the new component. This would take time, occupy valuable space and require impedance matching. The PCB-based interconnect 1 solves this problem by already providing the framework or building block for any number of possible experimental setups using different electrical components that can be fitted directly to the PCB 2.

[0055] One or more electronic filters (not shown) may be arranged along the interconnect 1 , typically along a rigid section of the PCB 2 formed by one of the thermal connectors 6, 7, 8, 9. The rigid copper PCB breakouts 6-9 enable electronics to be integrated at any temperature stage without the signal integrity losses introduced by connectors. Typically, the filters are added along the DC transmission lines at intermediate stages, such as the second thermal connector 7 that is thermally coupled to the second plate 11 and / or the third thermal connector 8 that is thermally coupled to the third plate 12 in use. For example, a resistor and capacitor may be used to form a low pass filter for the DC signal. RF electronics (such as mixer, amplifiers, voltage or current sources) may also be integrated onto the PCB 2. The exact arrangement of electronic components can be chosen or adapted by the user based on the experimental need. Existing design software may be used to plan the location of the electronic components.

[0056] An improved interconnect is therefore provided which allows for a higher density of electrical connections to be provided to a device for measurement in a cryogenic cooling system. Using large flexi-rigid PCBs allows the wiring and the electronics to be interfaced on the same medium, reducing the need for interfaces between them. The interconnect comprises a plurality of thermal connectors and flexible portions of the PCB for reducing thermal conduction across the PCB and facilitating low temperature cooling of the device for measurement. The folded portions also enable a reliable thermal and mechanical connection to be formed between the interconnect and components of the cryogenic cooling system during thermal cycling of the system. The folds in the PCB also extend the interconnect distance beyond the maximum dimensions of a standard manufacturable PCB panel. The use of the PCB, in contrast to cabling, typically reduces the number of electrical joints or connections that need to be formed, which may improve the fidelity of the electrical signals transmitted along the interconnect. Furthermore, the use of the PCB makes the interconnect highly customisable so that new electrical components can be included to the system at ease and without the need for additional cabling. Other advantages are apparent and discussed herein.

[0057] The invention provides benefit in any number of applications within the field of low temperature physics but it provides particular benefit in the context of QIP. Here the interconnect provides a turnkey solution for all the cryogenic parts to support operation of a quantum computer inside a dilution refrigerator.

[0058] Further embodiments of the invention are provided by the following numbered clauses.

[0059] Clause 1 : An electrical assembly for a cryogenic cooling system, comprising: a printed circuit board, PCB; and a plurality of thermal connectors arranged along the PCB; wherein each said thermal connector is distally separated from an adjacent thermal connector by a respective folded portion of the PCB.

[0060] Clause 2: An electrical assembly according to clause 1 , wherein each said folded portion of the PCB is deformable to allow a change in the separation between adjacent thermal connectors. Clause 3: An electrical assembly according to clauses 1 or 2, wherein each said folded portion of the PCB extends along two or more planes, which are preferably orthogonal.

[0061] Clause 4: An electrical assembly according to any of the preceding clauses, wherein the electrical assembly is elongate in a first axis along which the PCB substantially extends, the thermal connectors being separated from each other along the first axis, wherein each said folded portion comprises two overlapping regions of the PCB having a normal in the first axis, the two overlapping regions being mutually connected by a connection region of the PCB having a normal orthogonal to the first axis.

[0062] Clause 5: An electrical assembly according to clause 4, wherein the overlapping regions are connected to the adjacent thermal connectors by respective upper and lower regions of the PCB having a normal orthogonal to the first axis.

[0063] Clause 6: An electrical assembly according to any of the preceding clauses, wherein the assembly forms an insert for a cryogenic cooling system.

[0064] Clause 7: An electrical assembly according to any of the preceding clauses, wherein the PCB comprises electrical transmission lines formed from a coppernickel alloy.

[0065] Clause 8: An electrical assembly according to any of the preceding clauses, wherein the plurality of thermal connectors comprises a first thermal connector and a second thermal connector; the electrical assembly further comprising: a first set of electrical components provided at the first thermal connector; and a second set of electrical components provided at the second thermal connector; wherein the first set of electrical components is electrically connected to the second set of electrical components by the PCB.

[0066] Clause 9: An electrical assembly according to clause 8, wherein the second set of electrical components comprises a device for measurement. Clause 10: An electrical assembly according to any of the preceding clauses, wherein the PCB comprises an array of RF lines overlapping a mesh plane.

[0067] Clause 11 : An electrical assembly according to any of the preceding clauses, wherein the PCB further comprises a first array of DC lines arranged on a first layer of the PCB and a second array of DC lines arranged on a second layer of the PCB.

[0068] Clause 12: An electrical assembly according to clauses 10 and 11 , wherein the array of RF lines is arranged within a first segment of the PCB, and wherein the first array of DC lines and the second array of DC lines is arranged within a second segment of the PCB, the second segment being adjacent to the first segment.

[0069] Clause 13: An electrical assembly according to any of clauses 1 to 9, wherein the PCB comprises a first layer of electrically conductive material separated from a second layer of electrically conductive material by an electrically insulating layer; wherein, in a first segment of the PCB, the first layer forms an array of RF transmission lines and the second layer forms a backplane; and wherein, in a second segment of the PCB, the first layer forms a first array of DC transmission lines and the second layer forms a second layer of DC transmission lines.

[0070] Clause 14: An electrical assembly according to clause 13, wherein the backplane is hashed.

[0071] Clause 15: A cryogenic cooling system comprising: a vacuum chamber; a plurality of thermal stages arranged inside the vacuum chamber; a cryogenic cooling device configured to apply a thermal gradient across the plurality of thermal stages; and an electrical assembly according to any of the preceding clauses, wherein each said thermal connector is connected to a respective said thermal stage.

[0072] Clause 16: A cryogenic cooling system according to clause 15, wherein the plurality of thermal stages comprises a base temperature stage configured to be cooled to the lowest temperature of the plurality of thermal stages by the cryogenic cooling device, the base temperature stage being thermally coupled to a device for measurement, wherein the electrical assembly is electrically connected to the device for measurement. Clause 17: A quantum processing system, comprising: a cryogenic cooling system according to clauses 15 or 16; and a qubit array comprising one or more qubits communicatively coupled to the electrical assembly.

Claims

CLAIMS1 . An electrical assembly (1 ) for a cryogenic cooling system, comprising: a printed circuit board, PCB (2); and a plurality of thermal connectors (6, 7, 8, 9) arranged along the PCB (2), wherein each thermal connector is arranged on the outside of the PCB and comprises a region of high thermal conductivity material; wherein each said thermal connector (6, 7, 8, 9) is distally separated from an adjacent thermal connector by a respective folded portion (3, 4, 5) of the PCB (2); wherein the electrical assembly is elongate in a first axis along which the PCB (2) substantially extends, the thermal connectors (6, 7, 8, 9) being separated from each other along the first axis, wherein each said folded portion extends along two or more planes and comprises two overlapping regions of the PCB having a normal in the first axis, the two overlapping regions being mutually connected by a connection region of the PCB having a normal orthogonal to the first axis, wherein the overlapping regions are connected to the adjacent thermal connectors by respective upper and lower regions of the PCB having a normal orthogonal to the first axis.

2. An electrical assembly according to claim 1 , wherein each said folded portion (3, 4, 5) of the PCB (2) is deformable to allow a change in the separation between adjacent thermal connectors (6, 7, 8, 9).

3. An electrical assembly according to claims 1 or 2, wherein each thermal connector comprises a layer or block of high thermal conductivity material.

4. An electrical assembly according to any of the preceding claims, wherein the overlapping regions are mutually connected by a connecting portion of the PCB (2) having a normal orthogonal to the first axis, wherein the upper and lower portions have a plane normal to that of the connecting portion.

5. An electrical assembly according to any of the preceding claims, wherein the assembly forms an insert for a cryogenic cooling system.RECTIFIED SHEET (RULE 91) ISA / EP6. An electrical assembly according to any of the preceding claims, wherein the PCB comprises electrical transmission lines formed from a copper-nickel alloy.

7. An electrical assembly according to any of the preceding claims, wherein the plurality of thermal connectors comprises a first thermal connector (6) and a second thermal connector (7); the electrical assembly further comprising: a first set of electrical components (17) provided at the first thermal connector (6); and a second set of electrical components (19) provided at the second thermal connector (7); wherein the first set of electrical components is electrically connected to the second set of electrical components by the PCB (2).

8. An electrical assembly according to claim 7, wherein the second set of electrical components comprises a device for measurement.

9. An electrical assembly according to any of the preceding claims, wherein the PCB (2) comprises an array of RF lines (50) overlapping a mesh plane (52).

10. An electrical assembly according to any of the preceding claims, wherein the PCB further comprises a first array of DC lines (54) arranged on a first layer of the PCB and a second array of DC lines (55) arranged on a second layer of the PCB.

11. An electrical assembly according to claims 9 and 10, wherein the array of RF lines is arranged within a first segment (56) of the PCB (2), and wherein the first array of DC lines (54) and the second array of DC lines is arranged within a second segment (58) of the PCB, the second segment being adjacent to the first segment (56).

12. An electrical assembly according to any of claims 1 to 11 , wherein the PCB (2) comprises a first layer of electrically conductive material separated from a second layer of electrically conductive material by an electrically insulating layer.RECTIFIED SHEET (RULE 91) ISA / EP13. An electrical assembly according to any of claims 1 to 8, wherein the PCB (2) comprises a first layer of electrically conductive material separated from a second layer of electrically conductive material by an electrically insulating layer; wherein, in a first segment (56) of the PCB (2), the first layer forms an array of RF transmission lines (50) and the second layer forms a backplane (52); and wherein, in a second segment (58) of the PCB (2), the first layer forms a first array of DC transmission lines (54) and the second layer forms a second layer of DC transmission lines.

14. An electrical assembly according to claim 13, wherein the backplane (52) is hashed.

15. A cryogenic cooling system comprising: a vacuum chamber; a plurality of thermal stages (30, 31 , 32, 33) arranged inside the vacuum chamber; a cryogenic cooling device (40, 41 , 42) configured to apply a thermal gradient across the plurality of thermal stages (30, 31 , 32, 33); and an electrical assembly (1) according to any of the preceding claims, wherein each said thermal connector (6, 7, 8, 9) is connected to a respective said thermal stage (30, 31 , 32, 33).

16. A cryogenic cooling system according to claim 15, wherein the plurality of thermal stages comprises a base temperature stage (33) configured to be cooled to the lowest temperature of the plurality of thermal stages by the cryogenic cooling device, the base temperature stage being thermally coupled to a device for measurement (45), wherein the electrical assembly (1 ) is electrically connected to the device for measurement.

17. A quantum processing system, comprising: a cryogenic cooling system according to claims 15 or 16; and a qubit array (45) comprising one or more qubits communicatively coupled to the electrical assembly (1).RECTIFIED SHEET (RULE 91) ISA / EP

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