Ultra-low temperature cooling system

Abstract

Provided is a cryogenic refrigerator assembly and a cryogenic cooling system including two or more connected modules. The cryogenic refrigerator assembly includes one or more cryogenic refrigerators. Each connected module includes a housing having a plurality of sides defining an internal volume of the module and a plurality of stages disposed within the internal volume of the module, and one or more of the plurality of stages are thermally coupled to the cryogenic refrigerator assembly. Two or more modules are interconnected at respective sides, and a first stage of a first module is thermally coupled to a first stage of a second module.

Description

Technical Field

[0001] The present invention relates to the field of cryogenic cooling systems. A particularly desirable application of this technology is commercial-scale superconducting quantum computing.

Background Art

[0002] For the upscaling of quantum computing based on superconducting quantum information processing (QIP) devices, it is thought that a continuously low temperature of about 10 to 20 mK or less is required for the operation of the QIP devices. For this reason, a dilution refrigerator (DR) is assumed to be indispensable. Today's industrial QIP companies use commercially available cryogen-free ("dry") dilution refrigerators that follow a traditional form factor in which a series of vertically spaced copper plates having a circular cross-section are thermally separated from each other and a single dilution unit is installed between the lower stage plates. Together with the pipes connected to the inlet and outlet of the dilution unit and the hardware for the pump, a closed-cycle dilution refrigerator is formed. Each stage plate of the dilution refrigerator insert is surrounded by concentric cylindrical radiation shields to reduce the radiative heat load applied to the dilution refrigerator stage (to ~0.8 K at the still stage and less than 10 mK at the mixing chamber stage). The entire assembly is surrounded by a cylindrical vacuum vessel. In many cases, the assembly is frame-mounted so that the radiation shield and the outer vacuum chamber (OVC) can be removed.

[0003] Figures 1 and 2 show examples of the above-described prior art dilution refrigerators. Figure 1 is a perspective view of a cylindrical outer vacuum chamber 1 supported by a cryostat support frame 10. External components of the helium gas treatment system 2 and a non-refrigerant refrigerator in the form of a pulse tube refrigerator (PTR) 3 can also be seen. Figure 2 is a cross-sectional perspective view of the outer vacuum chamber 1. For clarity, PTR and the dilution unit are not shown in Figure 2. There is a stepped configuration of thermal stages 5 to 9, each having the shape of a circular plate and being cooled to its respective temperature during use. An assembly of nested cylindrical radiation shields 4 can also be seen, with each shield connected to its respective thermal stage. The first stage of PTR 3 is attached to the PT1 stage 5, and the second stage of PTR 3 is attached to the PT2 stage 6. A dilution unit including a fractionator and a mixing chamber connected by a series of heat exchangers forms part of the dilution refrigerator. The fractionator is attached to the fractionator stage 7, and the mixing chamber is attached to the mixing chamber stage 9. During operation, a working fluid formed of a helium-3 / helium-4 mixture circulates around the dilution unit. The fractionator and the mixing chamber cool the system as a result of the phase change or mixing of the working fluid. In the mixing chamber, cooling is obtained from the enthalpy of mixing when helium-3 is diluted in helium-4. As a result, the mixing chamber is operable to achieve the lowest temperature of any part of the dilution refrigerator. In the fractionator, helium-3 boils and energy is removed by the latent heat of vaporisation. A "cooling plate" 8 forming each thermal stage is disposed between the fractionator stage 7 and the mixing chamber stage 9 to achieve an intermediate temperature during use. A cylindrical thermal radiation shield is connected to each of the PT1 stage 5, the PT2 stage 6, and the fractionator stage 7 surrounding the low-temperature stage.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In cryogenic applications such as QIP, various dissipative elements are attached across different stages of a dilution refrigerator to ensure sufficient thermalization of the experimental wiring. The heat dissipated from the resistive elements and conductive wiring adds a heat load to the dilution refrigerator, i.e., additional cooling power is required to maintain a given reference temperature of the system. (Explained with reference to FIGS. 1 and 2) The above form factor functions very well in the academic environment where most dry DR systems have been installed. However, as QIP scales up in the industrial environment, this form factor is starting to become limiting. Large radiation shields are cumbersome to handle. Since such systems have a fixed size, the maximum experimental size and the available cooling power at each stage are limited. Although it is possible to design an extra-large vacuum chamber and a cryogenic refrigerator as a way to provide an upgradable physical space for experiments, such a system becomes commercially unattractive for both manufacturers and customers due to the huge capital investment. Therefore, a cryogenic cooling system that better suits the industrial or commercial environment is needed. For example, it is necessary to compensate for the even higher heat load that may arise from building a larger quantum computer than before without incurring the above problems and to facilitate a new mode of operation.

Means for Solving the Problems

[0006] A first aspect of the present invention includes a cryogenic refrigerator assembly including one or more cryogenic refrigerators, two or more connected modules, and each connected module includes a housing having a plurality of sides defining the internal volume of the module, a plurality of stages disposed within the internal volume of the module, including, wherein one or more of a plurality of stages are thermally coupled to a cryogenic refrigerator assembly, Two or more modules are interconnected at respective sides, A cryogenic cooling system in which a first stage of a first module is thermally coupled to a first stage of a second module.

[0007] In contrast to the prior art designs of FIGS. 1 and 2, the cryogenic cooling system is modularized. As a result, by connecting the interior of a first module to other similar modules to enable "scaling up" of the system, the shared interior volume and the available total cooling power are increased. The user can address concerns about the high equipment investment associated with owning a large cryogenic cooling system by incrementally building the size of the cryogenic cooling system by connecting additional modules. Also, by modularizing the design, any shield can be miniaturized, and the user can facilitate system repair since only a part of the housing of a given module needs to be removed to access the associated internal experimental space. By thermally coupling the stages of different connected modules, it becomes possible to share the cooling power of the cryogenic refrigerator across the entire system. Thus, the user can balance this heat load by connecting modules to other modules (e.g., n times) rather than typically replacing the entire system with a larger system, which would include additional heat dissipating components on or between different cooling stages of the system. Therefore, new applications such as the construction of a QIP server farm become possible through this technology.

[0008] The cryogenic refrigerator assembly can include a common cryogenic refrigerator, such as a liquid helium refrigeration plant, that cools each stage of two or more modules within the system. However, typically, the cryogenic refrigerator assembly is configured to include a cryogenic refrigerator in which each connected module is thermally coupled to one or more of its stages. As will be described later, these cryogenic refrigerators can take a plurality of forms, including cryocoolers, helium-3 or helium-4 refrigerators, and dilution refrigerators. Each module can have a cryogenic refrigerator attached to the stage of the module by a high thermal conductivity connection.

[0009] One or each module is preferably configured to operate as an independent cryogenic cooling system in a first configuration. Typically, independent operation requires that the internal volume of the module be hermetically sealed by a housing and preferably further evacuated. Thus, in the first configuration, a removable panel can be attached to the housing (and preferably the sides) of the module to surround the internal volume of the module. Each module is further configured to operate as a component of an integrated cryogenic cooling system that includes a plurality of modules whose respective sides are interconnected to form a shared internal volume in which the stages of each connected module are disposed, in a second configuration in which the removable panel is removed from the housing. The user can configure one or each module in the first configuration by separating the modules, for example, to perform different experiments using each module. Alternatively, the user can arrange the modules in the second configuration, for example, when a higher total cooling power is required for a target device inside the system. Typically, the housing of each module includes a framework that supports a plurality of sides. When providing a removable panel, the removable panel can be attached to the framework in the first configuration and removed from the framework in the second configuration.

[0010] As described above, the side surface of a module (e.g., the first module) can be connected to the opposing side surface of another module (e.g., the second module). Usually, the housing (e.g., the framework) of each module includes one or more housing connectors for connecting the first module to the second module. Usually, one or more housing connectors are arranged so as to be configured such that the outer surface of the housing of the first module conforms to the outer surface of the housing of the second module in order to form an integrated cryogenic cooling system. As a result, simple connection between different modules becomes easy. For example, the housing connector on the first side surface of the first module can be connected to the housing connector on the second side surface of the second module, and the second side surfaces of the respective modules face the first side surfaces. This front-to-back geometry enables an infinite number of modules to be connected within the system. Alternatively, two modules can also be connected to each other by connecting the housing connector on the first surface of the first module to the housing connector on the similar first surface of the second module. The first module can be connected to two modules (one on each side) by connecting the housing connector on the first surface of the first module to the housing connector on the second surface of the second module and connecting the housing connector on the second surface of the first module to the housing connector on the first surface of the third module. One or more housing connectors can preferably include a flange surrounding the outside of the removable panel or provided around the periphery of the outside of the removable panel. The housing connector can preferably further include fasteners in the form of, for example, an array of bolts and holes for joining the flanges of different modules around a seal member (such as an O-ring seal).

[0011] The modular cryogenic cooling system can have sides that form a plurality of different tessellating or non-tessellating structures. On the other hand, there is a particular advantage in that the housing of each module includes four orthogonal sides. This design allows the modules to easily conform to the shape of most rooms and enables such a plurality of modules to be connected without taking up space. Thus, more modules can be connected to increase the total cooling power of the system before the available floor area is filled up. Orthogonal symmetry also means that the connected modules can be densely packed without the need for spacers, for example, between adjacent modules. Therefore, the side of the first module is preferably directly connected to the side of the second module. For example, by bringing adjacent modules closer together so that the modules are directly connected to each other without using extension spacers, the total mass to be cooled is reduced, allowing for even higher cooling efficiency and lower temperatures to be achieved.

[0012] One, more than one, or potentially each thermal stage of the first module can be thermally coupled to the corresponding stage of the second module and optionally to each of the remaining modules within the system. Typically, the second stage of the first module is thermally coupled to the second stage of the second module. Typically, the connected stages are arranged to achieve similar operating temperatures / reference temperatures within a range, for example, between 1 and 2 K when in use. The system preferably further includes one or more extension plates arranged to connect adjacent stages of different modules within the system. Thus, the system can be arranged such that one or more stages of the first module are thermally coupled to each stage of the second module by one or more extension plates that mechanically connect the thermally coupled stages. Thus, the (single or plural) extension plates can increase the thermal conductivity between the stages of adjacent modules. In practice, for example, this results in an expansion of the thermal stage of the first module, enabling the sharing of experimental services between different modules. Similarly, to compensate for the high thermal loads that may be applied to the second module by the experimental services, the cooling power applied to the stages of the first module can be transferred to another stage of the adjacent second module. That is, for example, a high thermal load can be applied to the thermal stage by incorporating highly dissipative electrical cable wiring or components, which may be desirable depending on the application. For example, each extension plate can be configured such that the effective thermal conductivity between the connected stages exceeds 5000 W / mK at a temperature of 20 K.

[0013] Also, the (single or multiple) extension plate(s) can also perform a heat insulation function by blocking the direct radiation path between different temperature stages of adjacent modules. For example, the system can include multiple extension plates, and one or two or more extension plates are configured such that the effective thermal conductivity between the connected stages exceeds 5000 W / mK at a temperature of 20 K, and one or two or more extension plates are configured such that the effective thermal conductivity between the connected stages is less than 50 W / mK at a temperature of 20 K. The (single or multiple) low thermal conductivity plate(s) can function to enhance the structural rigidity or heat insulation of the assembly without providing an effective thermal coupling between the connected stages. As a result, new operating modes and experiments that were difficult to reproduce in conventional cryostat designs can be unlocked.

[0014] Normally, the connected stages come to the same height across different modules. Normally, the multiple stages of each module are arranged in a stepped manner. For example, the first stage extends along a plane parallel to the second stage and is spatially dispersed from the second stage along an axis perpendicular to the first and second stages. At this time, each stage of the first module is preferably coplanar or substantially coplanar with the corresponding stage of the second module. At this time, usually, one or two or more extension plates are arranged to mechanically connect one or two or more stages of the first module to the respective coplanar stages of the second module. Usually, the (single or multiple) extension plate(s) is planar and can fill the gap between the coplanar stages of adjacent connected modules and effectively expand these stages to form a common connection stage of the system. Depending on experimental needs, extension plates can be included between some or all of the coplanar stages of the connected modules.

[0015] Typically, one or more than one of the plurality of stages includes one or more stage connectors for connecting an extension plate between the coplanar (or same plane) or "respective" stages of the first and second modules. For example, the stage connectors can include specific means for connecting to the extension plate, such as an array of fastening members provided at the peripheral edge of each stage or holes for fastening members. At least one of the one or more extension plates preferably includes an extensible joint. Such a joint can maintain a mechanical connection between the connected stages during relative movement of the connected stages that can occur, for example, due to contraction and expansion of components during a thermal cycle. For example, at least one of the one or more extension plates can include a braided joint.

[0016] Typically, each stage forms a platform to which components of, for example, a cryogenic cooling system or heat dissipation elements within an electrical system can be attached. Typically, each stage is a plate formed of a material such as copper that has a very high thermal conductivity at cryogenic temperatures. Typically, each stage is arranged to obtain its respective reference temperature by the operation of a cryogenic refrigerator and can thus also be referred to as a "thermal stage". Typically, the cryogenic refrigerator is configured to cool each stage to cryogenic temperatures (temperatures below 100 Kelvin) during use. Generally, the stages are arranged such that their longitudinal axes extend perpendicular to the surface of each stage. Typically, the cryogenic refrigerator is configured to apply a thermal gradient across the stages, typically along the longitudinal axis. Thus, typically, the stages are arranged such that the reference temperature of each stage gradually decreases in the direction extending between the plurality of stages.

[0017] Typically, the system is configured such that a plurality of stages of each module share a common internal volume defined by the housing of the connected modules. Typically, the internal volume of the system is hermetically sealed from the ambient environment, and typically, the regions housing the stages of each module are evacuated. As a result, the system can operate without the need for a gas treatment device to expel any undesirable gases that may liquefy or solidify at low temperatures.

[0018] Each module can be provided with a radiation shield, and these can be interconnected to form a larger radiation shield assembly surrounding the cryogenic components of the system. Typically, each module includes one or more radiation shields, each radiation shield being thermally coupled to a respective stage of the module and surrounding one or more remaining stages, and each radiation shield including one or more shield surfaces. The system further includes one or more shield extension sections, each shield extension section connecting coplanar shield surfaces of adjacent connected modules and being maintained at generally the same temperature during use. In a module housing having four orthogonal sides and connected to another module within the system, each radiation shield can include one, two, or three orthogonal sides extending perpendicular to the respective thermally coupled stage. It should be understood that the plurality of stages of each module within the system are not completely surrounded by the radiation shield of that module, as there should typically be space to thermally connect adjacent stages of different modules and / or to perform experimental services between modules within the internal volume.

[0019] One or more radiation shields can further include a shield connector for connecting shield extension sections between adjacent radiation shields of the first module in the second configuration and the second module in the second configuration. The shield extension section may be required when there is a gap between coplanar sides that form part of the radiation shields of adjacent modules, and the shield extension section is provided to fill this gap and join the shields of adjacent connected modules to surround the cryogenic components of the system.

[0020] Different cryogenic refrigerators can be used, including "wet" refrigerators, refrigerant-free ("dry") cryocoolers (also referred to as mechanical refrigerators in this specification), or combinations thereof. For example, in small-scale facilities where the system includes up to three connected modules, the use of cryocoolers such as pulse tube refrigerators, Stirling refrigerators, Gifford-McMahon refrigerators, etc. is particularly cost-effective. In systems with four or more modules, it is often more efficient to cool one or more (single or multiple) stages of the module through thermal contact with a refrigerant such as liquid nitrogen or helium. The refrigerant can be stored internally, for example, in a dewar / reservoir within one or each module. Alternatively, the refrigerant can be stored in an external refrigeration plant and transported through conduits through the (single or multiple) modules. Therefore, the cryogenic refrigerator can include conduits for circulating the refrigerant through the module, including one or more heat exchangers that thermally couple the conduits to one or more of the multiple stages. Generally, especially when the refrigerant is helium, such conduits can be used to achieve temperatures below 4 Kelvin. Therefore, the cryogenic cooling assembly can include a helium refrigeration plant, which is preferably disposed outside the housing of each module. In this case, one or more modules preferably include a first helium heat exchanger thermally coupled to the stage of the module, and the helium heat exchanger is arranged to receive a flow of liquid helium from the helium refrigeration plant. One or more modules preferably further include a second helium heat exchanger thermally coupled to different stages of the module, and the second helium heat exchanger is arranged to receive a flow of gaseous helium at a temperature below 100 K, preferably between 40 and 80 K, more preferably between 50 and 70 K, from the helium refrigeration plant. In addition to or instead of this, the cryogenic cooling assembly can also include a nitrogen refrigeration plant, which is preferably disposed outside the housing of each module.In this case, it is preferable that one or more modules include a nitrogen heat exchanger thermally coupled to the stage of the module, and the nitrogen heat exchanger is arranged to receive a flow of liquid nitrogen from a nitrogen refrigeration plant. It will be understood that the nitrogen heat exchanger is arranged on a different thermal stage from the first helium heat exchanger.

[0021] To achieve cooling below millikelvin, it is preferable that one or more modules of the system include a dilution unit of a dilution refrigerator. As described above, the dilution unit includes a fractionator and a mixing chamber. In a particularly desirable implementation, the first module includes a first dilution unit and the second module includes a second dilution unit. For example, the first dilution unit can include a fractionator attached to the fractionator stage among the plurality of stages of the first module, and the second dilution unit can include a fractionator attached to the fractionator stage among the plurality of stages of the second module, and the fractionator stages of the first and second modules are mechanically connected by an extension plate. Usually, the fractionator stages are substantially coplanar, and an extension plate extends between the fractionator stages. Depending on the application, it may be useful to thermally couple the connected fractionator stages by an extension plate, in which case the extension plate should have a relatively high thermal conductivity. On the other hand, in some applications, it may be desirable to reduce the effective thermal conductivity between the connected fractionator stages so that, for example, the heat load applied to one fractionator stage does not transfer to the other fractionator stage. In such a situation, it is preferable that the effective thermal conductivity between the fractionator stages of the first and second modules is less than 50 W / mK at a temperature of 20 K. This arrangement allows the two fractionators to operate at different steady-state temperatures, which may be desirable depending on the application.

[0022] Similarly, the first dilution unit can include a mixing chamber attached to the mixing chamber stage among the plurality of stages of the first module, and the second dilution unit can include a mixing chamber attached to the mixing chamber stage among the plurality of stages of the second module. The mixing chamber stages of the first and second modules are typically mechanically connected by an extension plate extending between the coplanar mixing chamber stages. There may be a case where it is desirable to physically connect the mixing chamber stages of the first and second modules without performing effective thermal coupling. In this case, the extension plate connecting the mixing chamber plates can be configured such that the effective thermal conductivity between the mixing chamber stages of the first and second modules is less than 50 W / mK at a temperature of 20K.

[0023] In one preferred implementation, the first module includes two or more dilution units. The first dilution unit is preferably arranged such that the first fractionator is on the first stage and the first mixing chamber is on the second stage. The second dilution unit is preferably arranged such that the second fractionator is on the first stage and the second mixing chamber is on the third stage. The second stage is arranged between the first stage and the third stage. Therefore, an active cooling source (specifically, a fractionator or a mixing chamber) can be provided for each of the first stage, the second stage, and the third stage. Thus, for example, the second stage does not rely on the cooling power of the adjacent first and third stages to compensate for the heat load applied to the second stage.

[0024] The system can be configured to enable various experimental services to be performed between different modules. The system preferably includes a target assembly that includes an electrical circuit (e.g., cable wiring) that extends between one or more stages of a first module and one or more stages of a second module within the internal volume of the system. For example, the target assembly can form part of a quantum computer. Thus, each module can form a scalable platform for supporting a quantum information processing device.

[0025] Different connected modules can be provided with different cryogenic refrigerators. In a particularly desirable implementation, the first module includes a cryogenic refrigerator in the form of a dilution unit or a helium-4 refrigerator having a 1K pot, and the second module, or a third module having a stage thermally coupled to the corresponding stage of the first module, includes a cryogenic refrigerator in the form of a cryocooler and / or a reservoir for a liquid refrigerant. The reservoir can take the form of a dewar or any suitable tank / vessel for holding the liquid refrigerant. When a reservoir and a cryocooler are provided within a module, the reservoir can be arranged to recover the refrigerant liquefied by the operation of the cryocooler. The reservoir can be operable to cool one or more stages of the first module by the flow of the refrigerant along a conduit thermally coupled to one or more stages of the first module. The reservoir can function as a fail-safe mechanism for maintaining cryogenic temperatures within the module in the event of a power failure that could otherwise affect the operation of any cryocooler. Typically, the reservoir is fixed to one or more stages of the system, for example, to passively cool the components of the system through conduction in the event of a power failure. Alternatively, active control is also envisioned. For example, the system can further include a controller configured to operate the reservoir to cool one or more stages of the first module in response to a change in the state of the cryogenic refrigerator within the system. Typically, the controller is an electronic controller and can be battery-powered. The controller is preferably configured to monitor the operation of one or more cryocoolers within the system and to operate the reservoir to cool one or more stages of the first module in response to detecting a failure of the monitored cryocooler. The failure can be an unexpected interruption in the operation of the compressor due to, for example, a power shortage or a failure of mechanical components.

[0026] One, more than one, or preferably each module within the system further includes a heat switch assembly connected to one or more of a plurality of stages operable to selectively thermally couple adjacent stages of the plurality of stages. For example, the heat switch assembly can include one or more gas gap heat switches and / or one or more heat pipes. The heat switch assembly is operable to control the thermal conductivity across the assembly such that, for example, during an initial cooling process from room temperature, the stages are in a state of high thermal conductivity with respect to each other, and thermally isolate each stage from each other at a temperature below 4 Kelvin or during steady-state operation.

[0027] The QIP system may require that a high cooling power be applied to each of the lowest temperature stages. Thus, in a particularly desirable implementation, the plurality of stages of the first module includes a fractionator stage, a cooling stage, and a mixing chamber, the cooling stage is disposed between the fractionator stage and the mixing chamber stage, the first module further includes three dilution units, each dilution unit includes a fractionator thermally coupled to the fractionator stage of the first module, the first dilution unit further includes a mixing chamber thermally coupled to the cooling stage, and the second and third dilution units each include a mixing chamber thermally coupled to the mixing chamber stage of the first module.

[0028] One or more modules of the system are preferably configured to allow convenient access to and replacement of samples by use of a secondary insert that allows an experimental apparatus (typically including a sample) to be connected to the module. For example, one or more modules include a primary insert and a removable secondary insert, the primary insert includes a plurality of primary plates formed by respective stages of the module, the secondary insert includes a plurality of secondary plates connected in a self-supporting structure, and the system further includes one or more adjustment members configured to conductively and thermally contact the primary and secondary plates when the secondary insert is attached to the primary insert.

[0029] This system design is particularly suitable for QIP applications and for building server farms with 1000+ logical quantum bit systems including error correction. Such large-scale wiring requirements benefit not only from the expansion of the experimental space achieved by connecting multiple modules, but also from the high cooling power obtained, for example, by connecting multiple dilution refrigerators within the same cryostat. Also, as described above, the modular design allows various cooling technologies scattered across different modules to be implemented and facilitates new operating modes. A second aspect of the present invention is a quantum information processing apparatus including a cryogenic cooling system according to the first aspect. The features described in connection with the first aspect of the present invention can equally apply to the second aspect.

[0030] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

Brief Description of the Drawings

[0031]

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Embodiments for Carrying Out the Invention

[0032] First, with reference to FIGS. 3 to 5, a first module 100 that forms part of an ultra-low temperature cooling system according to a first embodiment of the present invention will be described. The module 100 includes a generally cubic housing 110 that includes six orthogonal faces, in contrast to the conventional cylindrical design shown in FIGS. 1 and 2. The housing has a framework 21 (see FIG. 4) that supports four orthogonal side faces that extend between the upper and lower faces of the housing 110. The upper face supports a stepped assembly consisting of cooling stages 105 to 109 and two ultra-low temperature refrigerators. The first ultra-low temperature refrigerator is a mechanical refrigerator (also referred to as a "cryocooler" / "non-refrigerant refrigerator") in the form of a pulse tube refrigerator (PTR) 103. Other mechanical refrigerators that can be used include a Gifford-McMahon (GM) refrigerator and a Stirling refrigerator. The second ultra-low temperature refrigerator is a dilution unit that includes a fractionator 30 and a mixing chamber 31 connected by a series of heat exchangers. The housing is configured to form a vacuum chamber 101 when fully assembled, and the interior of the vacuum chamber (which houses the ultra-low temperature refrigerators and cooling stages 105 to 109) is evacuated to a vacuum during use.

[0033] The assembly of cooling stages 105 - 109 includes a PT1 stage 105 and a PT2 stage 106. The first stage of the PTR 103 is attached to the PT1 stage 105 to cool the PT1 stage 105 to a reference temperature of 50 - 70K. The second stage of the PTR 103 is attached to the PT2 stage 106 to cool the PT2 stage 106 to a reference temperature of 3 - 5K. The assembly further includes a fractionator stage 107 and a mixing chamber stage 109 that are thermally coupled to a fractionator 30 and a mixing chamber 31, respectively. During operation, a working fluid formed of a mixture of helium-3 and helium-4 circulates around the dilution unit through a pump line. The fractionator stage 107 and the mixing chamber stage 109 achieve respective reference temperatures of 0.5 - 2K and 5 - 20mK during use. A "cooling plate" 108 that forms a further heat stage (also referred to herein as a "cooling stage") is disposed between the fractionator stage 107 and the mixing chamber stage 109 to achieve the temperature between the fractionator stage 107, which is typically 50 - 150mK, and the mixing chamber stage 109.

[0034] During use, a heat switch assembly (not shown) including a plurality of gas gap heat switches selectively thermally couples each of the cooling stages 106 - 109. The heat switch assembly is configured to allow heat exchange between the cooling stages 106 - 109 during an initial cooling procedure and to thermally isolate different stages at low temperatures. Optionally, for example, one or more heat pipes can be provided between the PT1 stage 105 and the PT2 stage 106. The heat pipes can further accelerate the pre-cooling process in conjunction with the PTR 103 and the gas gap heat switches. The combination of the gas gap heat switches, heat pipes, and low thermal conductivity support rods connected to the housing 110 holds the stages 105 - 109 in the illustrated stepped arrangement.

[0035] Inside module 100, a nested assembly of thermal radiation shields is provided, and each thermal radiation shield is connected to its respective thermal stage to surround a cooler component. The first thermal radiation shield 41 is connected to the PT1 stage 105, the second thermal radiation shield 42 is connected to the PT2 stage 106, the third thermal radiation shield 43 is connected to the fractionator stage 107, and the second thermal radiation shield 42 is disposed between the first thermal radiation shield 41 and the third thermal radiation shield 43. The first and second thermal shields 41, 42 are typically aluminum, while the third thermal shield 43 is typically copper. The thermal radiation shields 41 - 43 are configured to form a closed structure when fully assembled and attached to their respective thermal stages, and surround the components of a system configured to be cooled to a lower temperature. The thermal radiation shields 41 - 43 have four orthogonal sides configured to mate with each other. In this embodiment, as shown in FIG. 5, each side of each thermal radiation shield includes two removable panels that are coplanar to provide external access to the area enclosed by the thermal radiation shield. For convenience during assembly and disassembly, handles are attached to the outside of the sides.

[0036] FIG. 4 shows the appearance of module 100 in a first configuration with the housing 110 fully assembled. In the first configuration, module 100 is configured to operate as an independent cryogenic cooling system to achieve cryogenic temperatures inside it. In this embodiment, each side of the housing 110 has a removable panel that can be removed from the framework 21 to provide access to the internal volume of module 100 defined by the inside of the housing 110. In FIG. 5, the removable front panel 35 (shown in FIG. 4) is removed from the front side 22 of module 100 to expose the front side of the first thermal radiation shield 41, and this side can also be removed for maintenance etc. as required.

[0037] Another example where the removable panel can be removed from the side of module 100 is for connecting module 100 to an adjacent module. Module 100 is further configured to operate as part of an integrated cryogenic cooling system that includes a plurality of such modules connected by respective sides in a second configuration. Figures 6 and 7 show an example of combining two such modules according to Figures 3 - 5 to form an integrated cryogenic cooling system according to the first embodiment. As shown in Figure 6, remove the removable panel from the right side of the first module 100 and remove the removable panel from the left side of the second module 150. Then, arrange the first module 100 and the second module 150 such that the right side of the first module 100 abuts the left side of the second module 150. The frameworks 21 of the first module and the second module include a housing connector 120 that includes a flange on the outer surface of the housing 110 and a seal member disposed around the outside of the area of the removable panel. The flange on the right side of this first module 100 is arranged to align with the flange on the left side of the second module 150 by a fastening member such as a bolt that couples the flanges together.

[0038] When connected, the first and second modules 100, 150 form a common cryogenic cooling system having an internal volume that includes the sum of the internal volumes of the connected modules. This is shown in FIG. 7. The housings of the modules 100, 150 engage with each other to evacuate the airtight sealed internal volume of the system including the stages from each module. Remove the heat radiation shield sides on the right side of the first module 100 and the left side of the second module 150. During system assembly, additional radiation shield sides and the sides of the module housing can be removed to provide access and then reattached. In this configuration, a large sample space is formed for conducting much larger scale experiments than can be otherwise performed with a typical single-unit type cryostat. This configuration has the advantage that not only is there a large space, but the cooling power is greatly enhanced by thermally coupling one or more adjacent thermal stages of different modules.

[0039] In this embodiment, when the first and second modules 100 and 150 are connected such that the first module 100 is connected to the second module 150 by the housing connector 120, each thermal stage of the first module is configured to be coplanar with each thermal stage of the second module 150. This means that adjacent coplanar (same plane) thermal stages can be connected to each other by an extension plate 160 extending between these thermal stages. The extension plate 160 provides a structural advantage of enhancing the rigidity of the assembly and can have a heat shielding effect. The extension plate 160 can also be used for heat transfer purposes, for example, to couple the cooling power from one cryogenic refrigerator of the module to the thermal stage of another module. In this case, a material with high thermal conductivity such as copper can be used to form the extension plate. Around each of the stages 105 - 109, stage connectors 50 in the form of an array of holes and bolts for connection to the extension plate, or any other suitable fastening means, are provided. These are schematically shown in FIG. 3, and by providing stage connectors 50 at each end of each thermal stage, the stage can be extended in any direction for connection to an adjacent module on the left or right side of the module 100.

[0040] As described above, the radiation shields 41 - 43 surround the cryogenic components of the system and are respectively attached to the PT1 stage 105, PT2 stage 106, and the fractionator stage 107. Therefore, the width of the PT2 stage 106 is narrower than the width of the PT1 stage 105, the width of the fractionator stage 107 is narrower than the width of the PT2 stage 106, and the widths of the cooling plate 108 and the mixing chamber stage 109 are also narrower than the width of the fractionator stage 107. As a result, the lateral spacing between the coplanar thermal stages of two adjacent modules is not the same for each stage. Therefore, as shown in FIG. 8, extension plates 160 of different sizes are used to fill the gaps between the stage connectors 50 on each stage.

[0041] In the first embodiment, first, second, and third radiation shields 41-43 are configured to surround the cryogenic components of the system. In the configuration shown in FIG. 3, each radiation shield has four orthogonal side faces, but at least one of these side faces is removed when connecting to another module within the system. As a result, it becomes possible to extend experimental services such as cable wiring between different modules. To connect the coplanar surfaces of the radiation shields of the connected modules, a plurality of shield extension sections 180 in the form of plates are attached. Usually, direct physical contact between adjacent radiation shields is not essential, but it is preferable to provide a shield connector 55 at each end of each radiation shield so that adjacent shield extension sections 180 can be interlocked and preferably thermally coupled. The shield connector 55 can include any one of an interlocking feature for connecting the shield extension sections 180, a flexible joint, an array of holes and bolts, or any other suitable fastening means.

[0042] Although only two modules are coupled in the first embodiment, it will be understood that any number of modules can be connected to expand the size of the system. Further, the system is not limited to a one-dimensional array of connected modules, and a two-dimensional array is also envisioned. A particular advantage of the cubic module design is that the modules can be attached in close proximity to each other. As a result, the use of the available floor area becomes efficient. This also means that there is no need for a "spacer" between modules, which could potentially affect the cooling performance by increasing the size and mass of the system, as the housings of adjacent modules can be directly connected on top of each other.

[0043] In the first embodiment of FIG. 8, two modules 100, 150 each having the same cryogenic refrigeration configuration are coupled. Specifically, each module forms a non-refrigerant dilution refrigerator in which a PTR and a dilution unit as described with reference to FIG. 3 are arranged. However, a particular advantage provided by the modular design is the flexibility that different modules of the system can have different refrigeration configurations. FIG. 9 shows a second embodiment that is basically the same as the first embodiment, but in which the mixing chamber 31' of the second module 250 is attached on the cooling plate 208'. Therefore, the cooling power applied to the cooling plate 208' is significantly increased compared to the first embodiment. This cooling power is also transmitted to the cooling plate of the first module 200 by a copper stage extension plate that thermally couples the coplanar cooling plates. Therefore, the cooling plates of the first and second modules 200, 250 have dedicated cooling sources rather than relying entirely on the cooling power of adjacent stages. This means that the ability to withstand heat loads that may be imposed by components of the QIP system, etc., is increased without substantially raising the operating temperature of the system.

[0044] Figures 10 and 11 are perspective views of the first module 200 of the second embodiment. Similar to the first embodiment, housing connectors 220 for directly connecting the module 200 to adjacent modules are provided on each of two opposing side surfaces. The housing connector 220 includes a seal member 220 surrounded by an array of bolts and holes. In this embodiment, the housing connector on the right side surface of the first module 200 is configured to connect to the corresponding housing connector on the left side surface of the second module 250. Removable panels 135 can be attached by bolts to the left and right side surfaces of each housing to surround the internal volume of the module 200 and facilitate the independent operation of the module 200. Removing the removable panel 135 enables connection of the module 200 to one or more adjacent modules. The seal member 220 enables a vacuum-tight seal to be formed between the connected housings. In this embodiment, the seal member 220 includes an O-ring groove and an O-ring on the right side surface of the first module 200 configured to engage an O-ring seal provided on the left side surface of the second module 250. However, other types of seal members such as a knife-edge gasket seal can also be used.

[0045] The front surface of the module 200 includes a door 210 firmly attached to the housing by hinges and bolts so as to maintain a vacuum inside the housing. The door 210 can be opened, for example, to provide access to the inside of the module 200 during maintenance or to adjust any bolts such as bolts provided on the side surface of the housing or on the radiation shield.

[0046] FIG. 12 is a perspective view of a system according to a third embodiment, in which the doors 210 of each module 200, 250 are not shown to illustrate various connections between the two modules 200, 250. The third embodiment is basically the same as the second embodiment and uses the same general module design. However, in the third embodiment, an extension plate between the cooling plates 208, 208' is removed so that the cooling plate 208 of the first module 200 is thermally separated from the cooling plate 208' of the second module 250. In another embodiment, one of the thermal stages can be thermally separated without physically disconnecting by using an extension plate with low thermal conductivity, which is partially or completely formed of a material such as plastic or stainless steel. As an example, it may be desirable to thermally separate adjacent coplanar stages so that heat from electrical elements on the thermal stage of the first module does not transfer to the second module.

[0047] As each module of the system cools from room temperature, adjacent thermal stages contract and move apart from each other and can cool at different rates depending on the exact configuration of each module. There also appear to be manufacturing tolerances to consider. To address these issues, the thermal stages of each module can be "indirectly" joined to each other by flexible joints. An example of such a connection will now be described with reference to FIG. 13.

[0048] Figure 13 shows the connection between the coplanar thermal stages of the first and second modules 200, 250 of the third embodiment. Specifically, it shows the flexible mechanical connection formed between the PT1 stage 205 of the first module 200 and the PT1 stage 205' of the second module 250. Similar connections are formed between each of the remaining coplanar stages, except for the unconnected cooling plates 208, 208'. Stage connectors 251 that enable the connection between coplanar stages are provided on the left and right sides of each stage by one or more intermediate extension plates. In this embodiment, the PT1 stage 205 of the first module 200 is connected to the PT1 stage 205' of the second module 250 by the first and second extension plates 206a, 206b. The lower sides of the first and second extension plates 206a, 206b are joined together by an extensible joint 266 that includes soft braids of high thermal conductivity formed of copper or the like. The extensible joint 266 enables good heat conduction between the adjacent PT1 stages 205, 205' and relative movement between the stages that may occur during thermal cycling.

[0049] A labyrinth structure 265 is provided to assist in radiation shielding in the region between the ends of the two extension plates 260a, 260b. The labyrinth structure or labyrinthine structure 265 is formed by the downward lip of the end of the first extension plate 260a disposed inside the upward lip of the adjacent end of the second extension plate 260b. To further assist in heat insulation, additional light-absorbing material can be applied to the interface surface between the two lips inside the labyrinth structure 265. As a result of the labyrinth structure 266 enabling relative movement between the first extension plate 206a and the second extension plate 206b, the lips can come into contact with each other. Other things are also envisioned including using interlocking or alternating components that extend while maintaining physical (and preferably thermal) contact between the stages when the connected stages contract as a means of connecting adjacent thermal stages.

[0050] Using a structure similar to that shown in FIG. 13, a thermal radiation shield can be connected. As shown in FIG. 12, the coplanar thermal shields of the first and second modules 200, 250 are coupled to each other by two shield extension sections respectively. For example, the first shield extension section 280a is attached to the third thermal shield 143 of the first module 200 by a shield connector 155 in a form similar to the stage connector 251 shown in FIG. 13. The second shield extension section 280b is attached to the third thermal shield 143 of the second module 250 by a similar shield connector 155. As shown in FIG. 14, the first and second shield extension sections 280a, 280b are coupled to each other by a shield labyrinth structure (shield labyrinth structure) 275 similar to the labyrinth structure 266 described in connection with FIG. 13. In this case, the end of the first shield extension section 280a forms a lip that is disposed inside another lip formed at the end of the second shield extension section 280b. This engagement allows for some relative movement between the shield extension sections 280a, 280b that may occur during a thermal cycle, etc., and also allows for some misalignment due to manufacturing tolerances. A similar connection is made between each adjacent coplanar shield extension section.

[0051] Substantially, the system provides an extensible cryostat composed of an extensible vacuum chamber and an extensible temperature stage. This extensibility enables a plurality of independent cryostats (or "modules") to be strongly mechanically and thermally coupled to each other and operate at the same or similar temperatures at the corresponding / adjacent thermal stages of each cryostat as described with reference to FIGS. 7 - 9. Also, this extensibility enables a plurality of modules to be strongly (or weakly) mechanically coupled to each other and operate as a single entity at different temperatures at the corresponding / adjacent thermal stages of each module. FIG. 12 shows an example where the coplanar cooling plates 208, 208' are separated from each other and thus thermally isolated. Next, another example will be described.

[0052] FIG. 15 is a schematic diagram showing a system according to a fourth embodiment of the present invention having anisothermal connected stages. A first module 300 is connected to a second module 350 to form an integrated cryogenic cooling system. This configuration is basically the same as the first embodiment except that the fractionator stage 307, the cooling stage 308, and the mixing chamber stage 309 of the first module are not thermally coupled to the corresponding fractionator stage 307', cooling stage 308', and mixing chamber stage 309' of the second module 350. All other coplanar stages are thermally coupled in the same manner as in the first embodiment. The first stage extension plate mechanically connects the fractionator stage 307 of the first module 300 to the fractionator stage 307' of the second module 350, and the second stage extension plate mechanically connects the mixing chamber stage 309 of the first module to the mixing chamber stage 309' of the second module 350. As a result, the structural rigidity and heat insulation of the system are improved. However, the first stage and the second stage extension plates are preferably configured to have a low thermal conductivity of less than 50 W / mK at a temperature of 20K. This configuration allows the coplanarly connected stages to reach different operating temperatures and can be achieved in a plurality of ways. For example, the first stage and the second stage extension plates can be partially or entirely composed of materials having a relatively low thermal conductivity at their respective operating temperatures, such as plastic, silicon, or stainless steel. In this embodiment, copper stage extension plates are used as in the first embodiment, but these are connected to each other by stainless steel expandable joints 366. The cooling stage 308 of the first module 300 is separated from the cooling stage 308' of the second module 350 by a gap, and thus these adjacent cooling stages 308, 308' are substantially thermally separated from each other and can have different temperatures during use.

[0053] In this embodiment, the system forms part of a QIP system (represented schematically) in which a first module 300 includes an "input" line 381 and a second module 350 includes an "output" line 383. Conductors in the form of the input line 381 and the output line 383 are thermally coupled to respective cooling stages of the first and second modules 300, 350. The input line 381 transmits an electrical signal from a location external to the system, typically to a QIP device 382 (e.g., a quantum bit processor) that is thermally coupled to the lowest temperature stage of the system. The output line 383 transmits an electrical signal from the QIP device 382 to a location external to the system. Since the input line is one of the largest heat sources generated through active and passive heat dissipation at different stages of the cryocooler in a QIP experiment, it would be advantageous if the output line could be operated at a lower temperature than the input line. As a result, the signal-to-noise ratio of the output signal is maintained until the output signal can be amplified. Typically, the operating temperatures of the fractionator stage 307, the cooling stage 308, and the mixing chamber stage 309 of the first module 300 are each higher than the operating temperatures of the fractionator stage 307', the cooling stage 308', and the mixing chamber stage 309' of the second module 350. Optionally, an additional cryocooler at an extremely low temperature can be thermally coupled to the output line 383 with respect to the input line 381 to compensate for the high heat load imposed by the input line 381. For example, the configuration shown in FIG. 15 can be modified such that two PTRs and two dilution units are provided within the first module 300.

[0054] The ability to "split up" these cooling stages by thermally coupling them to each other weakly is useful in applications where the first cryogenic refrigerator on the cooling stage operates at one temperature to more effectively manage high heat loads, but this temperature is not advantageous for the correct operation of another cryogenic refrigerator connected to the cooling stage. Next, an example will be described with reference to FIG. 16, which is a schematic diagram showing a system according to a fifth embodiment of the present invention. A first module 400 is connected to a second module 450 to form an integrated cryogenic cooling system. This configuration is the same as that of the first embodiment, but the components of the QIP system are not shown for clarity. Similar to the first embodiment, the first module 400 has a pulse tube refrigerator and a dilution unit including a fractionator 430 and a mixing chamber 431. However, the only cryogenic refrigerator provided in the second module 450 is a helium-4 refrigerator called a "1K pot" 432, which is configured to receive a helium flow from an external location by a pump line. As described above, the 1K pot 432 is connected to a "1K stage" 407' that is thermally coupled to the fractionator stage 407 by one or more stage extension plates. A further difference is that the fractionator 430 is connected to the fractionator stage 407 by a weak thermal link 467 such as a plastic or stainless steel collar.

[0055] The assembly can form part of a QIP system having any number of connected electrical elements that can provide a heat source. The relatively high cooling power of the thermally coupled 1K pot operating at 1 - 1.5K or 1.2 - 2K can effectively manage the large heat load applied to the fractionator stage 407. The fractionator of the dilution unit ideally operates in the 750 - 850mK region. Thus, the weak thermal link 467 advantageously enables the dilution unit to operate in a more optimal form independent of the temperature of the 1K pot. As a result, the performance of QIP measurements can be enhanced by reducing the overall reference temperature of the mixing chamber 431.

[0056] Different types of modules that serve different purposes can also be connected to each other to form part of an integrated cooling system. FIG. 17 is a schematic diagram showing a system according to a sixth embodiment of the present invention. A first module 500 is connected to a second module 550 to form an integrated cryogenic cooling system. In this embodiment, the first module 500 includes a pulse tube refrigerator 503 having a first cooling stage connected to a PT1 stage 505 and a second cooling stage connected to a PT2 stage 506 (as described above). In this embodiment, the PT1 stage 505 and the PT2 stage 506 are thermally coupled to a first tank 590 and a second tank 592, respectively. The first tank 590 is attached to the PT1 stage 505, and the second tank 592 is attached to a radiation shield thermally coupled to the PT2 stage 506. Each tank is configured to store a liquid refrigerant during use. Specifically, the first tank 590 is configured to receive nitrogen that is introduced into the system along a first conduit 591 and that typically liquefies by thermal contact with the PT1 stage 505. The second tank 592 is configured to receive helium-4 that is introduced into the system along a second conduit 593. The second conduit 593 is thermally coupled to the PT1 stage 505 and the PT2 stage 506 by a heat exchanger, and thus the helium can liquefy when it comes into thermal contact with the PT2 stage 506 during cryogenic operation. The conduits 591, 593 not only allow these fluids to flow into their respective tanks 590, 592 of the first module 500, but also provide a path for discharging gas when it is necessary to warm the system or when the liquid boils.

[0057] The second module 550 includes PT1 stage 505’ and PT2 stage 506’ that are thermally coupled to the PT1 stage 505 and PT2 stage 506 of the first module 500, respectively. Optionally, the second module 550 can further include a cryocooler that facilitates cooling of the second module (specifically, PT1 stage 505’ and PT2 stage 506’) alone when there is no connection to the first module 500. Also, the second module 550 includes three cryogenic thermal stages: a fractionator stage 507’, a cooling plate 508’, and a mixing chamber stage 509’, and three dilution units connected to the cryogenic stages. The first dilution unit includes a first fractionator 530 and a first mixing chamber 531, the second dilution unit includes a second fractionator 530’ and a second mixing chamber 531’, and the third dilution unit includes a third fractionator 530’’ and a third mixing chamber 531’’. The first fractionator 530, the second fractionator 530’, and the third fractionator 530’’ are each thermally coupled to the fractionator stage 507’. The second mixing chamber 531’ and the third mixing chamber 531’’ are thermally coupled to the mixing chamber stage 509’, while the first mixing chamber 531 is thermally coupled to the cooling plate 508. The combination of three dilution units, including one dilution unit having a mixing chamber coupled to an intermediate cooling plate, ensures that high cooling power is provided to each of the lowest temperature stages 507’, 508’, 509’. This configuration of the second module 550 is typically particularly effective in managing the heat load introduced by the electrical components of a QIP system.

[0058] A particular advantage provided by the first module 500 in this embodiment is that the first module 500 functions as a fail - safe mechanism for cooling the second module 550 when needed. Specifically, the first and second tanks 590, 592 are configured to cool the PT1 stage 505’ and the PT2 stage 506’ of the second module 550 in the event of a failure of the PTR503 or any of the cryocoolers within the system that may occur due to an interruption in power supply or a mechanical failure. In this embodiment, the first tank 590 (containing liquid nitrogen) is configured to maintain the temperature of the PT1 stage 505’ at around 70K, and the second tank 592 (containing liquid helium) is configured to maintain the temperature of the PT2 stage at around 4K. The fail - safe mechanism operates passively by the thermal coupling between the tanks 590, 592 and the rest of the system components. In the absence of power, the temperature within the system may gradually increase at a rate depending on the insulation of the tanks 590, 592 and the mass of the liquid refrigerant inside them. Usually, the temperature of the components inside the system approaches and becomes constant at the boiling point of the liquid refrigerant, but any external headloads are absorbed by the latent heat of the liquid. When different tanks containing different refrigerants are provided, the temperature can stabilize at the higher boiling point of the refrigerant. When the liquid boils and evaporates, the temperature will start to rise further. The expected amount of liquid and the heat load should allow for a “hold - time” of several hours or days. Thus, the above - described system can maintain the interior of the system at a cryogenic temperature below 100K for at least 24 hours even in the absence of any power.

[0059] Alternatively, in some embodiments, the system includes an electronic controller (typically battery-powered) configured to detect a change in the state of the cryocooler within the system that may occur due to an unexpected power outage or mechanical failure. For example, the controller can monitor the operation of the compressor motor within the PTR503. In response to detecting a change in state, the controller can operate one or more pumps (forming part of the controller) to cool one or more thermal stages of the system, typically by conveying a refrigerant in liquid form along a conduit. The refrigerant can be conveyed from a location within the system (such as the first and second tanks 590, 592 shown in FIG. 17) or from an external dewar.

[0060] So far, the 4K cooling power of dry dilution refrigerators has been provided by cryocoolers. The demand for these systems has arisen precisely because they are dry technologies. This technology simplifies the infrastructure and experience support required for the operation of the system, making the system usable in an academic environment. However, in commercial-scale systems, there are concerns about reliability associated with relying on multiple cryocoolers. As the experimental size increases, the available cooling power at 4K rapidly becomes insufficient. Roughly speaking, assuming four separate units each providing 2W, realistically the cryocooler can provide ~8W of 4K cooling power. This relatively short lifespan is already a significant investment on its own. The components have strict duty cycles, and if ignored, the performance of the cryocooler will degrade. In an industrial environment, the 4K cooling power provided by a facility-scale 4He recirculation system is desirable (commercial suppliers include Air Liquide and Linde Kryotechnik). Such systems are implemented in medical equipment where high reliability and cooling power are essential (e.g., for MRI systems), or in large-scale physical and engineering experiments (such as CERN and ITER). Standard commercial liquefier units can easily liquefy 280l / hr of 4He, which is equivalent to 200W of available 4K cooling power (liquefiers are available with a capacity of >3500l / hr, which is equivalent to >2.5kW of 4K cooling power). Liquid helium can circulate through the heat exchanger piping inside one or more of the modules to provide the necessary thermal contact to the 4K cooling bus of the QIP system. The facility-level 4K cooling mechanism enables multiple dilution refrigerators to operate on the same 4K cooling loop. Such a 4K cooling mechanism can include separate quantum computers that share the same pre-cooling system while performing separate calculations. These quantum computers can be present within the same module or within separate modules connected within a common vacuum chamber.Next, an embodiment will be described in which components of the system are cooled by liquid refrigerant received from an external refrigeration plant.

[0061] FIG. 18 is a schematic diagram of a system 600 according to a seventh embodiment of the present invention. The system 600 includes a first module 610, a second module 620, a third module 630, and a fourth module 640. These modules are arranged as a one-dimensional array in which both side surfaces of the second module 620 are connected to the first and third modules 610 and 630, and both side surfaces of the third module 630 are connected to the second and fourth modules 620 and 640. The housings of the modules are connected to each other as described in the above embodiments.

[0062] The first module 610 has a first conduit 651 that receives a flow of liquid nitrogen from a refrigeration plant (not shown) external to the module. Alternatively, the first conduit 651 can receive a flow of cryogenic helium gas taken from a helium refrigeration plant before the final liquefaction stage. The first conduit 651 is thermally coupled to the first stage 601 of the first module 610 by a first heat exchanger 653. The first module 610 also has a second conduit 655 that receives a flow of liquid helium from the refrigeration plant. The second conduit 655 is thermally coupled to the second stage 602 of the first module 610 by a second heat exchanger 657. In use, the first and second conduits are configured to cool the first and second stages 601 and 602 to approximately 50 - 70 K and around 4 K, respectively, by the respective refrigerant flows through the first and second heat exchangers 653 and 657. This configuration provides an alternative to a configuration using a cryocooler and is particularly desirable in large-scale facilities having more than three modules for the reasons described above.

[0063] A variant of this scheme uses the flow of incoming liquid helium-4 to hold a substantial amount of liquid helium inside a reservoir tank thermally connected to the second stage. By using an internal helium reservoir, the system can continue to operate even if the operation of the liquefier is interrupted for a certain period. If the required cooling power at the second stage is only on the order of a few watts, a helium liquefaction system using a cryocooler (described in FIG. 17) can be used as well. In this case, by using an internal reservoir, the system can continue to operate while the cryocooler liquefier is being replaced with a spare unit, reducing concerns about the reliability of the cryocooler. In systems with low cooling power requirements where a cryocooler can be used instead of a facility-scale helium-4 recirculation system, methods for reducing concerns about the reliability of cryocoolers other than cryocooler-based liquefiers are well known in the art. However, usually, the number of cryocoolers attached to the system exceeds the number strictly required for operation, and thus the system can operate even if one or more cryocoolers fail to operate.

[0064] Referring again to the seventh embodiment, the first and second stages 601, 602 of the first module are thermally coupled by stage extension plates (as described above) to corresponding coplanar stages of the second, third, and fourth modules 620, 630, 640, respectively, forming a common isothermal stage throughout the system. Optionally, as shown by the dashed lines at the end of the first and second conduits 651, 655 described above, the first and second conduits 651, 655 can be repeatedly connected to corresponding isothermal stage plates on each module by respective heat exchangers. Appropriate ports can be provided on the upper surface of each module to facilitate fluid flow into and between the modules.

[0065] The second module 620 includes a helium-4 refrigerator 632 (also referred to as a "1K pot") configured to cool the third stage 603 to around 1K. The third stage 603 is thermally coupled to the corresponding coplanar stage of the third module 630, but is separated by a gap from the coplanar third stage 603' of the fourth module 640. As a result, the third stage 603' of the fourth module 640 can operate at a temperature different from that of the third stage 603 of the second and third modules 620, 630. This configuration may be desirable in some cases as described above with reference to the fourth embodiment.

[0066] The third module 630 has a fourth stage 604 thermally coupled to the corresponding thermal stage of the fourth module 640 and a fifth stage that forms the lowest temperature stage and is not connected to any corresponding stage of the connected modules. The third module 630 further includes a first dilution unit having a first fractionator 635 attached to the third stage 603 by a weak thermal link 667 (as described in connection with the fifth embodiment of FIG. 16) and a first mixing chamber 636 attached to and thermally coupled to the fifth stage 605. The fourth module 640 includes a second dilution unit having a second fractionator 635' attached to the third stage 603' of the fourth module 640 and a second mixing chamber 636' attached to the fourth stage 604. This configuration ensures that each thermal stage is thermally coupled to its respective cryogenic refrigerator and can be particularly advantageous for compensating for the thermal load that may be applied by connected electrical devices and the like.

[0067] FIG. 18 schematically depicts the components of an arbitrary QIP system. Input line 681 is sequentially connected to each of a first stage 601, a second stage 602, and a third stage 603 within a second module 620. And the input line 681 extends from the third stage 603 within the second module 620 to a third module 630. Thereafter, the input line 681 is connected to each of a fourth stage 604 and a fifth stage 605 within the third module 630, and to a QIP device within the third module 630, and the QIP device is thermally coupled to the fifth stage 605. As a result, the input line 681 is cooled by each of the connected thermal stages and is configured to transmit an electrical signal from a location external to the system 600 to the QIP device 682. Output line 683 is configured to transmit an electrical signal from the QIP device 682 to another location external to the system 600. The output line 683 extends from the QIP device 682 into a fourth module 640 and is then sequentially connected to each of a fifth stage 605, a fourth stage 604, a third stage 603', a second stage 602, and a first stage 601 within the fourth module 640.

[0068] The seventh embodiment illustrates how the common internal volume of the system 600 formed by the connected modules can be utilized in various parts of the QIP system. This embodiment also shows how the cooling components of different modules can be used to compensate for the heat load imposed by the connected electrical devices (e.g., caused by the operation of the RF attenuator forming part of the input line 681). In this example, the relatively large heat load brought about by the input line 681 of the third stage 603 is reduced by the cooling power of the dedicated 1K pot 632 rather than by the fractionator of the dilution unit, which may have a low cooling power. The first fractionator 635 is thermally isolated from the input line 681 and the 1K pot 632 by using a low thermal conductivity link 667. At the fourth stage 604, the heat load from the input line 681 is taken into the second mixing chamber 636', which forms part of a dedicated "cooling stage dilution refrigerator". As a result, the first dilution unit of the third module 630 is placed in a different "no-load" state, enabling the first mixing chamber 636 to operate optimally to lower the temperature of the QIP device 682 and reduce the overall reference temperature of the system 600. In use, the fourth stage 604 typically achieves a reference temperature of about 100 - 200 mK, the fifth stage 605 typically achieves a reference temperature of about 10 - 20 mK, or it may be possible to achieve an even lower reference temperature depending on the heat load imposed by the connected electrical devices.

[0069] It will be appreciated that the modularization approach described herein provides the user with the flexibility to combine different modules having respective cooling configurations according to experimental needs. For example, the operation of multiple closed-loop (or open-loop) cryogenic refrigerators (including dilution refrigerators, helium-3 or helium-4 refrigerators) within a cryostat enables the cooling powers of multiple cooling systems to be combined by mechanically and thermally strongly and isothermally coupling the cooling stages to each other. This configuration can be achieved by using a material with high thermal conductivity to link the mixing chambers of multiple dilution units of the cooling stages to each other. Alternatively, the cooling powers of these cooling systems can also be separated by mechanically strongly (or weakly), but thermally weakly coupling the cooling stages of the multiple cooling systems to be non-isothermal. This configuration can be achieved by having one mixing chamber of one dilution unit operating at one temperature and another mixing chamber of another dilution unit operating at another temperature. Further, cooling at the PT2 stage (which generally has an operating temperature of about 4K) can be performed by any one (or any combination) of a cryocooler, dry mechanical contact to the cooling section of a helium recondensing system, or connection to a liquid helium bath.

[0070] The stages of the modules within the system can be considered to form a primary insertable body that can be connected to a secondary insertable body that houses the target device. By facilitating the loading and replacement of the target device into the system through the connection of the secondary insertable body to the primary insertable body, the customizability of the system can be further enhanced. The secondary insertable body can include a plurality of secondary plates connected in a self-supporting structure, and usually, each secondary plate can be connected to the respective stage that forms the primary plate of the primary insertable body. One or more adjustment members can be configured to bring the primary and secondary plates into conductive and thermal contact when the secondary insertable body is attached to the primary insertable body. For example, (as described in International Publication No. WO 2021 / 170976 A1) the (single or plural) adjustment members can vary the separation distance between the primary plate and the secondary plate, or can include (single or plural) deformable members that form part of each primary or secondary plate. A plurality of secondary insertable bodies can be connected to the primary insertable body, and for example, (depending on the size and configuration of the module) two or more secondary insertable bodies can be connected to each of the two sides of the module. Access to the secondary insertable body can be conveniently enabled, for example, by a door on the side of the housing as shown in FIG. 10.

[0071] Each module can be regarded as forming a "unit cell" of an expandable system. The modules can be configured to operate independently or, when the complexity of the experiment increases, coupled to adjacent modules to expand the cooling capacity and internal physical space of the system and operate as a single large system. The connected modules within the system can perform different functions and, as described above, can have different sizes, shapes, or installation areas from other modules within the system when the housing enables interconnection. The expandable temperature stage facilitates, for example, the combination of a greater number of cryogenic cooling systems on a common temperature stage to compensate for a higher heat load. The option of thermally weakly coupling each temperature stage allows the stages to be subdivided into different temperature regions, enabling, for example, the optimal operation of the connected devices. The above describes further preferred features and advantages. Overall, the system described in this specification provides a scalable cryogenic system that can operate at millikelvin temperatures.

Explanation of Signs

[0072] 50 Stage connector 100 First module 150 Second module 160 Extension plate

Claims

1. A cryogenic cooling system comprising: a cryogenic refrigerator assembly including one or more cryogenic refrigerators; two or more connected modules; wherein each of the connected modules includes: a housing having a plurality of sides that define an internal volume of the module; a plurality of stages disposed within the internal volume of the module; wherein one or more of the plurality of stages are thermally coupled to the cryogenic refrigerator assembly; the two or more connected modules are interconnected at respective sides; a cryogenic cooling system in which a first stage of a first module is thermally coupled to a first stage of a second module.

2. The cryogenic cooling system according to claim 1, wherein the cryogenic refrigerator assembly is configured to include a cryogenic refrigerator in which each of the connected modules is thermally coupled to one or more stages of the module.

3. The cryogenic cooling system according to claim 1 or 2, wherein each module is configured to operate as an independent cryogenic cooling system in a first configuration in which a removable panel is attached to the housing of the module so as to surround the internal volume of the module.

4. The cryogenic cooling system according to claim 3, wherein each module is further configured to operate as a component of an integrated cryogenic cooling system including a plurality of modules whose respective sides are interconnected to form a shared internal volume of the system in which the stages of each of the connected modules are disposed in a second configuration in which the removable panel is removed from the housing.

5. The cryogenic cooling system according to any one of claims 1 to 4, wherein each of the connected modules includes four orthogonal sides.

6. The cryogenic cooling system according to any one of claims 1 to 5, wherein a side of the first module is directly connected to a side of the second module.

7. The cryogenic cooling system according to any one of claims 1 to 6, wherein a second stage of the first module is thermally coupled to a second stage of the second module.

8. The cryogenic cooling system according to any one of claims 1 to 7, wherein one or more stages of the first module are connected to respective stages of the second module by one or more extension plates. Claim 9 The cryogenic cooling system according to claim 8, wherein each extension plate is configured such that the effective thermal conductivity between the connected stages exceeds 5000 W / mK at a temperature of 20 K. Claim 10 The cryogenic cooling system according to claim 8, comprising a plurality of the extension plates, wherein one or more of the extension plates are configured such that the effective thermal conductivity between the connected stages exceeds 5000 W / mK at a temperature of 20 K, and one or more of the extension plates are configured such that the effective thermal conductivity between the connected stages is less than 50 W / mK at a temperature of 20 K. Claim 11 The cryogenic cooling system according to any one of claims 8 to 10, wherein the connected stages are generally coplanar. Claim 12 The cryogenic cooling system according to any one of claims 8 to 11, wherein at least one of the one or more extension plates preferably includes an extensible joint configured to maintain a mechanical connection between the connected stages during relative movement of the connected stages. Claim 13 The cryogenic cooling system according to any one of claims 1 to 12, wherein the plurality of stages of each module share a common internal volume defined by the housing of the connected module, and the internal volume of the system is hermetically sealed from the ambient environment. Claim 14 Each module includes one or more radiation shields, each radiation shield being thermally coupled to respective stages of the module and surrounding one or more remaining stages, and each radiation shield including one or more shield surfaces, The cryogenic cooling system according to any one of claims 1 to 13, further comprising one or more shield extension sections, each shield extension section connecting coplanar shield surfaces of adjacent connected modules. Claim 15 The cryogenic cooling system according to any one of claims 1 to 14, wherein the first module includes a first dilution unit and the second module includes a second dilution unit. Claim 16 The first dilution unit includes a fractionator attached to a fractionator stage among the plurality of stages of the first module, the second dilution unit includes a fractionator attached to a fractionator stage among the plurality of stages of the second module, and the fractionator stage of the first module and the fractionator stage of the second module are mechanically connected by an extension plate. The cryogenic cooling system according to claim 15.

17. The effective thermal conductivity between the fractionator stage of the first module and the fractionator stage of the second module is less than 50 W / mK at a temperature of 20 K. The cryogenic cooling system according to claim 16.

18. The first dilution unit includes a mixing chamber attached to a mixing chamber stage among the plurality of stages of the first module, the second dilution unit includes a mixing chamber attached to a mixing chamber stage among the plurality of stages of the second module, and the mixing chamber stage of the first module and the mixing chamber stage of the second module are mechanically connected by an extension plate. The cryogenic cooling system according to any one of claims 15 to 17.

19. The effective thermal conductivity between the mixing chamber stage of the first module and the mixing chamber stage of the second module is less than 50 W / mK at a temperature of 20 K. The cryogenic cooling system according to any one of claims 15 to 18.

20. The system further includes a target assembly including an electrical circuit extending between the stages of the two or more connected modules within the internal volume of the system. The cryogenic cooling system according to any one of claims 1 to 19.

21. The first module includes a cryogenic refrigerator in the form of a dilution unit or a 1K pot, and the second module, or a third module having a stage thermally coupled to a corresponding stage of the first module, includes a cryogenic refrigerator in the form of a cryocooler and / or a liquid refrigerant reservoir. The cryogenic cooling system according to any one of claims 1 to 20.

22. The reservoir is configured to recover the refrigerant liquefied by the operation of the cryocooler. The cryogenic cooling system according to claim 21.

23. The cryogenic cooling system according to claim 21 or 22, wherein the reservoir is operable to cool one or more stages of the first module.

24. The cryogenic cooling system according to any one of claims 1 to 23, wherein the cryogenic cooling assembly includes a helium refrigeration plant, and the helium refrigeration plant is preferably disposed outside the housing of each module.

25. The cryogenic cooling system according to claim 24, wherein one or more of the modules include a first helium heat exchanger thermally coupled to a stage of the module, and the first helium heat exchanger is arranged to receive a flow of liquid helium from the helium refrigeration plant.

26. The cryogenic cooling system according to claim 25, wherein one or more of the modules further include a second helium heat exchanger thermally coupled to a stage of the module, and the second helium heat exchanger is arranged to receive a flow of gaseous helium at a temperature of less than 100 K, preferably 40 to 80 K, more preferably 50 to 70 K, from the helium refrigeration plant.

27. The cryogenic cooling system according to any one of claims 1 to 26, wherein the cryogenic cooling assembly includes a nitrogen refrigeration plant, and the nitrogen refrigeration plant is preferably disposed outside the housing of each module.

28. The cryogenic cooling system according to claim 27, wherein one or more of the modules include a nitrogen heat exchanger thermally coupled to a stage of the module, and the nitrogen heat exchanger is arranged to receive a flow of liquid nitrogen from the nitrogen refrigeration plant.

29. The plurality of stages of the first module include a fractionator stage, a cooling stage, and a mixing chamber stage, the cooling stage is disposed between the fractionator stage and the mixing chamber stage, the first module further includes three dilution units, each dilution unit includes a fractionator thermally coupled to the fractionator stage of the first module, the first dilution unit further includes a mixing chamber thermally coupled to the cooling stage, and the second and third dilution units each include a mixing chamber thermally coupled to the mixing chamber stage of the first module. The cryogenic cooling system according to any one of claims 1 to 28.

30. A quantum information processing apparatus comprising the cryogenic cooling system according to any one of claims 1 to 29.

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