CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of United States Patent Application No. 63/307,347 filed on February 7, 2022, the contents of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

[0002] The present disclosure generally relates to the field of cryogenics. More specifically, the present disclosure relates to dilution refrigerators.

BACKGROUND

[0003] Dilution refrigerators, which comprise a cryostat and a gas handling system, provide cooling down to millikelvin temperatures. The cryostat is designed to host at least one device and cool down the device to cryogenic temperatures.

[0004] There are significant costs associated with the operation of a dilution refrigerator, and many scaling challenges to provide additional cooling for larger devices. Therefore, improvements are needed.

SUMMARY

[0005] In accordance with a first broad aspect, there is provided a dilution refrigerator comprising a plurality of cryostats each comprising a plurality of temperature-controlled flanges inside a vacuum chamber, the temperature-controlled flanges cooled to progressively lower temperatures. Dilution units are disposed inside each of the cryostats and operable to cool at least a subset of the temperature-controlled flanges. A gas handling system is fluidly coupled to the plurality of cryostats and comprises a flow circuit and flow control devices interconnecting a gas mixture reservoir with the dilution units. A controller is coupled to the gas handling system and configured for operating the flow control devices in accordance with a sequence of operations that open and close paths in the flow circuit that allow a gas mixture to circulate between the gas mixture reservoir and the dilution units.

[0006] The dilution refrigerator as defined above and described herein may further include one or more of the following additional features, in whole or in part, and in any combination. [0007] In some embodiments, the gas handling system comprises a vacuum generator having a first pumping arrangement operable to apply a vacuum to each of the vacuum chambers for evacuation thereof.

[0008] In some embodiments, the first pumping arrangement comprises an evacuation valve per cryostat, coupled to a respective one of the vacuum chambers, and a first set of pumps configured for evacuating the cryostats via a respective evacuation valve.

[0009] In some embodiments, the flow control devices comprise a second pumping arrangement; a first set of valves coupled between the mixture reservoir and the second pumping arrangement; and a second set of valves coupled between the second pumping arrangement and the dilution units of the cryostats.

[00010] In some embodiments, the second pumping arrangement comprises a high vacuum pump, a backing pump, and a condensing pump.

[00011 ] In some embodiments, the condensing pump is isolated from a main circulation loop of the flow circuit by isolation valves.

[00012] In some embodiments, the first set of valves comprises at least one mixture supply valve and at least one mixture storing valve operable to regulate flow in and out of the mixture reservoir, and wherein the second set of valves comprises at least one in-flow valve per cryostat and at least one outflow valve per cryostat operable to regulate flow in and out of the dilution units.

[00013] In some embodiments, the flow circuit and flow control devices define, for each cryostat a mixture storing path; a mixture admission path; and a mixture circulating path.

[00014] In some embodiments, at least two of the paths share at least one of the flow control devices.

[00015] In some embodiments, all of the paths share at least one of the flow control devices.

[00016] In accordance with another broad aspect, there is provided a method for operating a dilution refrigerator having a plurality of cryostats each comprising a vacuum chamber and a dilution unit disposed therein. The method comprises receiving a request to transition the dilution refrigerator from a first operating state to a second operating state; determining, in accordance with the request, a sequence of operations for flow control devices in a flow circuit of a gas handling system of the dilution refrigerator, the flow circuit interconnecting a gas mixture reservoir with the dilution units of the cryostats; and operating the flow control devices in accordance with the sequence of operations that open and close paths in the flow circuit that allow a gas mixture to circulate between the gas mixture reservoir and the dilution units. [00017] The method as defined above and described herein may further include one or more of the following additional steps and/or features, in whole or in part, and in any combination.

[00018] In some embodiments, first operating state comprises having at least a first one of the cryostats online, and the second operating state comprises having at least a second one of the cryostats online.

[00019] In some embodiments, the request comprises taking at least one of the cryostats offline.

[00020] In some embodiments, the request comprises bringing at least one of the cryostats online.

[00021] In some embodiments, determining the sequence of operations comprises determining a current operating state of all the cryostats and a requested operating state of all of the cryostats.

[00022] In some embodiments, determining the sequence of operations comprises retrieving the sequence of operations from a storage medium based on the current operating state of the cryostats and the requested operating state of the cryostats.

[00023] In some embodiments, determining the sequence of operations comprises selecting from a list of a mixture storing path, a mixture admission path, and a mixture circulating path associated with each one of the cryostats.

[00024] In some embodiments, the method further comprises determining that the request requires bringing at least one of the cryostats from an offline status to an online status, and applying a vacuum to the vacuum chamber of the at least one of the cryostats.

[00025] In accordance with yet another broad aspect, there is provided a non-transitory computer- readable medium having stored thereon program code executable by a machine for operating a dilution refrigerator having a plurality of cryostats each comprising a vacuum chamber and a dilution unit disposed therein. The program code is executable for receiving a request to transition the dilution refrigerator from a first operating state to a second operating state; determining, in accordance with the request, a sequence of operations for flow control devices in a flow circuit of a gas handling system of the dilution refrigerator, the flow circuit interconnecting a gas mixture reservoir with the dilution units of the cryostats; and operating the flow control devices in accordance with the sequence of operations that open and close paths in the flow circuit that allow a gas mixture to circulate between the gas mixture reservoir and the dilution units.

[00026] In some embodiments, the program code is further executable for determining that the request requires bringing at least one of the cryostats from an offline status to an online status, and applying a vacuum to the vacuum chamber of the at least one of the cryostats.

[00027] Features of the systems, devices, and methods described herein may be used in various combinations, in accordance with the embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[00028] Reference is now made to the drawings, in which:

[00029] FIG. 1 A is a schematic diagram of an example cryostat;

[00030] FIGS. 1 B-1C are block diagrams showing examples of dilution refrigerators;

[00031] FIGS. 2A-2B are schematic diagrams showing examples of vacuum generators;

[00032] FIGS. 3A-3B are schematic diagrams showing examples of gas circulators;

[00033] FIG. 4 is a schematic diagram showing an example of a pumping arrangement of a gas circulator;

[00034] FIGS. 5A-5C are flowcharts of example methods of operating multi-cryostat dilution refrigerators;

[00035] FIGS. 6A-6G are schematic diagrams showing examples of gas mixture flow during various stages of operation of the dilution refrigerator;

[00036] FIG. 7 is a block diagram of an example controller; and

[00037] FIG. 8 is a flowchart of an example method of operation of the controller.

DETAILED DESCRIPTION

[00038] A dilution refrigerator is a cryogenic system that provides continuous cooling from ambient temperature all the way down to millikelvin temperatures without any moving part at the low temperature stages (below 3 K). Dilution refrigerators are used in various applications, including low temperature detectors, superconductivity research, low temperature solid state physics, and quantum computing.

[00039] A cryostat comprises a number of temperature stages used to thermally anchor radiation shields and wiring in order to reduce the amount of heat leaking to the colder stages. Aside from a room-temperature (300 K) stage, there can be, for example, stages at 50 K, 10 K, and 4 K, and additional stages at temperatures below 1 K. The cooling of these stages, or “flanges”, may be achieved by one or more different cooling systems including, for example, dilution units and pulse tube cryocoolers.

[00040] The pulse tube cryocooler serves many purposes. It is used 1) to cool and maintain the 50 K, 10 K, and 4 K stages at their cold temperature, 2) to achieve the initial cool down of the coldest stages of the dilution refrigerator to around 4 K prior to activating the dilution unit (also referred to herein as “pre-cooling”), and 3) to cool the circulating gas mixture from ambient temperature to 4 K during the steady state operation of the dilution unit.

[00041 ] The present disclosure is directed to a dilution refrigerator having multiple cryostats. Each cryostat may comprise a vacuum chamber and one or more dilution units inside the vacuum chamber. A gas handling system is shared by the multiple cryostats and comprises a flow circuit and flow control devices interconnecting a gas mixture reservoir with the dilution units. A controller is coupled to the gas handling system and configured to selectively bring the multiple cryostats online and offline through operation of the flow control devices. The controller applies a sequence of operations to the flow control devices to open and close various paths of the flow circuit, in order to allow a gas mixture to circulate between the gas reservoir and the dilution units. In some embodiments, cooling in each cryostat is induced by the mixing of Helium-3 (3He) and Helium-4 (4He) isotopes.

[00042] With reference to FIG. 1A, there is illustrated an example embodiment of a cryostat 101 A. The cryostat 101 A comprises temperature-controlled flanges, illustratively five flanges 11 1 A, 1 11 B, 111C.111 D and 111 E defining five temperature stages of decreasing temperature. When the cryostat 101 A is in steady-state operation, the flange 111 A, also called an outer vacuum chamber, is at 300 K while flanges 11 1 B, 11 1 C, 1 11 D, 111 E can, for example, be at 50 K, 4 K, 800 mK, and 8 mK. These temperatures are exemplary, and other temperature stages may be contemplated. Flanges 111 B, 111 C, 11 1 D, 1 11 E need not be perfectly sealed. The flanges may serve for supporting radiation shields as well as for thermal anchoring of other components of the cryostat 101 A. The flanges 111 A, 1 11 B, 1 11 C, 111 D and 111 E can be made of plates (where components can be affixed). In some embodiments, the plates also act as radiation shields. Additional thermal anchoring stages which may not take the form of a flange may also be present, for example at the 100 mK stage. Other embodiments may have a different number of flanges at different temperatures. A device 102 resides inside the coldest flange 1 11 E. The device 102 can, for example, be a quantum processor. Note that some of the temperature stages illustrated as flanges in FIG. 1A, for example the flange 111 E, could also alternatively take the shape of an open structure. In some cases, the flanges 111A-111 E may take the form of nested enclosures, where the innermost flange is at a lowest temperature and the outermost flange is at a highest temperature, with intermediate flanges being maintained at progressively lower temperatures from the outermost flange towards the innermost flange.

[00043] In some embodiments, the cryostat 101 A is a dry cryostat in which cooling of upper stages (e.g. 50 K, 10 K, and 4 K stages) is provided by one or more pulse tube cryocooler 109. Other embodiments for cooling of upper stages may also apply, such as those described in U.S. Patent Application No. 18/051 ,992. In some embodiments, the pulse tube cryocooler 109 comprises a first pulse tube 105A providing cooling to the flange 111 B and a second pulse tube 105B providing cooling to the flange 111 C. The pulse tubes 105A and 105B are connected to regenerators 106A and 106B, respectively, through heat exchangers 107A and 107B, respectively. Note that the heat exchangers 107A and 107B are illustrated as if in direct contact with the flanges 111 B and 111 C, but they could also be in contact with the enclosures through thermally conductive elements such as heat straps. Other embodiments, for example having more or less than two pulse tubes 105A, 105B, may also apply. The pulse tube cryocooler 109 can be used to cool a supply line 104A if the latter is also coupled to the pulse tube cryocooler 109 through a heat exchanger, such as heat exchangers 107A and/or 107B.

[00044] Cooling of the lower temperature stages, such as those defined by flanges 11 1 D, 11 1 E, is provided by a dilution unit 103A, which is schematically represented by a simple rectangle in Fig. 1 A. The dilution unit 103A may comprise a mixing chamber, liquid counter flow (recuperative) heat exchangers, a still and a condenser, which can be a vapor counter flow (recuperative) heat exchanger, or a liquid 4He heat exchanger coupled to a liquid 4He bath supplied via a separate circulation circuit. The dilution unit 103A is supplied with the gas mixture through a supply line 104A, which is thermally anchored at different temperature stages. A pumping line 108A is used to extract gas from the dilution unit 103A.

[00045] The dilution unit 103A provides cooling power by the energy required to mix two isotopes of helium, liquid 3He and 4He. Indeed, at low temperature, the mixture of both isotopes separates into two phases: a 3He rich phase and a 4He rich phase. When liquid 3He is circulated into a mixing chamber containing 4He, it mixes with the 4He rich phase which requires energy and thus removes heat from the mixing chamber’s environment.

[00046] Continuous cooling power is produced by circulating 3He, for example using various pumps located in a gas handling system outside the cryostat, such that 3He evaporated from the 3He/4He mixture is returned to the dilution unit 103A to be condensed before entering the mixing chamber again. The dilution unit 103A comprises a condenser to turn the gaseous 3He supply to liquid 3He. The condenser can be a recuperative heat exchanger cooled by the enthalpy of the 3He evaporated from the still. Alternatively, the condenser can be a heat exchanger cooled by a liquid 4He bath at a temperature around 1 K.

[00047] In operation, the dilution unit 103A and the 3He supply are first cooled from room temperature, and then maintained at temperatures close to liquid helium temperature (4.2 K) to allow for the liquefaction of the incoming 3He supply in the condenser.

[00048] In cases where the device 102 employs electronic control from outside the cryostat 101 A, as is the case for quantum processors, the cryostat 101 A can be wired with control cables thermally anchored at each temperature stage through various heat exchangers similarly to the 3He supply line 104A.

[00049] Turning to FIG. 1 B, there is illustrated an example of a dilution refrigerator 100 having multiple cryostats, such as cryostats 101 A, 101 B. For simplicity, only outer vacuum flanges/cham- bers 111 A, 111 B and dilution units 103A, 103B are illustrated. The two cryostats 101 A, 101 B may be identical, as illustrated, or different, depending on practical applications. For example, one cryostat may comprise five temperature stages and the other cryostat may comprise four temperature stages. In another example, both cryostats may have the same number of temperatures stages, but the temperature values at each corresponding stage may differ. Other differences between the cryostats in the multi-cryostat dilution refrigerator 100 may also be present.

[00050] A gas handling system 112 is coupled to the cryostats 101A, 101 B and in fluid communication therewith. In some embodiments, the gas handling system 112 allows the evacuation of gas (such as air or other gases) from the cryostats 101 A, 101 B in order to form the vacuum needed inside. A vacuum generator 1 15 draws gas out of the cryostats 101A, 101 B through vacuum lines

I I OA, 110B along paths C and D, respectively. The gas handling system 1 12 also supplies the gas mixture to the dilution units 103A, 103B on startup (cool down) and retrieves it on shutdown (warmup), using, for example, a plurality of pumps and valves. A gas circulator 1 16 circulates gas through a first cryostat 101 A along path A and through a second cryostat 101 B along path B, through respective dilutions units 103A, 103B. The gas mixture enters the dilution units 103A, 103B through supply lines 104A, 104B, respectively, and exits the dilution units 103A, 103B through pumping lines 108A, 108B, respectively. The gas handling system 112 is operated via a controller 1 18, as will be explained in more detail below.

[00051 ] As shown in FIG. 1 C, the multi-cryostat dilution refrigerator 100 may be adapted for N cryostats 101A, 101 B, ... , 101 N, where N is a positive integer. Respective vacuum lines 110A,

I I OB, ... , 11 ON are provided between the vacuum generator 1 15 and each cryostat 101 A, 101 B, ... , 101 N. Supply lines 104A, 104B, ... , 104N and pumping lines 108A, 108B, ... , 108N are provided between the gas circulator 116 and each cryostat 101A, 101 B, ... , 101 N to allow gas to flow therethrough. Although the gas mixture is shown to flow counterclockwise through the gas circulator 1 16 and the cryostats 101 A, 101 B, ... , 101 N, this is for illustrative purposes only. It will be understood that the gas mixture may also flow in a clockwise direction, in accordance with practical implementations.

[00052] With reference to FIGS. 2A-2B, an example embodiment is shown for the vacuum generator 1 15 for a dilution refrigerator 100 having two cryostats 101 A, 101 B (FIG. 2A) and a dilution refrigerator 100 having N cryostats 101A, 101 B, ... , 101 N (FIG. 2B). In FIG. 2A, a pumping arrangement is composed of one or more pumps 202 coupled to a first evacuation valve 204A and a second evacuation valve 204B. The evacuation valves 204A, 204B, which may be the same or different, may be any type of on-off or modulated valve, operable to allow and prevent the flow of fluid therethrough. In some embodiments, the evacuation valves 204A, 204B are actuated valves, such as linear valves, rotary valves, and self-actuated valves, and may comprise a manual override. In some embodiments, the evacuation valves 204A, 204B are fast acting on/off valves with built-in actuators, such as but not limited to solenoid valves, coaxial valves, and angle seat valves. When the first cryostat 101 A needs to be evacuated, the evacuation valve 204A is activated (i.e. opened) and subsequently deactivated (i.e. closed). When the second cryostat 101 B needs to be evacuated, the evacuation valve 204B is activated (i.e. opened) and subsequently deactivated (i.e. closed). As shown in FIG. 2B, additional evacuation valves 204N may be added for each additional cryostat 101 N in the dilution refrigerator 100. More than one evacuation valve 204A, 204B, ... 204N may be activated concurrently to evacuate a corresponding cryostat 101A, 101 B, ... 101 N.

[00053] The pumping arrangement may comprise one or more pump 202. The pump(s) 202 may correspond to vacuum pump(s), such as a rotary vane vacuum pump(s), diaphragm vacuum pump(s), liquid ring vacuum pump(s), scroll vacuum pump(s), and turbomolecular vacuum pump(s). In some embodiments, the pumps 202 comprise a high vacuum pump, such as a turbo- molecular pump, and an appropriately sized backing pump, for example a scroll pump. Other embodiments may also apply based on a desired vacuum level and other specifications of the dilution refrigerator 100. In some embodiments, the pumping arrangement is only in use during an initial portion of the cool down of the cryostat.

[00054] With reference to FIGS. 3A-3B, an example embodiment is shown for a gas circulator 1 16 of a dilution refrigerator 100 having two cryostats 101 A, 101 B (FIG. 3A) and a dilution refrigerator 100 having N cryostats 101A, 101 B, ... , 101 N (FIG. 3B). The gas circulator 116 has a plurality of flow control devices, such as valves and pumps, interconnecting the dilution units 103A, 103B, ... 103N to a gas mixture reservoir 302 to define a flow circuit. As used herein, a set of pumps corresponds to one or more pumps, a set of valves corresponds to one or more valves, a pumping arrangement may comprise pumps and valves or just pump(s). In FIG. 3A, the flow control devices comprise a pumping arrangement 304 coupled between a first inflow valve 308A and a first outflow valve 306A to circulate gas through the first cryostat 101 A along path A. The pumping arrangement 304 is also coupled between a second inflow valve 308B and a second outflow valve 306B to circulate gas through the second cryostat 101 B along path B. Although shown as part of the gas circulator 116, inflow and/or outflow valves 308A, 308B, 306A, 306B may also reside inside the outer vacuum chambers/flanges 111 A, 111 B of the cryostats 101 A, 101 B.

[00055] The inflow valves 308A, 308B and outflow valves 306A, 306B may be the same types of valves as the evacuation valves 204A, 204B or different therefrom. The inflow valves 308A, 308B may be the same types of valves as the outflow valves 306A, 306B or different therefrom. The first inflow and outflow valves 308A, 306A may be the same types of valves as the second inflow and outflow valves 308B, 306B or different therefrom. Any type of on-off or modulated valve, operable to allow and prevent the flow of fluid therethrough, may be used. In some embodiments, the inflow valves 308A, 308B and/or outflow valves 306A, 306B are actuated valves, such as linear valves, rotary valves, or self-actuated valves, and may comprise a manual override. In some embodiments, the inflow valves 308A, 308B and/or outflow valve 306A, 306B are fast acting on/off valves with built-in actuators, such as but not limited to solenoid valves, coaxial valves, or angle seat valves.

[00056] During steady-state operation of the first cryostat 101 A, the valves 308A and 306A are open (i.e., allow flow therethrough) and the pumping arrangement 304 circulates a gas mixture continuously through the dilution unit 103A. During steady-state operation of the second cryostat 101 B, the valves 308B and 306B are open and the pumping arrangement 304 circulates the gas mixture continuously through the dilution unit 103B. The pumping arrangement 304 compresses the circulating gas mixture coming from respective outlets of the dilution units 103A, 103B, up to a desired pressure, for example 0.5 atmosphere. The pumping arrangement 304 circulates the gas mixture at a desired flow rate while maintaining a sufficiently low cryostat outlet pressure to allow the evaporating helium bath to remain at a desired temperature, for example 700 to 900 mK. The cryostat inlet pressure is held at a level sufficient to allow for the circulating gas mixture to satisfactorily re-condense as it is returned to the dilution units 103A, 103B. In some embodiments, one or more filtration device, such as a cold trap, may be used to remove contaminants that could cause blockages in the dilution units 103A, 103B. This filtration device may be placed downstream from the inflow valves 308A, 308B, inside the cryostat 101 A, 101 B or inside the gas handling system 112. Additional filtration devices may be placed within the gas handling system 112 and/or the gas circulator 116.

[00057] The gas circulator 116 comprises a gas mixture reservoir 302, a mixture storing valve 310, and a mixture supply valve 312, which are used during the cool down and warmup phases of the dilution refrigerator 100. The gas mixture reservoir 302, mixture storing valve 310, and mixture supply valve 312 are shown to form part of the gas circulator 1 16 but may be provided in whole or in part externally thereto, with the mixture storing valve 310 coupled to a pump node 316 and the mixture supply valve 312 coupled to a pump node 314. During warmup, the gas mixture enters the reservoir 302 by flowing through the mixture storing valve 310. During cool down, the gas mixture exits the reservoir 302 by flowing through the mixture supply valve 312. The pumping arrangement 304 is operated by the controller 118 to selectively direct the gas mixture into and out of the reservoir 302.

[00058] The pumping arrangement 304 may comprise one or more pumps, such as a rotary vane vacuum pump, diaphragm vacuum pump, liquid ring vacuum pump, scroll vacuum pump, and tur- bomolecular vacuum pump. In some embodiments, the pumping arrangement 304 comprises a high vacuum pump, such as a turbomolecular pump, and an appropriately sized backing pump, for example a scroll pump. To facilitate or accelerate the process of condensing the mixture during a cool down, the pumping arrangement 304 may contain an additional pump that can be operated in series with the other pumps to temporarily increase the pressure of the gas before it enters the dilution units, 103A, 103B.

[00059] As shown in FIG. 3B, additional inflow valves 308N and outflow valves 306N may be added to the gas circulator 116 for each additional cryostat 101 N in the dilution refrigerator 100.

[00060] FIG. 4 illustrates a more detailed example of the pumping arrangement 304, in accordance with a specific and non-limiting embodiment. The pumping arrangement 304 comprises three pumps: a high vacuum pump 402 (for example a turbomolecular pump), a backing pump 404 (for example a scroll pump) and a third pump 406 (for example a scroll pump) to increase the cryostat inlet pressure. In some embodiments, the pump 406 is a condensing pump, only used when initially condensing the gas mixture, and thus may be isolated from a main circulation loop of the flow circuit using a series of isolation valves 408, 410, 412. The mixture reservoir 302 connects to the pumping arrangement 304 through the valves 310 and 312. In this case, valve 310 connects to the pump node 316 while valve 312 connects to the pump node 314. Alternatively, the mixture reservoir 302 may be connected to the pumping arrangement 304 at alternate locations, as long as one or more connections are provided in a configuration that allows pumping the mixture both towards and from the gas mixture reservoir 302.

[00061 ] It will be understood that the gas handling system 112, vacuum generator 115, and/or gas circulator 116 may contain additional elements, such as valves, pressure indicators, flow meters, particle filters such as cold traps, and/or access ports to allow the monitoring and maintenance of the dilution refrigerator 100 and/or to facilitate its operation. In addition, one or more components may be shared between the vacuum generator 115 and the gas circulator 116, such as but not limited to pumps and valves. As shown in the embodiments of FIGS. 2B and 3B, each additional cryostat 101 N may be added by providing another evacuation valve 204N, inflow valve 308N, and outflow valve 306N. Mixture storing valve 310 and mixture supply valve 312 need not be duplicated and may be shared by all cryostats 101 N, although additional mixture storing and/or mixture supply valves may be provided. For example, if mixture reservoir 302 is not sufficiently large to accommodate enough gas mixture for the total number of cryostats present, additional mixture reservoir(s) may be added to the flow circuit with the appropriate number of additional mixture and/or storage valves. Although the cryostats 101 A, 101 B, 101 N are shown to share a single vacuum generator 1 15 and a single gas circulator 116, other embodiments may be used wherein multiple vacuum generators and/or multiple gas circulators are provided.

[00062] The controller 118 is coupled to the gas handling system 112 and configured for transitioning the dilution refrigerator 100 between operating states. Transition is effected through operation of the flow control devices, in accordance with a sequence of operations to open and close paths in the flow circuit that allow the gas mixture to circulate between the gas mixture reservoir 302 and the dilution units 103A, 103B, ... , 103N. The sequence is selected in accordance with a request to transition the dilution refrigerator from a first operating state to a second operating state.

[00063] A first example embodiment for transitioning the dilution refrigerator 100 between operating states is shown in FIG. 5A, where the controller 118 takes the dilution refrigerator 100 from a first operating state 500, where cryostat 101 A is online and cryostat 101 B is offline, to a second operating state 502, where cryostat 101 B is online and cryostat 101 A is offline.

[00064] At step 504, the gas mixture is directed from the first cryostat 101 A to the mixture reservoir 302. This implies stopping mixture circulation in the first cryostat 101 A and more specifically in the dilution unit 103A. The mixture is redirected into the mixture reservoir 302 via the mixture storing valve 310 once it exits the dilution unit 103A and flows through the outflow valve 306A. In some embodiments, the second cryostat 101 B is evacuated and pre-cooled at step 506 since it transitions from an offline status at 500 to an online status at 502. As used herein, “pre-cooling” refers to the initial cool down of the coldest stages of the dilution refrigerator to around 4 K prior to activating the dilution unit. Step 506 may be performed concurrently with step 504. In some embodiments, step 506 may be performed before or after step 504, as performing step 506 does not interrupt the operation of cryostat 101 A. In some embodiments, step 506 is omitted as it is controlled by a different process. At step 508, cryostat 101 B is ready to receive the gas mixture, and the controller 1 18 directs the gas mixture from the mixture reservoir 302 to the cryostat 101 B. In some embodiments, the controller 118 is also configured to provide heat, for example using one or more heat switches, to evaporate the gas mixture from the dilution unit 103A, as part of step 504. Alternatively, or in addition thereto, heat may be applied concurrently with step 508 in order to reduce the time needed to reach a target temperature.

[00065] Using the example embodiment for the gas circulator 116 of FIG. 3A, the operation of the flow control devices and flow of the gas mixture for transitioning from the first operating state 500 to the second operating state 502 is illustrated in FIGS. 6A-6D. FIG. 6A represents the starting position of the valves when the dilution refrigerator 100 is in the first operating state 500, where the pumping arrangement 304 circulates the gas mixture through the dilution unit 103A of the first cryostat 101 A via inflow and outflow valves 308A, 306A, respectively. The gas mixture is said to be flowing through a mixture circulating path 602 of the first cryostat 101 A, as defined by the position of the flow control devices in the flow circuit.

[00066] FIG. 6B represents the position of the flow control devices during step 504, where the gas mixture is directed from the dilution unit 103A of the first cryostat 101 A to the mixture reservoir 302. The mixture circulating path 602 of the first cryostat 101A has been closed and the gas mixture is now flowing through a mixture storing path 604 of the first cryostat 101A. To transition from the mixture circulating path 602 to the mixture storing path 604, the outflow valve 308A is closed and the mixture storing valve 310 is opened. In some embodiments, high vacuum pumps in the pumping arrangement 304 have been turned off as the pumping arrangement 304 participates in pumping the gas mixture through the outflow valve 306A and the mixture storing valve 310.

[00067] FIG. 6C represents the positioning of the valves during step 508, where the gas mixture is directed from the mixture reservoir 302 to the second cryostat 101 B through a mixture admission path 606 of the second cryostat 101 B. When transitioning from the mixture storing path 604 of the first cryostat 101A to the mixture admission path 606 of the second cryostat 101 B, the mixture storing valve 310 is closed and the mixture supply valve 312 and outflow valve 308B are opened. In some embodiments, pressure buildup is prevented by repeatedly and alternately opening and closing valves 312 and 306B as the dilution unit 103B is filled with the gas mixture from the mixture reservoir 302.

[00068] FIG. 6D represents the positioning of the valves when the dilution refrigerator 100 is in the second operating state 502, where the pumping arrangement 304 circulates the gas mixture through the second cryostat 101 B via inflow and outflow valves 308B, 306B, respectively, through a gas circulating path 608 of the second cryostat 101 B. When transitioning from the mixture admission path 606 of the second cryostat 101 B to the mixture circulating path 608 of the second cryostat 101 B, the mixture supply valve 312 is closed and the mixture outflow valve 306B is opened. [00069] A second example embodiment for transitioning the dilution refrigerator 100 between operating states is shown in FIG. 5B, where the controller 118 takes the dilution refrigerator 100 from the first operating state 500 having the first cryostat 101 A online and the second cryostat 101 B offline, to a third operating state 510 where the first and second cryostats 101 A, 101 B are online concurrently. Since the second cryostat 101 B is transitioning from an offline state at 500 to an online state at 510, the second cryostat 101 B may be evacuated and pre-cooled at step 506, in accordance with some embodiments. Also in some embodiments, and as illustrated in Fig. 5B, the flow control devices may be set to direct the gas mixture from the dilution unit 103A of the first cryostat 101A to the mixture reservoir 302 through the mixture storing path 604 prior to admitting the gas mixture into the dilution unit 103B of the second cryostat 101 B. It will be noted that in some embodiments, step 504 may be omitted and the gas mixture circulating through an online cryostat is not directed to the mixture reservoir 302 when the online cryostat is not being taken offline during the transition in operating states. Instead, the mixture circulating path of the online cryostat is closed prior to allowing the gas mixture to flow into the dilution unit of the previously offline cryostat.

[00070] At step 512, the gas mixture is directed from the mixture reservoir 302 to the second cryostat 101 B through the gas admission path 606 of the second cryostat 101 B, as per Fig. 6C. If the gas mixture is previously returned to the mixture reservoir 302 from the dilution unit 103A of the first cryostat 101 A, a mixture admission path 610 of the first cryostat 101 A is opened concurrently with the mixture admission path 606 of the second cryostat 101 B, as shown in Fig. 6E. In some embodiments, pressure buildup is prevented by repeatedly and alternately opening and closing valves 312, 306A, and 306B as the dilution units 103A, 103B are filled with the gas mixture from the mixture reservoir 302. Once the mixture reservoir 302 is emptied and/or the cryostats 101 A, 103A are filled, the mixture admission path 606, and in some embodiments the mixture admission path 610, is closed and mixture circulating paths 602, 608 of the first and second cryostats 101 A, 101 B, respectively, are opened, as shown in Fig. 6F. At this stage, the dilution refrigerator 100 operates in the third operating state 510.

[00071 ] A third example embodiment for transitioning the dilution refrigerator 100 between operating states is shown in FIG. 5C. The controller 118 takes the dilution refrigerator 100 from the third operating state 510, where the first and second cryostats are online, to the second operating state 502, where the second cryostat is online and the first cryostat is offline. When in the third operating state 510, the flow control devices are set to allow the gas mixture to flow through the first and second mixture circulating paths 602, 608, as per Fig. 6F. At step 514, the first and second mixture circulating paths 602, 608 are closed and the gas mixture is directed from the first and second dilution units 103A, 103B to the mixture reservoir 302 through the mixture storing path 604 of the first cryostat 101A and a mixture storing path 612 of the second cryostat 101 B. These two paths 604, 612 may be opened concurrently, as shown in Fig. 6G where outflow valves 306A, 306B and mixture storing valve 310 are opened, or sequentially. In some embodiments, high vacuum pumps in the pumping arrangement 304 are shut down during this process. When the dilution units 103A, 103B are empty, the mixture storing paths 604, 612 are closed and the gas mixture is returned to the second dilution unit 103B through the mixture admission path 606 of the second cryostat 101 B at step 508, as per Fig. 6C, where the mixture supply valve 312 and the inflow valve 308B are opened. In some embodiments, pressure buildup is prevented by repeatedly and alternately opening and closing valves 312 and 306B as the dilution unit 103B is filled with the gas mixture from the mixture reservoir 302. Once the dilution unit 103B is full, the mixture admission path 606 is closed and the mixture circulating path 608 of the second cryostat 101 B is opened, as per Fig. 6D. The second cryostat 101 B is then online while the first cryostat 101 A is offline, as per the first operating state 502.

[00072] In some embodiments, only the cryostat being taken offline has its dilution unit emptied by directing the gas mixture to the mixture reservoir. In this case, step 514 would only comprise directing the gas mixture from the first cryostat 101 A to the mixture reservoir 302 through the mixture storing path 604, as shown in Fig. 6B. Furthermore, step 508 would only comprise opening the mixture circulating path 608, as shown in Fig. 6D, so as to circulate the gas mixture in the second cryostat 101 B.

[00073] It will be understood that the examples shown in FIGS. 6A-6G are using one possible embodiment for the gas handling system 112 and/or gas circulator 116. Other embodiments for the gas handling system 112 and/or gas circulator 116 may also be used, which would result in a different configuration for the operation of controller 1 18. Furthermore, the examples shown are for two cryostats but may be readily applied to N cryostats, for example using the embodiment for the gas circulator 116 shown in FIG. 3B. Other embodiments for a gas circulator 116 for N cryostats may also be used.

[00074] In some embodiments, the transitioning procedure for bringing one cryostat either online or offline only causes a short interruption (typically a few hours) in the other cryostats while the gas mixture is removed and then re-condensed. In some embodiments, the cryostats 101A, 101 B, ... , 101 N are operated such that at least one of the cryostats 101A, 101 B, ... , 101 N is always online, thus reducing any downtime of the dilution refrigerator 100. An offline cryostat may be undergoing maintenance or a new device 102 may be placed therein while the online cryostat(s) are in operation.

[00075] In some embodiments, the controller 118 is configured to operate the dilution refrigerator 100 in a manner to minimize the downtime of the dilution refrigerator 100, by transitioning the dilution refrigerator 100 from one operating state where at least one first cryostat is online to another operating state where at least one second cryostat is online.

[00076] FIG. 7 is an example embodiment of the controller 118. The controller 1 18 can be provided as part of a computing unit of the dilution refrigerator 100, either separately therefrom or integrated therewith. The controller 118 can also be provided as part of a computing unit of a quantum computer comprising the dilution refrigerator 100. As depicted, the controller 118 can include one or more processor 702, memory 704, one or more I/O interface 706, and one or more network interface 708. Processor 702 may be an Intel or AMD x86 or x64, PowerPC, ARM processor, or the like.

[00077] Memory 704 may include a suitable combination of non-transitory computer-readable data storage media. The memory 704 may be located internally or externally to the controller 1 18 and include long term and/or short term storage of data. Memory 704 may be any data storage medium that is physical, transferable and reproducible. Examples for memory 704 include but are not limited to random-access memory (RAM), read-only memory (ROM), integrated memory, compact discs (CDs), CD read-only memory (CDROM), volatile memory, flash drives, digital versatile discs (DVDs), and the like.

[00078] One or more I/O interface 706 enables the controller 1 18 to interconnect with one or more input devices, such as a keyboard, mouse, camera, touch screen and a microphone, or with one or more output devices such as a display screen and a speaker. This interface can be used, for example, for receiving command controls from an operator of the dilution refrigerator 100.

[00079] One or more network interface 708 enables the controller 1 18 to communicate with other components, for example, through an API to exchange data with other components, to access and connect to network resources, to serve applications, and perform other computing applications by connecting to a network (or multiple networks) capable of carrying data including the Internet, Ethernet, plain old telephone service (POTS) line, public switch telephone network (PSTN), integrated services digital network (ISDN), digital subscriber line (DSL), coaxial cable, fiber optics, satellite, mobile, wireless (e.g., Wi-Fi, WiMAX), SS7 signaling network, fixed line, local area network, wide area network, and others.

[00080] In some embodiments, the controller 118 is a Programmable Logic Controller (PLC) which has the capacity for sending and receiving digital signals and analog signals, as well as communicating with devices using Ethernet and serial communication. In some embodiments, the controller 1 18 contains relay modules capable of switching low power DC components, such as heaters, on and off. The processor 702 may be a PLC Central Processing Unit (CPU). The inputs to the PLC may be, for example, pressure readings, temperature readings, status of devices (pumps, pulse tubes, valves, etc.), critical parameters (pump speed, pulse tube oil temperature, etc.), pneumatic switch status, and valve position. The PLC may be used to control, for example, valves, pumps, heaters, and pulse tube compressor.

[00081 ] In one specific and non-limiting example, the controller 1 18 is a PLC, and a user interacts with the PLC via a human-machine interface (HMI). Valves in the gas handling system 1 12 are pneumatically controlled and air pressure is controlled using a pneumatic valve terminal which communicates with the PLC. Pressures are monitored using vacuum gauges connected to a pressure controller. The pressure controller outputs a voltage signal that is sent to the PLC and converted to a pressure. Pressure measurements are used, for example, to determine whether the outer vacuum chamber has been sufficiently evacuated, or whether the gas mixture has been fully extracted from a dilution unit. Temperatures are monitored using a temperature controller, which communicates with the PLC. Temperature measurements are used, for example, to determine whether a dilution unit is ready for mixture condensation, whether a dilution unit has been filled with a sufficient amount of mixture, or whether the gas mixture has been fully extracted from a dilution unit. A pump (e.g. a scroll pump) used to evacuate a cryostat may be controlled using an output from the PLC that is connected to a relay, to turn the pump on and off. Another pump (e.g. a turbomolecular pump) used to evacuate the cryostat is controlled to turn the pump on and off and monitor its status (e.g. a rotational speed of the pump). Pulse tubes for the pre-cooling and/or cooling are turned on and off and their status is monitored (e.g. temperature of a cooling oil). Backing pumps and high-vacuum pumps used to circulate the gas mixture in the gas handling system and the dilution units are turned on and off and a pump status is monitored (e.g. rotational speed of the pump). One or more heaters used to supply heat for evaporating the gas mixture from a dilution unit may be controlled by the PLC.

[00082] With reference to Fig. 8, there is illustrated a flowchart of an example method 800 for operating a multi-cryostat dilution refrigerator, such as the dilution refrigerator 100 described herein. Each cryostat has a vacuum chamber and a dilution unit disposed therein. At step 802, a request to transition the dilution refrigerator from a first operating state to a second operating state is received. The request may include a new operating state for one or more of the cryostats of the dilution refrigerator, or may include an operating state for all of the cryostats of the dilution refrigerator, including cryostats that do not change operating state. For example, the request may be as simple as “turn on C2”, or as complete as “C1 on, C2 on, C3 on, C4 on, C5 on” for a five cryostat dilution refrigerator where all cryostats except for C2 are already on. [00083] In some embodiments, the first (or current) operating state comprises having a first cryostat, for example cryostat 101 A, online and the second (or newly requested) operating state comprises having a second cryostat, for example cryostat 101 B, online. In the first operating state, the second cryostat may be online or offline. In the second operating state, the first cryostat may be offline or online.

[00084] At step 804, a sequence of operations for flow control devices of the dilution refrigerator is determined in accordance with the request. The incoming request is processed to determine which sequence of operations are to be applied to the flow control devices to allow the dilution refrigerator to transition from its current operating state to the newly requested operating state. For a request that does not include all cryostats of the dilution refrigerator (e.g. “turn on C2”), the controller 118 may be configured to maintain all other cryostats not included in the request to a current operating state. For example, a request to bring cryostat 101 B online would result in no change to the operating state of cryostat 101 A.

[00085] In some embodiments, the flow circuit of the dilution refrigerator and the flow control devices disposed therein define a set of predetermined paths. An example is illustrated in Table 1 for two cryostats.

TABLE 1

[00086] As shown in the example of Table 1 , each cryostat has three predetermined paths: one for storing the gas mixture, one for admitting the gas mixture, and one for circulating the gas mixture. The mixture storing path takes the gas mixture from the dilution unit of a cryostat to the mixture reservoir. The mixture admission path takes the gas mixture from the mixture reservoir to the dilution unit of a cryostat being taken online. The mixture circulating path allows the mixture to flow in and out of the dilution unit of a cryostat for steady state operation.

[00087] The various paths of the flow circuit may share one or more flow control devices. In some embodiments, all of the paths share at least one flow control device, for example the pumping arrangement 304 of the example embodiment for the gas circulator 1 16 shown in Fig. 3A.

[00088] In some embodiments, the sequence of operations are retrieved from a storage medium, such as memory 704, in orderto be applied accordingly. An example is illustrated in Table 2, where the current operating state has cryostat 101 A online and cryostat 101 B offline, and the newly requested operating state has cryostat 101 A online and cryostat 101 B online. In this case, the request may be to bring cryostat 101 B online. The controller 118 may be configured to determine the current operating state of all cryostats, which in this example is cryostat 101A online, cryostat 101 B offline. The controller 1 18 can then determine the newly requested operating state to be cryostat 101 A online, cryostat 101 B online and retrieve the sequence of operations described in the example of Table 2.

TABLE 2

[00089] At step 806, the flow control devices are operated in accordance with the sequence of operations determined at step 804. As per the example of Table 2, each operation listed in the left hand column is associated with the opening or closing of various paths by operating the flow control devices in the manner defined in the right hand column. It will be understood that for ease of understanding, only the operation of the valves are recited in the example and the operation of the various pumps are omitted.

[00090] In some embodiments, the method 800 comprises determining that the request requires bringing at least one cryostat from an offline status to an online status, and applying a vacuum to a vacuum chamber of the cryostat being brought online at step 808. Alternatively, the vacuuming procedure is managed separately from the method 800, by the controller 118 or externally therefrom.

[00091] The described embodiments and examples are illustrative and non-limiting. Practical implementation of the features may incorporate a combination of some or all of the aspects, and features described herein should not be taken as indications of future or existing product plans. Applicant partakes in both foundational and applied research, and in some cases, the features described are developed on an exploratory basis.

[00092] The term "connected" or "coupled to" may include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements).

[00093] Although the embodiments have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the scope. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification.

[00094] As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

[00095] As can be understood, the examples described above and illustrated are intended to be exemplary only.