Persistent current switch

An improved persistent current switch design and method of operation are disclosed. By way of example, a persistent current switch circuit comprises a heating element and a switch element located proximate to the heating element, the switch element being substantially formed from a material (by way of example only, titanium) which exhibits a superconducting temperature value below a superconducting temperature value exhibited by a material (by way of example only, aluminum) used to provide a connection to the switch element. The switch element is responsive to the heating element such that the heating element is used to control whether or not the switch element is in a superconducting state. The switch element may also have a folded geometry. Such persistent current switches exhibit low power and low inductance.

FIELD OF THE INVENTION

The present invention relates to thin film superconducting persistent current switches and, more particularly, to low power, low inductance thin film superconducting persistent current switches.

BACKGROUND OF THE INVENTION

It is known that quantum computation devices (also referred to as “qubits”) need high precision magnetic bias signals to accurately set the operating point of the device. Control currents on the order of 10 milliAmps (mA) are required, with an accuracy of one part in a million, and the noise temperature of the control electronics must be low to avoid decohering the qubit. Because the qubit is operated at temperatures near 20 milliKelvin (mK), the usual way of directly coupling control signals to the quantum bit will heat the refrigerator and inject undesirable noise into the device. The simplest way to solve these problems is to use a thin film superconducting persistent current switch. Examples of such persistent current switches are disclosed in A. C. Leuthold, R. T. Wakai, G. K. G. Hohenwarter, and J. E. Nordman, “Characterization of a Simple Thin-Film Superconducting Switch,” IEEE Trans. Appl. Supercond., vol. 4, no. 3, pp. 181-183, 1994; and P. Balchandani, R. H. Torii, and R. Shile, “Thin-Film Persistent Current Switch,” IEEE Trans. Appl. Supercond., vol. 15, no. 3, pp. 3821-3826, 2005, the disclosures of which are incorporated by reference herein.

The standard switch design described in the existing literature involves joule heating a thin film superconducting niobium (Nb) line using a thin film heater which crosses the Nb line. To reduce the amount of power needed to operate the switch, the overlap area typically is 100 micrometers (μm)×100 μm. The amount of power needed to operate the switch is about 50 microWatts (μW).

SUMMARY OF THE INVENTION

Principles of the invention provide an improved persistent current switch design and method of operation.

In a first embodiment, a persistent current switch circuit comprises a heating element and a switch element located proximate to the heating element, the switch element being substantially formed from a material which exhibits a superconducting temperature value below a superconducting temperature value exhibited by a material used to provide a connection to the switch element. The switch element is responsive to the heating element such that the heating element is used to control whether or not the switch element is in a superconducting state. The switch element is preferably formed to minimize an inductance value associated therewith. The inductance value of the switch element may be minimized by forming the switch element in accordance with a folded geometry. The switch element is preferably substantially formed from titanium and the heating element is preferably substantially formed from a combination of gold and palladium.

In a second embodiment, an apparatus comprises a heating element and a switch element located proximate to the heating element, the switch element being substantially formed from a material which exhibits a superconducting temperature value below a superconducting temperature value exhibited by a material used to form a connection element that provides a connection to the switch element. The switch element is responsive to the heating element such that the heating element is used to control whether or not the switch element is in a superconducting state. Further, the apparatus preferably comprises a coupling element operatively coupled across the switch element, and a device operatively coupled to the coupling element, wherein the device comprises a quantum computation device. Still further, the heating element and the switch element may be formed on a first substrate and the coupling element and the device formed on a second substrate.

In a third embodiment, an integrated circuit comprises a substrate layer, a heating element layer formed on the substrate layer, and a switch element layer formed proximate to the heating element layer, the switch element layer being substantially formed from a material which exhibits a superconducting temperature value below a superconducting temperature value exhibited by a material used to provide a connection to the switch element. The switch element layer is responsive to the heating element layer such that the heating element layer is used to control whether or not the switch element layer is in a superconducting state. Preferably, the switch element is substantially formed from titanium, and the heating element layer is substantially formed from a combination of gold and palladium. An insulator layer may be formed between the heating element layer and the switch element layer. The insulator layer is preferably substantially formed from silicon nitride. Two balanced heater current wiring paths may be formed on the substrate. The two balanced heater current wiring paths are preferably substantially formed from niobium. The heating element layer and the switch element layer are preferably formed so as to allow external connections via aluminum wire bonds.

In a fourth embodiment, a method comprises the following steps. A first current is applied to a heating element located proximate to a substantially titanium-formed switch element such that the switch element is warmed above a superconducting transition temperature associated therewith. A control signal is introduced such that the control signal flows substantially entirely through a coupling element operatively coupled to the switch element. The first current applied to the heating element is removed such that the switch element closes when the switch element cools below the superconducting transition temperature thereby allowing a second current to flow persistently through the switch element and the coupling device, and thus allowing an operating characteristic associated with a device operatively coupled to the coupling element to be maintained. The method may further comprise the step of removing the control signal. The device may comprise a quantum computation device and the operating characteristic may comprise a magnetic flux bias point for the quantum computation device.

In a fifth embodiment, a design structure embodied in a machine readable medium comprises a heating element and a switch element located proximate to the heating element, the switch element being substantially formed from a material which exhibits a superconducting temperature value below a superconducting temperature value exhibited by a material used to provide a connection to the switch element, wherein the switch element is responsive to the heating element such that the heating element is used to control whether or not the switch element is in a superconducting state.

Advantageously, principles of the invention provide for low power, low inductance thin film superconducting persistent current switches.

DETAILED DESCRIPTION

In accordance with illustrative principles of the present invention, it has been realized that, for qubit experiments, the standard switch design (as described above in the background section) has three undesirable features. First, the cooling power of most dilution refrigerators at 20 mK is typically less than 10 μW. Therefore, a switch that dissipates 50 μW of power will warm the refrigerator and the qubit if it is “on” or “open” for too long. Second, the inductance of the typical switch is too large in comparison to the inductance of the current bias control line that sets the qubit. In this case, a large fraction of the noise current from the room temperature control electronics is coupled to the qubit when the switch is “off.” Finally, the mutual inductance between the heater circuit and the persistent current switch is large and therefore this limits ones ability to set the control current accurately.

A persistent current switch design according to illustrative principles of the invention described herein solves the above and other problems. For example, key design improvements in the inventive persistent current switch are as follows.

First, we address the heating problem. Illustrative principles of the invention propose to solve this problem by utilizing a switch element (or “strap”) made of a material which exhibits a superconducting temperature value (also known as a critical temperature abbreviated Tc) “below” a superconducting temperature value exhibited by a material used to provide a connection to the switch element. It is to be appreciated that “below” in this case is defined as being “at least about half.” Accordingly, the material of the switch element exhibits a superconducting temperature value that is at least about half of the superconducting temperature value exhibited by the material used to provide a connection to the switch element. It is to be appreciated that the lower the superconducting temperature value is, the less power is needed to drive the material normal.

In addition, the switch element should have a superconducting temperature value that is above the operating temperature of the qubit. This can mean marginally below or significantly below. By way of example, in an embodiment described below, the qubit operates at 20 mK (0.02K), while the superconducting temperature of various illustrative materials that may be used for the switch element are, e.g., between about 0.11 K and about 0.61 K. Thus, it is to be appreciated that the temperature of the experimental chip (in this case, the qubit) merely needs to be below the Tcof the switch element material (note that when the switch is closed, i.e., the titanium is superconducting, the experimental chip and the switch are both below the Tcof the Ti). There would only be a problem if the experiment produces so much heat that the switch element material goes normal. But this will not occur in accordance with principles described herein since, first, the experimental chip is very weakly coupled thermally to the switch chip; and, second, we would not put enough current into the experimental chip to heat it significantly because it would not then stay at the low temperature which we want it to be at.

While, in a preferred embodiment as will be described below, we use titanium (Ti) for the material of the switch element due to its Tcof about 0.4 Kelvin (0.39 K), as compared with the Tcof a connection material such as aluminum, there are other suitable alternatives. For example, other possible materials for the switch element include but are not limited to: Gold-Indium-AuIn2(0.21 K), Zirconium-Zr (0.61 K), Ruthenium-Ru (0.49 K), and Iridium-Ir (0.11 K).

Thus as mentioned above, in a preferred embodiment, principles of the invention utilize a switch element (or “strap”) made of Ti. It is to be understood that it is preferred that the entire switch element be made of Ti, but it is to be appreciated that benefits of the invention can be achieved with the switch element being substantially made (i.e., “substantially formed”) of Ti. Note that this point applies to the respective compositions of the various other elements of the persistent current switch of the invention.

The critical temperature Tc(also known as the superconducting temperature since it is the temperature below which the element acts as a superconductor) of Ti is 0.39 Kelvin (K) which is significantly less than the Tcof Nb (9.2 K), which is used for the switch element in existing designs. The much lower Tcmeans that we can drive the Ti into its resistive state using much less power. This also means that one must have a cryogenic system that cools to temperatures below 0.39 K. This can be accomplished using the same dilution refrigerator used to cool the qubits. One other important factor that we will discuss in more detail below is that the Tcof Ti is also much less than the Tc(1.2 K) of the aluminum (Al) wire bonds used to connect the heat switch to the current bias control circuit. If the Al bonds are resistive or in their “normal” state, the heat switch will then be cooled by the “cold” qubit chip and therefore more heat is needed to open the switch. With the inventive design, we can open the switch using only 0.6 μW. This is 80× less than existing designs.

Furthermore, the superconducting Ti “strap” is designed to have a very low inductance. This is done using a large width to length ratio and then folding the strap with as narrow a slot as possible to keep its inductance low. The folding allows the interconnect pads to be close to each other and therefore the wire bonds to the qubit chip can be short and low in inductance. An illustrative embodiment of the Ti strap will be described below in the context ofFIG. 2. The inductance of the heat switch is roughly 600 picoHenries (pH) which is about 1/10 the inductance of the current bias “coupling” coil plus wire bonds. When the switch is closed, any noise currents are now shunted mostly through the heat switch instead of the coupling coil.

Another design improvement concerns the heater circuit. Illustrative principles of the invention propose to use gold-palladium (AuPd) for the resistive material. More importantly, we use a balanced current feed that provides a gradiometric cancellation of the stray magnetic fields coupling the heater circuit to the switch strap.

FIG. 1Ashows an overall schematic view of a persistent current switch according to an embodiment of the invention.

In particular,FIG. 1Ashows the switch integrated circuit or chip, the qubit integrated circuit or chip and the room temperature electronics needed to supply currents to the heater R and the current bias loop L. The switch chip and the qubit chip (each denoted by dashed lines) are mounted on a sample holder that is cooled in a cryogenic system to temperatures T near 20 mK (0.02 K). Our goal is to apply a fixed control current through L. It is important that this current be very precise and free of spurious noise currents. L is a lithographically deposited planar Nb loop. Current in this loop couples magnetic flux to the qubit. We may therefore generally refer to loop L as coupling element L. This flux biases the qubit at the correct operating point of the device. R is a AuPd heater and S is a Ti strap. Both are deposited using photo-lithography. More details about the switch chip will be presented below. All connections to the two chips are made via Al wire bonds WB.

The current through L is persisted as follows. We first apply current to the heater R until the strap S is warmed above its superconducting transition temperature (Tc=0.39 K). We note that it is important that we do not apply so much heat that the Al wire bonds rise above their transition temperature (1.2 K). Otherwise there will be heat transferred between the switch chip and the qubit chip. This would cause the qubit chip to warm, and, it would cool the switch chip. This could then cause the switch to close unless we apply more heat. These problems are easily avoided since we have a large temperature window (0.39-1.2 K) with which to work.

The control current Isignalis then turned on. Since R is now resistive all of Isignalflows through L. At this point there is a large current flowing through L and the qubit is strongly coupled to noise sources in the room temperature electronics. While the control current is flowing, the current to the heater is turned off and the switch S closes when the strap cools below its superconducting transition temperature. Now the entire circuit including the switch strap S, the coupling coil L, and the interconnecting wire bonds WB comprise a completely superconducting circuit and as such will allow current flowing in it to persist indefinitely. The control signal current source can be turned off and disconnected if desired and the magnetic flux bias point for the qubit maintained.

While the switch S is closed, all currents, including noise currents from the signal source at room temperature, will be divided between the coupling coil L and the switch strap. Consider the schematic shown inFIG. 1B.

The division of current will occur according to the following formula: (iSW)(LSW)+(iL)(L)=0. This formula merely states that the magnetic flux in the superconducting loop cannot change. Because the inductance of the switch strap has been designed to be about ten times lower than the inductance of the coupling coil plus wire bonds, the noise currents going through L will be attenuated by that same factor as well. We have confirmed this result directly in our experimental setup. Also shown inFIG. 1Bis the stray mutual inductance Mstraybetween the heater circuit and the signal circuit. Any currents in the heater circuit will be partially coupled to the signal circuit and will appear as additional current noise in the superconducting loop. For this reason, the heater circuit should be designed such that Mstrayis minimized. An example of how this can be done in our case will be explained now in the context ofFIG. 2.

FIG. 2shows an exploded view of the switch integrated circuit or chip (denoted by the leftmost dashed-line box inFIG. 1A). The switch comprises three layers on top of an electrically insulating substrate. The substrate (20) is about 300 μm thick and is usually made of undoped silicon (Si). Next is a patterned AuPd resistor (22) that is about 300 Angstroms (A) thick. Even though AuPd is more difficult to evaporate, it has a higher resistivity which allows us to use lower heater bias currents. This is desirable due to Mstray. Also, for the same reason, we made the AuPd layer fairly thin so that R would be large. This is followed by about 1000 A of insulating Silicon Nitride (SiNi) layer (24). The final layer (26) is a patterned Ti layer about 2000 A thick. This is the superconducting strap and the “folded” geometry allows a low-inductance connection between the strap and the coupling coil L. The aluminum wire bonds (27) are connected to connection pads (26A). The return wiring (28) for the heater (connected to switch chip via heater connections29) is made using about 2000 A thick Nb. The return wiring is split into two paths (28A and28B) to form a balanced connection so that Mstrayis minimized. There is also a crossover (23) for the heater. Finally, even though only one switch is shown on the substrate inFIG. 2, one may fabricate many switches on the same substrate.

In an illustrative embodiment, the basic “folded” geometry of the Ti strap26is one which achieves a low inductance layout. Ultimately, the dimensions used in this embodiment were determined by the size of the bonding pads26A. To accommodate several bonds on each pad, if needed, the pads are about 230 microns× 190 microns. The length of the strap in this embodiment is determined by the size of the AuPd resistor22that is underneath the strap. Since one of our goals is to minimize the amount of current in the resistor and, given the heat leak through the wire bonds, this determined approximately the size of the heater (AuPd resistor22). The width of each arm of the Ti strap is about 150 microns and the length is about 1200 microns. The gap between the two arms is about 20 microns, a size that is achievable with optical lithography—given the length of the strap. The two parallel arms of the strap with the gap between them is what forms the “folded” geometry.

Also, it is to be understood that, in the illustrative embodiment ofFIG. 2, the widths of various components were determined based on the size of the two bond pads. Further, given that the AuPd resistor was 300 A thick, and given that we wanted about 300 ohms of resistance, these criteria determined the length of the resistor. Thus, the width and length of the Ti strap was determined based on these criteria.

We noted that the heat leak along the wire bonds27can be undesirable. Even when the wire bonds are superconducting, they still conduct some heat via phonon conduction. This problem becomes smaller the lower in temperature we go. While wire bond connections can use multiple wire bonds in order to give a lower inductance connection, we found that extra wire bonds could give an unacceptable heat leak. Thus, in a preferred embodiment, we use single wire bonds only.

The heat flow paths on the various substrates have been designed to accomplish several desired features.FIG. 3Ashows schematically the heat flow between the various components.

It is important that the qubit be well thermally connected to the thermal reservoir at temperature T0. Likewise, the coupling coil L must be superconducting at all times. Therefore, they are both placed on a substrate SUB2that is connected to the cryostat with a very high thermal conductance. It is to be appreciated that the thermal conductivity (at low temperatures) should be similar to that of fairly pure copper. At very low temperatures, the conductivity of pure copper is in the range of 10 W/Km. Note the thermal conductivity of a material is related to the thermal conductance as follows: k=(L/A)/K, where k is the thermal conductivity, K is the thermal conductance, L is the relevant length, and A is the relevant contact area.

The persistent current switch is deposited on a separate substrate SUB1(item20inFIG. 2). SUB1is attached to a material that has a low thermal conductance, e.g., FR4. The FR4 is screwed to the cryostat (shown inFIG. 3B). The persistent current switch is now thermally isolated from the cryostat and therefore when the heater warms the switch to a higher temperature T2in order to drive it into the normal state, the qubit and coupling coil are not warmed appreciably. At low temperatures, the FR4 has a thermal conductivity less than 10-4 W/Km. We use FR4 because it is readily available and inexpensive. Other materials would work just as well, for example, vespel or nylon.

Also shown inFIG. 3Ais the possibility that multiple switches may be deposited on SUB1and that some switches may independently be open or closed. This can occur if the thermal conductance of SUB1is small. Not shown inFIG. 3Aare the thermal paths between the two substrates due to the aluminum wire bonds. If the temperature of the bonds stays well below 1.2 K, then the amount of heat transferred along each bond will be negligible.

In addition, with respect toFIG. 3A, it is to be understood that the upper small cross-hatched box (switch1or2) is the titanium metal. The thermal impedance between it and SUB1is the silicon nitride. The thermal impedance between SUB1and the cryostat is the FR4.

We shall now discuss how we measure the amount of power needed to operate the inventive persistent current switch. To do this, we pass a small ac current through the Ti strap using an alternating current signal generator and a resistor. We also monitor the voltage across the Ti strap. If the heater power is low, the Ti strap is always superconducting and we observe no voltage drop across the Ti strap. We then increase the current through the AuPd heater. If the heater current is sufficiently large, then the temperature of the Ti will rise above its Tcand the measured IV curve of the strap will look like a simple resistor. If we adjust the heater current carefully, the temperature of the Ti strap will be just below Tcand the IV curve will exhibit a standard superconducting IV curve with a small critical current Ic. This is shown inFIG. 4.

FIG. 4shows an ac measurement of the current through the Ti strap ISWversus the voltage across the Ti strap VSWwhere “SW” refers to “switch.” In this case, the current through the AuPd heater is 33.74 μA. At low temperature, we measured R=479Ω and therefore roughly Pheater=(33.74×10−6)(33.74×10−6) (479)=0.545 μW. If we increase the power by less than 10%, we no longer observe a critical current. If we reduce the heater current to zero, then Icbecomes very large (greater than 4 mA). We were unable to measure Icin this case because the large current needed to do this measurement would cause the cryostat to warm.

We note that if we were to use Al instead of Ti for the switch strap, the amount of power needed to operate the switch is substantially larger. There are two reasons why. First, the Tcof Al is higher. More importantly, if we heat the Al strap enough to drive it normal, the Al wire bonds will also go normal, and now the switch chip is tightly coupled to the cold qubit chip. The qubit chip will then cool the switch chip and therefore more heater power is needed to keep the switch open. This extra power will heat both the qubit and the refrigerator which is undesirable. We have made persistent current switches using Al straps and the power needed is about 100 times more than when we use Ti straps.

There are several ways one might estimate the switching time of our persistent current switch. Since our switches are used in a cryogenic environment, the most reasonable measure is the thermal relaxation time. We have measured this time using the pulse scheme shown schematically inFIG. 5A.

First we apply a very long voltage pulse to the switch heater R. The amplitude of the pulse is a little larger than the minimum voltage Aoneeded to cause the Ti strap to become resistive. While we are doing the pulsing, we also monitor the voltage across the Ti strap. Using a direct current power supply and a resistor, we pass current through the Ti strap. When the Ti is superconducting, and therefore not resistive, we observe no voltage. If the Ti is resistive, we then observe a non-zero voltage. The pulse is then turned off at a time t0and then, at a later time t1, we pulse the switch again. The amplitude of the second pulse is adjusted until we just barely begin to see a voltage transient across the Ti strap. If t1−t0is small, then the Ti strap has not cooled much and therefore the amplitude A needed to cause the strap to go normal again is small. If A is less than Ao, then the Ti strap will again go superconducting if t is much larger than t1. In the limit t1−t0is very large, the amplitude A will approach Ao.

Our measurements of A versus t1−t0are shown inFIG. 5B.FIG. 5Bshows that the thermal relaxation time is about 4-5 μsec. This means that the switch can be used to set or reset the bias state of a qubit in about that time interval.

FIG. 6shows a block diagram of an exemplary design flow60used for example, in semiconductor design, manufacturing, and/or test. Design flow60may vary depending on the type of integrated circuit (IC) being designed. For example, a design flow60for building an application specific IC (ASIC) may differ from a design flow60for designing a standard component. Design structure62is preferably an input to a design process61and may come from an IC provider, a core developer, or other design company or may be generated by the operator of the design flow, or from other sources. Design structure62comprises an embodiment of the invention as shown in one or more ofFIGS. 1A-3Bin the form of schematics or HDL, a hardware-description language (e.g., Verilog, VHDL, C, etc.). Design structure62may be contained on one or more machine readable media. For example, design structure62may be a text file or a graphical representation of an embodiment of the invention as shown in one or more ofFIGS. 1A-3B. Design process61preferably synthesizes (or translates) an embodiment of the invention as shown in one or more ofFIGS. 1A-3Binto a netlist68, where netlist68is, for example, a list of wires, transistors, logic gates, control circuits, I/O, models, etc. that describes the connections to other elements and circuits in an integrated circuit design and recorded on at least one of the machine readable media. This may be an iterative process in which netlist68is resynthesized one or more times depending on design specifications and parameters for the circuit.

Design process61may include using a variety of inputs; for example, inputs from library elements63which may house a set of commonly used elements, circuits, and devices, including models, layouts, and symbolic representations, for a given manufacturing technology, design specifications64, characterization data65, verification data66, design rules67, and test data files69(which may include test patterns and other testing information). Design process61may further include, for example, standard circuit design processes such as timing analysis, verification, design rule checking, place and route operations, etc. One of ordinary skill in the art of integrated circuit design can appreciate the extent of possible electronic design automation tools and applications used in design process61without deviating from the scope and spirit of the invention. The design structure of the invention is not limited to any specific design flow.

Design process61preferably translates an embodiment of the invention as shown in one or more ofFIGS. 1A-3B, along with any additional integrated circuit design or data (if applicable), into a second design structure70. Design structure70resides on a storage medium in a data format used for the exchange of layout data of integrated circuits and/or symbolic data format (e.g. information stored in a GDSII (GDS2), GL1, OASIS, map files, or any other suitable format for storing such design structures). Design structure70may comprise information such as, for example, symbolic data, map files, test data files, design content files, manufacturing data, layout parameters, wires, levels of metal, vias, shapes, data for routing through the manufacturing line, and any other data required by a semiconductor manufacturer to produce an embodiment of the invention as shown in one or more ofFIGS. 1A-3B. Design structure70may then proceed to a stage71where, for example, design structure70: proceeds to tape-out, is released to manufacturing, is released to a mask house, is sent to another design house, is sent back to the customer, etc.