Patent Publication Number: US-11038093-B2

Title: Wirebond cross-talk reduction for quantum computing chips

Description:
BACKGROUND 
     The present invention relates generally to a device, a fabrication method, and fabrication system for wirebonds on superconducting quantum computing chips. More particularly, the present invention relates to a device, method, and system for wirebond cross-talk reduction for quantum computing chips. 
     Hereinafter, a “Q” prefix in a word or phrase is indicative of a reference of that word or phrase in a quantum computing context unless expressly distinguished where used. 
     Molecules and subatomic particles follow the laws of quantum mechanics, a branch of physics that explores how the physical world works at a more fundamental level. At this level, particles behave in strange ways, taking on more than one state at the same time, and interacting with other particles that are very far away. Quantum computing harnesses these quantum phenomena to process information. 
     The computers we use today are known as classical computers (also referred to herein as “conventional” computers or conventional nodes, or “CN”). A conventional computer uses a conventional processor fabricated using semiconductor materials and technology, a semiconductor memory, and a magnetic or solid-state storage device, in what is known as a Von Neumann architecture. Particularly, the processors in conventional computers are binary processors, i.e., operating on binary data represented in 1 and 0. 
     A quantum processor (q-processor) uses the odd nature of quantum bit devices (compactly referred to herein as “qubit,” plural “qubits”) to perform computational tasks. In the particular realms where quantum mechanics operates, particles of matter can exist in multiple states—such as an “on” state, an “off” state, and both “on” and “off” states simultaneously. Where classical computing using semiconductor processors is limited to using just the on and off states (equivalent to 1 and 0 in binary code), a quantum processor harnesses these quantum states of matter to output signals that are usable in data computing. 
     Conventional computers encode information in bits. Each bit can take the value of 1 or 0. These 1s and 0s act as on/off switches that ultimately drive computer functions. Quantum computers, on the other hand, are based on qubits, which differ from classical bits according to two key principles of quantum physics: superposition and entanglement. Superposition means that each qubit can represent both a 1 and a 0 at the same time. Entanglement means that qubits in superposition can be correlated with each other in a non-classical way; that is, the state of one (whether it is a 1 or a 0 or both) can depend on the state of another, and that there is more information that can be ascertained about the two qubits when they are entangled than when they are treated individually. 
     Using these two principles, qubits operate as more sophisticated processors of information, enabling quantum computers to function in ways that allow them to solve difficult problems that are intractable using conventional computers. IBM has successfully constructed and demonstrated the operability of a quantum processor using superconducting qubits (IBM is a registered trademark of International Business Machines corporation in the United States and in other countries.) 
     A superconducting qubit includes a Josephson junction. A Josephson junction is formed by separating two superconducting metal layers by a thin non-superconducting material. When the metal in the superconducting layers is caused to become superconducting—e.g. by reducing the temperature of the metal to a specified cryogenic temperature—pairs of electrons can tunnel from one superconducting layer through the non-superconducting layer to the other superconducting layer. In a qubit, the Josephson junction—which functions as a dispersive nonlinear inductor—is electrically coupled in parallel with one or more capacitive devices forming a nonlinear microwave oscillator. The oscillator has a resonance/transition frequency determined by the value of the inductance and the capacitance in the qubit circuit. Any reference to the term “qubit” is a reference to a superconducting qubit circuitry that employs a Josephson junction, unless expressly distinguished where used. 
     The information processed by qubits is carried or transmitted in the form of microwave signals/photons in the range of microwave frequencies. Information is carried or transmitted in a single photon. The microwave signals are captured, processed, and analyzed to decipher the quantum information encoded therein. A readout circuit is a circuit coupled with the qubit to capture, read, and measure the quantum state of the qubit. An output of the readout circuit is information usable by a q-processor to perform computations. 
     A superconducting qubit has two quantum states—|0&gt; and |1&gt;. These two states may be two energy states of atoms, for example, the ground (|g&gt;) and first excited state (|e&gt;) of a superconducting artificial atom (superconducting qubit). Other examples include spin-up and spin-down of the nuclear or electronic spins, two positions of a crystalline defect, and two states of a quantum dot. Since the system is of a quantum nature, any combination of the two states are allowed and valid. 
     For quantum computing using qubits to be reliable, quantum circuits, e.g., the qubits themselves, the readout circuitry associated with the qubits, and other parts of the quantum processor, must not unintentionally alter the states of the qubits, such as by injecting or dissipating energy in any significant manner, or influence the relative phase between the |0&gt; and |1&gt; states of the qubit. This operational constraint on any circuit that operates with quantum information necessitates special considerations in fabricating semiconductor and superconducting structures that are used in such circuits. 
     The connection lines or wirebonds between an external circuit and a q-circuit, e.g., an input line to the q-circuit or an output line from the q-circuit, or both, transfer signals between the q-circuit and the external circuit, and vice versa. Cross-talk is undesirable interference or unwanted transfer of signals in a range of frequencies of a signal carried by a neighboring signal line. A signal on a wirebond between the external circuit and a conductive pad on the q-circuit can generate cross-talk to nearby wirebonds. This cross-talk can be in the microwave frequency spectrum. A signal on a wirebond may correspond to information carried or transmitted by a single photon or only a few photons. Thus, quantum computing chips require a greater level of sensitivity than conventional computers. For the reasons described herein, cross-talk between nearby wirebonds is undesirable when the wirebonds and signal relate to quantum computing using q-circuits. Thus, there is a need to reduce and/or mitigate cross-talk between nearby wirebonds in a quantum computing chip. 
     The waveguide structures used in a q-circuit are designed to confine the microwave signals and minimize cross-talk between different parts of the circuit. This can be achieved, for example, by a large enough ground plane that separates two coplanar waveguides. On-chip waveguides can end in conductive pads, from where external circuits can be connected using wirebonds. At the location of the wirebonds, however, additional coupling between different signal lines is possible, which can lead to larger (undesirable) cross-talk. 
     The illustrative embodiments recognize that q-circuits are designed to minimize cross-talk on the q-circuit but ground planes and/or shielding are difficult between the wirebonds. Furthermore, the illustrative embodiments recognize as the number of qubits increase on the q-circuit, the number of conductors from where the qubits can be connected to external circuits also increase. Fabricating an increased number of qubits in the same size chip causes the conductor density on the chip to increase. Accordingly, the wirebonds from such conductors become physically closer to other wirebonds on the same chip. Thus, the wirebonds become more crowded and the crowding results in increased inter-wirebond cross-talk. Additionally, the illustrative embodiments recognize increasing the size of the q-circuit to accommodate for wirebond spacing results in inefficiencies and cost increases due to the large q-circuit size. 
     SUMMARY 
     The illustrative embodiments provide a wirebond cross-talk reduction device for quantum computing chips, and a method and system of fabrication therefor. An embodiment constructs a configuration of wirebonds for reducing cross-talk in a quantum computing chip to include a first wirebond coupling a first conductor of a quantum computing circuit with a first conductor of an external circuit. The embodiment further includes in the configuration a second wirebond coupling a second conductor of the quantum computing circuit with a second conductor of the external circuit, wherein the first wirebond and the second wirebond are separated by a first vertical distance in a direction of a length of the first conductor. 
     In one embodiment, an orientation of the second wirebond is angled from an orientation of the first wirebond. 
     In another embodiment, the orientation of the second wirebond is substantially orthogonal to the orientation of the first wirebond. 
     In another embodiment, the orientation of the second wirebond is substantially parallel to the orientation of the first wirebond. 
     In another embodiment, the configuration further includes a third wirebond coupling a third conductor of the quantum computing circuit with a third conductor of the external circuit, wherein the second wirebond and the third wirebond are separated by a second vertical distance. 
     In another embodiment, the first vertical distance and the second vertical distance are equal. 
     In another embodiment, an orientation of the third wirebond is angled from an orientation of the second wirebond. 
     In another embodiment, the orientation of the third wirebond is substantially similar to an orientation of the first wirebond. 
     In another embodiment, the configuration further includes a fourth wirebond coupling a fourth conductor of the quantum computing circuit with a fourth conductor of the external circuit, wherein the third wirebond and the fourth wirebond are separated by a third vertical distance. 
     In another embodiment, the third vertical distance is equal to a sum of the first vertical distance and the second vertical distance. 
     In another embodiment, the third vertical distance is equal to the first vertical distance. 
     In another embodiment, the third vertical distance is equal to the second vertical distance. 
     In another embodiment, an orientation of the fourth wirebond is angled from an orientation of the third wirebond. 
     In another embodiment, an orientation of the fourth wirebond is substantially parallel to an orientation of the second wirebond. 
     An embodiment includes a method for building the configuration of wirebonds. 
     An embodiment includes a system for constructing or assembly the configuration of wirebonds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of the illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  depicts a schematic diagram of an example wirebond spacing for a q-circuit in accordance with an illustrative embodiment; 
         FIG. 2  depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with an illustrative embodiment; 
         FIG. 3  depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with another illustrative embodiment; 
         FIG. 4  depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with another illustrative embodiment; 
         FIG. 5  depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with an illustrative embodiment; 
         FIG. 6  depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with an illustrative embodiment; and 
         FIG. 7  depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with an illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The illustrative embodiments used to describe the invention generally address and solve the above-described problem of cross-talk between the wirebonds for quantum computing chips. The illustrative embodiments provide for increasing wirebond separation distance while maintain q-circuit size, which address the above-described need or problem. 
     An operation described herein as occurring with respect to a frequency or frequencies should be interpreted as occurring with respect to a signal of that frequency or frequencies. All references to a “signal” are references to a microwave frequency signal unless expressly distinguished where used. 
     An embodiment provides a configuration for increased wirebond separation distance for q-circuits. Another embodiment provides a fabrication method for the increased wirebond separation configuration, such that the method can be implemented as a software application. The application implementing a fabrication method embodiment can be configured to operate in conjunction with an existing semiconductor fabrication system—such as a lithography system, or a circuit assembly system. 
     For the clarity of the description, and without implying any limitation thereto, the illustrative embodiments are described using some example configurations. From this disclosure, those of ordinary skill in the art will be able to conceive many alterations, adaptations, and modifications of a described configuration for achieving a described purpose, and the same are contemplated within the scope of the illustrative embodiments. 
     Furthermore, simplified diagrams of the example resistors, inductors, capacitors, waveguides, and other circuit components are used in the figures and the illustrative embodiments. In an actual fabrication or circuit, additional structures or component that are not shown or described herein, or structures or components different from those shown but for a similar function as described herein may be present without departing the scope of the illustrative embodiments. 
     Furthermore, the illustrative embodiments are described with respect to specific actual or hypothetical components only as examples. The steps described by the various illustrative embodiments can be adapted for fabricating a circuit using a variety of components that can be purposed or repurposed to provide a described function within a q-circuit, and such adaptations are contemplated within the scope of the illustrative embodiments. 
     The illustrative embodiments are described with respect to certain types of materials, electrical properties, steps, numerosity, frequencies, circuits, components, and applications only as examples. Any specific manifestations of these and other similar artifacts are not intended to be limiting to the invention. Any suitable manifestation of these and other similar artifacts can be selected within the scope of the illustrative embodiments. 
     The examples in this disclosure are used only for the clarity of the description and are not limiting to the illustrative embodiments. Any advantages listed herein are only examples and are not intended to be limiting to the illustrative embodiments. Additional or different advantages may be realized by specific illustrative embodiments. Furthermore, a particular illustrative embodiment may have some, all, or none of the advantages listed above. 
     With reference to  FIG. 1 , this figure depicts a schematic diagram of an example wirebond spacing for a q-circuit in accordance with an illustrative embodiment. Configuration  100  comprises a set of two or more wirebonds  102 ,  104 . Wirebonds  102 ,  104  connect an external circuit to q-circuit  106 . For example, wirebond  102  couples to conductor  103 A via conductive pad  103 B and wirebond  104  couples to conductor  105 A via conductive pad  105 B. Conductor  103 A, conductor  105 A, pad  103 B, and pad  105 B are fabricated on substrate  107 . A portion of wirebond  102  is separated from a corresponding portion of wirebond  104  by a lateral distance, Δx, and a vertical distance, Δy, as shown in the depicted example. 
     With reference to  FIG. 2 , this figure depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with an illustrative embodiment. Configuration  200  comprises a set of two or more wirebonds  202 ,  204 ,  206 ,  208  . . . . For example, wirebonds  202  and  204  are positioned in a vertically staggered manner, in a direction of a length of the conductor. The staggering causes the corresponding portions of wirebonds  202  and  204  to be separated from each other by a lateral distance and a vertical distance. Thus, where the corresponding portions of wirebonds  202  and  204  would only have lateral separation from one another in the prior-art, the staggered placement causes a diagonal distance larger than the lateral distance to separate the same corresponding portions of wirebonds  202  and  204 . Separation of adjacent wirebonds occurs in both the lateral distance and the vertical distance. 
     With reference to  FIG. 3 , this figure depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with an illustrative embodiment. The example configuration  300  in this figure comprises a set of two or more wirebonds  302 ,  304 ,  306 ,  308  . . . . Separation of adjacent wirebonds  302 ,  304  occurs in a first vertical distance  303 A and a first lateral distance  303 B. Separation of adjacent wirebonds  304 ,  306  occurs in a second vertical distance  305 A and a second lateral distance  305 B. Separation of adjacent wirebonds  306 ,  308  occurs in a third vertical distance  307 A and a third lateral distance  307 B. 
     The wirebonds can be arranged in a variety of arrangements within the scope of the illustrative embodiments. In one embodiment, the first lateral distance  303 B is equal to or different from the second lateral distance  305 B. In another embodiment, the first vertical distance  303 A is equal to or different from the second vertical distance  305 A. In another embodiment, the third vertical distance  307 A is equal to or different from the sum of the first vertical distance  303 A and the second vertical distance  305 A. In another embodiment, the third vertical distance  307 A is equal to or different from the first vertical distance  303 A. 
     These examples of distances are not intended to be limiting. From this disclosure, those of ordinary skill in the art will be able to conceive many other ways of configuring staggered separation distances between wirebonds and the same are contemplated within the scope of the illustrative embodiments. 
     With reference to  FIG. 4 , this figure depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with an illustrative embodiment. Configuration  400  includes wirebonds  402 ,  404 , and external circuit  406 . Orientation of wirebond  404  differs from orientation of wirebond  402 . Wirebond  402  faces and extends towards the external circuit  406  in a first direction. Wirebond  404  faces and extends towards the external circuit  406  in a second direction. In one arrangement, the second direction is substantially opposite of the first direction. For example, the second direction is 180 degrees, ±10 degrees, from the first direction. 
     The wirebonds can be arranged in a variety of other arrangements within the scope of the illustrative embodiments. In one embodiment, one arrangement of adjacent wirebonds includes a vertical separation distance in addition to differing orientations of the adjacent wirebonds. 
     With reference to  FIG. 5 , this figure depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with an illustrative embodiment. Configuration  500  includes wirebonds  502 ,  504 , and external circuit  506 . Orientation of wirebond  504  differs from orientation of wirebond  502 . In one arrangement, orientation of wirebond  504  is substantially orthogonal to orientation of wirebond  502 . For example, orientation of wirebond  504  is 90 degrees, ±10 degrees, from the orientation of wirebond  502 . 
     Wirebonds  502  and  504  may be oriented relative to each other at other suitable angles, such as at 30, 45, 60, or 135 degrees, to give a few non-limiting examples. Such angled orientations may or may not further include a vertical distance between corresponding portions of wirebonds  502  and  504 . 
     With reference to  FIG. 6 , this figure depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with an illustrative embodiment. Configuration  600  includes wirebonds  602 ,  604 ,  606 ,  608  . . . . Separation of adjacent wirebonds  602 ,  604  occurs in a first lateral distance and a first vertical distance. Orientation of wirebond  604  differs from orientation of wirebond  602 . In one embodiment, wirebond  604  is substantially orthogonal to orientation of wirebond  602 . In another embodiment, wirebond  604  is oriented at another suitable angle relative to wirebond  602 , e.g., at any angle between 10 and 170 degrees. 
     Separation of adjacent wirebonds  604 ,  606  occurs in a second lateral distance and a second vertical distance. Orientation of wirebond  606  differs from orientation of wirebond  604 . In one embodiment, wirebond  606  is substantially orthogonal to orientation of wirebond  604 . In another embodiment, wirebond  606  is oriented at another suitable angle relative to wirebond  604 , e.g., at any angle between 10 and 170 degrees. Separation of adjacent wirebonds  606 ,  608  occurs in a third lateral distance and a third vertical distance. In one embodiment, the third vertical distance is equal to the sum of the first vertical distance and the second vertical distance. The wirebonds can be arranged in a variety of other arrangements within the scope of the illustrative embodiments. For example, the pattern of the arrangement repeats, as shown in  FIG. 6 . 
     With reference to  FIG. 7 , this figure depicts an example set of wirebonds between a q-circuit and an external circuit in accordance with an illustrative embodiment. Configuration  700  includes wirebonds  702 ,  704 ,  706 ,  708 ,  710  . . . . Separation of adjacent wirebonds  702 ,  704  occurs in a first lateral distance and a first vertical distance. Orientation of wirebond  704  differs from orientation of wirebond  702 . In one embodiment, wirebond  704  is substantially orthogonal to orientation of wirebond  702 . In another embodiment, wirebond  704  is oriented at another suitable angle relative to wirebond  702 , e.g., at any angle between 10 and 170 degrees. Separation of adjacent wirebonds  704 ,  706  occurs in a second lateral distance and a second vertical distance. Orientation of wirebond  706  differs from orientation of wirebond  704 . In one embodiment, wirebond  706  is substantially orthogonal to orientation of wirebond  704 . In another embodiment, wirebond  706  is oriented at another suitable angle relative to wirebond  704 , e.g., at any angle between 10 and 170 degrees. In another embodiment, orientation of wirebond  706  is substantially similar to wirebond  702 . 
     Separation of adjacent wirebonds  708 ,  706  occurs in a third lateral distance and a third vertical distance. Orientation of wirebond  708  differs from orientation of wirebond  706 . In one embodiment, wirebond  708  is substantially orthogonal to orientation of wirebond  706 . In another embodiment, wirebond  708  is oriented at another suitable angle relative to wirebond  706 , e.g., at any angle between 10 and 170 degrees. 
     Separation of adjacent wirebonds  710 ,  708  occurs in a fourth lateral distance and a fourth vertical distance. Orientation of wirebond  710  differs from orientation of wirebond  708 . In one embodiment, wirebond  710  is substantially orthogonal to orientation of wirebond  708 . In another embodiment, wirebond  710  is oriented at another suitable angle relative to wirebond  708 , e.g., at any angle between 10 and 170 degrees. The wirebonds can be arranged in variety of other arrangements within the scope of the illustrative embodiments. In one embodiment, the lateral distance between a first pair of adjacent wirebonds is equal to or differs from the lateral distance between a second pair of adjacent wirebonds. In another embodiment, the vertical distance between a first pair of adjacent wirebonds is equal to or differs from the vertical distance between a second pair of adjacent wirebonds. 
     In another embodiment, a method for determining a vertical distance of separation between adjacent wirebonds is calculated based on a predetermined lateral distance between adjacent wirebonds and a predetermined minimum separation distance between adjacent wirebonds. For example, selection of the predetermined lateral distance is based on q-chip size and desired number of qubits. In some embodiments, the predetermined minimum separation distance of wirebonds is set to minimize cross-talk between different signal lines. In one embodiment, selection of the predetermined minimum separation distance is based on q-chip size and desired number of qubits. 
     The vertical distance of separation between adjacent wirebonds is calculated from the predetermined lateral distance and the predetermined minimum separation distance. For example, the lateral distance, Δx, in  FIG. 1  may be a predetermined lateral distance. As shown in  FIG. 1 , the lateral distance, Δx, and vertical distance, Δy, form a right angle. The hypotenuse of the right angle between Δx and Δy is the minimum separation distance between adjacent wirebonds. The vertical distance between adjacent wirebonds can be calculated based on the predetermined lateral distance, the minimum separation distance, and the orientation and configuration of the arrangement of wirebonds. For example, the Pythagorean theorem can be used to calculate the vertical distance between adjacent wirebonds in the arrangement shown in  FIG. 1 , based on the predetermined lateral distance and the minimum separation distance between adjacent wirebonds. 
     Various embodiments of the present invention are described herein with reference to the related drawings. Alternative embodiments can be devised without departing from the scope of this invention. Alternative embodiments which extend the number of wirebonds greater than the number shown in the various embodiments are contemplated within the scope of this invention. Although various connections and positional relationships (e.g., over, below, adjacent, etc.) are set forth between elements in the following description and in the drawings, persons skilled in the art will recognize that many of the positional relationships described herein are orientation-independent when the described functionality is maintained even though the orientation is changed. These connections and/or positional relationships, unless specified otherwise, can be direct or indirect, and the present invention is not intended to be limiting in this respect. Accordingly, a coupling of entities can refer to either a direct or an indirect coupling, and a positional relationship between entities can be a direct or indirect positional relationship. As an example of an indirect positional relationship, references in the present description to forming layer “A” over layer “B” include situations in which one or more intermediate layers (e.g., layer “C”) is between layer “A” and layer “B” as long as the relevant characteristics and functionalities of layer “A” and layer “B” are not substantially changed by the intermediate layer(s). 
     The following definitions and abbreviations are to be used for the interpretation of the claims and the specification. As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” “contains” or “containing,” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a composition, a mixture, process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such composition, mixture, process, method, article, or apparatus. 
     Additionally, the term “illustrative” is used herein to mean “serving as an example, instance or illustration.” Any embodiment or design described herein as “illustrative” is not necessarily to be construed as preferred or advantageous over other embodiments or designs. The terms “at least one” and “one or more” are understood to include any integer number greater than or equal to one, i.e. one, two, three, four, etc. The terms “a plurality” are understood to include any integer number greater than or equal to two, i.e. two, three, four, five, etc. The term “connection” can include an indirect “connection” and a direct “connection.” 
     References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described can include a particular feature, structure, or characteristic, but every embodiment may or may not include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     The terms “about,” “substantially,” “approximately,” and variations thereof, are intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. For example, “about” can include a range of ±8% or 5%, or 2% of a given value. 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments described herein.