Microfabricated air bridge for quantum circuits

Abstract

A method for fabricating a bridge structure in a quantum mechanical device includes providing a substructure including a substrate having deposited thereon a layer of a first superconducting material divided into a first portion, a second portion, and a third portion electrically insulated from one another; depositing a sacrificial layer over the substructure; electrically connecting the first portion and the second portion with a strip of a second superconducting material, the second superconducting material being different from the first superconducting material; and removing a portion of the sacrificial layer to form a bridge structure over the third portion between the first and second portions, the bridge structure electrically connecting the first portion to the second portion but not electrically connecting the third portion to the first portion and not electrically connecting the third portion to the second portion.

Description

Technical Field

[0001] The embodiments of the present invention claimed herein are related to quantum circuits, and more particularly, to bridge structures within quantum mechanical devices and methods of manufacturing quantum mechanical devices having bridge structures.

Background Art

[0002] Quantum computing is based on reliable control of quantum bits (referred to herein as qubits throughout). The basic operations required to implement a quantum algorithm are a series of single qubit operations and two qubit operations that establish a correlation between two separate quantum bits. In order to both reach the error threshold of quantum computing and reach reliable quantum simulation, it may be desirable to implement highly faithful two qubit operations.

[0003] (A superconducting quantum processor having one or more superconducting qubits) includes a superconducting metal (such as Al, Nb, etc.) on an insulating substrate (such as Si or high-resistance Si, Al 2 O 3 etc.). A superconducting quantum processor is typically a planar two-dimensional lattice structure or a circuit of individual qubits linked by couplers with various lattice symmetries (such as square, hexagonal, etc.), and a readout structure located on a flip chip. The coupler can be made of a capacitor, resonator, coil, or any microwave component that couples between qubits.

[0004] Superconducting microwave circuits based on coplanar waveguides (CPWs) are susceptible to the influence of parasitic slot-line modes. These modes can couple to circuit elements such as qubits and thus potentially cause signal loss and decoherence. To remove these spurious modes, cross-connections are typically made between ground planes blocked by coplanar waveguides (CPWs). Conventionally, free-standing crosses known as air bridges have been used for that purpose. However, there remain issues associated with the fabrication of these air bridges that lead to undesirable quantum mechanical devices or circuits. Summary of the Invention

[0005] Aspects of the present invention provide a method for fabricating a bridge structure within a quantum mechanical device. The method includes providing a lower structure that includes a substrate having a layer of a first superconducting material deposited thereon, the layer being divided into a first portion, a second portion, and a third portion that are electrically insulated from each other. The method further includes depositing a sacrificial layer over the lower structure and electrically connecting the first and second portions of the first superconducting material to a strip of a second superconducting material, the second superconducting material being different from the first superconducting material. The method also includes removing a portion of the sacrificial layer deposited over the lower structure so as to form a bridge structure that includes a strip of the second superconducting material over the third portion between the first and second portions, the bridge structure electrically connecting the first portion to the second portion but not electrically connecting the third portion to the first portion and not electrically connecting the third portion to the second portion.

[0006] In an embodiment, providing the lower structure includes providing a substrate having a surface and forming a layer of a first superconducting material on the surface of the substrate. In an embodiment, providing the lower structure further includes etching the layer of the first superconducting material to form a first groove and a second groove so that a first portion, a second portion, and a third portion of the layer of the first superconducting material are spaced apart from each other by the etched first groove and the etched second groove, thereby defining the first portion, the second portion, and the third portion of the layer of the first superconducting material.

[0007] In an embodiment, the substrate includes silicon or sapphire. In an embodiment, the first superconducting material includes niobium or aluminum. In an embodiment, the sacrificial layer includes titanium (Ti), titanium nitride (TiN), or tantalum (Ta), or any combination thereof.

[0008] In an embodiment, forming a sacrificial layer on the lower structure includes sputtering a superconducting sacrificial material on the lower structure. In an embodiment, electrically connecting the first portion and the second portion of the first superconducting material to a strip of a second superconducting material includes sputtering a superconducting material under compressive stress to form a strip of the second superconducting material. In an embodiment, electrically connecting the first portion and the second portion of the first superconducting material to a strip of the second superconducting material includes attaching a first base pad of the strip to the first portion and attaching a second base pad of the strip to the second portion. In an embodiment, electrically connecting the first portion and the second portion of the first superconducting material to a strip of the second superconducting material includes electrically connecting the first portion and the second portion of the first superconducting material to a strip of a porous second superconducting material.

[0009] In an embodiment, removing a part of the sacrificial layer formed on the substrate includes etching a part of the sacrificial layer under the strip so as to form a gap between the strip and the third part of the first superconducting material and define a bridge structure on the third part of the first superconducting material. In an embodiment, etching a part of the sacrificial layer under the strip includes etching a part of the sacrificial layer using acid etching. In an embodiment, etching a part of the sacrificial layer under the strip includes etching the sacrificial layer under the strip formed on the third part, etching the sacrificial layer formed in the first and second grooves separating the first, second, and third parts, and not etching the sacrificial layer at both ends of the strip.

[0010] In an embodiment, providing a lower structure includes providing a substrate having a surface, depositing a layer of sacrificial material on the surface of the substrate, selectively etching the layer of sacrificial material to form first and second spaced-apart portions of the sacrificial material, selectively etching the substrate except for the first and second portions of the sacrificial material, depositing a layer of a first superconducting material on the etched substrate and the first and second portions of the sacrificial material, and removing the deposited layer of the first superconducting material and the first and second portions of the sacrificial material to obtain a layer of the first superconducting material divided into a first portion, a second portion, and a third portion electrically insulated from each other by the substrate material.

[0011] In an embodiment, depositing a sacrificial layer on the lower structure includes sputtering a superconducting sacrificial material on the layer of the first superconducting material. In an embodiment, removing a part of the sacrificial layer formed on the substrate includes etching a part of the sacrificial layer under the strip so as to form a gap between the strip and the third part of the first superconducting material and define a bridge structure on the third part of the first superconducting material, and etching the substrate material separating the first, second, and third parts of the superconducting material.

[0012] Another aspect of the present invention provides a quantum mechanical device including a substrate and a layer of a first superconducting material formed on the substrate, the layer being divided into a first portion, a second portion, and a third portion that are electrically insulated from each other. The quantum mechanical device further includes a bridge structure connected to the first portion and the second portion over the third portion located between the first portion and the second portion, the bridge structure having a strip of a second superconducting material configured to electrically connect the first portion and the second portion of the first superconducting material. The strip of the second superconducting material is different from the first superconducting material.

[0013] In an embodiment, the strip of the second superconducting material is porous at least in a portion crossing the third portion of the layer of the first superconducting material. In an embodiment, the first portion and the second portion of the first superconducting material are connected to the same ground potential. In an embodiment, the third portion of the first superconducting material is a signal line configured to carry an electromagnetic signal to and from a qubit. In an embodiment, the bridge structure is configured to substantially remove spurious modes of a planar microwave circuit. In an embodiment, the first portion and the second portion of the layer of the first superconducting material are first and second signal lines configured to carry a first electromagnetic signal to and from a first qubit, and the third portion is a third signal line configured to carry a second electromagnetic signal to and from a second qubit.

[0014] In an embodiment, the quantum mechanical device further includes a plurality of regularly spaced bridge structures for electrically connecting the first portion and the second portion of the layer of the first superconducting material at a plurality of locations of the layer of the first superconducting material. In an embodiment, the plurality of bridge structures are configured to connect the first portion and the second portion of the layer of the first superconducting material to the same ground potential.

[0015] The bridge structure can be used, for example, to remove spurious modes of a planar microwave circuit by connecting ground planes or to cross signal lines. This enables access to qubits surrounded by obstacles and thus enables splitting the plane of a superconducting two-dimensional (2D) quantum circuit. The bridge structure can include a superconducting material and is manufactured using a different process compared to the manufacturing process of the first layer in which the qubits and other parts of the circuit are fabricated. This provides flexibility in using different materials. Additionally, this method and the quantum mechanical device achieve better yields due to the prestress and bendable nature of the bridge structure. Further, there is no need to connect signal lines by performing different manufacturing, thus reducing or lowering signal loss.

[0016] In addition to the present disclosure, the operation and function methods of combinations of related elements and parts of the structure and the economics of manufacturing will become even more apparent when considering the following description and the appended claims with reference to the accompanying drawings. All of those form a part of this specification which designates corresponding parts in different figures with similar reference numerals. However, it should be clearly understood that those drawings are for illustrative and explanatory purposes only and are not intended as a definition of the limitations of the present invention.

Brief Description of the Drawings

[0017]

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DETAILED DESCRIPTION OF THE INVENTION

[0018] In an embodiment of the present invention, a method for manufacturing a bridge structure in a quantum mechanical device is provided. This method includes providing a lower structure 100 having a substrate 102 with a layer of a first superconducting material 104 formed on top. FIG. 1A is a schematic side view of a lower structure 100 including a substrate 102 on which a first superconducting material 104 is formed on top and described in an embodiment of the present invention. FIG. 1B is a schematic top view of a lower structure 100 including a substrate 102 on which a first superconducting material 104 is formed on top and described in an embodiment of the present invention. In the embodiment, the substrate 102 can be, for example, silicon or sapphire. In the embodiment, the first superconducting material 104 may include niobium (Nb), aluminum (Al), etc.

[0019] As shown in FIG. 1B, the first superconducting material layer 104 is divided into a first portion 104A, a second portion 104B, and a third portion 104C that are electrically insulated from each other. As shown in FIG. 1B, the first portion 104A, the second portion 104B, and the third portion 104C are electrically insulated from each other by the presence of a first groove 106A between the first portion 104A and the third portion 104C and a second groove 106B between the second portion 104B and the third portion 104C, and the substrate 102 can be seen through these grooves.

[0020] In an embodiment, as shown in FIG. 1A, the substrate 102 has a surface 102A, and a layer of the first superconducting material 104 is formed on the surface 102A. In an embodiment, providing the lower structure 100 includes etching the layer of the first superconducting material 104 to form the first groove 106A and the second groove 106B so that the first portion 104A, the second portion 104B, and the third portion 104C of the layer of the first superconducting material 104 are spaced apart from each other by the etched first groove 106A and the etched second groove 106B, and defining the first portion 104A, the second portion 104B, and the third portion 104C of the layer of the first superconducting material 104. In an embodiment, as shown in FIG. 1B, the third portion 104C is located between the first portion 104A and the second portion 104B.

[0021] In an embodiment, the first portion 104A and the second portion 104B of the first superconducting material 104 are connected to the same ground potential. In an embodiment, the third portion 104C of the first superconducting material 104 is a signal line configured to carry an electromagnetic signal to and from a qubit (not shown).

[0022] Figure 2 is a schematic top view of a substrate with a superconducting material deposited thereon, as described in another embodiment of the present invention. In an embodiment, the method may include providing a lower structure 200 having a substrate 202 with a layer 204 of a first superconducting material deposited thereon, as shown in Figure 2. In this embodiment, the first superconducting material layer 204 has a first portion 204A, a second portion 204B, and a third portion 204C. In an embodiment, the first portion 204A and the second portion 204B of the first superconducting material layer 204 are connected to the same ground potential. In an embodiment, the third portion 204C of the first superconducting material 204 is a signal line configured to carry an electromagnetic signal to and from a qubit (not shown). As shown in Figure 2, the first portion 204A, the second portion 204B, and the third portion 204C are electrically insulated from each other by the presence of a first groove 206A between the first portion 204A and the third portion 204C and a second groove 206B between the second portion 204B and the third portion 204C, and the substrate 202 can be seen through these grooves.

[0023] Similar to the previous embodiment, providing the lower structure 200 includes etching the layer of the first superconducting material layer 204 to form the first groove 206A and the second groove 206B to define the first portion 204A, the second portion 204B, and the third portion 204C of the layer 204 of the first superconducting material such that the first portion 204A, the second portion 204B, and the third portion 204C are spaced apart from each other by the etched first groove 206A and the etched second groove 206B.

[0024] In addition to the first portion 204A, the second portion 204B, and the third portion 204C, the first superconducting material layer 204 further includes a fourth portion 204D and a fifth portion 204E. The fourth portion 204D is electrically insulated from the first portion 204A, the second portion 204B, and the third portion 204C. Similarly, the fifth portion 204E is also electrically insulated from the first portion 204A, the second portion 204B, the third portion 204C, and the fourth portion 204D. The fourth portion 204D is electrically insulated from the first portion 204A by the presence of a groove 206C etched within the layer of the first superconducting material 204. The groove 206C forms a U-shape to insulate the fourth portion 204D from the first portion 204A. The fifth portion 204E is electrically insulated from the second portion 204B by the presence of a groove 206D etched within the layer of the first superconducting material 204. The groove 206D forms a U-shape to insulate the fifth portion 204E from the second portion 204B.

[0025] In an embodiment, both the first portion 204A and the second portion 204B are electrically connected to a ground potential. In an embodiment, the third portion 204C is a first signal line configured to carry a first electromagnetic signal between the first qubit. In an embodiment, when the fourth portion 204D and the fifth portion 204E are connected to each other using a bridge structure, they form a second signal line configured to carry a second electromagnetic signal between the second qubit.

[0026] FIG. 3 is a schematic side view of substrates 102, 202 on which first superconducting materials 104, 204 and a sacrificial resist layer 402 are deposited according to an embodiment of the present invention. In the embodiment, the method further includes depositing a sacrificial layer 402 on the lower structures 100, 200. In the embodiment, the sacrificial layer includes titanium (Ti), titanium nitride (TiN), or tantalum (Ta), or any combination thereof. In the embodiment, depositing the sacrificial layer 402 includes sputtering a superconducting sacrificial material on the lower structures 100, 200. In the embodiment, the sacrificial layer 402 is deposited on the first superconducting material layers 104, 204 and on the substrates 102, 202 within the grooves 106A, 106B, 206A, 206B, 206C, 206D.

[0027] In the embodiment, the method includes electrically connecting first portions 104A, 204A and second portions 104B, 204B of the first superconducting materials 104, 204 to a strip 502 of a second superconducting material 404, the second superconducting material 404 being different from the first superconducting materials 104, 204. FIG. 4A is a schematic top view of lower structures 100, 200 on which a strip 502 of a second superconducting material 404 is formed over third portions 104C, 204C according to an embodiment of the present invention.

[0028] In an embodiment, as shown in FIG. 3, a second superconducting material 404 is formed on a sacrificial layer 402. The second superconducting material 404 is different from the first superconducting materials 104 and 204. In an embodiment, as shown in FIG. 4A, forming the second superconducting material 404 on the lower structures 100 and 200 includes sputtering a superconducting material 404 that has been subjected to compressive stress to form a strip 502 of the second superconducting material 404. In an embodiment, as shown in FIG. 4A, the strip 502 crosses or intersects the third portions 104C and 204C. The strip 502 is shown intersecting the third portions 104C and 204C at substantially a right angle, but of course, the strip 502 can be made to intersect the third portions 104C and 204C at any other angle. In an embodiment, electrically connecting the first portions 104A and 204A and the second portions 104B and 204B of the first superconducting materials 104 and 204 to the strip 502 of the second superconducting material 404 includes attaching a first base pad 502A at an end of the strip 502 to the first portions 104A and 204A, and attaching a second base pad 502B at an end of the strip 502 to the second portions 104B and 204B.

[0029] FIG. 4B is a schematic top view of lower structures 100, 200 having formed thereon a strip 503 of a second superconducting material 404 that electrically connects a fourth portion 204D and a fifth portion 204E over a third portion 204C, as described in an embodiment of the present invention. In an embodiment, the method can also include electrically connecting a fourth portion 204D and a fifth portion 204E of a first superconducting material 204 to the strip 503 of a second superconducting material 404, where the second superconducting material 404 is different from the first superconducting material 204. In an embodiment, electrically connecting a fourth portion 204D and a fifth portion 204E of the first superconducting material 204 to the strip 503 of the second superconducting material 404 includes, for example, attaching a first base pad 503A at an end of the strip 503 to the fourth portion 204D and attaching a second base pad 503B at an end of the strip 503 to the fifth portion 204E so as to form a line for transmitting an electromagnetic signal between the qubit. In this case, the strip 503 crosses the first portion 204A, the second portion 204B, and the third portion 204C, as shown in FIG. 4B. In an embodiment, the strip 503 of the second superconducting material 404 includes a strip of a porous second superconducting material 404.

[0030] In an embodiment, the method includes removing a portion of the sacrificial layer 402 formed on the lower structures 100, 200 so as to form a bridge structure 602 that includes strips 502, 503 of a second superconducting material 404 over a third portion 104C, 204C between the first portions 104A, 204A and the second portions 104B, 204B. FIG. 5 is a schematic side view of the lower structures 100, 200 with the bridge structure 602 including the strips 502, 503 formed thereon, as described in an embodiment of the present invention. In an embodiment, a portion of the sacrificial layer 402 near and below the strips 502, 503 of the bridge structure 602 is removed so as to form a space or gap "G" between the bridge structure 602 of the first superconducting materials 104, 204 and the third portions 104C, 204C. In an embodiment, the gap "G" can be in the range of, for example, about 2 μm to 4 μm. In an embodiment, the bridge structure 602 passes over the third portions 104C, 204C of the first superconducting materials 104, 204. The bridge structure 602 electrically connects the first portions 104A, 204A to the second portions 104B, 204B, but does not electrically connect the third portions 104C, 204C to the first portions 104A, 204A and does not electrically connect the third portions 104C, 204C to the second portions 104B, 204B.

[0031] In an embodiment, removing a portion of the sacrificial layer 402 formed on the lower structures 100, 200 includes etching a portion of the sacrificial layer 402 under the strip 502 to form a gap "G" between the strip 502 and the third portions 104C, 204C of the first superconducting materials 104, 204, and to define a bridge structure 602 over the third portions 104C, 204C of the first superconducting materials 104, 204. In an embodiment, etching a portion of the sacrificial layer 402 under the strip 502 includes etching the sacrificial layer 402 under the strips 502, 503 formed over the third portions 104C, 204C, etching the sacrificial layer formed in the first grooves 106A, 206A and the second grooves 106B, 206B that separate the first portions 104A, 204A, the second portions 104B, 204B, and the third portions 104C, 204C, and not etching the sacrificial layer 402 at both ends of the strips 502, 503 including under the base pads 502A, 503A and 502B, 503B. In an embodiment, etching a portion of the sacrificial layer 402 under the strips 502, 503 includes etching a portion of the sacrificial layer 402 using an acid etchant.

[0032] FIG. 6 is a scanning electron microscope (SEM) image showing a bridge structure 602 including a strip 502 and base pads 502A and 502B, as described in an embodiment of the present invention. The strip 502 of the bridge structure 602 is made of a superconducting material 404 that is porous. The porosity of the strip 502 enables chemical etching of the sacrificial layer 402 under the strip 502 by allowing a fluid chemical etchant to reach the sacrificial layer 402 in order to form the bridge structure 602.

[0033] FIG. 7 is a scanning electron microscope (SEM) image showing a plurality of bridge structures 602 arranged along the length of an electromagnetic transmission line, as described in an embodiment of the present invention. Each bridge structure 602 electrically connects a first portion 104A, 204A of a first superconducting material 104, 204 (e.g., connected to a ground potential) to a second portion 104B, 204B while passing over a third portion 104C, 204C of the first superconducting material 104, 204. The third portions 104C, 204C form part of a transmission line for carrying an electromagnetic signal, for example, between qubits. In an embodiment, the spacing between the plurality of bridge structures 602 can be regularly spaced to reduce or substantially eliminate spurious modes, which may otherwise occur.

[0034] FIG. 8A is a schematic side view of a substrate 802 having a sacrificial layer 804 deposited thereon, as described in another embodiment of the present invention. FIG. 8B is a schematic top view of the substrate 802 having an etched sacrificial layer 804 deposited thereon, as described in another embodiment of the present invention. FIG. 8C is a schematic cross-sectional view of the substrate 802 having the etched sacrificial layer 804 deposited thereon, taken along line 8C-8C shown in FIG. 8B, as described in another embodiment of the present invention.

[0035] Similar to the method of manufacturing the bridge structure described above, the following method also includes providing a substrate 802 with a layer of sacrificial material layer 804 formed thereon, as shown in FIG. 8A. In an embodiment, the substrate 802 can be, for example, silicon or sapphire. In an embodiment, the sacrificial material layer 804 can be, for example, an oxide. However, other materials can also be used. The substrate 802 has a surface 802A. This method includes depositing a layer of sacrificial material 804 on the surface 802A of the substrate 802, as shown in FIG. 8A. In an embodiment, depositing the sacrificial material layer 804 on the substrate 802 includes sputtering a superconducting sacrificial material 804 on the surface 802A of the substrate 802. This method includes selectively etching the layer of sacrificial material 804 to form spaced-apart portions 804A and 804B of the sacrificial material 804, as shown in the top view of FIG. 8B and the cross-sectional view of FIG. 8C.

[0036] This method further includes selectively etching the substrate 802 except for the first portion 804A and the second portion 804B of the sacrificial material 804. FIG. 8D is a schematic cross-sectional view of the substrate 802 including the etched sacrificial material layer 804 and the selectively etched substrate 802, as described in another embodiment of the present invention. As shown in FIG. 8D, after etching the substrate 802, the first and second portions 804A and 804B are located on two respective protrusions 812A and 812B of the substrate 802.

[0037] This method further includes depositing a layer of a first superconducting material 806 over an etched substrate 802 and over first and second portions 804A and 804B of a sacrificial material 804, as shown in FIGS. 9A and 9B. In an embodiment, the first superconducting material can be, for example, niobium (Nb). However, other superconducting materials can also be used. FIG. 9A is a schematic top view of a substrate 802 with a layer of a first superconducting material 806 further deposited thereon, as described in another embodiment of the present invention. FIG. 9B is a schematic cross-sectional view of the substrate 802 with a layer of a first superconducting material 806 further deposited thereon, taken along line 9B-9B shown in FIG. 9A. As shown in FIG. 9B, in an embodiment, two protrusions 806A and 806B within the layer of the first superconducting material 806 are formed over respective protrusions 812A and 812B of the substrate 802, and first and second portions 804A and 804B of the sacrificial material 804 are deposited over the protrusions 812A and 812B.

[0038] This method also includes removing the layer of the first superconducting material 806, as well as the first portion 804A and the second portion 804B of the sacrificial material 804, to obtain a layer of the first superconducting material 806 divided into a first portion 1002A, a second portion 1002B, and a third portion 1002C that are electrically insulated from each other by the substrate material 802, as shown in FIGS. 10A and 10B. FIG. 10A is a schematic top view of a substrate 802 having a layer of the first superconducting material 806 divided into a first portion 1002A, a second portion 1002B, and a third portion 1002C, as described in an embodiment of the present invention, formed thereon. As shown in FIG. 10A, the layer of the first superconducting material 806 and the first portion 804A and the second portion 804B of the sacrificial material 804 are removed up to the line 10B-10B shown in FIG. 9B to obtain a layer of the first superconducting material 806 divided into a first portion 1002A, a second portion 1002B, and a third portion 1002C. In an embodiment, the layer of the first superconducting material 806 is removed by using, for example, chemical or mechanical polishing, also called a damascene process, in which the superconducting material 806 is planarized. After this removal, the first portion 804A and the second portion 804B of the sacrificial material 804 are subsequently removed using, for example, a wet or dry etching process.

[0039] The first portion 1002A, the second portion 1002B, and the third portion 1002C are electrically insulated from each other by the protrusions 812A and 812B of the substrate 802, as shown in FIG. 10B. FIG. 10B is a schematic cross-sectional view of a substrate 802 having a layer of the first superconducting material 806 divided into a first portion 1002A, a second portion 1002B, and a third portion 1002C that are electrically insulated from each other by the protrusions 812A and 812B of the substrate 802, as described in an embodiment of the present invention, formed thereon, and this cross-section is taken along the line 10B-10B shown in FIG. 10A. The resulting lower structure 1000 shown in FIG. 10B includes the substrate 802 having a layer of the first superconducting material 806 divided into a first portion 1002A, a second portion 1002B, and a third portion 1002C formed thereon.

[0040] Following the above steps, to obtain the bridge structure, steps similar to those described above are performed with reference to FIGS. 1A-5. FIG. 11 is a schematic cross-sectional view of a bridge structure 1100 formed on a lower structure 1000 according to another embodiment of the present invention. To obtain the bridge structure 1100, the method further includes depositing a superconducting sacrificial layer 1102 on the lower structure 1000. In an embodiment, the sacrificial superconducting layer can be made of, for example, titanium (Ti), titanium nitride (TiN), or tantalum (Ta), or any combination thereof. In an embodiment, depositing the sacrificial layer 1102 includes sputtering a superconducting sacrificial material on the lower structure 1000. In an embodiment, the superconducting sacrificial layer 1102 is deposited on the first superconducting material layer 806 and on the protrusions 821A and 812B of the substrate 802.

[0041] In an embodiment, the method includes electrically connecting a first portion 1002A and a second portion 1002B of the first superconducting material 806 to a strip 1104 of a second superconducting material 1106, the second superconducting material 1106 being different from the first superconducting material 806.

[0042] In an embodiment, the second superconducting material 1106 is deposited on the superconducting sacrificial layer 1102. In an embodiment, as shown in FIG. 11, depositing the second superconducting material 1106 on the lower structure 1000 includes sputtering a superconducting material under compressive stress to form a strip 1104 of the second superconducting material 1106. In an embodiment, as shown in FIG. 11, the strip 1104 crosses a third portion 1002C. In an embodiment, electrically connecting the first portion 1002A and the second portion 1002B of the first superconducting material 806 to the strip 1104 of the second superconducting material 1106 includes attaching a first base pad 1104A of the strip 1104 to the first portion 1002A and attaching a second base pad 1104B of the strip 1104 to the second portion 1002B.

[0043] In an embodiment, this method includes removing a portion of the superconducting sacrificial layer 1102 formed on the lower structure 1000 so as to form a bridge structure 1100 including a strip 1104 of a second superconducting material 1106 on a third portion 1002C between a first portion 1002A and a second portion 1002B. In an embodiment, a portion of the superconducting sacrificial layer 1102 near and below the strip 1104 of the bridge structure 1100 is removed so as to form a space or gap "G" between the bridge structure 1104 of the first superconducting material 806 and the third portion 1002C. In an embodiment, the gap "G" can be, for example, in the range of about 2 μm to 4 μm. In an embodiment, the bridge structure 1100 passes over the third portion 1002C of the first superconducting material 806. The bridge structure 1100 electrically connects the first portion 1002A to the second portion 1102B, but does not electrically connect the third portion 1002C to the first portion 1002A and does not electrically connect the third portion 1002C to the second portion 1002B.

[0044] In an embodiment, as shown in FIG. 11, removing a portion of the superconducting sacrificial layer 1102 formed on the lower structure 1000 includes etching a portion of the superconducting sacrificial layer 1102 under the strip 1104 so as to form a gap "G" between the strip 1104 and the third portion 1002C of the first superconducting material 806 and to define the bridge structure 1100 over the third portion 1002C of the first superconducting material 806. In an embodiment, etching a portion of the superconducting sacrificial layer 1102 under the strip 1104 includes etching the sacrificial layer 1102 under the strip 1104 formed on the third portion 1002C, but not etching the sacrificial layer 1102 at both ends of the strip 1104 including under the base pads 1104A and 1104B. In an embodiment, etching a portion of the superconducting sacrificial layer 1102 under the strip 1104 includes, for example, etching a portion of the sacrificial layer 1102 using acid etching.

[0045] As can be understood from the above paragraphs, a quantum mechanical device is provided. The SEM images of FIGS. 5 and 11, and FIGS. 6 and 7 schematically show the components of the quantum mechanical device. In an embodiment, the quantum mechanical device includes substrates 102, 202, 802. The quantum mechanical device also includes layers of a first superconducting material 104, 204, 806 formed on the substrates 102, 202, 802. The layers of the first superconducting material 104, 204, 806 are divided into a first portion 104A, 204A, 1002A, a second portion 104B, 204B, 1002B, and a third portion 104C, 204C, 1002C that are electrically insulated from each other. The quantum mechanical device also includes a bridge structure 602, 1100 on the third portion 104C, 204C, 1002C located between the first portion 104A, 204A, 1002A and the second portion 104B, 204B, 1002B and connected to the first portion 104A, 204A, 1002A and the second portion 104B, 204B, 1002B. The bridge structure 602, 1100 includes strips 502, 1104 of a second superconducting material 404, 1106 configured to electrically connect the first portion 104A, 204A, 1002A and the second portion 104B, 204B, 1002B of the first superconducting material 104, 204, 806. The second superconducting material 404, 1106 of the strips 502, 1104 is different from the first superconducting material 104, 204, 806.

[0046] In an embodiment, the second superconducting material 404, 1106 of the strips 502, 1104 is porous at least in a portion crossing the third portion 104C, 204C, 1002C of the layer of the first superconducting material 104, 204, 806. In an embodiment, the bridge structure 602, 1100 is configured to substantially remove spurious modes of a planar microwave circuit.

[0047] In an embodiment, as shown, for example, in FIG. 7, a quantum mechanical device includes a plurality of regularly spaced bridge structures 602, 1100 for electrically connecting first portions 104A, 204A, 1002A and second portions 104B, 204B, 1002B of layers of a first superconducting material 104, 204, 806 at a plurality of locations of the layers of the first superconducting material 104, 204, 806. In an embodiment, the plurality of bridge structures 602, 1100 are configured to connect the first portions 104A, 204A, 1002A and the second portions 104B, 204B, 1002B of the layers of the first superconducting material 104, 204, 806 to the same ground potential.

[0048] The description of the various embodiments of the present invention has been presented for purposes of illustration but is not intended to be exhaustive or to be limited to the disclosed embodiments. 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 terms used herein have been chosen to best explain the principles of the embodiments, the practical application, or a technical improvement over technologies found in the marketplace, or to enable other practitioners of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

Claim 1 A method for manufacturing a bridge structure in a quantum mechanical device, comprising: providing a lower structure including a substrate having formed thereon a layer of a first superconducting material divided into a first portion, a second portion, and a third portion that are electrically insulated from each other; depositing a sacrificial layer on the lower structure including the first portion and the second portion of the first superconducting material; electrically connecting the first portion and the second portion of the first superconducting material to a strip of a second superconducting material, the second superconducting material being different from the first superconducting material; removing a portion of the sacrificial layer deposited on the lower structure so as to form a bridge structure including the strip of the second superconducting material over the third portion between the first portion and the second portion, the bridge structure electrically connecting the first portion to the second portion but not electrically connecting the third portion to the first portion and not electrically connecting the third portion to the second portion; removing the portion of the sacrificial layer deposited on the substrate forms a gap between the strip and the third portion of the first superconducting material, and etching a portion of the sacrificial layer under the strip so as to define the bridge structure over the third portion of the first superconducting material; etching the sacrificial layer under the strip deposited over the third portion, etching the sacrificial layer deposited in first and second grooves separating the first, second, and third portions, and not etching the sacrificial layer at both ends of the strip; A method. Claim 2 The method according to claim 1, wherein providing the lower structure includes providing the substrate having a surface and depositing the layer of the first superconducting material on the surface of the substrate. Claim 3 Providing the lower structure comprises etching the layer of the first superconducting material to form a first groove and a second groove, and the first part, the second part, and the third part of the layer of the first superconducting material are spaced apart from each other by the etched first groove and the etched second groove, and further comprises etching the layer of the first superconducting material to form a first groove and a second groove to define the first part, the second part, and the third part of the layer of the first superconducting material, the method according to claim 2.

4. The method according to any one of claims 1 to 3, wherein the substrate comprises silicon or sapphire.

5. The method according to any one of claims 1 to 4, wherein the first superconducting material comprises niobium or aluminum.

6. The method according to any one of claims 1 to 5, wherein the sacrificial layer comprises titanium (Ti), titanium nitride (TiN), or tantalum (Ta), or any combination thereof.

7. The method according to any one of claims 1 to 6, wherein forming the sacrificial layer on the lower structure comprises sputtering a superconducting sacrificial material on the lower structure.

8. The method according to any one of claims 1 to 7, wherein electrically connecting the first part and the second part of the first superconducting material to the strip of the second superconducting material comprises sputtering a superconducting material under compressive stress to form the strip of the second superconducting material.

9. The method according to any one of claims 1 to 8, wherein electrically connecting the first part and the second part of the first superconducting material to the strip of the second superconducting material comprises attaching a first base pad of the strip to the first part and attaching a second base pad of the strip to the second part.

10. The method according to any one of claims 1 to 9, wherein electrically connecting the first part and the second part of the first superconducting material to the strip of the second superconducting material comprises electrically connecting the first part and the second part of the first superconducting material to a strip of a porous second superconducting material.

11. Etching a portion of the sacrificial layer under the strip includes etching the portion of the sacrificial layer using acid etching, the method according to claim 10.

12. Providing the lower structure includes providing the substrate having a surface, forming a layer of sacrificial material on the surface of the substrate, selectively etching the layer of sacrificial material to form spaced-apart first and second portions of the sacrificial material, selectively etching the substrate except for the first and second portions of the sacrificial material, forming a layer of the first superconducting material on the etched substrate and the first and second portions of the sacrificial material, removing the formed layer of the first superconducting material and the first and second portions of the sacrificial material to obtain a layer of the first superconducting material divided into a first portion, a second portion, and a third portion electrically insulated from each other by substrate material, the method according to any one of claims 1 to 11.

13. Forming the sacrificial layer on the lower structure includes sputtering a superconducting sacrificial material on the layer of the first superconducting material, the method according to claim 12.

14. Removing a portion of the sacrificial layer formed on the substrate includes forming a gap between the strip and the third portion of the first superconducting material, and etching a portion of the sacrificial layer under the strip and etching the substrate material separating the first, second, and third portions of the superconducting material so as to define the bridge structure on the third portion of the first superconducting material, the method according to claim 13.

15. A quantum mechanical device comprising a substrate, a layer of a first superconducting material formed on the substrate, the layer being divided into a first portion, a second portion, and a third portion electrically insulated from each other, a bridge structure connected to the first portion and the second portion over the third portion located between the first portion and the second portion, the bridge structure comprising a strip of a second superconducting material configured to electrically connect the first portion and the second portion of the first superconducting material, the bridge structure A quantum mechanical device in which the second superconducting material of the strip is different from the first superconducting material and is porous at least in a portion crossing the third portion of the layer of the first superconducting material.

16. The quantum mechanical device according to claim 15, wherein the first portion and the second portion of the first superconducting material are connected to the same ground potential.

17. The quantum mechanical device according to claim 15 or 16, wherein the third portion of the first superconducting material is a signal line configured to carry an electromagnetic signal to and from a qubit.

18. The quantum mechanical device according to any one of claims 15 to 17, wherein the bridge structure is configured to substantially remove spurious modes of a planar microwave circuit.

19. The layer of the first superconducting material further includes a fourth portion and a fifth portion that are electrically insulated from the first portion, the second portion, and the third portion of the layer of the first superconducting material, and the fourth portion and the fifth portion are first and second signal lines configured to carry a first electromagnetic signal to and from a first qubit, and the third portion is a third signal line configured to carry a second electromagnetic signal to and from a second qubit. The quantum mechanical device according to any one of claims 15 to 18.

20. The quantum mechanical device according to any one of claims 15 to 19, further including a plurality of regularly spaced bridge structures for electrically connecting the first portion and the second portion of the layer of the first superconducting material at a plurality of positions of the layer of the first superconducting material.

21. The quantum mechanical device according to claim 20, wherein the plurality of bridge structures are configured to connect the first portion and the second portion of the layer of the first superconducting material to the same ground potential.

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