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EP-4091206-B1 - MICROFABRICATED AIR BRIDGES FOR QUANTUM CIRCUITS

EP4091206B1EP 4091206 B1EP4091206 B1EP 4091206B1EP-4091206-B1

Inventors

  • ADIGA, VIVEKANANDA
  • YAN, HONGWEN
  • PAPALIA, JOHN
  • RATH, DAVID
  • PATEL, JYOTICA

Dates

Publication Date
20260513
Application Date
20201214

Claims (14)

  1. A method for fabricating a bridge structure in a quantum mechanical device, comprising: providing a substructure (100) comprising a substrate (102) having deposited thereon a layer of a first superconducting material (104) divided into a first portion (104A), a second portion (104B) and a third portion (104C) that are electrically insulated from each other; depositing a sacrificial layer (402) on the substructure (100); electrically connecting the first portion (104A) and the second portion (104B) of the first superconducting material with a strip (502) of a second superconducting material (404), the second superconducting material being different from the first superconducting material; and removing a portion of the sacrificial layer (402) deposited on the substructure (100) so as to form a bridge structure (602) with the strip of the second superconducting material over the third portion (104C) between the first portion and the second portion (104B), the bridge structure electrically connecting the first portion to the second portion (104B) while not electrically connecting the third portion (104C) to the first portion (104A) and not electrically connecting the third portion (104C) to the second portion (104B), wherein removing the portion of the sacrificial layer deposited on the substrate comprises etching a portion of the sacrificial layer under the strip so as to form a gap (G) between the strip and the third portion (104C) of the first superconducting material to define the bridge structure (602) over the third portion (104C) of the first superconducting material, and wherein etching the portion of the sacrificial layer under the strip comprises etching the sacrificial layer under the strip that is deposited on the third portion (104C) and etching the sacrificial layer deposited within a first trench (106A) and a second trench (106B) separating the first, second and third portion (104C) and not etching the sacrificial layer at both ends of the strip.
  2. The method according to claim 1, wherein the sacrificial layer comprises titanium, Ti, titanium nitride, TiN, or tantalum, Ta, or any combination thereof.
  3. The method according to any of the preceding claims, wherein depositing the sacrificial layer on the substructure (100) comprises sputtering superconducting sacrificial material on the substructure (100).
  4. The method according to any of the preceding claims, wherein electrically connecting the first portion (104A) and the second portion (104B) of the first superconducting material with the strip (502) of the second superconducting material comprises sputtering compressively stressed superconducting material to form the strip of the second superconducting material.
  5. The method according to any of the preceding claims, wherein electrically connecting the first portion (104A) and the second portion (104B) of the first superconducting material with the strip (502) of the second superconducting material comprises attaching a first base pad (502A) of the strip to the first portion and attaching a second base pad (502B) of the strip to the second portion (104B).
  6. The method according to any of the preceding claims, wherein electrically connecting the first portion (104A) and the second portion (104B) of the first superconducting material with the strip (502) of the second superconducting material comprises electrically connecting the first portion (104A) and the second portion (104B) of the first superconducting material with a strip of porous second superconducting material.
  7. The method according to claim 1, wherein etching the portion of the sacrificial layer under the strip comprises etching the portion of the sacrificial layer using an acid etch.
  8. A method for fabricating a bridge structure in a quantum mechanical device, comprising: providing a substructure (1000) comprising a substrate (802) having deposited thereon a layer of a 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; depositing a sacrificial layer (1102) on the substructure (1000); electrically connecting the first portion (1002A) and the second portion (1002B) of the first superconducting material (806) with a strip (1104) of a second superconducting material (1106), the second superconducting material being different from the first superconducting material; and removing a portion of the sacrificial layer (1102) deposited on the substructure (1000) so as to form a bridge structure (1100) with the strip of the second superconducting material over the third portion (1002C) between the first portion (1002A) and the second portion (1002B), the bridge structure electrically connecting the first portion (1002A) to the second portion (1002B) while not electrically connecting the third portion (1002C) to the first portion (1002A) and not electrically connecting the third portion (1002C) to the second portion (1002B), wherein providing the substructure (1000) comprises: providing the substrate (802) having a surface; depositing a layer of sacrificial material (804) on the surface (802A) of the substrate; selectively etching said layer of sacrificial material to form first (804A) and second (804B) spaced apart portions of the sacrificial material; selectively etching the substrate except at said first and second portions of the sacrificial material; depositing a layer of the first superconducting material (806) 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 sacrificial material to obtain a layer of the first superconducting material divided into the first portion (1002A), the second portion (1002B) and the third portion (1002C) that are electrically insulated from each other by substrate material.
  9. The method according to the preceding claim, wherein depositing the sacrificial layer on top of the substructure (1000) comprises sputtering superconducting sacrificial material on the layer of the first superconducting material.
  10. The method according to the preceding claim, wherein removing the portion of the sacrificial layer deposited on the substrate comprises etching a portion of the sacrificial layer under the strip and etching the substrate material separating the first (1002A), second (1002B) and third portion (1002C) of the superconducting material so as to form a gap (G) between the strip (1104) and the third portion (1002C) of the first superconducting material to define the bridge structure (1100) over the third portion (1002C) of the first superconducting material.
  11. A quantum mechanical device, comprising: a substrate (102); a layer of a first superconducting material (104) deposited on the substrate, said layer being divided into a first portion (104A), a second portion (104B) and a third portion (104C) that are electrically insulated from each other; and a bridge structure (602) connected to the first portion (104A) and the second portion (104B) over the third portion (104C) that is located between the first portion (104A) and the second portion (104B), the bridge structure comprising a strip (502) of a second superconducting material (404) configured to electrically connect the first portion (104A) and the second portion (104B) of the first superconducting material, wherein the second superconducting material of the strip is different from the first superconducting material, and wherein the first portion (104A) and the second portion (104B) of the layer of the first superconducting material are first and second signal lines configured to carry a first electromagnetic signal to or from a first qubit and the third portion (104C) is a third signal line configured to carry a second electromagnetic signal to or from a second qubit.
  12. The quantum mechanical device according to the preceding claim, wherein the second superconducting material of the strip is porous at least in a portion that traverses the third portion (104C) of the layer of the first superconducting material.
  13. The quantum mechanical device according to any of the two preceding claims, wherein the first portion (104A) and the second portion (104B) of the first superconducting material are connected to a same ground potential.
  14. The quantum mechanical device according to any of the three preceding claims, wherein the bridge structure is configured to substantially eliminate spurious modes of planar microwave circuits.

Description

BACKGROUND The currently claimed embodiments of the present invention relate to quantum circuits, and more specifically, to methods of fabricating a bridge structure in a quantum mechanical device and a quantum mechanical device having the bridge structure. Quantum computation is based on the reliable control of quantum bits (referred to herein throughout as qubits). The fundamental operations required to realize quantum algorithms are a set of single-qubit operations and two-qubit operations which establish correlations between two separate quantum bits. The realization of high fidelity two-qubit operations may be desirable both for reaching the error threshold for quantum computation and for reaching reliable quantum simulations. The superconducting quantum processor (having one or more superconducting qubits) includes superconducting metals (e.g., Al, Nb, etc.) on an insulating substrate (e.g., Si or high resistivity Si, Al2O3, etc.). The superconducting quantum processor is typically a planar two-dimensional lattice structure or circuit of individual qubits linked by a coupler in various lattice symmetry (for example, square, hexagonal, etc.), and a readout structure located on a flip-chip. The couplers can be made of a capacitor, a resonator, a coil or any microwave component that provides a coupling between qubits. Superconducting microwave circuits based on coplanar waveguides (CPW) are susceptible to parasitic slot-line modes. These modes can couple to elements of the circuit such as qubits and thus can lead to signal loss and decoherence. In order to eliminate these spurious modes, cross-over connections are usually made between ground planes that are interrupted by the coplanar waveguides (CPW). Free standing crossovers known as airbridges are conventionally used for that purpose. However, problems remain in the fabrication of these airbridges leading to less than desirable quantum mechanical devices or circuits. Y.J.Y. Lankwarden ET AL: Development of NbTiN-Al Direct Antenna Coupled Kinetic Inductance Detectors (J Low Temp Phys (2012) 167:367-372) relates to coplanar waveguide (CPW) Kinetic Inductance Detector consisting of Al and NbTiN, coupled at its shorted end to a planar antenna. US 2016/0322693 A1 relates to a superconducting airbridge on a structure wherein a first ground plane, resonator, and second ground plane that are formed on a substrate. SUMMARY An aspect of the present invention is to provide a method for fabricating a bridge structure in a quantum mechanical device. The method includes providing a substructure comprising a substrate having deposited 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. The method further includes depositing a sacrificial layer on the substructure, and electrically connecting the first portion and the second portion of the first superconducting material with 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 on the substructure so as to form a bridge structure with 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 while not electrically connecting the third portion to the first portion and not electrically connecting the third portion to the second portion. Furthermore, the method according to the invention includes the steps as defined in independent claims 1 and 8. In an embodiment, providing the substructure includes providing the substrate having a surface and depositing the layer of the first superconducting material on the surface of the substrate. In an embodiment, providing the substructure further includes etching the layer of the first superconducting material to form a first trench and a second trench so as to define the first portion, the second portion and the third portion of the layer of the first superconducting material such that the first portion, the second portion and the third portion of the layer of the first superconducting material are spaced apart from each other by the etched first trench and the etched second trench. 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. In an embodiment, depositing the sacrificial layer on the substructure includes sputtering superconducting sacrificial material on the substructure. In an embodiment, electrically connecting the first portion and the second portion of the first superconducting material with the strip of the second supercon