Combined dolan bridge and quantum dot josephson junction in series

A method of producing a quantum circuit includes forming a mask on a substrate to cover a first portion of the substrate, implanting a second portion of the substrate with ions, and removing the mask, thereby providing a nanowire. The method further includes forming a first lead and a second lead, the first lead and the second lead each partially overlapping the nanowire. In operation, a portion of the nanowire between the first and second leads forms a quantum dot, thereby providing a quantum dot Josephson junction. The method further includes forming a third lead and a fourth lead, one of the third and fourth leads partially overlapping the nanowire, wherein the third lead is separated from the fourth lead by a dielectric layer, thereby providing a Dolan bridge Josephson junction. The nanowire is configured to connect the quantum dot Josephson junction and the Dolan bridge Josephson junction in series.

BACKGROUND

The present invention relates to Josephson junctions, and more specifically, to a Dolan bridge Josephson junction and a quantum dot Josephson junction connected in series.

Quantum computers require large numbers of qubits, and combining different types of qubits in a single quantum processor may be advantageous. For example, the ability to combine different types of qubits may have applications in frequency tuning, quantum memory, sensing of qubit states, error correction, and redundancy.

SUMMARY

According to an embodiment of the present invention, a method of producing a quantum circuit includes forming a mask on a substrate to cover a first portion of the substrate, implanting a second portion of the substrate not covered by the mask with ions, and removing the mask, thereby providing a nanowire comprising the first portion of the substrate. The method further includes forming a first lead and a second lead on top of the substrate, the first lead being spaced apart from the second lead, the first lead and the second lead each partially overlapping the nanowire, wherein, in operation, a portion of the nanowire between the first and second leads forms a quantum dot, thereby providing a quantum dot Josephson junction. The method further includes forming a third lead and a fourth lead on top of the substrate, one of the third lead and the fourth lead partially overlapping the nanowire, wherein the third lead is separated from the fourth lead by a dielectric layer, thereby providing a Dolan bridge Josephson junction. The nanowire is configured to connect the quantum dot Josephson junction and the Dolan bridge Josephson junction in series.

According to an embodiment of the present invention, a quantum circuit includes a substrate, the substrate including a first portion forming a nanowire and a second portion surrounding the first portion. The quantum circuit includes a first lead and a second lead formed on top of the substrate, the first lead being spaced apart from the second lead, the first lead and the second lead each partially overlapping the nanowire, wherein, in operation, a portion of the nanowire between the first and second leads forms a quantum dot, thereby providing a quantum dot Josephson junction. The quantum circuit includes a third lead and a fourth lead formed on top of the substrate, one of the third lead and the fourth lead partially overlapping the nanowire, wherein the third lead is separated from the fourth lead by a dielectric layer, thereby providing a Dolan bridge Josephson junction. The nanowire is configured to connect the quantum dot Josephson junction and the Dolan bridge Josephson junction in series.

According to an embodiment of the present invention, a quantum computer includes a refrigeration system under vacuum comprising a containment vessel, and a qubit chip contained within a refrigerated vacuum environment defined by the containment vessel, wherein the qubit chip includes a quantum circuit. The quantum computer includes an electromagnetic waveguide arranged within the refrigerated vacuum environment so as to direct electromagnetic energy to and receive electromagnetic energy from the quantum circuit. The quantum circuit includes a substrate, the substrate including a first portion forming a nanowire and a second portion surrounding the first portion. The quantum circuit includes a first lead and a second lead formed on top of the substrate, the first lead being spaced apart from the second lead, the first lead and the second lead each partially overlapping the nanowire, wherein, in operation, a portion of the nanowire between the first and second leads forms a quantum dot, thereby providing a quantum dot Josephson junction. The quantum circuit includes a third lead and a fourth lead formed on top of the substrate, one of the third lead and the fourth lead partially overlapping the nanowire, wherein the third lead is separated from the fourth lead by a dielectric layer, thereby providing a Dolan bridge Josephson junction. The nanowire is configured to connect the quantum dot Josephson junction and the Dolan bridge Josephson junction in series.

DETAILED DESCRIPTION

FIG. 1is a flowchart that illustrates a method100of producing a quantum circuit according to an embodiment of the current invention. The method100includes forming a mask on a substrate to cover a first portion of the substrate102, and implanting a second portion of the substrate not covered by the mask with ions104. The method100includes removing the mask, thereby providing a nanowire comprising the first portion of the substrate106. The method100includes forming a first lead and a second lead on top of the substrate, the first lead being spaced apart from the second lead, the first lead and the second lead each partially overlapping the nanowire, wherein, in operation, a portion of the nanowire between the first and second leads forms a quantum dot, thereby providing a quantum dot Josephson junction108. The method100includes forming a third lead and a fourth lead on top of the substrate, one of the third lead and the fourth lead partially overlapping the nanowire, wherein the third lead is separated from the fourth lead by a dielectric layer, thereby providing a Dolan bridge Josephson junction110. The nanowire is configured to connect the quantum dot Josephson junction and the Dolan bridge Josephson junction in series.

According to an embodiment of the present invention, forming the first lead and the second lead may include forming the first lead and the second lead to be substantially perpendicular to the nanowire.

According to an embodiment of the present invention, forming the third lead and the fourth lead includes forming the third lead on the substrate such that the third lead partially overlaps the nanowire, oxidizing the third lead to form the dielectric layer, and forming the fourth lead in contact with the dielectric layer.

The third lead may extend substantially perpendicular to the fourth lead, and to the nanowire. However, the embodiments of the invention are not limited to this configuration.

According to an embodiment of the present invention, the method100further includes forming a nanowire source lead and a nanowire drain lead on the substrate. The nanowire source lead may be formed to overlap the nanowire at a first end of the nanowire, and the nanowire drain lead may be formed to overlap the nanowire at a second end of the nanowire opposing the first end.

According to an embodiment of the present invention, at least one of the forming the first and second leads and the forming the third and fourth leads includes lift off processing.

FIG. 2is a schematic illustration of a quantum circuit200according to an embodiment of the present invention. The quantum circuit200includes a substrate202. The substrate202includes a first portion forming a nanowire204and a second portion206surrounding the first portion. The quantum circuit200includes a first lead210and a second lead212formed on top of the substrate202. The first lead210is spaced apart from the second lead212, and the first lead210and the second lead212each partially overlap the nanowire204. In operation, a portion220of the nanowire204between the first lead210and the second lead212forms a quantum dot, thereby providing a quantum dot Josephson junction208. The quantum circuit200includes a third lead216and a fourth lead218formed on top of the substrate202. One of the third lead216and the fourth lead218partially overlaps the nanowire204. The third lead216is separated from the fourth lead218by a dielectric layer, thereby providing a Dolan bridge Josephson junction214. The nanowire204is configured to connect the quantum dot Josephson junction208and the Dolan bridge Josephson junction214in series.

According to an embodiment of the present invention, the first and second leads210,212of the quantum dot Josephson junction208extend substantially perpendicular to the nanowire204. However, embodiments of the invention are not limited to this configuration. The first and second leads210,212of the quantum dot Josephson junction208may have other orientations with respect to each other, and to the nanowire204.

According to an embodiment of the present invention, the first portion of the substrate202forming the nanowire204includes indium arsenide (InAs), and the second portion206of the substrate202includes InAs implanted with ions. The substrate may include other materials besides or in addition to InAs, for example, 3-5 materials, gallium arsenide (GaAs), or indium gallium arsenide (InGaAs). The ions may be, for example, hydrogen, oxygen, helium, or argon. The nanowire204according to an embodiment of the present invention has a width less than 50 nm, and a length between 100 nm and 1000 nm. The nanowire204according to an embodiment of the present invention has a length between 500 nm and 1000 nm.

According to an embodiment of the present invention, the third lead216of the Dolan bridge Josephson junction214extends substantially perpendicular to the fourth lead218. The third lead216of the Dolan bridge Josephson junction214may also extend substantially perpendicular to the nanowire204. However, embodiments of the invention are not limited to these orientations of the third lead216with respect to the fourth lead218and the nanowire204.

According to an embodiment of the present invention, the quantum circuit200includes a nanowire source lead222and a nanowire drain lead224formed on the substrate202. The nanowire source lead222overlaps the nanowire204at a first end of the nanowire204, and the nanowire drain lead224overlaps the nanowire204at a second end of the nanowire204. The nanowire source lead222and nanowire drain lead224can be run out to pads, and can contact the pads directly with wire bonding or other methods. The pads can be used to control the current through the nanowire to connect the quantum dot Josephson junction208and the Dolan bridge Josephson junction214in series.

FIGS. 3A-10Bare schematic illustrations of a process that can be used to form a quantum circuit according to an embodiment of the present invention. InFIGS. 3A-10B, like reference numerals refer to like features, for example, reference numeral300inFIG. 3B and 400inFIG. 4Bboth refer to a substrate. The process schematically illustrated inFIGS. 3-12may employ liftoff processing techniques.

FIGS. 3A and 3Bare schematic illustrations of a plan view and a cross-sectional view of a substrate300. The substrate300may include, for example, InAs. The substrate300may include a capping layer302. The process of forming a quantum circuit may include forming a nanowire in the substrate300. The process of forming the nanowire may include forming a photoresist304to cover a first portion306of the substrate300, and then implanting a second portion308of substrate300not covered by the mask304with ions, and removing the mask304. According to an embodiment of the present invention, the second portion308of the substrate300is implanted with helium or hydrogen ions. The photoresist304may have a size that is substantially the size of the nanowire to be formed. The photoresist304may be formed, for example, using a 193 expose tool and off-axis illumination, such as dipole illumination, or quadrupole illumination. The pattern may be formed in resist and used as such, or it may be etched into a hard mask, such as Si or Ti, and a reactive ion etching process can be used to shrink the size of the resist space, if needed, for example, from 70 nm to 50 nm or smaller.

FIGS. 4A and 4Bare schematic illustrations of a plan view and a cross-sectional view of a substrate400with a nanowire408formed therein. The first portion306of the substrate300inFIG. 3Bforms the nanowire408.

The process of forming a quantum circuit includes forming a quantum dot Josephson junction on the substrate.FIGS. 5A and 5Bare schematic illustrations of a plan view and a cross-sectional view of a substrate500with a liftoff mask510formed thereon. The liftoff mask510is patterned for formation of the quantum dot, as well as source and drain leads for the nanowire508. Patterning the resist510may include depositing the resist, exposing it, and developing it.

FIGS. 6A and 6Bare schematic illustrations of a plan view and a cross-sectional view of a substrate600with a liftoff mask610formed thereon, and a metal612deposited on the substrate600and liftoff mask610. The metal612may be deposited by evaporation, for example, which can be directional enough not to coat the sidewalls of the lift off stack. Alternatively, the metal612may be deposited by, for example, molecular-beam epitaxy, sputter deposition, or chemical vapor deposition with a directional ion control method. The deposition methods described herein are provided as examples, and the embodiments of the invention are not limited to these deposition methods. The metal612may be any metal used for quantum dot Josephson junction wiring, as long as it does not have sufficient stress to distort the liftoff mask610. The metal612may include, for example, aluminum, lead, titanium, tungsten, or vanadium. After deposition of the metal612, the process includes lifting off the liftoff mask610.

FIGS. 7A and 7Bare schematic illustrations of a plan view and a cross-sectional view of a substrate700with a leads of a quantum dot Josephson junction714formed thereon. The quantum dot Josephson junction714includes a first lead716and a second lead718spaced apart from the first lead716. The first lead716and the second lead718each partially overlap the nanowire708. A portion of the nanowire708between the first and second leads716,718of the quantum dot Josephson junction714forms a quantum dot720.

The process of forming a quantum circuit may include forming a nanowire source lead722and a nanowire drain lead724on the substrate700. The nanowire source722lead may be formed to overlap the nanowire708at a first end, and the nanowire drain lead724may be formed to overlap the nanowire708at a second end opposing the first end.

The process of forming a quantum circuit includes forming a Dolan bridge Josephson junction on top of the substrate.FIGS. 8A and 8Bare schematic illustrations of a plan view and a cross-sectional view of a substrate800with a liftoff mask having a first layer826and a second layer828formed thereon. The first layer826and the second layer828are patterned to exposed portions of the substrate800on which the Dolan bridge Josephson junction will be formed. The first layer826may include, for example, an organic polymer. The second layer828may include, for example, titanium or silicon. The first layer826and second layer828may be chosen such that etching exposes a portion of the substrate800that is larger than the area of the opening in the second layer828. For example, the first layer826and second layer828may be etched using reactive ion etching. The etching may etch the first layer826more quickly than the second layer828.

FIGS. 9A and 9Bare schematic illustrations of a plan view and a cross-sectional view of a substrate900with a metal layer930deposited on the second layer928of the liftoff mask. The metal layer930may include, for example, aluminum, lead, titanium, tungsten, vanadium, or niobium, for example, and the deposition method may be directional. The metal layer930according to an embodiment of the present invention may be deposited in two steps: 90 degree metal evaporation and 45 degree metal evaporation. The 90 degree evaporation results in a third lead932of the Dolan bridge Josephson junction. The 45 degree evaporation results in a fourth lead934. The third lead932may be exposed to oxygen prior to formation of the fourth lead934, forming an oxide layer between the third lead932and the fourth lead934. The oxide layer acts as the dielectric layer of the Dolan bridge Josephson junction. However, embodiments of the invention are not limited to the dielectric layer being an oxide layer. Alternative methods for forming the dielectric layer may be used. One of the third lead932and the fourth lead934partially overlaps the nanowire. As shown inFIG. 9A, the third lead932partially overlaps the nanowire.

FIGS. 10A and 10Bare schematic illustrations of a plan view and a cross-sectional view of the device ofFIGS. 9A and 9Bafter removal of the liftoff mask. The nanowire1008connects the quantum dot Josephson junction1014and the Dolan bridge Josephson junction1036in series.

FIG. 11is a schematic illustration of a quantum computer1100according to an embodiment of the present invention. The quantum computer1100includes a refrigeration system under vacuum including a containment vessel1102, and a qubit chip1104contained within a refrigerated vacuum environment defined by the containment vessel1102. The qubit chip1104includes a quantum circuit. The quantum computer1100includes an electromagnetic waveguide1106arranged within the refrigerated vacuum environment so as to direct electromagnetic energy to and receive electromagnetic energy from the quantum circuit. The quantum circuit includes a substrate, the substrate including a first portion forming a nanowire1108and a second portion1110surrounding the first portion.

The quantum circuit includes a first lead1114and a second lead1116formed on top of the substrate. The first lead1114is spaced apart from the second lead1116. The first lead and the second lead each partially overlap the nanowire1108. In operation, a portion of the nanowire1108between the first and second leads1114,1116forms a quantum dot, thereby providing a quantum dot Josephson junction1112.

The quantum circuit includes a third lead1120and a fourth lead1122formed on top of the substrate. One of the third lead1120and the fourth lead1122partially overlaps the nanowire1108. The third lead1120is separated from the fourth lead1122by a dielectric layer, thereby providing a Dolan bridge Josephson junction1118. The nanowire1108is configured to connect the quantum dot Josephson junction1112and the Dolan bridge Josephson junction1118in series.

According to an embodiment of the present invention, the first and second leads1114,1116of the quantum dot Josephson junction1112extend substantially perpendicular to the nanowire1108. However, embodiments of the invention are not limited to this configuration. The first and second leads1114,1116of the quantum dot Josephson junction1112may have other orientations with respect to each other, and to the nanowire1108.

According to an embodiment of the present invention, the first portion of the substrate forming the nanowire1108includes indium arsenide (InAs), and the second portion of the substrate includes InAs implanted with ions. The ions may include, for example, helium ions or hydrogen ions. The nanowire1108according to an embodiment of the present invention has a width less than 50 nm, and a length between 100 nm and 1000 nm. The nanowire1108according to an embodiment of the present invention has a length between 500 nm and 1000 nm.

According to an embodiment of the present invention, the third lead1120of the Dolan bridge Josephson junction1118extends substantially perpendicular to the fourth lead1122. The third lead1120of the Dolan bridge Josephson junction1118may also extend substantially perpendicular to the nanowire1108. However, embodiments of the invention are not limited to these orientations of the third lead1120with respect to the fourth lead1122and the nanowire1108.

According to an embodiment of the present invention, the quantum circuit200includes a nanowire source lead222and a nanowire drain lead224formed on the substrate202. The nanowire source lead222overlaps the nanowire204at a first end of the nanowire204, and the nanowire drain lead224overlaps the nanowire204at a second end of the nanowire204.

According to an embodiment of the present invention, method of producing a nanowire includes forming a mask on a substrate to cover a first portion of the substrate, implanting a second portion of the substrate not covered by the mask with ions, and removing the mask, thereby providing a nanowire comprising the first portion of the substrate. An example of the method of producing a nanowire is schematically illustrated inFIGS. 3A-4B. The ions according to an embodiment of the present invention may include helium ions or hydrogen ions.