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2023-002: Single Step Fabrication of Incorporating Constriction Josephson Junctions in Superconducting Qubits

Invention: 2023-002

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Superconducting qubit consists of one or more superconductor/insulator/superconductor Josephson junctions, which are typically formed from a thin film sandwich structure of aluminum, aluminum oxide, aluminum (Al/AlOx/Al). The most common and promising implementation is the transmon qubit, in which the Josephson junction is shunted by a large capacitor to minimize the sensitivity to charge noise. Although the Josephson junctions are most often based on aluminum, a superconductor with critical temperature (Tc) typically around 1.4 K, the shunting capacitors and the microwave resonators are often made from a different superconductor, including, for example, niobium, tantalum, and titanium nitride. The fabrication of a full superconducting qubit device typically involves at least two lithography steps. In the first step, the microwave resonator and/or the shunting capacitor are patterned and fabricated using photolithography or electron beam lithography. After that, an unusual lithography step is used to define and fabricate the Al/AlOx/Al sandwich structure that is coupled to the capacitor. Different methods have been developed for this, but generally involve depositing the top and bottom aluminum layers from two different angles relative to the substrate. After depositing the first aluminum layer, the structure is exposed to a controlled level of oxygen to form a thin, insulating aluminum oxide layer. This method for Josephson junction creation has been refined over many years and is a primary reason that aluminum is the choice for the qubit Josephson junction element. However, use of this non-standard, double-angle metallization step has made it challenging to scale up the fabrication of transmon qubits to the quantities that will be needed in larger-scale quantum computers. This invention describes superconducting qubit architecture(s) that can be fabricated in a greatly simplified technique that involves only standard lithographical patterning step.

Tags: quantum