Three-dimensional niobium coaxial cavity with 0.1-second lifetime

We report on the internal quality factor of a three-dimensional niobium quarter-wave coaxial cavity that has undergone mid-temperature annealing, exhibiting <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msub> <a:mi>Q</a:mi> <a:mi>int</a:mi> </a:msub> <a:mo>≳</a:mo> <a:mn>3</a:mn> <a:mo>×</a:mo> <a:msup> <a:mn>10</a:mn> <a:mn>9</a:mn> </a:msup> </a:math> at the single-photon level below 20 mK, which corresponds to an internal photon lifetime of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msub> <c:mi>τ</c:mi> <c:mi>int</c:mi> </c:msub> <c:mo>∼</c:mo> <c:mn>90</c:mn> <c:mspace width="0.2em"/> <c:mi>ms</c:mi> </c:math> . Moreover, <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:msub> <f:mi>Q</f:mi> <f:mi>int</f:mi> </f:msub> </f:math> of the mid-temperature-annealed cavities remains almost unchanged even after several cooldown cycles and air exposure. These results suggest that stable low-loss niobium oxides are formed by mid-temperature annealing on the surface of a three-dimensional niobium cavity. This surface treatment could be applicable to the fabrication of two-dimensional superconducting circuits and help improve the lifetime of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mi>Nb</h:mi> </h:math> -based superconducting qubits.