Superconducting qubits have been used in the most advanced demonstrations of quantum information processing, and they can be manufactured at-scale using proven semiconductor techniques.This makes them one of the leading technologies in the race to demonstrate useful quantum computers. Since their initial demonstration, advances in design, fabrication, and materials have extended the timescales over which fragile quantum information can be stored and manipulated on superconducting qubits. Ubiquitous atomic-scale material defects have been identified as a primary cause of qubit energy-loss and decoherence. Here we study transmon qubits that exhibit energy relaxation times exceeding 2.5 ms. Even at these long timescales, our qubit energy loss is dominated by two level systems (TLS). We observe large variations in these energy-loss times that would make it extremely difficult to accurately evaluate and compare qubit fabrication processes and to perform studies that require precise measurements of energy loss. To address this issue, we present a technique for characterizing qubit quality factor. In this method, we apply a slowly varying electric field to TLS near the qubit to stabilize the measured energy relaxation time, enabling us to replace hundreds of hours of measurements with ones that span several minutes.
We have demonstrated a novel type of superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-elementtransmon (MET) potentially offers a smaller footprint and simpler fabrication than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces relative electric field participation from other interfaces. By combining micrometer-scale Al/AlOx/Al junctions with long oxidations and novel processing, we have produced functional devices with EJ/EC in the low transmon regime (EJ/EC ≲30). Cryogenic I-V measurements show sharp dI/dV structure with low sub-gap conduction. Qubit spectroscopy of tunable versions show a small number of avoided level crossings, suggesting the presence of two-level systems (TLS). We have observed mean T1 times typically in the range of 10-90 microseconds, with some annealed devices exhibiting T1 > 100 microseconds over several hours. The results suggest that energy relaxation in conventional, small-junction transmons is not limited by junction loss.