We measured the intrawell energy relaxation time τ d between macroscopic quantum levels in the double well potential of a Nb persistent-current qubit. Interwell population transitions were generated by irradiating the qubit with microwaves. Zero population in the initial well was then observed due to a multi-level decay process in which the initial population relaxed to the lower energy levels during transitions. The qubit's decoherence time, determined from τ d , is longer than 20 µs, holding the promise of building a quantum computer with Nb-based superconducting qubits.

Measurements of thermal activation are made in a superconducting, niobium Persistent-Current (PC) qubit structure, which has two stable classical states of equal and opposite circulating current. The magnetization signal is read out by ramping the bias current of a DC SQUID. This ramping causes time-ordered measurements of the two states, where measurement of one state occurs before the other. This time-ordering results in an effective measurement time, which can be used to probe the thermal activation rate between the two states. Fitting the magnetization signal as a function of temperature and ramp time allows one to estimate a quality factor of 10 6 for our devices, a value favorable for the observation of long quantum coherence times at lower temperatures.

dc measurements are made in a superconducting, persistent current qubit structure with a time-ordered meter. The persistent-current qubit has a double-well potential, with the two minima corresponding to magnetization states of opposite sign. Macroscopic resonant tunneling between the two wells is observed at values of energy bias that correspond to the positions of the calculated quantum levels. The magnetometer, a superconducting quantum interference device, detects the state of the qubit in a time-ordered fashion, measuring one state before the other. This results in a different meter output depending on the initial state, providing different signatures of the energy levels for each tunneling direction.

We discuss an anodization technique for a Nb superconductive-electronics-fabrication process that results in an improvement in critical-current-density uniformity across a 150-mm-diameter wafer. We outline the anodization process and describe the metrology techniques used to determine the NbO thickness grown. In the work described, we performed critical current measurements on Josephson junctions distributed across a wafer. We then compared the uniformity of pairs of wafers, fabricated together, differing only in the presence or absence of the anodization step. The cross-wafer standard deviation of was typically 5% for anodized wafers but 15% for unanodized wafers. This difference in uniformity is suggestive of an in-process modification from an unknown cause that is blocked by the anodic oxide. It is interesting that small junctions do not see an improvement in uniformity-apparently the anodization improves only the uniformity and not the variation in junction size. Control of is important for all applications of superconductive electronics including quantum computation and rapid single-flux quantum (RSFQ) circuitry.

We present experiments to characterize a Nb persistent-current qubit as a two-state system. The magnetization signal from the qubit is read-out by a DC-SQUID with near single-shot efficiency. Experiments varying SQUID ramp-rate and temperature suggest thermal activation occurs between the two circulating current states. Such data can be used to fit the parameters of the system, in order to characterize its performance as a potential quantum bit.