Impact of time-ordered measurements of the two states in a niobium superconducting qubit structure

Abstract

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 of macroscopic quantum levels in a superconducting qubit structure with a time-ordered meter

“…I c and ␣ can be determined experimentally from our previous thermal activation studies, which give ␣ϭ0.63 and I c ϭ1.2 A. 10 These values are within the range of estimated values from the process parameters.…”

“…In our previous experiments, 10 we observed the rate of thermal activation of the qubit's phase particle ͑the term for the wave function of the qubit's phase͒ above the barrier between the two wells. At low temperatures, thermal activation is insufficient to overcome this barrier within the measurement time scale when f q ϭ0.5.…”

“…10 In those experiments, we studied the classical, thermally driven regime of operation. In the present paper, we detail the effects of a time-ordered meter on the dc measurements of the PC qubit in the quantum regime.…”

Electromagnetically induced transparency in superconducting quantum circuits: Effects of decoherence, tunneling, and multilevel crosstalk

“…The relative depth of the two wells can be adjusted by detuning the flux bias to either side of the symmetric point f = 1/2. The potential wells exhibit quantized energy levels corresponding to the quantum states of the macroscopic circulating current [38,39,40,41], with the number of levels on each side determined by the depth and frequencies of the wells. In this basis the Hamiltonian can be written…”

Quantum computing with superconductors

“…The process is capable of yielding submicrometer Josephson junctions critical for the persistent-current qubit approach using i-line photolithography on 150-mm-diameter wafers [33]. Devices developed under this process have been used by multiple groups to realize superconductive quantum circuits [98], [99], [100].…”