Abstract
In this experiment, we fabricated the three-Josephson-junction(3JJ) superconducting °ux qubits with a large loop size(50¹m £ 70¹m) that the self-inductance(150pH) is much larger than previous studies from other groups(10pH). The reason for choosing a large inductance is to facilitate the on-chip bias scheme through the on-chip superconducting lines to control the qubit's energy levels and to increase the SQUID's readout sensitivity. The on-chip bias scheme we proposed[1] here is a ‾rst step of realizing scalability by utilizing the quantum engineering of Josephson devices. In this dissertation, we present several single qubit measurements at 50mK bath temper- ature. First, the quantum superposition of macroscopically distinct °ux states is observed by measuring the level anti-crossing energy spectrum. The measured tunneling splitting is 4GHz. This is an indirect prove of macroscopic quantum coherence(MQC). The single qubit manipulation is also demonstrated by measuring the Rabi oscillation which is driven by microwave photons. Through this measurements, we can de‾ne the pulse con‾gurations of ¼-pulse and ¼=2-pulse, or more speci‾cally, any single qubit unitary oper- ation. The measured Rabi decay time can vary from a few nanosecond to several tens of nanosecond depending on the qubit energy level spacing. A clear Rabi oscillation with 40ns decay time was observed around 10GHz energy level spacing. The life time of the excited state, or the relaxation time Tr, is about 150ns at 10GHz level spacing. The phase coherence time, or the dephasing time TÁ, is measured in several di®erent ways. One method is to measure the Ramsey fringe which is similar to free-induction decay(FID) in NMR system. The other one is to apply the spin echo technique to see the decay of echo signal. The former one gives the dephasing time 7ns and the latter one is 30ns. Both Tr and TÁ are much shorter than the estimation of the formula derived from spin-boson model.