库珀对盒谱中的反常避免能级交叉
Anomalous avoided level crossings in a Cooper-pair box spectrum
摘要：
我们在Al/AlOx/Al库珀对盒（CPB）的激发态光谱中观察到了一些明显的反常的避免能级交叉和电压依赖跃迁。该器件在15 - 50 GHz频率范围内测量为40 mK。我们发现，一个给定的水平交叉CPB频谱在两个不同的栅极电压，两个交叉点的频率和分裂的大小不同，分裂的大小取决于约瑟夫森能量的CPB。我们表明，这种行为不仅是一致的CPB耦合到离散带电的“两级”量子系统，移动原子的距离在CPB结，但光谱提供了新的信息的波动，这是不可避免的异常水平的相位量子比特光谱。特别是通过拟合模型哈密顿量，我们的数据，我们提取每个波动的微观参数，包括以及不对称性，隧穿振幅，和每个波动的最小跳跃距离。隧穿率的范围从小于3.5到13 GHz，这代表了5%和150%之间的良好的不对称性的值，和偶极矩给出了0.3至0.8埃的最小跳跃距离。我们还发现，这些离散的二能级系统对CPB的量子态的弛豫时间（T1）有显著的影响，因此可以成为超导量子比特的耗散源。
英文摘要：
We have observed a few distinct anomalous avoided level crossings and voltage dependent transitions in the excited state spectrum of an Al/AlOx/Al Cooper-pair box (CPB). The device was measured at 40 mK in the 15 - 50 GHz frequency range. We find that a given level crosses the CPB spectrum at two different gate voltages; the frequency and splitting size of the two crossings differ and the splitting size depends on the Josephson energy of the CPB. We show that this behavior is not only consistent with the CPB being coupled to discrete charged "two-level" quantum systems which move atomic distances in the CPB junctions but that the spectra provide new information about the fluctuators, which is not available from phase qubit spectra of anomalous avoided levels. In particular by fitting a model Hamiltonian to our data, we extract microscopic parameters for each fluctuator, including well asymmetry, tunneling amplitude, and the minimum hopping distance for each fluctuator. The tunneling rates range from less than 3.5 to 13 GHz, which represent values between 5% and 150% of the well asymmetry, and the dipole moments give a minimum hopping distance of 0.3 to 0.8 Anstrom. We have also found that these discrete two-level systems have a pronounced effect on the relaxation time (T1) of the quantum states of the CPB and hence can be a source of dissipation for superconducting quantum bits.
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