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arXiv 1807.04849quant-ph
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超导量子比特腔衰减器

Cavity Attenuators for Superconducting Qubits

Z. Wang, S. Shankar, Z. K. Minev, P. Campagne-Ibarcq, A. Narla, M. H. Devoret

摘要:

在电路QED结构中,由读出腔中的剩余热光子引起的退相是限制量子比特相干时间的主要因素。这种数量级为$10^{-1}$ - $10^{-3}$的残余热粒子数被怀疑是由从谐振器的输入和输出端口撞击谐振器的噪声引起的。为了解决这个问题,我们设计并测试了一种新型的带通微波衰减器,它由一个耗散腔以及热化到混合室阶段的稀释制冷机。通过将这样的腔衰减器与容纳transmon量子比特的3D超导腔串联,我们已经可重复地测量了增加的量子比特相干时间。在基础温度下,通过哈恩回波实验,我们测量了两个量子比特在多个冷却时间内的$T_{2\mathrm{e}}/2T_1 = 1.0\,({+0.0}/{-0.1})$。通过噪声诱导的退相测量,我们得到了读出腔基模剩余光子布居的上界$2\times 10^{-4}$,据我们所知,这是迄今为止报道的最低值。这些结果验证了保护量子比特免受光子噪声影响的有效方法,该方法可以发展成为量子线路实验的标准技术。

英文摘要:

Dephasing induced by residual thermal photons in the readout resonator is a leading factor limiting the coherence times of qubits in the circuit QED architecture. This residual thermal population, of the order of $10^{-1}$--$10^{-3}$, is suspected to arise from noise impinging on the resonator from its input and output ports. To address this problem, we designed and tested a new type of band-pass microwave attenuator that consists of a dissipative cavity well thermalized to the mixing chamber stage of a dilution refrigerator. By adding such a cavity attenuator inline with a 3D superconducting cavity housing a transmon qubit, we have reproducibly measured increased qubit coherence times. At base temperature, through Hahn echo experiment, we measured $T_{2\mathrm{e}}/2T_1 = 1.0\,({+0.0}/{-0.1})$ for two qubits over multiple cooldowns. Through noise-induced dephasing measurement, we obtained an upper bound $2\times 10^{-4}$ on the residual photon population in the fundamental mode of the readout cavity, which to our knowledge is the lowest value reported so far. These results validate an effective method for protecting qubits against photon noise, which can be developed into a standard technology for quantum circuit experiments.

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