Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
MIT Lincoln Laboratory, 244 Wood Street, Lexington, Massachusetts 02420, USA.
Phys Rev Lett. 2018 Jun 29;120(26):260504. doi: 10.1103/PhysRevLett.120.260504.
In the cavity-QED architecture, photon number fluctuations from residual cavity photons cause qubit dephasing due to the ac Stark effect. These unwanted photons originate from a variety of sources, such as thermal radiation, leftover measurement photons, and cross talk. Using a capacitively shunted flux qubit coupled to a transmission line cavity, we demonstrate a method that identifies and distinguishes coherent and thermal photons based on noise-spectral reconstruction from time-domain spin-locking relaxometry. Using these measurements, we attribute the limiting dephasing source in our system to thermal photons rather than coherent photons. By improving the cryogenic attenuation on lines leading to the cavity, we successfully suppress residual thermal photons and achieve T_{1}-limited spin-echo decay time. The spin-locking noise-spectroscopy technique allows broad frequency access and readily applies to other qubit modalities for identifying general asymmetric nonclassical noise spectra.
在腔 QED 体系中，由于交流斯塔克效应，残余腔光子的光子数涨落会导致qubit 退相。这些不需要的光子来自各种来源，如热辐射、剩余的测量光子和串扰。我们使用电容分流通量量子比特与传输线腔耦合，展示了一种基于时域自旋锁定弛豫测量从噪声谱重建来识别和区分相干和热光子的方法。使用这些测量，我们将系统中限制退相的源归因于热光子而不是相干光子。通过改善通向腔的线的低温衰减，我们成功地抑制了残余热光子，并实现了 T_1 限制的自旋回波衰减时间。自旋锁定噪声谱技术允许广泛的频率访问，并易于应用于其他量子比特模式，以识别一般的非对称非经典噪声谱。
