1.College of Electronics and Information Engineering, Sichuan University, Chengdu 610065, China 2.AVIC Xi’an Automatic Control Research Institute, Xi’an 710065, China
Fund Project:Project supported by the National Basic Research Program of China (Grant No. 2011CBA00304) and the Key Program of the National Natural Science Foundation of China (Grant No. 60836001)
Received Date:27 May 2020
Accepted Date:16 July 2020
Available Online:17 November 2020
Published Online:05 December 2020
Abstract:The superconducting quantum bit(qubit) based on Josephson junction is a macroscopic artificial atom. The basic parameters of the artificial atom can be changed by micro and nano machining. The three-dimensional (3D) Transmon qubit is a kind of qubit with the longer decoherence time. It is coupled with a 3D superconducting cavity by means of capacitance. It is a man-made coupling system between atom and cavity field, which can verify the effects of atomic physics, quantum mechanics, quantum optics and cavity quantum electrodynamics. In this paper, transmon qubits are prepared by the double angle evaporation method, and coupled with aluminum based 3D superconducting resonator to form 3D transmon qubits. The basic parameters of 3D transmon are characterized at an ultra-low temperature of 10 mK. The 3D transmon parameters are EC = 348.74 MHz and EJ = 11.556 GHz. The coupling coefficient g2/Δ between qubit and the 3D cavity is 43 MHz, which is located in the dispersive regime. The first transition frequency of qubit is f01 = 9.2709 GHz, and the second transition frequency is f12 = 9.0100 GHz. The 3D resonator is made of the material 6061T6 aluminum, the loaded quality factor is 4.8 × 105, and the bare frequency of the resonator is 8.108 GHz. The Jaynes-Cummings readout method is used to find the optimal readout power to distinguish among the qubit in the ground state $ \left| {\rm{0}} \right\rangle $, qubit in the superposition state of $ \left| {\rm{0}} \right\rangle $ and $ \left| {\rm{1}} \right\rangle $, and qubit in the superposition state of $ \left| {\rm{0}} \right\rangle $, $ \left| {\rm{1}} \right\rangle $ and $ \left| {\rm{2}} \right\rangle $. Then, the Aulter-Townes splitting (ATS) experiment can be fulfilled in this system. Unlike the method given by Novikov et al. [Novikov S, Robinson J E, Keane Z K, et al. 2013 Phys. Rev. B88 060503], our method only needs to apply continuous microwave excitation signal to the qubit, and does not need to carry out precise timing test on the qubit, thus reducing the test complexity of observing ATS effect. The ATS effect in resonance and non-resonance regime are observed. In the resonance ATS experiment, in order to obtain the peak value and frequency of resonance peak, Lorentz curve can be used for fitting peaks, and the ATS curve of double peak can be fitted by adding two Lorentz curves together. In the non-resonance ATS experiment, the detection signal is scanned, and the ATS double peak will shift with the different coupling signal detuning, forming an anti-crossing structure. The two curves formed by crossing free structure give two eigenvalues of Hamiltonian. By solving the equation, the experimental results can also be found to be consistent with the theoretical results. Keywords:3-dimensional transmon/ Jaynes-Cummings readout method/ Aulter-Townes splitting
3.Jaynes-Cummings方法寻找最佳读出功率对3D Transmon量子比特的量子态读取常用的方法有色散读出法[6]以及Jaynes-Cummings读出法. 色散读出法给超导谐振腔施加的读出信号功率较低, Jaynes-Cummings方法基于三维超导谐振腔奇异的非线性效应, 可直接在谐振腔的本征模态${f_{\rm{C}}}$处进行读出. 这种读出方法主要基于三维超导谐振腔对量子比特状态的“继承性”. 3D Transmon的频域测试系统如图4所示, 总体的测试系统设计按照输入衰减、输出放大的原则进行, 这样能够提供最大的信噪比. 3D Transmon安装在10 mK级. 微波源与网络分析仪的输出端口1通过功分器进入稀释制冷机, 稀释制冷机在不同的温度区域安装了衰减器, 输入信号从室温环境进入到样品衰减了39 dB, 在输出端安装了两个隔离器用于隔离输出放大器的噪声对3D Transmon的影响, 在4 K和室温下分别进行了两级放大. 图 4 3D Transmon的频域测试系统 Figure4. Frequency domain measurement system of 3D Transmon.
用图4所示的测试系统对3D Transmon量子比特的频域特性进行了扫描, 得到图5所示的结果. 图 5 网络分析仪变功率扫描S21曲线 Figure5.S21 curve of variable power scanning of network analyzer.