1.Quantum Physics and Quantum Information Division, Beijing Computational Science Research Center, Beijing 100193, China 2.Department of Physics, Zhejiang University, Hangzhou 310027, China 3.Center for Quantum Information Frontier Science, Institute of Microelectronics, Tsinghua University, Beijing 100084, China 4.Beijing Academy of Quantum Information, Beijing 100193, China
Abstract:In recent years, quantum Rabi model has aroused considerable interest because of its fundamental importance and potential applications in quantum technologies. For a conventional cavity-quantum-electrodynamic (cavity-QED) system involving the interaction between an atom and photons in a cavity, the atom-photon coupling frequency is much smaller than the transition frequency of the atom and the frequency of the cavity mode. This cavity-QED system is usually described by the Jaynes-Cummings model in which the rotating-wave approximation can be adopted by neglecting the counter-rotating coupling terms in the Hamiltonian of the system. However, by designing the unique structure of the superconducting circuit, the ultrastrong-coupling regime can be achieved in a circuit-QED system in which the counter-rotating coupling terms become as important as the rotating terms. Thus, the rotating-wave approximation cannot be used in the ultrastrongly coupled circuit-QED system. Owing to the ultrastrong coupling, this circuit-QED system is described by the standard quantum Rabi model when a superconducting qubit is coupled only to a single resonator mode. In this work, we experimentally study an ultrastrongly coupled circuit-QED system consisting of a four-junction superconducting flux qubit and a muti-mode coplanar-waveguide resonator. The transmission-spectrum measurement and numerical simulations show that the system is in the ultrastrong-coupling regime. By changing the photon number in the resonator, we observe the frequency shift of the flux qubit via the spectroscopic measurement. This frequency shift contains the contributions from not only the rotating-coupling terms but also the counter-rotating terms, which is in good agreement with the theory. The result indicates that this ultrastrongly-coupled quantum system can be used as a good platform to investigate the quantum Rabi model and has potential applications in various aspects of quantum technology, such as quantum simulation, ultrafast quantum gates, entangled-state preparation and protected qubits. Keywords:superconducting flux qubit/ ultrastrong coupling/ quantum Rabi model/ Bloch-Siegert shift
实验系统如图1(a)所示, 样品置于一台He3He4稀释制冷机的最低温区, 工作时该温区的温度约为20 mK. 样品处于一个小超导磁体中, 磁体可以产生最大约10 G (1 G = 10–4 T)的磁场. 主要测试仪器包括一台矢量网络分析仪和一台微波信号源, 将矢量网络分析仪输出的信号记为探测信号${\omega _{\rm{p}}}$, 微波信号源输出记为驱动信号${\omega _{\rm{d}}}$. ${\omega _{\rm{p}}}$和${\omega _{\rm{d}}}$经过一个功率合成器混合进入制冷机内部, 再经过逐级衰减后进入样品; 混合信号与样品相互作用后再经过隔离器、放大器返回网络分析仪, 网络分析仪通过比较返回信号相比于信号${\omega _{\rm{p}}}$幅度和相位的变化, 就可以得到样品的信息. 图 1 实验装置 (a)测量系统, 其中信号由网络分析仪和微波源发出, 经过衰减合成后进入稀释制冷机内部, 样品处在制冷机的最低温区, 测试时温度为20 mK; (b)共面波导谐振腔示意图, 磁通量子比特处于谐振腔中心, 黄色、蓝色曲线分别为第一、三模式电流分布; (c)磁通量子比特扫描电子显微镜图像 Figure1. Experimental setup: (a) Measurement system: signals from both the PNA and the PSG generator are combined and attenuated before entering the dilution refrigerator; the sample is placed in the sample chamber of the refrigerator whose working temperature is 20 mK; (b) the schematic of the coplanar-waveguide resonator, with a four-junction flux qubit located at the center of the resonator; yellow and blue curves are current distributions of the first and third modes of the resonator, respectively; (c) the scanning electron microscope images of the flux qubit.
3.实验结果和理论分析前文分析过, 磁通量子比特主要有两个参数, 分别是能量偏置$\varepsilon = 2{I_{\rm{p}}}\left( {\varPhi {\rm{ - }}{\varPhi _{\rm{0}}}{\rm{/2}}} \right)$和隧穿能Δ, 其中${I_p}$为磁通量子比特持续电流, 和Δ一样由制备条件所确定. 磁通量子比特工作在外加磁通为N/2 (N = 1, 2, 3, ···)个磁通量子附近, 而根据我们的制备工艺参数, 隧穿能$\varDelta /{\text{2π}} \approx 6\;{\rm{GHz}}$, 谐振腔谐振频率${\omega _n}/\left( {2{\text{π}}} \right) \approx 3 n$GHz. 实验上, 通过调节外加磁通使磁通量子比特与谐振腔第三模式共振, 观测真空Rabi劈裂. 通过探测到的真空Rabi劈裂, 可以寻找量子比特的工作磁场位置并确定量子比特的基本信息. 图2(a)给出用网络分析仪测试腔第三模式传输谱随外加磁场的变化, 可以看到两个清晰的拉比劈裂. 同时还测试了第一模式随磁场的变化, 见图2(b). 虽然第一模式与磁通量子比特始终不共振, 但由于他们之间有很强的色散相互作用, 谐振峰同样受到了明显的影响. 这里需要说明的是, 在图2中将横坐标磁场换算成了外加磁通量偏置, 换算方法是根据能量偏置的周期性, 测量相邻的两个拉比劈裂, 这两个相邻拉比劈裂对应的电流就是产生一个磁通量量子所需的电流. 图 2 系统传输谱随磁通量偏置的变化 (a)腔第三模式传输谱随磁通量偏置的变化; (b)腔第一模式传输谱随磁通量偏置的变化; 图中拟合红色虚线为哈密顿量(2)式数值解的结果 Figure2. The transmission |S21| as a function of the flux bias: (a) The transmission |S21| of the third mode of the resonator as a function of the flux bias; (b) the transmission |S21| of the first mode of the resonator as a function of the flux bias. Red dashed curves are the fitting results numerically obtained from the Hamiltonian (2) equation.
可以发现腔频率会根据量子比特状态的不同而改变, 变化的大小为$ \pm \left( {\dfrac{{g_n^2{{\sin }^2}\theta }}{{{\delta _ - }}} + \dfrac{{g_n^2{{\sin }^2}\theta }}{{{\delta _ + }}}} \right)$. 根据这个原理, 可以通过探测腔场的变化得到量子比特能谱. 具体的测试方法如下: 将${\omega _{\rm{p}}}$固定在谐振腔的谐振频率处, 同时使用微波源施加一个扫描信号${\omega _{\rm{d}}}$, 当扫描信号频率与量子比特频率共振时, 量子比特状态改变, 导致腔谐振频率发生变化, 这时${\omega _{\rm{p}}}$的幅度和相位也会发生变化. 图3是用腔第一模式色散读出得到的能谱图. 图中黑色箭头所示的是由于谐振腔模式之间的交叉克尔效应导致的谐振频率变化, 他们对应的频率分别为腔第一、二、三模式. 抛物线型曲线对应的是磁通量子比特基态到激发态的跃迁频率, 大小为${\omega _{\rm{q}}} = \sqrt {{\varepsilon ^2} + {\varDelta ^2}} {\rm{/}}\hbar$. 根据图2参数, 对图3中的谱线做了数值拟合, 结果见图中红色虚线. 图3中除了量子比特本身的谱线, 还可以看到量子比特的边带跃迁. 这种边带跃迁是反旋波项和驱动场共同作用的结果, 增加驱动场的功率会看到更高阶的边带跃迁[26]. 图 3 腔第一模式测量的系统能谱随外加磁通偏置的变化, 红色拟合虚线代表量子比特基态到激发态跃迁频率, 测量时腔内的平均光子数为${\bar n_1} = {\rm{4}}{\rm{.7}} \times {\rm{1}}{{\rm{0}}^{ - 3}}$ Figure3. The spectrum measured using the first mode of the resonator as a function of the flux bias. The red dashed curve is the numerical result for the qubit transition and the average photon number in the resonator is ${\bar n_1} = $4.7 × 10–3.
对于量子比特频率位移的测量, 选择将外磁场固定在信噪比较好的量子比特频率$\dfrac{\omega _{\rm{q}}}{2{\text{π}}} =7.42\;{\rm{ GHz}}$位置, 用谐振腔第一模式做能谱测量. 根据之前的分析, 量子比特的频率会随着光子数的增加而增加, 通过增加探测信号的输出功率的方法来不断增加腔内光子数. 随着腔内光子数越来越多, 如图4所示, 量子比特的频率发生了显著的变化, 变化幅度约为$200\;{\rm{ MHz}}$. 这种频率变化产生的根源是哈密顿量(3)式中的 图 4 量子比特频率随光子数的变化, 红色实线表示包含交流斯塔克和布洛赫-西格特效应的拟合曲线, 蓝色虚线只包含交流斯塔克效应 Figure4. The qubit transition frequency as a function of the average photon number. The red solid curve denotes the simulation results when both the ac Stark and Bloch-Siegert shifts are included. The blue dashed curve denotes the simulation results when only the Bloch-Siegert shift is included.