基于FPGA的微波相干源线性检测和特性表征

微波光子的检测和表征对于量子信息具有重要意义。通常使用Hanbury Brown-Twiss(HBT)干涉仪对光子源性质进行检测和表征,然而,由于微波光子信噪比较低,在对其进行检测和表征过程中常常需要多次计算才能移除噪声影响。本工作基于现场可编程门阵列(FPGA),提出一种快速、可扩展的测量方案用于分析微波光子源性质,可更快速地实现该计算过程。利用该方案对频率为3 MHz的相干源进行了模拟测量,分析了二阶相关函数所具有的特性,发现测量结果与理论具有良好的一致性。该系统的实现对未来进行单光子源检测具有重要意义。

行波微波光子探测

单光子探测器在量子光学中是必不可少的工具,但是在微波领域仍然没有合适的探测器。本文利用腔量子电动力学原理,在超导量子比特和微波光子之间建立了纠缠门,最终对微波光子数的状态进行了非破坏性探测。

Frequency-tunable transmon in a three-dimensional copper cavity

We have realized a frequency-tunable transmon in a three-dimensional cooper cavity using a direct current supercon- ducting quantum interference device. Both the transition frequency of the transmon and the frequency of the dressed cavity can be varied with the applied external flux bias, which are well consistent with the theoretical model. The range of the variable transition frequency is from 5.188 GHz to 7.756 GHz. The energy relaxation time of the transmon is hundreds of nanoseconds.

An easily-prepared impedance matched Josephson parametric amplifier

An impedance matched parametric amplifier(IMPA)with Josephson junctions is fabricated and characterized.A hybrid structure containing coplanar and strip structures is implemented to realize an impedance taper line and a plate capacitor in an LC nonlinear resonator based on Josephson junctions.The upper plate of the capacitor is isolated with SiNx without grounding as well as the strips.Such easily-prepared designs greatly reduce the requirements for lithography alignment and precision,which makes the fabrication process more reliable.The experimental results show that in such IMPA a gain higher than 25 dB with a bandwidth of about 100 MHz can be obtained.This broadband amplifier operates close to the quantum limit.By adjusting the working point,a higher bandwidth of about 400 MHz can be obtained with a gain of about 17 dB.

Compact NbN resonators with high kinetic inductance

We design and fabricateλ/2 coplanar waveguide NbN resonators,the thickness and length of which are only several nanometers and hundred microns,respectively.The quality factor of such compact resonators can reach up to 7.5×10~4 at single photon power level at 30 m K with the resonance frequency around 6.835 GHz.In order to tune the resonant frequency,the resonator is terminated to the ground with a dc-SQUID.By tuning the magnetic flux in the dc-SQUID,the effective inductance of the dc-SQUID is varied,which leads to the change in the resonant frequency of the resonator.The tunability range is more than 30 MHz and the quality factor is about 3×10~3.These compact and tunable NbN resonators have potential applications in the quantum information processing,such as in the precision measurement,coupling and/or reading out the quantum states of qubits.

Superconducting membrane mechanical oscillator based on vacuum-gap capacitor

Using the diluted S1813 UV photoresist as a sacrificial layer, we successfully fabricate a superconducting suspended parallel-plate capacitor, in which the top layer of aluminum film acts as a membrane mechanical resonator. Together with a superconducting octagonal spiral inductor, this parallel-plate capacitor constitutes a superconducting microwave resonator. At m K temperature, the transmission characteristic and spectrum of the microwave resonator are measured.Sideband frequencies caused by the vibration of the membrane mechanical resonator are clearly demonstrated. By downconverting with a mixer, the dependence of fundamental frequency and its harmonics on the input microwave power are clearly demonstrated, which is consistent with the numerical simulation.

三维传输子量子比特制备研究

研究了三维传输子(3D-transmon)量子比特的制备技术。采用电子束光刻技术制备亚微米双层胶悬空掩模,电子束斜蒸发技术制备Al/AlOx/Al超导隧道结。将隧道结与三维谐振腔耦合,成功实现3D-transmon。在20 mK下测试了量子比特的能级跃迁频率、拉比振荡、能量弛豫时间t1等性能,t1约566 ns。t1较短的原因是量子比特跃迁频率与三维谐振腔频率接近,耦合过紧。