Research Fields

Quantum Optoelectronics Laboratory explores the world of superconducting mesoscopic electronics and focuses on the development of superconducting quantum devices for detector and quantum information applications:

• Single photon detection with superconducting detectors (such as kinetic inductance detectors)

• Superconducting resonators, microwave circuits and superconducting quantum devices

• Superconducting detector array for THz (sub-millimeter) imaging

• Microfabrication of superconductive devices

• Quantum optics experiments (using cryogenic detectors)

• Quantum computing with superconducting Josephson devices

Selected Experiments

7. High-absorption optical stack for kinetic inductance detectors

We present a high-absorption optical stack design for aluminum (Al) kinetic inductance detectors (KIDs). Aluminum can be easily processed in micro-fabrication and is the most conventional superconducting material for KIDs.However, it is challenging to achieve high absorption in the Al absorber because of its high reflection at optical wavelengths. By embedding the thin Al film between an anti-reflection (AR) coating layer and a dielectric-based distributed Bragg reflector, we show that close-to-unity absorption can be achieved around a single wavelength (e.g., ~ 98.9% at 1518 nm). The reflection and transmission measurements agreewell with the calculation based on the transmission matrix model.We also show our preliminary results of absorption70% in a broader wavelength range (~ 230 nm) with multilayer AR coatings. The absorber design in a lumped-element KID is discussed. Our work paves the way to high-efficiency photon-counting and energy-resolving Al-based KIDs in the optical to NIR range. Appl. Opt. 62, 5294 (2023) & J Low Temp Phys 194, 361 (2019).

6. Fitting method for nonlinear superconducting resonators

We present a new fitting method that can robustly and accurately fit the complex transmission curve of a superconducting resonator in the nonlinear regime. This method takes into account the varying internal current in the resonator at different frequencies and the nonlinear dependence of the inductance on the internal current. We demonstrate using this method to retrieve important resonator parameters, such as the quality factor and the resonance frequency, from resonators driven below, near, and above bifurcation. We further use this method to retrieve I* for lumped-element TiN resonators with various inductor designs. By fitting the resonance frequency shift at different readout powers of each resonator, we can determine the characteristic current I*, which is found to be linearly related to the cross-sectional area of the narrow inductor strip. Our method has wide applications in superconducting detector, superconducting qubit and parametric amplifier data analysis where the resonator is driven in the nonlinear regime. Supercond. Sci. Technol. 36, 015003 (2023) & Supercond. Sci. Technol. 33, 075004 (2020).

5. Study of quasi-particle dynamics using the optical pulse response of a superconducting resonator

We study the optical pulse response of a superconducting half-wavelength coplanar waveguide (CPW) resonator. We apply a short optical pulse to the center strip of the CPW resonator, where the current distribution shows antinodes or nodes for different resonance modes, and measure the frequency response. We develop a time-dependent variable inductance circuit model with which we can simulate the optical pulse response of the resonator. By fitting this model to experimental data, we extract the temporal kinetic inductance variations, which directly reflect the quasi-particle recombination with time and diffusion in space. We also retrieve the spatial size of the quasi-particle distribution and the quasi-particle diffusion constant. Our study is very useful for the design of photon-counting kinetic inductance detectors, and the method developed in this work provides a useful way to study the quasi-particle dynamics in the superconductor. [Appl. Phys. Lett. 119, 022601 (2021)]

4. Superconducting micro-resonator arrays with ideal frequency spacing

We present a wafer trimming technique for producing superconducting micro-resonator arrays with highly uniform frequency spacing. With the light-emitting diode mapper technique demonstrated previously, we first map the measured resonance frequencies to the physical resonators. Then, we fine-tune each resonator’s frequency by lithographically trimming a small length, calculated from the deviation of the measured frequency from its design value, from the interdigitated capacitor. We demonstrate this technique on a 127-resonator array made from titanium-nitride and show that the uniformity of frequency spacing is greatly improved. The array yield in terms of frequency collisions improves from 84% to 97%, while the quality factors and noise properties are unaffected. The wafer trimming technique provides an easy-to-implement tool to improve the yield and multiplexing density of large resonator arrays, which is important for various applications in photon detection and quantum computing. [Appl. Phys. Lett. 111, 252601 (2017)] & [J. of Appl. Phys. 122, 034502 (2017)]

3. Counting near infrared photons with microwave kinetic inductance detectors

We demonstrate photon counting at 1550 nm wavelength using microwave kinetic inductance detectors (MKIDs) made from TiN/Ti/TiN trilayer films with superconducting transition temperature Tc ~ 1.4K. The detectors have a lumped-element design with a large interdigitated capacitor covered by aluminum and inductive photon absorbers. The energy resolution improves as the absorber volume is reduced. We achieved an energy resolution of 0.22 eV and resolved up to 7 photons per optical pulse, both greatly improved from previously reported results at 1550nm wavelength using MKIDs. Further improvements are possible by optimizing the optical coupling to maximize photon absorption into the inductive absorber. [Appl. Phys. Lett. 110, 212601 (2017)]

2. A tunable coupler for superconducting microwave resonators

We present a tunable coupler scheme that allows us to tune the coupling strength between a feedline and a superconducting resonator in situ over a wide range. In this scheme, we shunt the feedline with a 50 ohm lumped-element nonlinear transmission line made from a 20 nm NbTiN film. By injecting a DC current, the nonlinear kinetic inductance changes and the effective impedance shunting the resonator periodically varies from a short to an open, which tunes the coupling strength and coupling quality factor Qc. We have demonstrated Qc tuning over a factor of 40, for a 4.5 GHz resonator by applying a DC current less than 3.3 mA. Our tunable coupler scheme is easy to implement and may find broad applications in superconducting detector and quantum computing/information experiments. [Appl. Phys. Lett. 108, 222604 (2016)]

1. Experimental demonstrations of high-Q superconducting coplanar waveguide resonators

We successfully designed and fabricated an absorption-type of superconducting coplanar waveguide (CPW) resonators. The resonators are made from a niobium film (about 160 nm thick) on a high-resistance Si substrate, and each resonator is fabricated as a meandered quarter-wavelength transmission line (one end is short to the ground and another end is capacitively coupled to a through feedline). With a vector network analyzer we measured the transmissions of the applied microwave through the resonators at ultra-low temperature. The obtained loaded quality factors are significantly high, i.e. up to ~ 10^6. In principle, this type of device can integrate a series of CPW resonators with a common feedline, making it a promising candidate as the data bus for coupling distant solid-state qubits and the sensitive detector of single photons. [Chinese Science Bulletin 58(20), 2413 (2013).] & [J. of Appl. Phys. 114, 153109 (2013)]

Funded Projects

主持国家自然科学基金面上项目和青年项目各1项;

主持科技部国家重点研发计划课题1项;

主持四川省科技厅面上项目1项;

主研过多项国家自然科学基金项目.