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Superconducting quantum computing is currently one of the most promising platforms for universal quantum information processing. The readout resonator is an essential integral part of a superconducting qubit, while its size is much larger compared to the Josephson junction. We propose and realize a new readout resonator using space-filling curves, specifically Hilbert space-filling curves. We introduce the frequency analysis method and demonstrate a qubit sample, in which the Hilbert-space-filling-curves resonator (HSFCR) is used to read out the qubit states. We also propose to fabricate the HSFCRs and Josephson junctions simultaneously in the same processes of E-beam lithography and E-beam evaporation. Our design reduces the resonator area sufficiently and, thus, will help to improve the integration of superconducting qubits, as well as to design other superconducting quantum devices.

We acquire tantalum thin film in its α phase ( α-Ta) using direct-current magnetron sputtering. According to x-ray diffraction results, 110-Ta is dominant. Quarter-wavelength coplanar waveguide resonators are fabricated with the α-Ta film and characterized at millikelvin in a dilution refrigerator. In the single photon regime, an intrinsic quality factor ( Qi) up to [Formula: see text] is obtained in these resonators. At high power, Qi rises to [Formula: see text]. Moreover, we also fabricate an array with 7 × 7 lumped element resonators using the α-Ta film. The array shows excellent uniformity. At high power, Qis of all pixels exceed [Formula: see text]. In the single photon regime, Qis of over 90% pixels exceed [Formula: see text]. Superconducting quantum computing and ultrasensitive electromagnetic wave detectors will benefit a lot from devices based on the α-Ta film.

We present a fishbone-like lumped-element artificial transmission line to overcome impedance mismatch in a reflection-type Josephson parametric amplifier between a nonlinear resonator and an external transmission line. Using this easily prepared architecture, we design and fabricate a broadband Josephson parametric amplifier, which has gain in an excess of 20 dB with a bandwidth of hundreds of MHz. Furthermore, by varying the working point of the device, the operating frequency of amplification can be tuned in a wide frequency range of 1 GHz while the amplifier operates in the mode of either three-wave mixing or four-wave mixing. Such a parametric amplifier is suitable for engineering applications of superconducting circuit quantum electrodynamics.

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 SiN x 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.

We design and fabricate λ/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 × 104 at single photon power level at 30 mK 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 × 103. 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.

By adjusting the frequency of the cavity, we perform a microwave reflection measurement and directly probe the coherence and interference effects in a phase-sensitive Josephson parametric amplifier. We demonstrate the shift in the peak and the dip in the reflection spectra of the amplifier, which operates in the phase-sensitive mode. The behavior of the shift can be precisely controlled by tuning the pump power, the frequency of the cavity, and the relative phase between the incident signal and pump field. Theoretical simulations are in good agreement with the experimental results. These results provide an alternative way of controlling the parametric process by adjusting the frequency of the cavity.

Phenomenally, Cooper pairs can be broken up by external energy and thus the Cooper-pair density in the superconducting electrodes of a Josephson junction (JJ) under radiation can be lowered accordingly. Therefore, by probing the shift of the switching current through the junction, the radiation power absorbed by the superconductors can be detected. Here, we experimentally demonstrate weak optical detections in two types of JJs: Al/AlO x /Al junction (Al-J) and Nb/AlO x /Nb junction (Nb-J), with the superconducting transition temperatures T c ≈ 1.2 K and 6.8 K respectively. The photon-induced switching current shifts are measured at ultra-low temperature ( T ≈ 16 mK ) in order to significantly suppress thermal noises. It is observed that the Al-J has a higher sensitivity than the Nb-J, which is expected since Al has a smaller superconducting gap energy than Nb. The minimum detectable optical powers (at 1550 nm) with the present Al-J and Nb-J are measured as 8 pW and 2 nW respectively, and the noise equivalent power (NEP) are estimated to be 7 × 10 - 11 W / Hz (for Nb-J) and 3 × 10 - 12 W / Hz (for Al-J). We also find that the observed switching current responses are dominated by the photon-induced thermal effects. Several methods are proposed to further improve the device sensitivity, so that the JJ based devices can be applicable in photon detections.

By applying a coupling microwave field gated by random-telegraph-noise (RTN), we report the experimental demonstration of dynamically modulated Autler-Townes effect (ATE) in an artificial atom---a superconducting transmon qubit---and obtain the distinguished characteristics when a system is affected by RTN. By further applying another coupling microwave filed on the transmon, we use an intense bichromatic field to couple the energy levels. Depending on the detuning of the couplers, remarkable interference patterns emerge in the spectrum, which come from the dressed states induced by the modulated ATE. Numerical calculations agree well with the experimental data.

Both system detection efficiency (DE) and dark count rate (DCR) are critical parameters of single-photon detectors for practical applications. For a superconducting nanowire single-photon detector (SNSPD), DE is always achieved with high bias current, which also produces high dark count rate. In this paper, we analyzed the DE of SNSPD with 50 nm NbN nanowires patterned on the films with a thickness of 4 and 6 nm. The maximum DE for communication wavelengths is 10.3% at 1310 nm with a 50 nm-wide and 4 nm-thick device. The chip with 6 nm thickness shows high DE at ultra-low dark count rate. For example, DEs of 32% for 404 nm photons and 21% for 660 nm photons were achieved at a low dark count rate of 0.01 Hz. Comparing with the SNSPD with 4 nm-thick films, the improved DE for visible photons with the 6 nm-thick SNSPD is caused by the increased absorbance of NbN films, which also reduced the dark count rate produced by thermal activation around film defects. By optimizing the micro-fabrication process and further decreasing the nanowire width while retaining uniformity, the wavelength response of the 6 nm-thick SNSPD may be extended to infrared photons.

This paper describes the design and fabrication of superconducting hot electron bolometer (HEB) mixer based on ultra-thin superconducting NbN films. The high quality films were epitaxially grown on high resistance Si substrates. The device was fabricated by magnetron sputtering, electron beam lithography (EBL), reactive ion etching (RIE), lithography, and so on. The device’s resistance-temperature (R-T) curves and current-voltage (I-V) curves were studied. The results of THz response of the device are presented. Y-factor technique was used to measure the device’s noise temperature. When the device was irradiated with a laser radiation of 2.5 THz, the obtained lowest noise temperature of the device was 2213 K.

Single photon detectors based on superconducting niobium nitride (NbN) nanowires over areas of 30 μm×30 μm are fabricated using E-beam lithography (EBL) and reactive ion etching (RIE). SEM images of the NbN nanowires indicate that the widths are uniformly 200 nm over the whole lengths and the fill factors are about 66.7%. The detectors are characterized at optical wavelengths of 810, 1310 and 1550 nm, respectively. Maximum detector efficiencies (DE) of 0.3% at 1550 nm and 0.6% at 1310 nm are obtained, and the resetting times are just about 10 ns. Additionally we demonstrated the broadband response properties of the devices as well as linear relations between the count rates and incident photon numbers.

We report the extraordinary terahertz (THz) transmission through subwavelength hole array in superconducting NbN film. As the temperature drops below the superconducting transition temperature, the transmission spectra experience distinct changes. The extraordinary transmission is greatly enhanced in superconducting state due to the enhancement of surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs). We have also observed temperature-dependent resonance frequency shift, which mainly depends on the coupling between SPPs and LSPs.

A fiber-coupled superconducting nanowire single photon detector is presented considering practical applications. The system performances, such as dark count rate, system efficiency and noise equivalent power, are discussed. A system efficiency of 3% (1550 nm) and a dark count rate of 10 Hz at 4.2 K were achieved in experiment.

We have realized a frequency-tunable transmon in a three-dimensional cooper cavity using a direct current superconducting 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.

We experimentally investigated the optical responses of a superconducting niobium resonator. It was found that, with increasing radiation power, the resonance frequency increases monotonically below around 500 mK, decreases monotonically above around 1 K and exhibits a nonmonotonic behavior at around 700 mK. These observations show that one can operate the irradiated resonator in three temperature regimes, depending on whether two-level system (TLS) effects or kinetic inductance effects dominate. Furthermore, we found that the optical responses at ultra-low temperatures can be qualitatively regarded as a photon-induced thermalization effect of TLSs, which could be utilized to achieve thermal sensitive photon detections., Comment: Accepted by the Journal of Applied Physics

We design and fabricate two types of superconducting niobium coplanar waveguide microwave resonators with different coupling capacitors on high purity Si substrates. Their microwave transmissions are measured at 20 mK. It is found that these two types of resonators possess significantly different loaded quality factors; one is 5.6 × 103 and the other is 4.0 × 104. The measured data are fitted well by classical ABCD matrix approach and consequently the coupling capacitances are determined. It is found that the transmission peak deviates from the standard Lorentizian with a frequency broadening.

We report on fractal-featured square and ring-shaped apertures with a Sierpinski carpet pattern (SCP) on metallic and superconducting NbN films. Multiple extraordinary terahertz (THz) transmission peaks are studied in the transmission spectra using both THz time-domain spectroscopy and numerical simulation. The characteristic transmission peaks are found to be associated with the interaction of surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) for ring-shaped apertures. The effect of LSPs is less remarkable in the square apertures. For the superconducting NbN film, when the temperature is slightly lower than the critical transition temperature Tc, the peak magnitude of SPP resonances is most prominent due to the non-monotonic temperature dependence of kinetic inductance. These results provide a new way to design compact and efficient THz devices.

We designed and fabricated planar terahertz (THz) metamaterials made from superconducting NbN films to mimic electromagnetically induced transparency (EIT) system. They are characterized using THz time domain spectroscopy over a temperature range from 8 to 300 K. High transmittance and large delay-bandwidth product at transparency window are demonstrated, which mainly arise from the enhanced coupling and decreased damping in superconducting state. The EIT-like spectral response could be tuned in a wide frequency range. By applying two dark resonators with different resonance frequencies coupled with a radiative resonator, we experimentally demonstrated the planar metamaterials mimicking four-level EIT system.

For high-quality superconducting tunnel junctions (STJs), it is necessary to reduce leakage current as much as possible. We describe the fabrication of niobium STJs using the selective niobium (Nb) etching process and various ways to minimize the leakage current. The experiment shows that the leakage current mainly comes from shorts in the tunnel barrier layer rather than those around the junction edges. Through systematic analysis of the thin film stress, surface morphology and modified junction structures, we fabricate high-quality Nb STJs with a gap voltage of 2.8 mV and a leakage current at 1 mV as low as 8.1% and 0.023% at 4.2 K and 0.3 K, respectively.

Superconducting planar terahertz (THz) metamaterials (MMs), with unit cells of different sizes, are fabricated on 200 nm-thick niobium nitride (NbN) films deposited on MgO substrates. They are characterized using THz time domain spectroscopy over a temperature range from 8.1 K to 300 K, crossing the critical temperature of NbN films. As the gap frequency (f(g) = 2Δ0/h, where Δ0 is the energy gap at 0 K and h is the Plank constant) of NbN is 1.18 THz, the experimentally observed THz spectra span a frequency range from below f(g) to above it. We have found that, as the resonance frequency approaches f(g), the relative tuning range of MMs is quite wide (30%). We attribute this observation to the large change of kinetic inductance of superconducting film.

Superconducting terahertz (THz) metamaterial (MM) made from superconducting Nb film has been investigated using a continuous-wave THz spectroscopy with a superconducting split-coil magnet. The obtained quality factors of the resonant modes at 132 GHz and 450 GHz are about three times as large as those calculated for a metal THz MM operating at 1 K, which indicates that superconducting THz MM is a very nice candidate to achieve low loss performance. In addition, the magnetic field-tuning on superconducting THz MM is also demonstrated, which offer an alternative tuning method apart from the existed electric, optical and thermal tuning on THz MM.

The superconductive Josephson junction is the key device for superconducting quantum computation. We have fabricated Al/ Al2O3/Al tunnel junctions using a double angle evaporation method based on a suspended shadow mask. The Al2O3 junction barrier has been formed by introducing pure oxygen into the chamber during the fabrication process. We have adjusted exposure conditions by changing either the oxygen pressure or the oxidizing time during the formation of tunnel barriers to control the critical current density Jc and the junction specific resistance Rc . Measurements of the leakage in Al/Al2O3/Al tunnel junctions show that the devices are suitable for qubit applications.