Yirong Jin

Fabrication of Nb Superconducting Nanowires by Nanoimprint Lithography

Nanoimprint lithography (NIL) is considered to be an attractive nonconventional lithographic technique in the fabrication of nanostructures with many advantages including low cost, high throughput, and high resolution on relatively large areas. In this paper, NIL was used to pattern superconducting nanowires with meander structures based on ultrathin (~4 nm) Nb films deposited by dc-magnetron sputtering at room temperature. A combination of thermal-NIL and UV-NIL was exploited to transfer the meander pattern from the imprint hard mold to Nb films. The hard mold, etched into a Si wafer, was defined by e-beam lithography (EBL), which was nonexpendable due to the application of IPS as a soft mold to transfer the pattern to the imprint resist in the NIL process. Superconducting properties such as transition temperature T c and critical current density Jc were measured on the NIL-made Nb nanowires. The results are compared with those of EBL-made nanowires.

Detection of nuclear magnetic resonance in the microtesla range using a high Tc dc-SQUID

We have detected the ultra-low field nuclear magnetic resonance signal from water samples using a high-Tc dc-SQUID sensor. The measurements were carried out in a homemade magnetically shielded room. Resonance spectra of 1H from tap water and other substance samples were obtained in the field range from 7-110μT corresponding to resonance frequency 300-4.68kHz. Two kind of experimental systems were built, the first one is a directly coupled system, its signal to noise ratio in a single-shot measurement is around 4 for about 15 ml water. The second one used a Cu coil to transfer the flux to the SQUID sensor. Signal to noise ratio was improved to about 20 in a single-shot measurement for 5ml water, which benefits from the improvement of coupling efficiency. The effect of residual gradient in the magnetically shielded room was also investigated. J-coupling of 2,2,2-Trifluoroethyl alcohol was measured, the peaks are consistent with high field results.

The Effect of Low Frequency External Field Disturbance on the SQUID Based Ultra-Low Field NMR Measurements

We have carried out NMR experiments in the microtesla range using a high-Tc DC-SQUID sensor. The measurements were carried out in a simple magnetically shielded room. Resonance spectra of 1H nuclei from tap water and other substance samples were obtained in the field range 7-70 μT. The effect of frequency, strength, direction and phase of a low frequency external disturbance field on the NMR spectra was investigated. A sinusoidal field was applied along or perpendicular to the measurement field to simulate the external disturbance. The results were compared with the numerical calculations based on the Bloch equation and good agreement was obtained. In all cases, the field disturbance less than a few Hz are proved to influence the NMR spectra more severely, suppressing the resonance peak and driving it into split bands. On the other hand, the influence of the disturbance field at the frequency range around 50 Hz on the resonance peak position and profile is small, even though some additional small peaks appear in the spectra due to frequency mixing. The effect is more significant when the disturbance field is along the direction of the measurement field. The phase difference between the disturbance field and the free-induction-decay signal could change the NMR peak position and profile drastically.

Fabrication and properties of the meander nanowires based on ultra-thin Nb films

We report the fabrication and the study of superconducting properties of ultra-thin Nb superconducting meander nanowires, which can be used as superconducting nanowire single-photon detectors (SNSPDs). The ultra-thin (about 7-nm thick) Nb films are patterned into micro-bridges, and 100-nm wide meander nanowires by using e-beam lithography (EBL). The average transition temperature (T-c) of the nanowires is about 4.8 K and the critical current density j(c) is about 2.8 x 10(6) A/cm(2). Superconducting characteristics of the specimens at different applied magnetic fields up to 8 T (parallel or perpendicular to the specimen) are systematically investigated. The normalized temperature t (= T/T-c) dependences of the parallel critical field (H-c parallel to) for both the micro-bridge and the meander nanowire are almost the same, following the Ginzburg and Landau (GL) formalism for ultra-thin films. However, in perpendicular field and in the vicinity of T-c (> 0.95T(c)), the critical field H-c perpendicular to of the nanowire exhibits a down-turn curvature nonlinear temperature dependence while the micro-bridge displays a linear temperature dependence. The nonlinear behavior of H-c perpendicular to in the nanowire is believed to be due to the fact that in the vicinity of T-c the coherence length becomes larger than the line width. Additionally, the localization of carriers in the nanowire could also contribute to the nonlinear behavior. The resistive transitions could be described by the phase-slip model for quasi-one-dimensional system. Moreover, the hysteresis in I-V curve of the meander nanowires can be illustrated by a simple model of localized normal hotspot maintained by Joule heating.

High-Quality Stepped-Impedance Resonators Suitable for Circuit-QED Measurement of Superconducting Artificial Atoms

High-quality factor coplanar resonators are critical elements in superconducting quantum circuits. We describe the design, fabrication, and measurement of stepped impedance resonators (SIRs), which are more compact in size than commonly used uniform impedance resonators (UIRs). With properly chosen impedance ratio, SIRs can be 27% shorter than UIRs. As a result, the area occupied by SIRs can be reduced. Two kinds of designs containing both SIRs and UIRs are fabricated and measured. The power dependence of the extracted internal quality factors (Qi ) for all the resonators showed that SIRs and UIRs have comparable performance with Qi around half-million under single-photon level excitation. These results indicate that SIRs could be used in superconducting quantum circuits. The reduced size of SIRs may also lead to reduced overall circuit area and increased integration level.

Fabrication and characterization of ultra-low noise narrow and wide band Josephson parametric amplifiers*

We have fabricated two types of lumped-element Josephson parameter amplifiers (JPAs) by using a multilayer micro-fabrication process involving wet etching of Al films. The first type is a narrow band JPA which shows typical gain above 14 dB in a bandwidth around 35 MHz. The second type is a wideband JPA which is coupled to an input 50 Ω transmission line via an impedance transformer that changes the impedance from about 15 Ω on the non-linear resonator side to 50 Ω on the input transmission line side. The wideband JPA could operate in a 200 MHz range with a gain higher than 14 dB. The amplifiers were used for superconducting qubit readout. The results showed that the signal to noise ratio and hence the readout fidelity were improved significantly.

Fabrication of superconducting NbN meander nanowires by nano-imprint lithography*

Superconducting nanowire single photon detector (SNSPD), as a new type of superconducting single photon detector (SPD), has a broad application prospect in quantum communication and other fields. In order to prepare SNSPD with high performance, it is necessary to fabricate a large area of uniform meander nanowires, which is the core of the SNSPD. In this paper, we demonstrate a process of patterning ultra-thin NbN films into meander-type nanowires by using the nano-imprint technology. In this process, a combination of hot embossing nano-imprint lithography (HE-NIL) and ultraviolet nano-imprint lithography (UV-NIL) is used to transfer the meander nanowire structure from the NIL Si hard mold to the NbN film. We have successfully obtained a NbN nanowire device with uniform line width. The critical temperature (Tc) of the superconducting NbN meander nanowires is about 5 K and the critical current (Ic) is about 3.5 μA at 2.5 K.

Working Point Adjustable DC-SQUID for the Readout of Gap Tunable Flux Qubit

In flux qubits, qubit states are commonly read out by a dc superconducting quantum interference device (SQUID), which is inductively coupled to the qubit and works as a persistent current detector. However, neither the bottom nor the top regions of the critical current modulation curve of the SQUID can be used for the readout. In this paper, we show that a flux-trap loop inserted to the readout SQUID provides the ability to adjust the working point, which helps to avoid this constraint. This improvement is useful for some complicated flux qubit experiments.

Ultra-Low-Field MRI and Spin-Lattice Relaxation Time of

We have carried out ultra-low-field nuclear magnetic resonance (NMR) and magnetic resonance imaging experiments in microtesla magnetic fields using a high-Tc dc-SQUID sensor. The measurements were carried out in a home-made magnetically shielded room. NMR spectra of 1H from tap water and other substance samples were obtained in the field range from 10-200 μT. By adapting a transformer coupled detection system, the signal-to-noise ratio of NMR peak in a single-shot measurement for 10 ml tap water was increased significantly as compared with our previously used direct detection method. The improvement was attributed to the increased coupling efficiency and reduced SQUID noise. By applying magnetic field gradient in three directions, one- and two-dimensional imaging results were obtained from water phantom samples. Furthermore, the effect of Fe3O4 magnetic nanoparticles on the spin-lattice relaxation time T1 was studied.

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We carry out an ultra-low-field nuclear magnetic resonance (NMR) experiment based on high-Tc superconducting quantum interference devices (SQUIDs). The measurement field is in a micro-tesla range (~10 μT−100 μT) and the experiment is conducted in a home-made magnetically-shielded-room (MSR). The measurements are performed by the indirect coupling method in which the signal of nuclei precession is indirectly coupled to the SQUID through a tuned copper coil transformer. In such an arrangement, the interferences of applied measurement and polarization field to the SQUID sensor are avoided and the performance of the SQUID is not destroyed. In order to compare the detection sensitivity obtained by using the SQUID with that achieved using a conventional low-noise-amplifier, we perform the measurements using a commercial room temperature amplifier. The results show that in a wide frequency range (~1 kHz−10 kHz) the measurements with the SQUID sensor exhibit a higher signal-to-noise ratio. Further, we discuss the dependence of NMR peak magnitude on measurement frequency. We attribute the reduction of the peak magnitude at high frequency to the increased field inhomogeneity as the measurement field increases. This is verified by compensating the field gradient using three sets of gradient coils.