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Chinese Physics B

Flux-to-voltage characteristic simulation of superconducting nanowire interference device*

Xingyu Zhang, Yongliang Wang, Chaolin Lv, Lixing You, Hao Li, Zhen Wang, Xiaoming Xie

Published May 7, 2020• 1 citation
Publisher

Abstract

Inspired by recent discoveries of the quasi-Josephson effect in shunted nanowire devices, we propose a superconducting nanowire interference device in this study, which is a combination of parallel ultrathin superconducting nanowires and a shunt resistor. A simple model based on the switching effect of nanowires and fluxoid quantization effect is developed to describe the behavior of the device. The current–voltage characteristic and flux-to-voltage conversion curves are simulated and discussed to verify the feasibility. Appropriate parameters of the shunt resistor and inductor are deduced for fabricating the devices.

DOI
10.1088/1674-1056/ab90f4

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Inspired by recent discoveries of the quasi-Josephson effect in shunted nanowire devices, we propose a superconducting nanowire interference device in this study, which is a combination of parallel ultrathin superconducting nanowires and a shunt resistor. A simple model based on the switching effect of nanowires and fluxoid quantization effect is developed to describe the behavior of the device. The current–voltage characteristic and flux-to-voltage conversion curves are simulated and discussed to verify the feasibility. Appropriate parameters of the shunt resistor and inductor are deduced for fabricating the devices.
Recent experiments on superconducting flux qubits, consisting of a superconducting loop interrupted by Josephson junctions, have demonstrated quantum coherence between two different quantum states. The state of the qubit is measured with a superconducting quantum interference device (SQUID). Such measurements require the SQUID to have high resolution while exerting minimal backaction on the qubit. By designing shunts across the SQUID junctions appropriately, one can improve the measurement resolution without increasing the backaction significantly. Using a path-integral approach to analyze the Caldeira-Leggett model, we calculate the narrowing of the distribution of the switching events from the zero-voltage state of the SQUID for arbitrary shunt admittances, focusing on shunts consisting of a capacitance Cs{C}_{s}Cs​ and resistance Rs{R}_{s}Rs​ in series. To test this model, we fabricated a dc SQUID in which each junction is shunted with a thin-film interdigitated capacitor in series with a resistor, and measured the switching distribution as a function of temperature and applied magnetic flux. After accounting for the damping due to the SQUID leads, we found good agreement between the measured escape rates and the predictions of our model. We analyze the backaction of a shunted symmetric SQUID on a flux qubit. For the given parameters of our SQUID and realistic parameters for a flux qubit, at the degeneracy point we find a relaxation time of $113\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$, which limits the decoherence time to $226\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$. Based on our analysis of the escape process, we determine that a SQUID with purely capacitive shunts should have narrow switching distributions and no dissipation.
The voltage modulation depths, ΔV of high-Tc direct current superconducting quantum interference devices (SQUIDs) with resistors connected in parallel with their inductances were investigated. Both the junctions and resistors in the SQUIDs were fabricated using focused electron-beam irradiation. The effect of varying the resistor value (using focused ion-beam trimming) and the screening parameter βL (by varying the temperature and hence the junction critical current) were studied. Significant enhancement of ΔV relative to an equivalent unshunted SQUID for βL values up to 50 was observed, and the most effective shunt resistor value was found to be approximately equal to the junction resistance.
The experimental results obtained with low noise dc superconducting quantum interference devices (dc-SQUIDs) are sometimes not well understood. In particular, when the critical current is high, very irregular voltage versus flux characteristics can be measured. On the other hand, when the critical current is low, the characteristics follow perfectly the resistively shunted junction model. We have studied the dynamics of a dc-SQUID in the presence of stray capacitance and shunting resistors imbalance. The equations describing a circuit model for this physical system are integrated numerically and the results are compared with voltage versus flux characteristics obtained from fabricated devices. We find that the theoretical analysis accounts very well for previously misunderstood experimental features of the dc-SQUID. This model can give more physical sense to the various regimes of behavior of the dc-SQUID giving more insight into the experimental results.
We propose a superconducting microwave relaxation oscillator based on a nanowire shunted by a resistor and an inductor controlled by quasiparticle injection from a tunnel junction positioned on it: the QUISTRON. This device exhibits relaxation oscillator behavior with DC voltage-controlled frequency tuning and DC current bias. The device frequency is modulated via the tunnel junction, which induces localized heating by injecting quasiparticles. This heating mechanism modulates the nanowire switching current, enabling relaxation oscillations when it falls below the bias current. We demonstrate the device operating principles and characterize its performance across various parameters, including different choices of shunt resistor, shunt inductance, and bath temperature ranging from 20 mK to 1 K. The device showed oscillation with a frequency range approximately between 1 GHz and 10 GHz and total energy dissipation per cycle of ∼100\sim100∼100 zJ. Our results suggest that this design offers a promising platform for compact, tunable superconducting oscillators in the microwave spectrum with potential applications in quantum information processing, microwave technology, and ultra-low-power electronics. The straightforward frequency control mechanism and integration potential make this device an attractive candidate for superconducting microwave local oscillators.
We propose a superconducting microwave relaxation oscillator based on a nanowire shunted by a resistor and an inductor controlled by quasiparticle injection from a tunnel junction positioned on it: the QUISTRON. This device exhibits relaxation oscillator behavior with dc voltage--controlled frequency tuning and dc bias. The device frequency is modulated via the tunnel junction, which induces localized heating by injecting quasiparticles. This heating mechanism modulates the nanowire switching current, enabling relaxation oscillations when it falls below the bias current. We simulate the device operating principles and its performance across various parameters, including different choices of shunt resistor, shunt inductance, and bath temperature ranging from 20 mK to 1 K. The device showed oscillation with a frequency range approximately between 1 and 10 GHz and total energy dissipation per cycle of approximately 100100100 zJ. Our results suggest that this design offers a promising platform for compact, tunable superconducting oscillators in the microwave spectrum with potential applications in quantum information processing, microwave technology, and ultralow-power electronics. The straightforward frequency control mechanism and integration potential make this device an attractive candidate for superconducting microwave local oscillators.