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Chinese Physics B

Characterization of barrier-tunable radio-frequency-SQUID for Maxwell’s demon experiment

Gang Li, Suman Dhamala, Hao Li, Jianshe Liu, Wei Chen

Published Jun 1, 2018• 2 citations
Publisher

Abstract

We present the design, fabrication, and characterization of a barrier-tunable superconducting quantum interference device (SQUID) qubit for the study of Maxwell’s demon experiment. In this work, a compound Josephson junction (CJJ) radio-frequency (RF)-SQUID qubit with an overdamped resistively shunted direct-current (DC)-SQUID magnetometer is used to continuously monitor the state of the qubit. The circuit is successfully fabricated with the standard Nb/Al-AlO x /Nb trilayer process of our laboratory and characterized in a low noise measurement system, which is capable of measuring coherent dynamics of superconducting qubits, in an Oxford dilution refrigerator. All circuit parameters are determined accurately by fitting experimental data to theoretical analysis and simulation, which allows us to make a quantitative comparison between the results of the experiment and theory.

DOI
10.1088/1674-1056/27/6/068501

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Acta Physica Sinica•Jan 1, 2015•2 citations
We present the design, fabrication, and characterization of a barrier-tunable superconducting quantum interference device (SQUID) qubit for the study of Maxwell’s demon experiment. In this work, a compound Josephson junction (CJJ) radio-frequency (RF)-SQUID qubit with an overdamped resistively shunted direct-current (DC)-SQUID magnetometer is used to continuously monitor the state of the qubit. The circuit is successfully fabricated with the standard Nb/Al-AlO x /Nb trilayer process of our laboratory and characterized in a low noise measurement system, which is capable of measuring coherent dynamics of superconducting qubits, in an Oxford dilution refrigerator. All circuit parameters are determined accurately by fitting experimental data to theoretical analysis and simulation, which allows us to make a quantitative comparison between the results of the experiment and theory.
A practical strategy for synchronizing the properties of compound Josephson junction (CJJ) radio frequency monitored superconducting quantum interference device (rf-SQUID) qubits on a multi-qubit chip has been demonstrated. The impact of small (∼1%) fabrication variations in qubit inductance and critical current can be minimized by the application of a custom-tuned flux offset to the CJJ structure of each qubit. This strategy allows for a simultaneous synchronization of the qubit persistent current and tunnel splitting over a range of external bias parameters that is relevant for the implementation of an adiabatic quantum processor.
Superconducting quantum interference devices (SQUIDs) with 3 μm × 3 μm or 4 μm × 4 μm self-shunted Nb/NbxSi1-x/Nb Josephson junctions are designed and fabricated. By adjusting the Nb content and thickness of the NbxSi1-xbarrier, the parameters of the SQUIDs can be easily tuned. The optimal parameters for the barrier are found to be a thickness of around 15 nm and 7% Nb content. The I-V curve, V-Φ curve, and noise spectrum are measured for the SQUID. The high frequency noise for the SQUID is only about several hundred nΦ0/√Hz. Another advantage for these devices is found, that it is not sensitive to the static electricity.
Abstract The detailed characteristics of classical states in a radio-frequency superconducting quantum interference device (rf-SQUID) with well-designed energy-potential shape are investigated at 4.2 K. Using Nb/AlO x standard process which enables precise circuit-parameter implementation, the rf-SQUID is designed in its energy potential. Directions of persistent current can be distinguished in the rf-SQUID we have fabricated. Analyses in circuit simulations indicate the device design based on enough high critical current provides available parameters to construct the device for 10 mK measurement by circuit-parameter scaling. This knowledge may contribute to the precise design of the rf-SQUID for superconducting qubit through device process with lower critical current density.
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.
At low temperature and low damping, the magnetic flux of a rf Superconducting QUantum Interference Device (SQUID) acts as a macroscopic quantum variable whose dynamic is described by a double well potential. A rf SQUID consists of a superconducting loop interrupted by a thin insulating barrier, known as a Josephson junction. The modified rf SQUID design discussed in this thesis has independent, in situ, controls that allow varying the barrier height between the potential wells and their relative tilt. This high level of control of the potential together with the capability to implement an experimental setup optimized to reduce coupling to external sources of noise, made this system ideal not only for tests of quantum transition at a macroscopic level, but also as a scalable solid state device for quantum computation. A variety of measurements of quantum phenomena are reported in this thesis, including microwave level spectroscopy, coherent oscillations, and macroscopic resonant tunneling (MRT). These measurements are compared with theoretical models to study the possible sources of decoherence in a rf SQUID qubit. The analysis confirmed that the main source of decoherence is not due to coupling to the external environment, but it can be linked to intrinsic low frequency flux noise. Work from many groups over the last several years has shown that the performance of a superconductive qubit is limited by coupling to spurious defects in the materials used for the device fabrication. In this work we analyze the possibility of using the MRT as a tool for future studies of effects of materials and fabrication process in the realization of improved qubits, and we also explore the feasibility for measurements of quantum phenomena that involve fast pulsing of the potential barrier height.
Superconducting quantum interference device (SQUID) amplifier is best known for its low input impedance, low noise and low power consumption. Nowadays it is widely used for detecting the weak signals. Compared with other methods, the Nb/Al-AlOx/Nb structure Josephson junction based SQUID has the advantages of high transition temperature, high voltage flux modulation index and good heat recycle ability, wide critical voltage range, so it is a very good option for making SQUID amplifier. In this work, we fabricate the overdamped Josephson junction and washer dc SQUID, and test the I-V characteristics at He3 3 K stage temperature and calculate the current resolution of SQUID. The result of SQUID modulation property is good. The magnification becomes larger after increasing the input line number of loops, and the system noise becomes smaller after the join of the LC filter. This work is very important for designing and manufacturing transition edge sensor readout circuits.