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Review of Scientific Instruments

A cryogen-free dilution refrigerator based Josephson qubit measurement system

Ye F. Tian, Hui Yu, Hui Deng, G. M. Xue, D. T. Liu, Yanjun Ren, G. H. Chen, D. N. Zheng, X.N. Jing, Li Lü, S. P. Zhao, Siyuan Han

Published Mar 1, 2012• 18 citations
Publisher

Abstract

We develop a small-signal measurement system on cryogen-free dilution refrigerator which is suitable for superconducting qubit studies. Cryogen-free refrigerators have several advantages such as less manpower for system operation and large sample space for experiment, but concern remains about whether the noise introduced by the coldhead can be made sufficiently low. In this work, we demonstrate some effective approaches of acoustic isolation to reduce the noise impact. The electronic circuit that includes the current, voltage, and microwave lines for qubit coherent state measurement is described. For the current and voltage lines designed to have a low pass of dc-100 kHz, we show that the measurements of Josephson junction's switching current distribution with a width down to 1 nA, and quantum coherent Rabi oscillation and Ramsey interference of the superconducting qubit can be successfully performed.

DOI
10.1063/1.3698001

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We develop a small-signal measurement system on cryogen-free dilution refrigerator which is suitable for superconducting qubit studies. Cryogen-free refrigerators have several advantages such as less manpower for system operation and large sample space for experiment, but concern remains about whether the noise introduced by the coldhead can be made sufficiently low. In this work, we demonstrate some effective approaches of acoustic isolation to reduce the noise impact. The electronic circuit that includes the current, voltage, and microwave lines for qubit coherent state measurement is described. For the current and voltage lines designed to have a low pass of dc-100 kHz, we show that the measurements of Josephson junction's switching current distribution with a width down to 1 nA, and quantum coherent Rabi oscillation and Ramsey interference of the superconducting qubit can be successfully performed.
Two dilution refrigerators have been installed and made operational for the purpose of having dedicated measurement systems one at MIT and the other at the University of Rochester for carrying out quantum computing experiments. Both systems are a Kelvinox 400 model from Oxford instruments, equipped with both dc and rf lines for measurements; and both systems are vibrationally isolated and magnetically and electrically shielded for low noise measurements. Within the first year these systems have been fitted with custom- designed sample spaces and wiring. Within less than a year, this equipment has enabled the researchers to demonstrate: quantum levels in a persistent current qubit, rf and dc spectroscopy of these energy levels, and control circuits using Single Flux Quantum circuits. This equipment is enhancing our research effort to study, design, demonstrate, and develop algorithms and quantum-coherent circuitry using magnetic-flux-based superconducting qubits.
Abstract Superconducting Josephson structures play a significant role in quantum‐state engineering. Achieving high‐fidelity quantum state measurements in superconducting Josephson structures requires ultra‐low noise environments and robust signal purification techniques. Here, the advanced low‐noise signal measurement system designed for dilution refrigerators is presented, integrating multi‐stage cryogenic filtering and electromagnetic shielding strategies to suppress noise sources across a broad frequency spectrum. The effectiveness of low‐pass RC filters is demonstrated, silver‐epoxy microwave absorbers, and optimized ground isolation to achieve an unprecedented noise reduction, enabling sub‐nanoampere switching current distribution measurements superior to commerical systems at mK temperatures. The system is optimized for precision studies of superconductor‐insulator‐superconductor, superconductor‐ferromagnet‐superconductor, and superconductor‐normal metal‐superconductor Josephson junctions with low critical currents. This approach establishes a reliable framework for next‐generation quantum electronic experiments, ensuring that observed switching phenomena are governed by intrinsic device physics rather than environmental perturbations.
Cryogen-free low-temperature setups are becoming more prominent in experimental science due to their convenience and reliability, and concern about the increasing scarcity of helium as a natural resource. Despite not having any moving parts at the cold end, pulse tube cryocoolers introduce vibrations that can be detrimental to the experiments. We characterize the coupling of these vibrations to the electrical signal observed on cables installed in a cryogen-free dilution refrigerator. The dominant electrical noise is in the 5-10 kHz range and its magnitude is found to be strongly temperature dependent. We test the performance of different cables designed to diagnose and tackle the noise, and find triboelectrics to be the dominant mechanism coupling the vibrations to the electrical signal. Flattening a semi-rigid cable or jacketing a flexible cable in order to restrict movement within the cable, successfully reduces the noise level by over an order of magnitude. Furthermore, we characterize the effect of the pulse tube vibrations on an electron spin qubit device in this setup. Coherence measurements are used to map out the spectrum of the noise experienced by the qubit, revealing spectral components matching the spectral signature of the pulse tube.
The energy states of a well-isolated hysteretic Josephson junction in the "phase" regime can be used as a qubit. The state of the junction can be determined by measuring when the junction switches from the zero-voltage (qubit) state to the running voltage state, since different levels have different switching rates. The experimental challenge is to provide sufficient electrical isolation of the junction from wires that must be attached to provide bias current and to measure switching events. This isolation must be effective at frequencies around the energy level spacing, in our case from 3-8 GHz. We report on the design and measurement of isolated moderate-to-high-Q junctions using a resonant isolation scheme of a series inductance and capacitive shunt. Microwave activation measurements of the Al-AlO/sub x/-Al junctions at 70 mK were used to gauge the isolation effectiveness and to show the effect of current noise from the isolation resonance on the junction coherence.
The superconducting quantum computer with strong data storage capability and fast execution speed has attracted a lot of attention in recent years and is finding practical applications in fields such as finance, military, biomedicine, and artificial intelligence. To suppress the thermal noise and to achieve high coherence, the quantum chips require millikelvin temperature, which poses a significant challenge for the suitable cryogenic system. The dilution refrigerator (DR) with ultra-low vibration and electromagnetic interference becomes an appropriate candidate for this purpose. To achieve the aimed millikelvin temperature, a cryogen-free DR composed of a Gifford-McMahon type pulse tube refrigerator (GM-type PTR), Joule-Thompson refrigerator (JTR), and dilution unit is investigated. This article presents the structural design of the DR and the coupling system with superconducting quantum chips. The cooling performance of the designed DR is estimated based on the developed cryogenic system. The minimum deduced temperature of 0.76 K in the still is achieved and the next steps to obtain the final temperature of 5 mK are underway.
We developed a quantum noise measurement system in a dilution refrigerator by using three kinds of cryogenic low pass filters. One of them is a commercial low pass filter inserted into the noise measurement lines instead of the conventional powder filter, which assures well-defined circuit parameters necessary for the noise measurement at a finite frequency. We checked that this filter gives sufficiently large attenuation up to 20 GHz at room temperature, 77 and 4.2 K. The electron temperature of the mesoscopic device placed in the present system was confirmed to be down to around 20 mK by measuring the thermal noise of the device.