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NTT technical review

Characterization of Metal Ions in Neurons Using a Superconducting Flux Qubit

Hiraku Toida, Koji Sakai, Tetsuhiko Teshima, Kosuke Kakuyanagi, Imran Mahboob, Shiro Saito

Published May 1, 2024• 1 citation
Publisher

Abstract

In the race to develop superconducting quantum computers, superconducting quantum circuit technology has made remarkable progress.The technology has also been applied to quantum sensing, especially highly sensitive magnetometers.The target of on-chip measurement by superconducting quantum sensors has thus far been limited to impurities in semiconductors.However, the range of applications is not limited to such hard materials.This article describes the detection of ferric ions in neurons with a high-sensitivity, high-spatial-resolution magnetometer using a superconducting flux qubit.

DOI
10.53829/ntr202405fa5

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In the race to develop superconducting quantum computers, superconducting quantum circuit technology has made remarkable progress.The technology has also been applied to quantum sensing, especially highly sensitive magnetometers.The target of on-chip measurement by superconducting quantum sensors has thus far been limited to impurities in semiconductors.However, the range of applications is not limited to such hard materials.This article describes the detection of ferric ions in neurons with a high-sensitivity, high-spatial-resolution magnetometer using a superconducting flux qubit.
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Superconducting circuits are promising components for future quantum information technologies such as quantum computing, quantum simulation, and quantum sensing. Superconducting quantum interference devices (SQUIDs) and superconducting flux qubits can be used as a sensitive magnetic field sensor to detect local electron spin resonance (ESR). These types of sensor are characterized by good sensitivity and high spatial resolution for electron spins. In fact, we have achieved a sensitivity of 20 spins/Hz\sqrt{\text {Hz}}Hz​ with a sensing volume of 6 fl combining a flux qubit and qubit readout using a Josephson bifurcation amplifier. As a demonstration, we measured the ESR spectrum of erbium electron spins in a Y2_{2}2​SiO5_{5}5​ crystal and nitrogen vacancy centers in diamond. In both cases, we obtained reasonable electron spin parameters by fitting a simulation to the spectrum. In this way, we proved superconducting circuits to be promising magnetic field sensors for evaluating the properties of solid materials.
We use a superconducting flux qubit for the sensing application of electron spin resonance (ESR) spectroscopy.The superconducting flux qubit is a highly sensitive magnetometer with micrometer-scale spatial resolution.The sensitivity of our spectrometer is 400 spins/√Hz, and the sensing volume is 0.05 pL.This high sensitivity and high spatial resolution enable us to investigate a small number of electron spins in a microscale area.The spectrometer also obtains a wider ESR spectrum range with twoparameter (frequency and magnetic field) scanning.This enables us to refine the material parameters.
A challenge with a superconducting quantum bit (qubit)-the basic element of a quantum computer-is its short lifetime.To overcome this challenge for developing a quantum computer, approaches, such as elucidating the mechanisms that affect the lifetime of a qubit and extending its lifetime by correcting errors that occur when it reaches the end of its life, have been taken.Extending the lifetime of a qubit is expected to improve the accuracy of quantum sensing.A superconducting flux qubit has two quantum states, which correspond to the direction of the superconducting current, and by controlling these states with a magnetic field, the qubit can be used as a highly sensitive magnetometer.We interviewed Shiro Saito, a senior distinguished researcher at NTT Basic Research Laboratories, who aims to apply a hybrid combination of high-performance magnetometers and biological samples to pathological diagnosis.He talked about the detection of iron ions in neurons by using superconducting flux qubits and shared his research approach that looks beyond the red ocean of a research field.
Abstract Iron plays important physiological and pathological roles in the human body. However, microscopic analysis including redox status by a conventional electron spin resonance (ESR) spectrometer is difficult due to limited spatial resolution and sensitivity. Here we demonstrate magnetometry of cultured neurons on a polymeric film using a superconducting flux qubit that works as a sensitive magnetometer in a microscale area towards realizing ESR spectroscopy. By changing temperature (12.5–200 mK) and a magnetic field (2.5–12.5 mT), we observe a clear magnetization signal from the neurons that is well above the control magnetometry of the polymeric film itself. From ESR spectrum measured at 10 K, the magnetization signal is identified to originate from electron spins of iron ions in neurons. This technique to detect a bio-spin system can be extended to achieve ESR spectroscopy at the single-cell level, which will give the spectroscopic fingerprint of cells.
The quantum-technology revolution is reshaping computing, sensing, and communication. In magnetometry, recent advances leverage precise control of spin qubits and color centers in solid-state crystals for mesoscopic-scale sensing. Yet at very low temperatures, superconducting sensing technology remains unrivaled because of its non-invasiveness and higher sensitivity. Here we describe a class of superconducting sensors that offers low loss and quantum non-demolition measurement characteristics. We designed and fabricated a superconducting flux-tunable resonator (tRes) in a superconducting chip foundry and matured it to a level that combines the speed of an inductor-capacitor circuit with the flux sensitivity of a superconducting quantum interference device (SQUID) to perform magnetometry at milli-kelvin temperature to investigate targets. We introduce its fundamental functionality readily at MHz magnetic sampling rate, showcase two measurement modalities, and investigate three circuits with gradually increasing complexity to extract target-specific information. The combination of high sensitivity and fast readout characteristics make tRes an attractive and versatile magnetometer.