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arXiv:2003.09131 · Applied Physics Letters

Electron spin resonance with up to 20 spin sensitivity measured using a superconducting flux qubit

Rangga P. Budoyo, Kosuke Kakuyanagi, Hiraku Toida, Yuichiro Matsuzaki, Shiro Saito

Published May 11, 2020• 24 citations
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Abstract

We report on electron spin resonance spectroscopy measurements using a superconducting flux qubit with a sensing volume of 6 fl. The qubit is read out using a frequency-tunable Josephson bifurcation amplifier, which leads to an inferred measurement sensitivity of about 20 spins in a 1 s measurement. This sensitivity represents an order of magnitude improvement when compared to flux-qubit schemes using a direct current-superconducting quantum interference device switching readout. Furthermore, noise spectroscopy reveals that the sensitivity is limited by flicker (1/f) flux noise.

DOI
10.1063/1.5144722

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We report on electron spin resonance spectroscopy measurements using a superconducting flux qubit with a sensing volume of 6 fl. The qubit is read out using a frequency-tunable Josephson bifurcation amplifier, which leads to an inferred measurement sensitivity of about 20 spins in a 1 s measurement. This sensitivity represents an order of magnitude improvement when compared to flux-qubit schemes using a direct current-superconducting quantum interference device switching readout. Furthermore, noise spectroscopy reveals that the sensitivity is limited by flicker (1/f) flux noise.
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.
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 performed magnetic field and frequency tunable electron paramagnetic resonance spectroscopy of an Er3+{\mathrm{Er}}^{3+}Er3+-doped Y2SiO5{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Y2​SiO5​ crystal by observing the change in flux induced on a direct current superconducting quantum interference device (dc SQUID) loop of a tunable Josephson bifurcation amplifer. The observed spectra show multiple transitions which agree well with the simulated energy levels, taking into account the hyperfine and quadrupole interactions of 167Er^{167}\mathrm{Er}167Er. The sensing volume is about 0.15 pl, and our inferred measurement sensitivity (limited by external flux noise) is approximately $1.5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ electron spins for a 1-s measurement. The sensitivity value is two orders of magnitude better than similar schemes using a dc SQUID switching readout.
We have implemented a resonant circuit that uses a SQUID as a flux-sensitive Josephson inductor for qubit readout. In contrast to the conventional switching current measurement that generates undesired quasiparticles when the SQUID switches to the voltage state, our approach keeps the readout SQUID biased along the supercurrent branch during the measurement. By incorporating the SQUID inductor in a high-Q resonant circuit, we can distinguish the two flux states of a niobium persistent-current (PC) qubit by observing a shift in the resonant frequency of both the magnitude and the phase spectra. The readout circuit was also characterized in the nonlinear regime to investigate its potential use as a nonlinear amplifier.
Abstract Electron paramagnetic resonance (EPR) spectroscopy is an important technology in physics, chemistry, materials science, and biology. Sensitive detection with a small sample volume is a key objective in these areas, because it is crucial, for example, for the readout of a highly packed spin based quantum memory or the detection of transition metals in biomaterials. Here, we demonstrate a novel EPR spectrometer using a single artificial atom as a sensitive detector of spin magnetization. The artificial atom, a superconducting flux qubit, provides advantages in terms of its strong coupling with magnetic fields. We estimate a sensitivity of ~400 spins·Hz−1/2 with a magnetic sensing volume around 10−14λ3 (50 femtoliters), where λ is the wavelength of the irradiated microwave. Our artificial atom works as a highly sensitive EPR spectrometer with micron-scale area with future opportunity for measuring single spins on the nanometer scale.