Electron paramagnetic resonance spectroscopy using a single artificial atom

Abstract

Electron paramagnetic resonance (EPR) spectroscopy is an important technology in physics, chemistry, materials science, and biology [1]. 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 unlabeled metalloproteins in a single cell. In conventional EPR spectrometers, the energy transfer from the spins to the cavity at a Purcell enhanced rate [2] plays an essential role [1,3,4] and requires the spins to be resonant with the cavity, however the size of the cavity (limited by the wavelength) makes it difficult to improve the spatial resolution. 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 both in terms of its quantum properties and its much stronger coupling with magnetic fields. We have achieved a sensitivity of ∼400 spins/ √ Hz with a magnetic sensing volume around 10 −14 λ 3 (50 femto-liters). This corresponds to an improvement of two-order of magnitude in the magnetic sensing volume compared with the best cavity based spectrometers while maintaining a similar sensitivity as those spectrometers [5,6]. Our artificial atom is suitable for scaling down and thus paves the way for measuring single spins on the nanometer scale.EPR spectroscopy is an essential tool for characterizing the properties of electron spins in materials. Due to the wide variety of EPR applications, significant efforts have been devoted to improving both its sensitivity and spatial resolution. A conventional EPR spectrometer relies on energy exchange (transverse) coupling, where the spins and detector should be resonant. In particular, in a leaky cavity limit, the spins mainly emits photons to the measurement chain at the Purcell enhanced relaxation rate [3], and the detector absorbs the photon energy as a signal. Recently, sensitive EPR spectrometers based on a superconducting resonator have been realized [4][5][6][7] with a measurement chain that uses a quantum limited amplifier. This approach limits the size of the device according to the wavelength, and so such spectrometers may not scale well at a smaller size. On the other hand, it is also possible to observe the EPR phenomenon without a cavity and magnetization detection [8] is one such example. Magnetically induced force detection [9] has recently been demonstrated that achieves high sensitivity and spatial resolution. In these cases, energy transfer between spins and the detector is suppressed due to the large detuning, thus the signal is detected without significant disturbance to the spin system. However, such non-resonant methods still require improved in their sensitivity.In this paper, we demonstrate sensitive local EPR spectroscopy using an artificial atom (a superconducting flux qubit [10]) as a magnetic field sensor [11,12]. The superconducting flux qubit has two distinct states correspo...

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