Reaching the quantum limit of sensitivity in electron spin resonance

Abstract

1 arXiv:1507.06831v1 [quant-ph] Jul 2015The detection and characterization of paramagnetic species by electron-spin resonance (ESR) spectroscopy is widely used throughout chemistry, biology, and materials science [1], from in-vivo imaging [2] to distance measurements in spinlabeled proteins [3]. ESR typically relies on the inductive detection of microwave signals emitted by the spins into a coupled microwave resonator during their Larmor precession -however, such signals can be very small, prohibiting the application of ESR at the nanoscale, for example, at the single-cell level or on individual nanoparticles. In this work, using a Josephson parametric microwave amplifier combined with high-quality factor superconducting micro-resonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly 4 orders of magnitude. We demonstrate the detection of 1700 bismuth donor spins in silicon within a single Hahn [4] echo with unit signal-to-noise (SNR) ratio, reduced to just 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence [5]. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance. The detection volume of our resonator is ∼0.02 nl, and our approach can be readily scaled down further to improve sensitivity, providing a new and versatile toolbox for ESR at the nanoscale.A wide variety of techniques are being actively explore to push the limits of sensitivity of ESR to the nanoscale, including approaches based on optical [6,7] or electrical [8,9] detection, as well as scanning probe methods [10,11]. Our focus in this work is to maximise the sensitivity of inductively detected pulsed ESR, in order to maintain the broad applicability to different spin species as well as fast high-bandwidth detection. Pulsed ESR spectroscopy proceeds by probing a sample coupled to a microwave resonator of frequency ω 0 and quality factor Q with sequences of microwave pulses that perform successive spin rotations, triggering the emission of a microwave signal called a spin-echo whose amplitude and shape contain the desired information about the number and properties of paramagnetic species. The spectrometer sensitivity is conveniently quantified by the minimal number of spins N min that can be detected within a single echo [4]. Conventional ESR spectrometers use 3D resonators with moderate quality factors in which the spins are only weakly coupled to the microwave photons and thus obtain a sensitivity of N min ∼ 10 13 spins at T = 300 K and X-band fre-2 quencies (ω 0 /2π ∼ 9 − 10 GHz). To increase the sensitivity, micro-fabricated metallic planar resonators with smaller mode volumes have been used, resulting in larger spin-microwave coupling [12,13]. Combined with operation at T = 4 K and the use of low-noise cryogenic amplifiers and superconducting high-Q thin-film resonators, sensitivities up to N min ∼ 10 7 spins have ...

Molecular Spins in the Context of Quantum Technologies

“…Experiments are currently testing different solutions since the detection of tiny signals requires extraordinary sensitivity. By using Josephson junctions amplification, the detection of a small ensemble of about 10 3 spins has been reported and this is, at present, the best performance of EPR nano-detection [65]. The use of electric field component to manipulate spins is a further attractive alternative and several mechanisms have been proposed [66] and they are currently under investigation [67].…”

Magnetic Resonance with Squeezed Microwaves

“…The noise present in these signals determines the spectrometer sensitivity and is ultimately limited by the fluctuations in the microwave field at the cavity output. The thermal contribution to these fluctuations can be removed by lowering the temperature T of the sample and cavity such that k B T ≪ ℏω s , where ω s is the spin resonance frequency and k B is Boltzmann's constant [2]. However, even at these cryogenic temperatures, quantum fluctuations of the electromagnetic field remain and pose a fundamental limitation to the achievable sensitivity.…”

Sensitive spin detection using an on-chip SQUID-waveguide resonator

“…Multi-level systems are promising for implementing few-qubits algorithms 12 or as quantum memories [13][14][15][16] . For quantum technology applications, a higher sensitivity electron spin resonance (ESR) measurement is needed to be able to manipulate spins in mesoscopic crystals placed on superconducting chips [17][18][19][20][21] .Compared to other ultra-high sensitivity ESR measurements 22,23 , the use of Josephson junctions can increase significantly the spatial resolution of the magnetic detection while allowing an on-chip implementation. For instance, the magnetic signal of one nanoparticle is detectable if placed on the junction of a micrometer sized superconducting quantum interference device (micro-SQUID) 24 .…”