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Reaching the quantum limit of sensitivity in electron spin resonance


The detection and characterization of paramagnetic species by electron spin resonance (ESR) spectroscopy is widely used throughout chemistry, biology and materials science1, from in vivo imaging2 to distance measurements in spin-labelled proteins3. ESR 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). Here, using a Josephson parametric microwave amplifier combined with high-quality-factor superconducting microresonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly four orders of magnitude4,5. We demonstrate the detection of 1,700 bismuth donor spins in silicon within a single Hahn6 echo with unit signal-to-noise ratio, reduced to 150 spins by averaging a single Carr–Purcell–Meiboom–Gill sequence7. 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 versatile toolbox for ESR at the nanoscale.

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Figure 1: Experimental set-up and spin system.
Figure 2: Sample characterization.
Figure 3: Spectrometer sensitivity.
Figure 4: Further sensitivity improvement with the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence.


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The authors acknowledge technical support from P. Sénat, D. Duet, J.-C. Tack, P. Pari, P. Forget, as well as useful discussions within the Quantronics Group. The authors also acknowledge support from the European Community's Seventh Framework Programme (FP7/2007-2013) through European Research Council grants nos. 615767 (CIRQUSS), 279781 (ASCENT) and 630070 (quRAM) and through the QIPC project SCALEQIT, and from C'Nano IdF through the QUANTROCRYO project. J.J.L.M. is supported by the Royal Society. C.C. Lo is supported by the Royal Commission for the Exhibition of 1851. B. Julsgaard and K. Mølmer acknowledge support from the Villum Foundation. C.D.W. and T.S. acknowledge support from the Office of Science of the US Department of Energy under contract no. DE-AC02-05CH11231.

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Authors and Affiliations



A.B., J.J.P., J.J.L.M. and P.B. designed the experiment. X.Z. and D.V. designed and fabricated the Josephson parametric amplifier. C.C.L., C.D.W., T.S. and M.L.W.T. provided the Bi-implanted isotopically purified Si sample. A.B., J.J.P., and Y.K. fabricated the sample and performed the measurements. A.B., J.J.P., Y.K., J.J.L.M. and P.B. analysed the data. A.B., J.J.P., B.J. and K.M. performed the numerical simulations. J.J.L.M., D.E., D.V. and P.B. supervised the project. A.B., J.J.P., Y.K., M.S., D.V., D.E., B.J., K.M., J.J.L.M. and P.B. all contributed to the writing of the paper.

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Correspondence to P. Bertet.

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The authors declare no competing financial interests.

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Bienfait, A., Pla, J., Kubo, Y. et al. Reaching the quantum limit of sensitivity in electron spin resonance. Nature Nanotech 11, 253–257 (2016).

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