We report electron spin resonance spectroscopy measurements performed at millikelvin temperatures in a custom-built spectrometer comprising a superconducting micro-resonator at 7 GHz and a Josephson parametric amplifier. Owing to the small ∼10 −12 λ 3 magnetic resonator mode volume and to the low noise of the parametric amplifier, the spectrometer sensitivity reaches 260 ± 40 spins/echo and 65 ± 10 spins/ √ Hz, respectively.PACS numbers: 07.57. Pt, Electron spin resonance (ESR) is a well-established spectroscopic method to analyze paramagnetic species, utilized in materials science, chemistry and molecular biology to characterize reaction products and complex molecules 1 . In a conventional ESR spectrometer based on the so-called inductive detection method, the paramagnetic spins precess in an external magnetic field B 0 and radiate weak microwave signals into a resonant cavity, whose emissions are amplified and measured.Despite its widespread use, ESR has limited sensitivity, and large amounts of spins are necessary to accumulate sufficient signal. Most conventional ESR spectrometers operate at room temperature and employ three-dimensional cavities. At X-band 2 , they require on the order of ∼10 13 spins to obtain sufficient signal in a single echo 1 . Enhancing this sensitivity to smaller spin ensembles and eventually the singlespin limit is highly desirable and is a major research subject. This has been achieved by employing alternative detection schemes including optically detected magnetic resonance (ODMR) 3,4 , scanning probe based techniques 5-9 , SQUIDs 10 and electrically detected magnetic resonance 11,12 . For instance, ODMR achieves single spin sensitivity through optical readout of the spin state. However, this requires the presence of suitable opa) sebastian.probst@cea.fr tical transitions in the energy spectrum of the system of interest, which makes it less versatile.In recent years, there has been a parallel effort to enhance the sensitivity of inductive ESR detection [13][14][15][16][17][18][19][20] . This development has been triggered by the progress made in the field of circuit quantum electrodynamics (cQED) 21 , where high fidelity detection of weak microwave signals is essential for the measurement and manipulation of superconducting quantum circuits. In particular, it has been theoretically predicted 22 that single-spin sensitivity should be reachable by combining high quality factor superconducting micro-resonators and Josephson Parametric Amplifiers (JPAs) 23 , which are sensitive microwave amplifiers adding as little noise as allowed by quantum mechanics to the incoming spin signal.Based on this principle, ESR spectroscopy measurements 18 demonstrated a sensitivity of 1700 spins/ √ Hz. In this work, we build on these efforts and show that, by optimizing the superconducting resonator design, the sensitivity can be enhanced to the level of 65 spins/ √ Hz. Figure 1(a) shows a schematic design of the spectrometer consisting of a superconducting LC resonant circuit capacitively coupled to the...