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 ...

Spontaneous emission of radiation is one of the fundamental mechanisms by which an excited quantum system returns to equilibrium. For spins, however, spontaneous emission is generally negligible compared to other non-radiative relaxation processes because of the weak coupling between the magnetic dipole and the electromagnetic field. In 1946, Purcell realized that the spontaneous emission rate can be strongly enhanced by placing the quantum system in a resonant cavity -an effect which has since been used extensively to control the lifetime of atoms and semiconducting heterostructures coupled to microwave or optical cavities, underpinning single-photon sources. Here we report the first application of these ideas to spins in solids. By coupling donor spins in silicon to a superconducting microwave cavity of high quality factor and small mode volume, we reach for the first time the regime where spontaneous emission constitutes the dominant spin relaxation mechanism. The relaxation rate is increased by three orders of magnitude when the spins are tuned to the cavity resonance, showing that energy relaxation can be engineered and controlled on-demand. Our results provide a novel and general way to initialise spin systems into their ground state, with applications in magnetic resonance and quantum information processing. They also demonstrate that, contrary to popular belief, the coupling between the magnetic dipole of a spin and the electromagnetic field can be enhanced up to the point where quantum fluctuations have a dramatic effect on the spin dynamics; as such our work represents an important step towards the coherent magnetic coupling of individual spins to microwave photons.Comment: 8 pages, 6 figures, 1 tabl

We report measurements of spin-dependent scattering of conduction electrons by neutral donors in accumulation-mode field-effect transistors formed in isotopically enriched silicon. Spin-dependent scattering was detected using electrically detected magnetic resonance where spectra show resonant changes in the source-drain voltage for conduction electrons and electrons bound to donors. We discuss the utilization of spindependent scattering for the readout of donor spin-states in silicon based quantum computers. Neutral impurity scattering is spin-dependent because different spin configurations of the conduction and donor electrons (singlet or triplet) imply a different spatial distribution of the two-electron wavefunction, which translates into a difference in scattering crosssections. This SDS process by phosphorus impurities in an accumulation-mode fieldeffect transistor (aFET) was first observed by Ghosh and Silsbee using electrically detected magnetic resonance (EDMR) [9]. In an EDMR experiment, a static magnetic field induces a Zeeman splitting in the electron energy levels, and in thermal equilibrium, triplet scattering is favored as more spins are aligned with the static field. Singlet scattering can be enhanced by inducing spin flips with a resonant microwave field. This increase in singlet content then registers as an effective channel resistance change of the 3 aFET. Ghosh and Silsbee used large-area aFETs (1×0.1 mm 2 ) formed in bulk-doped silicon with about 2×10 17 phosphorus/cm 3 [9]. The number of donors close (~10 nm) to the aFET channel that contribute to SDS was estimated to be ~10 8 . However, bulk donors far away from the channel that did not contribute to SDS caused an undesired bolometric signal due to resonant microwave absorption, and substantial efforts were undertaken to resolve interfering bolometric effects and to isolate the SDS signal.In the present work, we demonstrate SDS by neutral 121 Sb donors in silicon aFETs. In order to avoid bolometric signals, aFETs were formed in undoped silicon and ~6×10 6 donors were implanted into the transistor channel. While most donor-based silicon quantum computer proposals have suggested spins of 31 P as qubits, 121 Sb is used in our experiments due to its smaller straggling in the channel implantation process, lower diffusion rates in silicon, and to avoid spurious signals arising from residual background 31 P atoms in the silicon substrate or from the polycrystalline silicon gate.