We experimentally demonstrate the first inductive readout of optically hyperpolarized phosphorus-31 donor nuclear spins in an isotopically enriched silicon-28 crystal. The concentration of phosphorus donors in the crystal was 1.5 x 10 15 cm −3 , three orders of magnitude lower than has previously been detected via direct inductive detection. The signal-to-noise ratio measured in a single free induction decay from a 1 cm 3 sample (≈ 10 15 spins) was 113. By transferring the sample to an X-band ESR spectrometer, we were able to obtain a lower bound for the nuclear spin polarization at 1.7 K of ∼ 64 %. The 31 P-T2 measured with a Hahn echo sequence was 420 ms at 1.7 K, which was extended to 1.2 s with a Carr Purcell cycle. The T1 of the 31 P nuclear spins at 1.7 K is extremely long and could not be determined, as no decay was observed even on a timescale of 4.5 hours. Optical excitation was performed with a 1047 nm laser, which provided above bandgap excitation of the silicon. The build-up of the hyperpolarization at 4.2 K followed a single exponential with a characteristic time of 577 s, while the build-up at 1.7 K showed bi-exponential behavior with characteristic time constants of 578 s and 5670 s.Nuclear spin defects are archetype models of qubits in solid state systems. We expect them to have long coherence times and to be well controlled [4, 5]. However, to date they have mainly been studied via their interaction to a neighboring electron spin [2, 4, 5, 7]. Such experiments are indirect probes of the local fields seen by the nuclear spins. Here, we directly observe nuclear spin defects in a dilute sample of silicon and through a combination of FID and echo measurements we characterize the local field and its fluctuations.The phosphorus donor impurity in silicon is a potentially promising candidate for a hybrid quantum information processor [5]. In natural abundance bulk silicon, the 300-600 µs coherence time of the donor electron spin at low temperatures has been shown to be limited primarily by spectral diffusion due to the 29 Si nuclei (4.7 % natural abundance) [6]. Similar coherence times have also been measured at the level of individual donors [2, 7]. In the bulk, this coherence time has been extended to 0.6 s by isotopically engineering the silicon lattice to reduce the 29 Si nuclear spin concentration and simultaneously reduce the donor concentration to minimize the dipolar coupling between electron spin donors (thus reducing instantaneous diffusion effects) [8]. The 31 P donor nuclear spin has also been shown to have extremely long coherence times (180 s at low temperature and B=845 G) [9], limited primarily by electron spin fluctuations. By ionizing the donors with below-gap narrow-line laser excitation and using dynamical decoupling techniques, the phosphorus nuclear spin coherence times were extended to 39 minutes at room temperature and 3 hours at 4.2 K in a silicon-28 lattice, at ∼ 845 G [10].It has recently been shown that it is possible to optically hyperpolarize the 31 P donor nuclear spins in...
