Quantum entanglement is among the most fascinating aspects of quantum theory1. Entangled optical photons are now widely used for fundamental tests of quantum mechanics2 and applications such as quantum cryptography1. Several recent experiments demonstrated entanglement of optical photons with trapped ions3, atoms4, 5 and atomic ensembles6, 7, 8, which are then used to connect remote long-term memory nodes in distributed quantum networks9, 10, 11. Here we realize quantum entanglement between the polarization of a single optical photon and a solid-state qubit associated with the single electronic spin of a nitrogen vacancy centre in diamond. Our experimental entanglement verification uses the quantum eraser technique5, 12, and demonstrates that a high degree of control over interactions between a solid-state qubit and the quantum light field can be achieved. The reported entanglement source can be used in studies of fundamental quantum phenomena and provides a key building block for the solid-state realization of quantum optical networks13, 14.
Figures at a glance
Figure 1: Scheme for spin-photon entanglement. 
a, Following selective excitation to the |A2
state, the Λ-type three level system decays to two different spin states through the emission of orthogonally polarized photons, resulting in spin–photon entanglement. b, Schematic of the optical set-up. Individual NV centres are isolated and addressed optically using a microscope objective. Two resonant lasers at 637 nm and an off-resonant laser at 532 nm address various optical transitions. Fluorescence emitted from the NV centre passes through a quarter-wave plate (QWP) and is spectrally separated into PSB and ZPL channels, and detected with avalanche photodiodes (APDs). The latter channel contains entangled photons and is sent using a beam splitter (BS) through a polarization analysis stage consisting of a half-wave plate (HWP) and a polarizing beam splitter (PBS). See text for details.
Figure 2: Characterization of NV centres. 
a, Energy levels of the NV centre under strain. Solid lines are based on a theoretical model23 and dots are data from seven NV centres. The dashed line indicates the NV centre used in this paper. b, Excitation spectrum of the NV centre under continuous wave (c.w.) microwave radiation. c, Polarization properties of the |±1
|A2 transition in absorption. The system is initially prepared in |+1 (blue) or |−1 (red). We then apply a laser pulse of varying polarization to the |A2 state while collecting fluorescence. Oscillations with visibility 77 ± 10% indicate that the transitions linking |±1 to |A2 are circularly polarized and mutually orthogonal (see Supplementary Information for details).
Figure 3: Experimental procedure for entanglement generation. 
a, After spin polarization into |0
, population is transferred to |+1 by a microwave π-pulse (Ω+1). The NV is excited to |A2 with a 637.19-nm π-pulse and the ZPL emission is collected. b, If a σ+ or σ− photon is detected, the population in |+1 or |−1 is transferred to |0. If an |H or |V photon is detected, a τ–2π–τ echo sequence (see Supplementary Information) is applied with Ω+1 and Ω−1, followed by a π-pulse which transfers the population in |M (see text) to |0. c, The population in |0 is measured using the 637.20-nm optical readout transition. d, Pulse sequence for the case where an |H or |V ZPL photon is detected (time axis not to scale). If a σ± photon is detected instead, only a π-pulse on either Ω+1 or Ω−1 is used for spin readout. Inset, detection time of ZPL channel photons, showing reflection from diamond surface and subsequent NV emission (blue) and background counts (purple).
Figure 4: Measurement of spin-photon correlations in two bases. 
a, Conditional probability of measuring |±1
after the detection of a σ+ or σ− photon. b, Conditional probability of measuring |± after the detection of an H or V photon, extracted from a fit to data shown in c and d. c, d, Measured conditional probability of finding the electronic spin in the state |M after detection of a V (c) or H (d) photon at time td. Blue shaded region is the 68% confidence interval for the fit (solid line) to the time-binned data (Supplementary Information). Errors bars on data points show ±1 s.d. Combined with the data shown in a, oscillations with amplitude outside of the yellow regions result in fidelities greater than 0.5. The visibility of the measured oscillations are 0.59 ± 0.18 (c) and 0.60 ± 0.11 (d).
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