Quantum entanglement between an optical photon and a solid-state spin qubit

Journal name:
Date published:

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.

At a glance


  1. Scheme for spin-photon entanglement.
    Figure 1: Scheme for spin-photon entanglement.

    a, Following selective excitation to the |A2right fence 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 637nm and an off-resonant laser at 532nm 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.

  2. Characterization of NV centres.
    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 |±1right fence right arrow |A2right fence transition in absorption. The system is initially prepared in |+1right fence (blue) or |−1right fence (red). We then apply a laser pulse of varying polarization to the |A2right fence state while collecting fluorescence. Oscillations with visibility 77±10% indicate that the transitions linking |±1right fence to |A2right fence are circularly polarized and mutually orthogonal (see Supplementary Information for details).

  3. Experimental procedure for entanglement generation.
    Figure 3: Experimental procedure for entanglement generation.

    a, After spin polarization into |0right fence, population is transferred to |+1right fence by a microwave π-pulse (Ω+1). The NV is excited to |A2right fence 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 |+1right fence or |−1right fence is transferred to |0right fence. If an |Hright fence or |Vright fence photon is detected, a τ–2πτ echo sequence (see Supplementary Information) is applied with Ω+1 and Ω−1, followed by a π-pulse which transfers the population in |Mright fence (see text) to |0right fence. c, The population in |0right fence is measured using the 637.20-nm optical readout transition. d, Pulse sequence for the case where an |Hright fence or |Vright fence 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).

  4. Measurement of spin-photon correlations in two bases.
    Figure 4: Measurement of spin-photon correlations in two bases.

    a, Conditional probability of measuring |±1right fence after the detection of a σ+ or σ− photon. b, Conditional probability of measuring |±right fence 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 |Mright fence 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 ±1s.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).


  1. Nielsen, M. A. & Chuang, I. L. Quantum Computation and Quantum Information (Cambridge Univ. Press, 2000)
  2. Aspect, A., Grangier, P. & Roger, G. Experimental realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: a new violation of Bell's inequalities. Phys. Rev. Lett. 49, 9194 (1982)
  3. Blinov, B. B., Moehring, D. L., Duan, L. M. & Monroe, C. Observation of entanglement between a single trapped atom and a single photon. Nature 428, 153157 (2004)
  4. Volz, J. et al. Observation of entanglement of a single photon with a trapped atom. Phys. Rev. Lett. 96, 030404 (2006)
  5. Wilk, T., Webster, S. C., Kuhn, A. & Rempe, G. Single-atom single-photon quantum interface. Science 317, 488490 (2007)
  6. Yuan, Z.-S. et al. Experimental demonstration of a BDCZ quantum repeater node. Nature 454, 10981101 (2008)
  7. Matsukevich, D. et al. Entanglement of a photon and a collective atomic excitation. Phys. Rev. Lett. 95, 040405 (2005)
  8. Sherson, J. F. et al. Quantum teleportation between light and matter. Nature 443, 557560 (2006)
  9. Cabrillo, C., Cirac, J. I., Garcia-Fernandez, P. & Zoller, P. Creation of entangled states of distant atoms by interference. Phys. Rev. A 59, 10251033 (1999)
  10. Chou, C. W. et al. Measurement-induced entanglement for excitation stored in remote atomic ensembles. Nature 438, 828832 (2005)
  11. Moehring, D. L. et al. Entanglement of single-atom quantum bits at a distance. Nature 449, 6871 (2007)
  12. Scully, M. O. & Drühl, K. Quantum eraser: a proposed photon correlation experiment concerning observation and “delayed choice” in quantum mechanics. Phys. Rev. A 25, 22082213 (1982)
  13. Kimble, H. J. The quantum internet. Nature 453, 10231030 (2008)
  14. Childress, L., Taylor, J. M., Sørensen, A. S. & Lukin, M. D. Fault-tolerant quantum communication based on solid-state photon emitters. Phys. Rev. Lett. 96, 070504 (2006)
  15. Duan, L.-M. & Monroe, C. Robust quantum information processing with atoms, photons, and atomic ensembles. Adv. At. Mol. Opt. Phys. 55, 419464 (2008)
  16. Neumann, P. et al. Multipartite entanglement among single spins in diamond. Science 320, 13261329 (2008)
  17. Ansmann, M. et al. Violation of Bell's inequality in Josephson phase qubits. Nature 461, 504506 (2009)
  18. DiCarlo, L. et al. Demonstration of two-qubit algorithms with a superconducting quantum processor. Nature 460, 240244 (2009)
  19. de Riedmatten, H., Afzelius, M., Staudt, M. U., Simon, C. & Gisin, N. A solid-state light-matter interface at the single-photon level. Nature 456, 773 (2008)
  20. Balasubramanian, G. et al. Ultralong spin coherence time in isotopically engineered diamond. Nature Mater. 8, 383387 (2009)
  21. Fuchs, G. D., Dobrovitski, V. V., Toyli, D. M., Heremans, F. J. & Awschalom, D. D. Gigahertz dynamics of a strongly driven single quantum spin. Science 326, 15201522 (2009)
  22. Dutt, M. V. G. et al. Quantum register based on individual electronic and nuclear spin qubits in diamond. Science 316, 13121316 (2007)
  23. Tamarat, P. et al. Spin-flip and spin-conserving optical transitions of the nitrogen-vacancy centre in diamond. N. J. Phys. 10, 045004 (2008)
  24. Manson, N., Harrison, J. & Sellars, M. Nitrogen-vacancy center in diamond: model of the electronic structure and associated dynamics. Phys. Rev. B 74, 104303 (2006)
  25. Santori, C. et al. Coherent population trapping of single spins in diamond under optical excitation. Phys. Rev. Lett. 97, 247401 (2006)
  26. Kaiser, F. et al. Polarization properties of single photons emitted by nitrogen-vacancy defect in diamond at low temperature. left fencehttp://arXiv.org/abs/0906.3426right fence (2009)
  27. Englund, D., Faraon, A., Fushman, I., Stoltz, N. & Petroff, P. Controlling cavity reflectivity with a single quantum dot. Nature 450, 857861 (2007)
  28. Schietinger, S., Schröder, T. & Benson, O. One-by-one coupling of single defect centers in nanodiamonds to high-Q modes of an optical microresonator. Nano Lett. 8, 39113915 (2008)
  29. Wang, C. F. et al. Fabrication and characterization of two-dimensional photonic crystal microcavities in nanocrystalline diamond. Appl. Phys. Lett. 91, 201112 (2007)
  30. Fleischhauer, M., Imamoğlu, A. & Marangos, J. P. Electromagnetically induced transparency: optics in coherent media. Rev. Mod. Phys. 77, 633673 (2005)

Download references

Author information

  1. These authors contributed equally to this work.

    • E. Togan &
    • Y. Chu


  1. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA

    • E. Togan,
    • Y. Chu,
    • A. S. Trifonov,
    • L. Jiang,
    • J. Maze,
    • L. Childress,
    • M. V. G. Dutt,
    • A. S. Zibrov &
    • M. D. Lukin
  2. Department of Physics, California Institute of Technology, Pasadena, California 91125, USA

    • L. Jiang
  3. Institute for Quantum Information, California Institute of Technology, Pasadena, California 91125, USA

    • L. Jiang
  4. Department of Physics and Astronomy, Bates College, Lewiston, Maine 04240, USA

    • L. Childress
  5. Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

    • M. V. G. Dutt
  6. QUANTOP, The Niels Bohr Institute, University of Copenhagen, DK2100 Copenhagen, Denmark

    • A. S. Sørensen
  7. Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843, USA

    • P. R. Hemmer


All authors contributed extensively to the work presented in this paper.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Author details

Supplementary information

PDF files

  1. Supplementary Information (1003K)

    This file contains 1 Supplementary Methods, 2 Level structure and polarization properties of the NV centre, 3 Spin readout, 4 Verification of polarization selection rules for A2 state, 5 Effects of magnetic environment, detunings, and echo, 6 Fidelity estimates, Supplementary Figures S1-S7 with legends and References.

Additional data