Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km

In his seminal work^[1], John Bell proved that no theory of nature that obeys locality, realism and free will can reproduce all the predictions of quantum theory. In the past decades, numerous ingenious Bell inequality tests have been reported. However, because of experimental limitations, all experiments to date required additional assumptions to obtain a contradiction with local realism, resulting in loopholes. I will present the main result^[2] of my PhD, a Bell experiment that is free of any such additional assumption and thus directly tests the principles underlying Bell’s inequality. We employ an event-ready scheme that enables the generation of high-fidelity entanglement between distant electronic spins^[3]. Efficient spin readout^[4] avoids the fair sampling assumption (detection loophole), while the use of fast random basis selection and readout combined with a spatial separation of 1.3 km ensure the required locality conditions. This result rules out large classes of local realist theories, and paves the way for implementing device-independent quantum-secure communication and randomness certification. In combination with the control of nuclear spins around the NV centre^[5,6], it establishes the NV centre as a prime candidate for the node of a future quantum network^[7]. I will discuss the set of optical and microwave control techniques used, discuss the limitations of the NV centre, and possible improvements.

1. Bell, J. S. On the Einstein-Podolsky-Rosen paradox. Physics 1, 195 (1964).
2. B. Hensen, H. Bernien, A.E. Dréau, A. Reiserer, N. Kalb, M.S. Blok, J. Ruitenberg, R.F.L. Vermeulen, R.N. Schouten, C. Abellán, W. Amaya, V. Pruner, M. W. Mitchell, M. Markham, D.J. Twitchen, D. Elkouss, S. Wehner, T.H. Taminiau, R. Hanson. Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km. arXiv:1508.05949 (2015).
3. H. Bernien, B. Hensen, W. Pfaff, G. Koolstra, M. S. Blok, L. Robledo, T. H. Taminiau, M. Markham, D. J. Twitchen, L. Childress, R. Hanson. Heralded entanglement between solid-state qubits separated by three meters. Nature 497, 86 (2013)
4. L. Robledo, L. Childress, H. Bernien, B. Hensen, P.F. A. Alkemade, R. Hanson. High-fidelity projective read-out of a solid-state spin quantum register. Nature 477, 547 (2011).
5. W. Pfaff, B. Hensen, H. Bernien, S. B. van Dam, M. S. Blok, T. H. Taminiau, M. J. Tiggelman, R. N., Schouten, M. Markham, D. J. Twitchen, R. Hanson. Unconditional quantum teleportation between distant solid-state qubits. Science 345, 532 (2014).
6. J. Cramer, N. Kalb, M. A. Rol, B. Hensen, M. S. Blok, M. Markham, D. J. Twitchen, R. Hanson & T. H. Taminiau. Quantum error correction by stabilizer measurements and feedback. Submitted (2015).
7. H. J. Kimble, The quantum internet. Nature 453, 1023 (2008).