This year’s Bell Prize could not go to a more appropriate set of experiments. John Bell, of course, derived the theorem in 1964 which opened the door to experimental tests of the nonlocality of quantum mechanics; from Einstein, Podolsky, and Rosen’s famous 1935 paper up until that point, this issue had appeared to be a metaphysical one that could not be addressed in the laboratory. While over the two decades following the publication of Bell’s theorem, pioneering experiments by John Clauser, Alain Aspect, and others demonstrated to the satisfaction of most that the behaviour of entangled particles could not be explained by any local realistic theory, there remained certain “loopholes.” Aspect’s 1982 experiment, and an improved version carried out by Zeilinger’s group in 1998, had closed the “locality loophole” by ensuring that the choices of the two analyzer settings were made in a spacelike-separated fashion, yet there remained the “detection loophole”: photon detectors being so inefficient, only a small fraction of the pairs were being studied, and additional assumptions were required before an inequality could be violated based only on the detected sample. In 2001, Dave Wineland’s group succeeded in violating a Bell Inequality with no detection loophole, by using entangled ions, which could be detected with very high efficiency – but in this case it was impossible to simultaneously close the locality loophole. Thus as of a few years ago, there was no experiment that could unequivocally demonstrate, with no supplementary assumptions, the nonlocality of nature.
This changed just over half a century after Bell’s original paper, with the nearly simultaneous publication of the results of three different experiments, led by Ronald Hanson at the Delft University of Technology; Anton Zeilinger at the University of Vienna; and Sae Woo Nam at NIST in Boulder, Colorado. Using two very different systems – diamond nitrogen-vacancy (NV) centres in Delft, and photons in Vienna and Boulder – all three groups succeeded in violating Bell’s inequalities without the need for any supplementary assumptions. These experiments involved developing new quantum technologies and pushing them to the very limit. The photon experiments were enabled by a new type of superconducting photodetector, pioneered by Nam’s group, and able to achieve efficiencies well in excess of 90%. The NV-centre experiment implemented Bell’s “event-ready” scheme, entangling diamonds in laboratories 1.3 km apart. All three experiments relied on newly developed quantum random number generators. Together, they represent a tour de force, in which a 50-year quest to resolve this deep mystery finally reached its conclusion.
In addition to their role in addressing a deep fundamental question about the nature of reality, such loophole-free experiments are of profound importance for practical applications. Notably, they play a central role in the most secure quantum-cryptography protocols. But more broadly, they constitute a dramatic demonstration of the degree of control it is now possible to wield over a variety of quantum systems, and the rapid advances in the basic building blocks of future quantum technology. Awarding these medals to Ronald Hanson, Sae Woo Nam, and Anton Zeilinger epitomizes the goal of this prize in John Bell’s name: to honour outstanding advances relating both to the foundations of quantum mechanics and to the applications of these principles.
References to the work for which the prize is being awarded:
• “Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres,” 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. Pruneri, M. W. Mitchell, M. Markham, D. J. Twitchen, D. Elkouss, S. Wehner, T. H. Taminiau & R. Hanson, Nature 526, 682 (2015)
- “Strong Loophole-Free Test of Local Realism,” Lynden K. Shalm, Evan Meyer-Scott, B. G. Christensen, Peter L. Bierhorst, Michael A. Wayne, Deny Hamel, Martin J. Stevens, Thomas Gerrits, Scott C. Glancy, Michael S. Allman, Kevin J. Coakley, Shellee D. Dyer, Adriana E. Lita, Varun B. Verma, Joshua C. Bienfang, Alan L. Migdall, Yanbao Zhang, William Farr, Francesco Marsili, Matthew D. Shaw, Jeffrey Stern, Carlos Abellan, Waldimar Amaya, Valerio Pruneri, Thomas Jennewein, Morgan Mitchell, P. G. Kwiat, Richard P. Mirin, Emanuel H. Knill, Sae Woo Nam, Phys. Rev. Lett. 115, 250402 (2015)
- “Significant-Loophole-Free Test of Bell’s Theorem with Entangled Photons,” Marissa Giustina, Marijn A. M. Versteegh, Sören Wengerowsky, Johannes Handsteiner, Armin Hochrainer, Kevin Phelan, Fabian Steinlechner, Johannes Kofler, Jan-Åke Larsson, Carlos Abellán, Waldimar Amaya, Valerio Pruneri, Morgan W. Mitchell, Jörn Beyer, Thomas Gerrits, Adriana E. Lita, Lynden K. Shalm, Sae Woo Nam, Thomas Scheidl, Rupert Ursin, Bernhard Wittmann, and Anton Zeilinger, Phys. Rev. Lett. 115, 250401 (2015)
For further reading:
“Viewpoint: Closing the Door on Einstein and Bohr’s Quantum Debate,” Alain Aspect, Physics 8, 123 (2015)
- Last updated: February 2019