Exciting news from the Ronald Hanson Lab (Delft/QN/QT): 'Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km'


Nature News reports:

"Entanglement swapping

In the latest paper, which was submitted to the arXiv preprint repository on 24 August and has not yet been peer reviewed, a team led by Ronald Hanson of Delft University of Technology reports the first Bell experiment that closes both the detection and the communication loopholes. The team used a cunning technique called entanglement swapping to combine the benefits of using both light and matter. The researchers started with two unentangled electrons sitting in diamond crystals held in different labs on the Delft campus, 1.3 kilometres apart. Each electron was individually entangled with a photon, and both of those photons were then zipped to a third location. There, the two photons were entangled with each other — and this caused both their partner electrons to become entangled, too.

This did not work every time. In total, the team managed to generate 245 entangled pairs of electrons over the course of nine days. The team's measurements exceeded Bell’s bound, once again supporting the standard quantum view. Moreover, the experiment closed both loopholes at once: because the electrons were easy to monitor, the detection loophole was not an issue, and they were separated far enough apart to close the communication loophole, too.

“It is a truly ingenious and beautiful experiment,” says Anton Zeilinger, a physicist at the Vienna Centre for Quantum Science and Technology. “I wouldn’t be surprised if in the next few years we see one of the authors of this paper, along with some of the older experiments, Aspect’s and others, named on a Nobel prize,” says Matthew Leifer, a quantum physicist at the Perimeter Institute in Waterloo for Theoretical Physics, Ontario. “It’s that exciting.”

A loophole-free Bell test also has crucial implications for quantum cryptography, says Leifer. Companies already sell systems that use quantum mechanics to block eavesdroppers. The systems produce entangled pairs of photons, sending one photon in each pair to the first user and the other photon to the second user. The two users then turn these photons into a cryptographic key that only they know. Because observing a quantum system disrupts its properties, if someone tries to eavesdrop on this process it will produce a noticeable effect, setting off an alarm."

Click here to read more on this on Nature News, or click here for more information in Dutch

..Or click here to visit the Hansonlab website, where you can also find the link to the article published on archXiv.


Abstract of the article 'Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km' by B. Hensen, H. Bernien, A.E. Dréau, A. Reiserer, N. Kalb, M.S. Blok, J. Ruitenberg, R.F.L. Vermeulen, R.N. Schouten, D. Elkouss, S. Wehner, T.H. Taminiau, R. Hanson et al., submitted on arXiv on 24 August 2015.

For more than 80 years, the counterintuitive predictions of quantum theory have stimulated debate about the nature of reality. In his seminal work, John Bell proved that no theory of nature that obeys locality and realism can reproduce all the predictions of quantum theory. Bell showed that in any local realist theory the correlations between distant measurements satisfy an inequality and, moreover, that this inequality can be violated according to quantum theory. This provided a recipe for experimental tests of the fundamental principles underlying the laws of nature. 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. Here we report on 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 electron spins. Efficient spin readout 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. We perform 245 trials testing the CHSH-Bell inequality S≤2 and find S=2.42±0.20. A null hypothesis test yields a probability of p=0.039 that a local-realist model for space-like separated sites produces data with a violation at least as large as observed, even when allowing for memory in the devices. This result rules out large classes of local realist theories, and paves the way for implementing device-independent quantum-secure communication and randomness certification. 


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