The experiment has remarkable practical value as well, as entanglement allows for a form of secure communication. The measurement outcomes can be used as an encryption key which is fundamentally impossible to eavesdrop on as it doesn’t travel between two points, but is created through the instantaneous entanglement link.
At issue is proving that quantum entanglement does not occur due to some strange unexplainable communication factor, or variable as Einstein suggested—a task that has proved exceptionally challenging—so much so that despite nearly a century of trying, no one, until now apparently, has been able to do it.
One of the ways to "prove" that entanglement does not occur due to some unknown factor that allows for communication to move between two entanglement particles, is to cause entanglement to come about between two particles that are far enough apart that any unknown force allowing them to communicate, would have to travel faster than light, which everyone agrees cannot happen. That was one of the loopholes described by John Bell, who famously came up with a way to prove mathematically that it should be possible to distinguish between quantum mechanics and so-called hidden variables. If such variables existed, he noted, measurements of certain results would have to be less than a critical value. If an experiment could be run that violated that inequality, that would "prove" that quantum mechanics has at least some non-local characteristics. Another loophole, it has been noted, occurs because single photons are difficult to measure—some get lost during transmission, particularly if sending them at a great enough distance to overcome the first loophole, making experimental results difficult to verify.
In this new experiment, led by Ronald Hanson, the researchers set about closing both loopholes, which would theoretically shut the door on local realism.
Arxiv version of full paper - Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometers
Nature - Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometers
More than 50 years ago, John Bell proved that no theory of nature that obeys locality and realism can reproduce all the predictions of quantum theory: in any local-realist theory, the correlations between outcomes of measurements on distant particles satisfy an inequality that can be violated if the particles are entangled. Numerous Bell inequality tests have been reported; however, all experiments reported so far required additional assumptions to obtain a contradiction with local realism, resulting in ‘loopholes’. Here we report a Bell experiment that is free of any such additional assumption and thus directly tests the principles underlying Bell’s inequality. We use an event-ready scheme that enables the generation of robust entanglement between distant electron spins (estimated state fidelity of 0.92 ± 0.03). Efficient spin read-out avoids the fair-sampling assumption (detection loophole), while the use of fast random-basis selection and spin read-out combined with a spatial separation of 1.3 kilometres ensure the required locality conditions. We performed 245 trials that tested the CHSH–Bell inequality S less than 2 and found S = 2.42 ± 0.20 (where S quantifies the correlation between measurement outcomes). A null-hypothesis test yields a probability of at most P = 0.039 that a local-realist model for space-like separated sites could produce data with a violation at least as large as we observe, even when allowing for memory in the devices. Our data hence imply statistically significant rejection of the local-realist null hypothesis. This conclusion may be further consolidated in future experiments; for instance, reaching a value of P = 0.001 would require approximately 700 trials for an observed S = 2.4. With improvements, our experiment could be used for testing less-conventional theories, and for implementing device-independent quantum-secure communication and randomness certification
26 pages of supplemental information
SOURCES - Arxiv, Nature, Delft University, Youtube