Researchers in the Netherlands, Spain, and the UK have confirmed in the lab that the weird instant correlations between remote "entangled" particles are real. The experimental result has important implications for both fundamental physics and future cryptography.
The research is published in the arXiv preprint repository with the title "Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km." The paper, freely available online, hasn't been published yet in a peer-reviewed journal, but the result is spreading virally on the Internet.
A summary by Zeeya Merali on the Foundational Questions Institute (FQXi) website is trending on Redditwith thousands of upvotes and comments. Merali has also written an article for Nature News titled "Quantum 'spookiness' passes toughest test yet."
History-Making Confirmation of 'Spooky Action at a Distance'
Quantum physics as it's currently understood doesn't predict the values of a particle's observable properties such as position and spin, but only their probabilities, encoded in the "quantum state" of the particle. It's only after a measurement has been made that the particle snaps randomly into a state with a definite value of the property that has been measured. What constitutes "a measurement," and how ghostly quantum states "collapse," is tentatively answered by the different interpretations of quantum physics that have been proposed.
Quantum entanglement is a phenomenon where two remote particles share a unique quantum state in a way that we are unable to visualize. For example, the shared quantum state may be such that the spins are always measured in opposite direction (1 up and 2 down, or 1 down and 2 up). The researchers measured the spins of hundred of entangled particles in two University of Delft labs, located 1.3 km apart, and confirmed that the entangled correlations are still observed when there is not enough time for light to travel from the first lab to the second, which means that entanglement isn't limited by the speed of light.
Einstein argued against the "spooky action at a distance" between entangled particles, but experimental results seem to side with entanglement.
Instant entanglement has been observed in the lab since Alain Aspect's experiment in the early 80s, but this is the strongest experimental confirmation to date.
“It is a truly ingenious and beautiful experiment,” said renowned University of Vienna quantum physicist Anton Zeilinger. "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," added Perimeter Institute quantum physicist Matthew Leifer.
Nicolas Gisin, a quantum physicist at the University of Geneva in Switzerland, said: From a fundamental point of view, this is truly history-making.
Gisin is on the Board of Directors of Id Quantique, a company in Geneva specialized in quantum cryptography. The new experimental result confirms the core idea of quantum cryptography: entangled particles can be used to generate secure encryption keys between two remote locations, and malicious agents trying to eavesdrop to intercept the keys would break the entanglement, which could be detected.
"This may enable the realization of large-scale quantum networks that are secured through the very same counter-intuitive concepts that inspired one of the most fundamental scientific debates for 80 years," conclude the researchers.
Many commenters on the Reddit thread speculate that perhaps instant entanglement could be used for instant communications, not limited by the speed of light. But, according to our current understanding of quantum physics, that doesn't seem to be the case. Measuring the spin of one of a pair of entangled particles always gives a random result - even if the results of the two measurements are correlated - and any attempt to preset the spin of a particle would break the entanglement. A good analogy is two decks of "magic" cards that are always in the same order, but the magic only works if both decks are well shuffled first, and cheating breaks the magic.
Images from Wikimedia Commons and Pixabay.