Quantum entanglement: A simple explanation

Quantum entanglement is when two particles link together in a certain way no matter how far apart they are in space. Their state remains the same.
Quantum entanglement is when two particles link together in a certain way no matter how far apart they are in space. Their state remains the same. (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY via Getty Images.)

Quantum entanglement is a bizarre, counterintuitive phenomenon that explains how two subatomic particles can be intimately linked to each other even if separated by billions of light-years of space. Despite their vast separation, a change induced in one will affect the other. 

Related: How quantum entanglement works (infographic) (opens in new tab)

In 1964, physicist John Bell posited that such changes can be induced and occur instantaneously, even if the particles are very far apart. Bell's Theorem is regarded as an important idea in modern physics, but it conflicts with other well-established principles of physics. For example, Albert Einstein (opens in new tab) had shown years before Bell proposed his theorem that information cannot travel faster than the speed of light (opens in new tab). Perplexed, Einstein famously described this entanglement phenomenon as "spooky action at a distance."

How to test quantum entanglement

For more than 50 years, scientists around the world experimented with Bell's Theorem but were never able to fully test the theory. In 2015, however, three different research groups were able to perform substantive tests of Bell's Theorem, and all of them found support for the basic idea.

This cartoon helps explain the idea of entangled particles. Alice and Bob represent photon detectors, which NASA's Jet Propulsion Laboratory and the National Institute of Standards and Technology developed. (Image credit: NASA/JPL-Caltech)

One of those studies was led by Krister Shalm, a physicist with the National Institute of Standards and Technology (NIST) in Boulder, Colorado. Shalm and his colleagues used special metal strips cooled to cryogenic temperatures, which makes them superconducting, meaning they have no electrical resistance. A photon hits the metal and turns it back into a normal electrical conductor for a split second, and scientists can see that happen. This technique allowed the researchers to see how, if at all, their measurements of one photon affected the other photon in an entangled pair.

Related: 10 mind-boggling things you should know about quantum entanglement (opens in new tab)

The results, which were published in the journal Physical Review Letters, strongly backed Bell's Theorem. "Our paper and the other two published last year show that Bell was right: any model of the world that contains hidden variables must also allow for entangled particles to influence one another at a distance," co-author Francesco Marsili, of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, said in a statement (opens in new tab).

What is quantum entanglement used for?

In addition to proving Bell's Theorem, there are practical applications to this work as well. The "superconducting nanowire single photon detectors'' (SNSPDs) used in that experiment, could be used in cryptography and in deep-space communications (opens in new tab), NASA officials said.

NASA's Lunar Atmosphere Dust and Environment Explorer (LADEE) mission, which orbited the moon from October 2013 to April 2014, helped demonstrate some of this communications potential. LADEE's Lunar Laser Communication Demonstration used components on the spacecraft and a ground-based receiver similar to SNSPDs. The experiment showed that it might be possible to build sensitive laser communications arrays that would enable much more data to be up- and downloaded to faraway space probes, NASA officials said. 

Quantum entanglement experiments may help develop powerful quantum computers that could be used in deep space.  (Image credit: Getty Images)

Latest quantum entanglement research

Quantum entanglement continues to puzzle researchers around the globe. 

In 2019, researchers from the University of Glasgow published the first-ever photo of quantum entanglement (opens in new tab), captured with a sophisticated system of lasers and crystals. 

In late 2021, an international group of researchers reported they had successfully subjected a tardigrade to temporary quantum entanglement (opens in new tab). Despite critical reviews, the team said their experiment represents the first time a living animal was quantum entangled. 

And in March 2022, NASA announced (opens in new tab) it would be sending a quantum entanglement experiment to space. The experiment, called the Space Entanglement and Annealing Quantum Experiment, or SEAQUE, will test two quantum computers in the harsh environment of space.

Additional resources

For a more in-depth definition and exploration of quantum entanglement, check out Jed Brody's "Quantum Entanglement (The MIT Press Essential Knowledge series) (opens in new tab)" (Knopf, 2008). Read the fascinating stories about what life was like at the time of quantum entanglement's discovery in Louisa Gilder's "The Age of Entanglement: When Quantum Physics Was Reborn (opens in new tab)" (Deckle Edge, 2008). Or, take a broader look at quantum physics as a whole in this book, "Quantum Physics for Beginners: From Wave Theory to Quantum Computing. Understanding How Everything Works by a Simplified Explanation of Quantum Physics and Mechanics Principles (opens in new tab)" by Carl J. Pratt (Independently published, 2021). 


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Jesse Emspak
Space.com Contributor

Jesse Emspak is a freelance journalist who has contributed to several publications, including Space.com, Scientific American, New Scientist, Smithsonian.com and Undark. He focuses on physics and cool technologies but has been known to write about the odder stories of human health and science as it relates to culture. Jesse has a Master of Arts from the University of California, Berkeley School of Journalism, and a Bachelor of Arts from the University of Rochester. Jesse spent years covering finance and cut his teeth at local newspapers, working local politics and police beats. Jesse likes to stay active and holds a fourth degree black belt in Karate, which just means he now knows how much he has to learn and the importance of good teaching.

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