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The theory of quantum entanglement has been around for over half a century, but until recently this very strange property of some subatomic particles has been difficult to prove. Now experiments are showing it’s a very real phenomenon.
Quantum entanglement is a property that allows those particles to share characteristics, so that when they’re put into close proximity, they become indistinguishable. Two or more particles can become entangled, so that when scientists measure the state of one particle, other particles appear to exhibit the same state.
When talking about the state of some particle, that means determining some characteristic such as the spin of the particle. Because of Heisenberg’s Uncertainty principle, you can never know all of the characteristics of a particle, such as its speed and location.
If you measure its speed, you can’t measure its location and vice versa. But once you measure one characteristic of one particle, the uncertainty of the others collapses and they display the same characteristic.
What’s important is that change from uncertainty to knowing a specific state of the particle happens instantaneously, without regard to distance. This means that if you were to separate the particles by sending one to Pluto while keeping the other on Earth, the change in state would be detected instantly instead of taking the average 4.5 hours it takes a signal to reach the planet at the speed of light.
The idea that information could be transmitted faster than the speed of light deeply troubled physicist Albert Einstein, who famously labeled it as “spooky action at a distance.” But in reality, the communication between the entangled particles apparently takes place outside of the bounds of normal time and space as we experience them.
So does this mean that we’re suddenly able to carry on faster-than-light communications, like the ones you see in movies such as Star Trek? Not exactly. This is because you first must physically take one of the entangled particles to the remote location before the communication can take place.
Then while the state change of the particle may be instantaneous, you can’t transmit much data. Effectively, you can think of each particle as holding one bit of information, and you must entangle each of them.
Experiments confirming the ability to transmit information using quantum entanglement were carried out in August in Calgary, Alberta, and Heifi, China. The results were published on Sept. 20, in Nature Photonics here and here. These experiments were successful, but limited. The Chinese team was more successful and was able to transmit 17 entangled photons over a distance of several miles.
An earlier experiment in the Canary Islands was able to transmit entangled photons over a longer distance of about 80 miles using lasers.
Scientists Demonstrate Long Distance Quantum Communication
The August experiments both used fiber optic cables to transmit the entangled photons, solving the problem of how to get them to the remote location. In addition, the researchers were able to test another property of those entangled particles, which is that they can provide powerful, perhaps uncrackable encryption. This is because when the entanglement takes place, it generates what is effectively a key, and the key must be transmitted to the remote site before the results can be read.
While it’s possible to intercept the entangled particle, doing so without the key is useless. In addition, even if you send the intercepted particle on its way to its intended recipient, the fact that its state was observed cannot be concealed. Ultimately, the recipient will know that the particle was intercepted, revealing the hack attempt.
While all of this may seem far-fetched, it’s not. The concept of quantum communications is in its early infancy. But it’s promising enough that the Chinese government has already launched its first satellite designed to test whether the signals it transmits cannot be hacked and to test other properties of quantum communications in space. It will be no problem to use that satellite to determine for sure whether faster than light communications are possible.
Other work needs to be done before the full capabilities of quantum communications can be determined and then turned into real uses. Among other things, the scientists working on the communications problem are also trying to see if there are limits to distances, and how long an entangled particle can be preserved before it’s used.
Right now the particles are being sent over lengths of fiber that are identical, so that they will appear at the other end at exactly the same time.
What’s not known is whether this precision is really necessary to send an encryption key to the other end to determine the state of the entangled particle and whether there are other factors that will affect how the entanglement works.
If some or all of those limitations can be overcome, then quantum communications could revolutionize communications as we know it. Right now, most scientists will say that some limits, such as the need to send a key, cannot be overcome. But as I said, it’s still early and it wasn’t very long ago when this whole area of study was dismissed as impossible.
The use of quantum encryption is most likely to become available in the near term because it’s effectively become an engineering problem, not a problem of theoretical physics. Beyond that, more research is necessary. Nobody knows how to do it just yet, but researchers are working on the problems.