They had to wait about forty years for it, but this year’s Nobel Prize in Physics goes to three quantum pioneers who in the 1970s and 1980s were the first to demonstrate with experiments that entanglement – a property from quantum mechanics in which individual particles mysteriously interact with each other connected – really exists. It concerns the Frenchman Alain Aspect (1947), the American John Clauser (1942) and the Austrian Anton Zeilinger (1945). With their experiments, they laid the foundation for promising quantum technology.
The Nobel Committee announced the awarding of the prize Tuesday morning at the Royal Swedish Academy of Sciences in Stockholm. The three physicists will receive their share of the prize of ten million Swedish kronor (about 918,000 euros) on December 10, the anniversary of Alfred Nobel’s death.
Ronald Hanson, professor at QuTech at TU Delft, is pleasantly surprised. “I am at a meeting about quantum internet in Delft and nobody here was busy with the announcement. A few years ago we always thought these three guys would get the prize, but now I didn’t expect it. Their experiments have been carried out so long ago. It’s great that they still get the Nobel Prize. They have done groundbreaking research and their work on quantum entanglement has had a lot of impact.”
Travel faster than light
According to the laws of quantum mechanics (the physics behind the tiniest particles), individual particles can be mysteriously connected to each other, even when they are at a distance from each other. This phenomenon is called entanglement. It means that a certain particle influences the properties of another particle, even though there is a large distance between them. Such a close bond was predicted by physicist Erwin Schrödinger, among others.
Albert Einstein thought this was impossible, because in such an entanglement information is exchanged between the particles without time passing. That means that information travels faster than light. And according to Einstein, nothing can travel faster than light.
In the 1960s, theoretical physicist John Bell devised an experiment that would make it possible to see who was right, Einstein or quantum physicists. “That Bell test works something like this,” says Ronald Hanson. “Suppose you have two light particles. They vibrate in a certain direction. Take a scenario where there is a fifty percent chance that they vibrate horizontally or vertically. Then when you measure the vibration direction you would always measure an arbitrary vibration direction. But entangled particles share properties. If one light particle vibrates horizontally, the other vibrates vertically. If you measure a horizontal vibration during the first measurement, you can be 100% sure that the second particle vibrates vertically according to the laws of quantum mechanics. By repeating these kinds of measurements, you can show that entanglement exists.”
John Clauser gave this experiment a twist, making it easier to perform. Bell’s version made unfeasible demands on the detectors used to measure the particles. Clauser managed to use simpler detectors. The American was also the first to conduct Bell’s experiment himself, together with his now deceased colleague Stuart Freedman. The results proved Einstein wrong. Hanson: “But then measuring equipment was not so good, and people were not yet convinced”. That conviction only came in the early 1980s when Frenchman Alain Aspect repeated the experiment with better measuring equipment.
Quantum Teleportation
Freedman, Clauser and Aspect were the first to demonstrate that quantum entanglement really existed. What is special is that with their experiments they rowed against the current, says Hanson. “Here at the meeting, we don’t have to argue that quantum entanglement experiments are worth doing. In the days of Clauser and Aspect it was not clear at all that this was all real and what kind of applications were going to get entangled. Bell has already passed away, otherwise he would certainly have received a Nobel Prize as well.”
An important application of this discovery is the secure quantum internet, based on quantum teleportation. When two particles are entangled, information can be sent between those particles without that information having to travel a distance. That way, the information cannot be tapped along the way.
Anton Zeilinger was the first to demonstrate that quantum teleportation was possible in this way. He also expanded Bell’s experiment. Instead of one pair of particles, the Austrian worked with two pairs at the same time.
Professor of physics Alexander Brinkman of the University of Twente explains that quantum teleportation does not mean that quantum information travels faster than light. “The quantum information itself may arrive at the entangled particle immediately, but you also have to send information that tells you how to read that quantum information. And that accompanying information can never travel faster than light.”
Chinese satellite
In Delft, Hanson’s group has created the longest entanglement to date from computer chips. In 2015, his team brought two chips to different labs at the ends of the campus, about a kilometer apart. They could entangle both chips with a light particle. They fired those light particles at each other, after which they intertwined somewhere in the middle. In this way, the two chips also became entangled.
In 2017, a Chinese satellite sent entangled particles over a distance of 1,000 kilometers. Hanson: “But those light particles usually don’t arrive. And the great thing about experiments with chips is that you know exactly when the entanglement was created, which is essential for applications.”
The technological development surrounding quantum internet is now moving very fast worldwide, says Alexander Binkman. “The practical applications of quantum entanglement are getting closer and closer. Perhaps that is why the three quantum pioneers will now receive the Nobel Prize.”