💡 The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics 2022 jointly to Alain Aspect, John F. Clauser, and Anton Zeilinger “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”.
This comes at a moment when quantum technologies are becoming a subject of great interest as we envision a future with quantum computers and a quantum Internet.
📰 The story of the latest Nobel Prize in Physics started with the discovery of quantum mechanics. Continued with Einstein identifying a new feature, entanglement, while trying to demonstrate that quantum mechanics was an incomplete theory.
Followed by John S. Bell identifying a way in which Einstein’s dilemma could be tested: a way to prove if entanglement really exists.
And finished with the newly laureates demonstrating experimentally that, arguably, the weirdest outcome from quantum mechanics is correct and Einstein was wrong. Their success in creating entanglement experimentally opened the gates for the development of quantum computers, quantum cryptography, and the quantum Internet.
Is Quantum Mechanics correct? 🤔
Quantum mechanics is regarded as a weird theory because many of its conclusions conflict with our everyday experience and intuition, not only for the common people but also for trained minds. One of the weirdest behaviors in quantum mechanics is called entanglement. 🧬.
According to quantum mechanics, a pair of particles can share a property (quantum state) belonging to the two of them as if they were single, no matter how far apart they are. Someone measuring the property of one particle can immediately know the property of the other particle, no matter how far it is.
In 1935 Albert Einstein, Boris Podolsky, and Nathan Rosen postulated the so-called EPR paradox discussing the apparent conflict between entanglement and relativity. According to quantum mechanics, the shared property of the particles is not defined until it is measured. Einstein, Podolsky, and Rosen thought that there should be some kind of hidden variable or information, yet unknown to us that predetermined the result.
🧐 Imagine that our particles are balls with the property that if one is black, the other is white and vice versa, that correlation is what we call entanglement. We launch the balls to two different observers, Alice and Bob. Quantum mechanics says that the ball’s color is not defined until it is measured. When Alice observes the color of her ball, she will know that if her ball is white, Bob’s will be black. If her ball results black, Bob’s will be white.
Einstein, Podolsky, and Rosen suggested that the colors of the balls were defined from the very beginning. According to them, quantum mechanics was incomplete and still had not identified some kind of hidden variable that could reconcile its conclusions with our everyday experience.
How to tell who was right? 👇👇
In 1964, Irish physicist John Stewart Bell designed a test that would provide different results depending on which of the theories was actually right. The so-called Bell inequality yields one result if the world behaves with hidden variables. On the contrary, if quantum mechanics applies, the experiment will violate the Bell inequality providing a different result.
John Clauser designed a way to run the Bell test in practice. In 1972, with his student, Stuart Freedman was the first to run a Bell test in practice. Their result proved Bell’s inequality violation.
What if the decisions taken in the experiment influenced the result? 🤨
In 1982 Alain Aspect improved Clauser’s experiments to eliminate this risk. Particularly, he was able to take decisions about how to measure the photons independently on each end after the photons had been emitted to avoid any spurious correlation in the setup.
Anton Zeilinger went further testing Bell’s inequalities, even taking measurement decisions based on photons sourced 600 years before somewhere in the Milky Way.
John Clauser, Alain Aspect, and Anton Zeilinger demonstrated empirically that quantum mechanics is correct: The world is not deterministically bound by hidden variables. Entanglement is a real feature and randomness influences the world as we know it.
Towards a quantum Internet 🚀
Quantum states get affected when measured. So, is it possible to transfer a quantum state without measuring it? This would be really useful! The Internet can transfer information from one computer to another.
This is done by copying bits across devices and networks, and it is possible because bits can be measured. However, quantum computers work with qubits (quantum bits). How do you transfer a quantum state if you can’t measure or clone it?
In 1997 Anton Zeilinger started to experiment with so-called “quantum teleportation”. Don’t think of this as any science-fiction transfer of matter. This is a misleading name to express transferring a quantum state from one particle to a different one belonging to an entangled pair of particles.
We can imagine using quantum teleportation to transfer information about a qubit to a distant quantum computer. However, the two quantum computers would need to be close enough to the source of entangled particles, as entanglement can’t be maintained over large distances on optical fiber, for instance.
After demonstrating quantum teleportation, in 1998 Zeilinger and his team were able to create entanglements across distant particles, with the so-called entanglement swapping. Take two pairs of entangled particles. If you entangle one from each pair, the remaining two particles will be entangled between themselves even if they never met! By achieving this, they demonstrated that entanglement can be created at large distances.
By combining entanglement swapping and teleportation we can envision a future quantum Internet over which distant quantum computers will be able to transfer information in the future Quantum Internet.
A prize for making the future possible 👏👏
The ability to manipulate matter to a level in which we can create and manage entanglement is enabling the development of quantum technologies. Particularly, entanglement is fundamental to develop quantum computers, communications, cryptography and sensors. The Nobel Prize in Physics 2022 recognizes some of the researchers that made future possible.
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