The key ideas

Quantum Chance: Nonlocality, Teleportation and Other Quantum Marvels

Nicolas Gisin, 2012

Two boxes at opposite ends of a city agree on their answers more often than any pre-arranged collusion would allow. Yet neither can pass a single word to the other. Physicist Nicolas Gisin, who ran such experiments himself near Geneva, explains this puzzle through a simple game and without formulas. We will sort out what experiment has proved and what remains the author's interpretation.

Who it is for

For those who have heard of “quantum entanglement” and want to understand what has been measured in it, what is interpretation, and what is just a loud word.

The most interesting thing in physics is not the mathematics but the ideas.

About this book

Does God play dice? Einstein, as is well known, thought not. Nicolas Gisin begins the book with this question and adds his own: what do physicists play at?

Imagine two boxes at opposite ends of a city. Their answers agree more often than any pre-arranged collusion would allow. Yet each box on its own produces answers that look like coin tosses. How can this be? Gisin, a physicist at the University of Geneva, ran such experiments himself and explains them without formulas.

First, a clear frame. The book has three layers: what has been measured in experiments, how Gisin himself interprets it, and what remains open. From here on we will mark which layer is which.

Idea 1. The game of Bell

Gisin explains the main point through a game. Alice and Bob sit far apart. Each has a box with a joystick. Each freely pushes the joystick left or right, and a 0 or a 1 appears on the screen.

The team wins a round by simple rules. If both pushed right, the answers must differ. In all other cases the answers must match.

Can they win every time? Suppose the boxes were given lists of ready answers in advance, like crib sheets. Going through all such strategies, Gisin shows that however the lists are drawn up, you can win no more than three rounds out of four. This ceiling for any pre-prepared answers is called Bell's inequality.

Two friends sit an oral exam in different rooms and agree in advance how to answer. Whatever agreement they reach, on one of the four types of question it will fail. This is a Livria analogy.

The game turns a philosophical dispute into a count of points. If someone consistently wins more than three rounds out of four, the explanation through prepared answers stops working.

Read in Livria

About the author

Nicolas Gisin is a Swiss physicist and professor at the University of Geneva. His group carried out experiments with entangled photons over distances of more than ten kilometres, and experiments in quantum teleportation. Co-founder of ID Quantique, winner of the John Stewart Bell Prize (2009).

All books by this author: Nicolas Gisin

Terms from the book

  • Bell's inequality: A mathematical ceiling on correlations that can be explained by answers prepared in advance. In Gisin's game this is no more than three wins out of four. Its violation has been measured many times in experiments.
  • Entanglement: A special connection between quantum particles, in which the results of measurements on them can agree more strongly than any prepared answers allow.
  • Nonlocality: A property of correlations that cannot be explained either by a common predetermined cause or by a signal between the parties. It does not allow messages to be sent faster than light.
  • True randomness: Randomness which, according to Gisin, arises at the moment of measurement and is not reducible to our ignorance. This is the author's interpretation, not the only one accepted in physics.
  • Quantum teleportation: Transfer of a particle's quantum state with the help of an entangled pair and an ordinary message. Matter is not transferred, and the original state is destroyed.
  • Quantum cryptography: A way to create a shared secret key in which an attempt at eavesdropping can be detected. The reliability of real systems also depends on engineering.

All terms

Similar books