Our daily life seems deterministic: if you drop a pen, it falls. But at the quantum level, this certainty collapses in favor of probabilistic chaos. For decades, scientists have asked a fundamental question: can we domesticate this chaos to generate perfect randomness, utterly unpredictable? Thanks to an experiment using quantum entanglement, a team from ETH Zurich has answered in the affirmative, proving that nature hides within itself a portion of absolute unpredictability that no supercomputer will ever be able to predict.
What you will learn
The role of Bell’s theorem in demonstrating quantum unpredictability.
How entanglement (“spooky action at a distance”) enables the creation of random numbers.
Why this breakthrough marks an insurmountable boundary for classical computing.
The Challenge of the “Perfect Die”
Bell’s theorem has long served to prove that the quantum world is not governed by deterministic hidden variables. Renato Renner and his colleagues inverted this logic: if the world is fundamentally random, can we use this randomness to manufacture numbers whose sequence no one, not even the creator of the system, could guess?
To achieve this, they used two entangled qubits, separated by a 30-meter tunnel where microwave photons travel. In this setup, the quantum state of one instantly influences the other. By measuring these particles, the team generated raw data unpredictable, which they then purified using a computational technique called a two-source extractor. The result is a amplification of randomness: an output stream of numbers whose unpredictability is guaranteed by the laws of physics.
An Unassailable Quantum Advantage
What makes this research exceptional is its “device-independent” nature. Unlike classical random-number generators, often based on algorithms whose inner workings could be compromised, this quantum protocol relies on no assumptions about the device’s internal functioning. It is a pure demonstration of quantum advantage: no conventional technology, however powerful, can ever match this capability.
If this advance has immediate applications in cryptography — where the absolute security of a key depends on its unpredictability — or in lottery and gaming systems, the researchers’ motivation was primarily philosophical and fundamental. They have shown that some natural processes are inherently unpredictable, ending the old debate about whether a Universe entirely determined is possible.
The Implications of a Non-Deterministic World
“It is simply impossible to predict the future,” summarizes Renato Renner. This work forces us to accept a disconcerting reality: at the deepest level of matter, the future is not written. There exist phenomena for which we cannot even formulate reliable probabilistic estimates.
By reinserting these questions into the public debate, this study does more than improve cybersecurity. It reminds us that science is not merely a quest for predictability, but an exploration of the limits of what can be known. We now possess a “perfect die,” a tool that does not grant us the power to predict the future, but which confirms with mathematical elegance that, in the fabric of the Universe, there remains room for the unexpected.
L’étude est publiée dans Nature.
