One of the most challenging problems in physics is generating true, provably unpredictable randomness.
This is because it is impossible to determine randomness based on results alone.
Dice may have nicks and imperfections that affect the outcome of a roll.
Computer random number generators usually operate based on algorithms.
Even the toss of a coin is subject to physical forces that can theoretically be predicted.
The challenge is not in generating numbers that appear random, but in showing that no one could have predicted the outcome – that the system is not subject to hidden, subtle rules or biases.
Now, a team of physicists from the Swiss Federal Institute of Technology in Zurich has overcome this problem by exploiting one of the strangest phenomena in quantum mechanics: quantum entanglement.
YouTube thumbnail
«"The resulting sequence of zeros and ones is now truly completely random, and we can even confirm this," says physicist Renato Renner of the Zurich Institute of Technology.
Randomness is crucial to modern security.
This is a key feature that makes passwords, authentication codes, and encryption keys more difficult to guess.
This is why password generators produce a meaningless sequence of jumbled characters, rather than something like YourFirstPet123.
But there's more at stake than just a Flickr password – international security is at stake.
Recent examples of security vulnerabilities include the 2024 PuTTY vulnerability, in which one of the world's most widely used SSH clients had a flaw in generating random numbers for cryptographic signatures.
And don't forget about the RDSEED bug in AMD Zen 5 2025, which caused the hardware random number generation instruction to produce predictable values but falsely report success.
A 30-meter tube connecting qubits. Photons can travel between the two chips, enabling quantum entanglement. (Kilian Kessler/ETH Zurich)
If the code is not completely random, it is easier for attackers to guess it.
«"Any normal electronic device, like a phone or a computer, is completely deterministic," Renner told Adam Kovacs of Scientific American, "so it's actually very difficult for a computer or any other electronic device to generate a random value.".
To try to find a solution to this problem, researchers turned to a quantum experiment known as a Bell test.
They created a pair of entangled quantum bits, or qubits, separated by 30 meters (98 feet), and cooled them to temperatures close to absolute zero.
An image of a sheep (left) encrypted using partial randomness (middle) and complete randomness (right). (ETH Zurich)
Entangled particles are particles that, when measured, exhibit similarities that cannot be explained by classical physics alone.
Measurements performed on qubits revealed correlations so strong that they could not be explained by ordinary hidden rules or pre-programmed behavior.
Achieving this goal required significant technical improvements in both the stability and speed of the experiment, allowing the team to run over a billion Bell-test runs in approximately nine hours.