We finally discovered how to create truly random numbers

Eeny, meeny, miny, mo, grab a tiger by the toe – so the rhyme goes. But even children know that this kind of nursery rhyme doesn’t help them make a truly random choice. Perhaps you remember the first time you realized you could influence the outcome by choosing your starting point carefully?

It might be better to flip a coin or roll a die, but try to prove that the outcome of your toss or roll is random and you will be stuck. Indeed, these things aren’t really random: if you knew the precise position of the die or coin in your hand, the trajectory of the throw, the force of gravity, and subtle factors like air resistance or friction of the landing surface, you could predict the outcome. True chance is hard to find.

The fact is that we now know that chance is real, embedded in the very structure of the universe in the form of quantum mechanics. Given the choice between two paths, a quantum entity – such as an electron or a photon of light – will take one at random: there is no predictable cause behind a quantum effect. The University of Colorado’s Randomness Beacon, affectionately known as CURBy, is taking advantage of this phenomenon. It went online this year as the world’s first publicly available source of traceable, verifiable, and truly random numbers.

You might be wondering who needs such radical coincidence. After all, people have been happily rolling dice and flipping coins for millennia. But there are applications where it is essential to generate as much randomness as possible. “People don’t realize it, but without randomness, digital life would not be safe or fair,” says Nemitari Ajienka, a computer scientist who studies verifiable randomness at Nottingham Trent University in the United Kingdom. Every time you log into a secure web page or generate a secure password, a certain degree of randomness is involved, he says. And machine learning builds randomness into training.

Another use is to support democracy. In Chile, for example, politicians and civil servants are subject to random tax audits, and those chosen tend to object that the system is targeting them for a nefarious reason. “Everyone is complaining that it’s a witch hunt,” says Krister Shalm, one of CURBy’s creators at the US National Institute of Standards and Technology (NIST). These claims are much more difficult to make if the system uses a random tag whose numbers come from truly random sources.

Currently, the Chilean government draws its luck from the analysis of, among other things, seismic activity and the radio production of the University of Chile. But it’s still not completely random: Seismic activity happens for a reason, after all, and someone decides the radio station’s playlist. It’s also not entirely traceable, since people can’t routinely access seismic data. CURBy, however, is both.

The quantum random generator

Ten years ago, the system was “held together by duct tape and prayers,” according to Shalm. That’s when the researchers behind the project first presented their careful proof of principle for CURBy. In the meantime, they worked to make the system fast, automated, and ready for use – at any time – by anyone with access to the Internet.

CURBy is now a state-of-the-art facility that processes thousands of user requests every day. It could help consolidate democracy, improve trust in justice systems, and even bring harmony to a family game night. “CURBy represents a functional and publicly accessible quantum technology. For me, this is an exciting development,” says Peter Brown, a physicist at the Institut Polytechnique de Paris.

Creating truly random numbers is difficult. Very few things in the universe work by true chance because, unless you’re dealing with quantum things, there is always a mechanism behind the generation of numbers. Even computers that generate “pseudo-random” numbers to create secure passwords can be manipulated. Passwords are generated from a “seed” number, and if you know the seed and the algorithm, there is nothing random about them.

You could go further and use random “high entropy” sources, like the unpredictable timing of the radioactive decay of a piece of matter – cobalt-60 or strontium-90, perhaps. This is a random quantum event, but difficult to make user-friendly. And unless someone is in the room with you, you can’t always prove that you didn’t make up the numbers.

This also makes it a rather dangerous game of Yahtzee – and with CURBy now available, there’s simply no need to expose family members to radiation. Instead, CURBy relies on pairs of photons connected by a quantum phenomenon called quantum entanglement.

When two entities are entangled, they behave as if they were, in some respects, one and the same thing. This weird situation appears when you make a measurement on one of the entities and then make a similar measurement on the other. In some circumstances, the first measurement affects the result of the second, even if the quantum objects have been moved to opposite sides of the universe and cannot have exchanged any information. It’s like rolling two dice and finding that if one rolls a 6, the other always rolls a 1.

The entanglement between quantum objects, nicknamed “spooky action at a distance” by Albert Einstein, defies common sense: it occurs without any signal being sent between the two. No one has ever found a physical mechanism for how this happens.

Inside CURBy, entanglement shows up in measurements of a property called polarization. Pairs of entangled photons are separated and sent via optical fibers to two destinations 100 meters apart. At each location, the device measures the polarization, with a very short time passing between the two measurements.

Then, the measurement results are “correlated”: there is a subtle relationship between the results, the extent of which CURBy can analyze. Under “classical” conditions there is an upper limit to this level, but if the behavior is truly quantum, and therefore random, the limit is exceeded and can be used to produce random numbers. This is done by “purifying” the inherent randomness using a technique called Treviso extraction. CURBy can make about 250,000 polarization measurements per second, and it takes about 15 million measurements to produce its final product: a string of 512 truly random binary digits, or bits, that users can use as they see fit.

If you want to know exactly how random these bits are, there is an algorithm for that. Since there are 512 bits in a string and each bit can be 0 or 1, that means there are 2⁵¹² possible combinations. “There are a huge number of possibilities,” says Shalm.

All should have the same probability of appearing, and Shalm and his colleagues were able to measure the probability of a particular string of bits appearing. It’s not perfectly equal, but it might as well be. Think of it like you want a completely flat road. If the slope is 1 in 10, it is a steep slope. Even 1 in 100 – 1 meter of elevation per 100 meters of road – is noticeable. The gradient equivalent to CURBy’s randomness is 1 in over 184 quintillion: as random as anyone needs.

Prove randomness

Randomness isn’t CURBy’s only selling point: In fact, the whole point is that anyone can trace the numbers back to their origin and prove that they’re random, Shalm says. “There is currently no effective way to accomplish this, regardless of the type of random number generator,” he says.

To make their randomness trackable, CURBy researchers borrowed from blockchain mathematics used to ensure the security of digital assets like NFTs and cryptocurrencies. It’s essentially a way to verify what was done, when and by whom – in a scenario where no one trusts anyone – and everything can be traced back to the original result of the experiment.

The other factor that makes it difficult for anyone to game the system is that the entire process is spread across a series of institutions. NIST transmits the quantum data to a device at the University of Colorado at Boulder for processing, then an independent cryptographic service known as the Distributed Randomness Beacon Daemon adds its own set of ingredients to extract the true randomness contained in the measurement data and convert it into the final, uniform binary string.

“It’s almost like a spider’s web of connected, temporally ordered objects,” says Shalm. “No party has complete control over the nature of the random bits, and you can go back and see if anyone cheated or tried to change things.”

The integration of all the necessary physics with high-level security analyzes is “quite remarkable,” Brown says. Quantum technologies are generally still in the development phase, he points out, and few complete products are available. But will CURBy be useful? Without a doubt, says Brown – although there are applications where you absolutely should not use traceable randomness. “You don’t want to choose your passwords based on some random public source,” he says.

But selecting juries and judges for cases, generating lottery results, and randomized sampling in clinical trials are a few examples of where traceable randomness would be a boon. Oxford University mathematician Artur Ekert is also impressed. The way the CURBy team blended quantum and classical physics to create cutting-edge yet accessible technology is a sign of things to come, he says.

In fact, Shalm explains, CURBy itself is explicitly designed to be compatible with other upcoming technologies. In other words, true chance will be integrated into our future, making the world a fairer and safer place. It’s certainly better than a draw.