EPJ QT Highlight - Generating true randomness with quantum measurements

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Published on 10 February 2025

A new approach can generate random information with extremely high efficiency through a process involving the emission and subsequent detection of single photons

From simulation to cryptography, randomness is a vital resource in many areas of technology. Ideally, random sequences can be created by measuring nondeterministic processes, whose outcomes are inherently unpredictable. Currently, many systems rely on pseudorandom processes such as thermal noise or chaotic oscillation, which exhibit some unpredictability, but are still fundamentally deterministic.

Through new research published in EPJ Quantum Technology, Jonas Almlöf and colleagues at Ericsson Research and the KTH Royal Institute of Technology, Sweden, show how these challenges can be overcome by exploiting the inherently random principles of quantum mechanics.

Their approach offers a realistic route to generating completely unbiased random sequences of information, and also enables far greater efficiency compared to existing methods.

A truly random sequence of information should be impossible to predict. In principle, this can be achieved by observing quantum systems, such as a photon passing through a beam splitter: an approach which has been widely explored in previous studies.

While a perfectly unbiased beam splitter divides an incoming laser beam into two equal parts, each of which travels along a separate path, the laws of quantum mechanics show that a single photon will initially travel along both paths at the same time. When the photon is measured, however, its quantum state collapses, revealing which of the two paths it took. This collapse is inherently random and provides a robust source of random information.

This method has limitations, however. Since each photon produces just one random bit, it is not particularly efficient. But if the beam splitter is even slightly imbalanced, it will introduce a bias which will need to be addressed in post-processing – limiting efficiency even further.

To address these issues, Almlöf’s team turned to an alternative approach involving the timing of photons emitted by a laser and subsequently detected. The timing of these detection events is governed by quantum uncertainty, making them inherently unpredictable.

Crucially, the researchers demonstrated that all possible orderings of the time intervals between detected photons occur with exactly equal probability, regardless of any initial non-uniformity in the emission times.

By detecting photons in sequence and recording the time differences between events, the team described how lists of inherently random time intervals could be generated. Each possible ordering of these lists was equally probable, meaning multiple random sequences could be generated from a single set of photon detections. This dramatically improves efficiency compared to traditional methods, since many random bits can be derived from each photon.

In one setup considered by the team, a block of 66 photon detections could occur in 65!, or 8.2 x 10⁹⁰ possible orderings, each with an equal probability. To put this into perspective, this value vastly exceeds the number of atoms in the entire observable universe. As a result, the method could extract 302 bits of randomness from each block – vastly improving on the efficiency of previous techniques.

While their study lays the theoretical groundwork, Almlöf and colleagues are optimistic that this approach could soon be implemented practically, offering a powerful new tool for generating randomness in various applications.