Physicists have made a major leap in our understanding of quantum entanglement by fully mapping out the statistics it can produce – essentially decoding the language of the quantum world.
This breakthrough reveals how the bizarre but powerful correlations in quantum systems can be used to test, secure, and certify the behavior of quantum devices, all without knowing their inner workings. The ability to self-test even partially entangled systems now opens doors to more robust quantum communication, encryption, and computing methods. It’s a game-changer for both fundamental physics and real-world quantum tech.
Cracking the Code of Quantum Entanglement
For the first time, theoretical physicists at the Institute of Theoretical Physics (IPhT) in Paris-Saclay have fully determined the range of statistical outcomes that can arise from systems using quantum entanglement. This breakthrough lays the foundation for comprehensive and reliable testing methods for quantum devices.
The findings appear in Nature Physics.
Following transformative inventions like transistors, lasers, and atomic clocks, quantum entanglement is now driving a new wave of innovation, often called the second quantum revolution. This revolution is bringing technologies like quantum communication and quantum computing closer to reality.
So what is quantum entanglement? Imagine two particles, such as photons, created in a shared quantum state. Even when separated by large distances, they retain a connection to their common origin. When you measure a property like polarization on one photon, the result is correlated with the measurement on the other, no matter how far apart they are.
Measuring the Quantum World
What does this correlation depend on? First, the degree of entanglement between the two objects may vary, depending on the nature of the source of the entangled quantum objects—in the example, horizontally polarized photons may be produced more frequently than vertically polarized ones. Then, a choice of measurement must be made—such as selecting a direction in which to measure the polarization—which may impact its result.
In order to generate meaningful quantum correlations, it is indeed essential that each object may be measured using a minimum of two distinct measurements, each offering at least two potential outcomes.
In the simplest experiment revealing the extent of quantum entanglement, five parameters can thus affect the measurement statistics: the degree of entanglement between the objects and the two directions in which both apparatuses perform their measurements. Generally speaking, however, quantum physics allows for intricate systems with numerous degrees of freedom, leading to a wide variety of correlations.
Understanding the Quantum Black Box
Quantum correlations have remarkable characteristics, notably their ability to pass a Bell test. When this happens, the results of a quantum experiment are “non-local” in the sense that they cannot be explained in terms of local hidden variable models, which capture our intuitive understanding of correlations. The experimental demonstration of this striking property was celebrated by the physics Nobel Prize awarded in 2022 to Alain Aspect, John F. Clauser, and Anton Zeilinger. But quantum correlations have more than one trick up their sleeve.
It turns out that physical attributes can often be estimated directly from the statistics obtained upon measurement of an entangled quantum state. For instance, observed correlations can certify that the observed measurement results are random. Importantly, this conclusion is attainable from the measurement results alone, without any assumption on the behavior of the quantum devices at hand, considered as “black boxes.” Ultimately, some quantum statistics have the property of fully identifying the physical model describing the entangled objects.
Self-Testing: Quantum Devices with No Assumptions
This stunning property, referred to as “self-testing,” plays a crucial role in device-independent quantum information protocols. Because these protocols do not rely on any assumption regarding the proper functioning of the source and measurement apparatuses, they offer unparalleled reliability. So far, several self-testing results have been obtained. For example, it is known that all qubit states can be self-tested, although all possible self-tests are not known yet. Indeed, only self-tests corresponding to maximally entangled states of two qubits have been fully characterized.
IPhT theoretical physicists Victor Barizien and Jean-Daniel Bancal have now demonstrated that it is also possible to describe exactly and completely the statistics obtained when measuring partially entangled objects.
“The idea, which is cute but hard to explain, was to describe the statistics from partially entangled states using what we understand of maximally entangled ones. We found a mathematical transformation that allows for a fruitful physical interpretation,” state the researchers.
In turn, identifying all correlations that can self-test partially entangled two-qubit states provided a complete description of the quantum statistics.
Implications for Quantum Security and Beyond
Having a complete knowledge of the quantum statistics achievable when entanglement is involved has wide consequences. On one hand, it identifies limits of quantum theory itself. In doing so, it bounds the extent of experimental results one can expect to observe provided that nature abides by the rules of quantum physics. On the other hand, it offers exceptionally effective test procedures, applicable to all types of entangled objects and measurements, and therefore to many different types of systems.
In particular, the security of devices using quantum entanglement can be enhanced by tests based on the results of observations made at each instant, rather than on the physical properties of the apparatuses, which are likely to evolve over time. More generally, the way is open to new protocols for quantum testing, communications, cryptography, and computation.
Reference: “Quantum statistics in the minimal Bell scenario” by Victor Barizien, and Jean-Daniel Bancal, 26 March 2025, Nature Physics.
DOI: 10.1038/s41567-025-02782-3
Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.
2 Comments
Click bait. I failed to see the relationship between the title of this article and its contents.
LOL yeah I confirm that notion. I too self tested my entanglement to the click bait title and found no correlation to its paired article contents..