The fragility and laws of quantum physics generally make the characterisation of quantum systems time-consuming. Furthermore, when a quantum system is measured, it is destroyed in the process. A recent breakthrough by researchers at the University of Vienna demonstrates a novel method for quantum state certification that efficiently verifies entangled quantum states in real time without destroying all available states - a decisive step forward in the development of robust quantum computers and quantum networks. The work was carried out in Philip Walther's laboratories at the Faculty of Physics and the Vienna Centre for Quantum Science and Technology (VCQ) and published in the journal Science Advances.
Entangled quantum states are the fundamental building blocks of many new quantum technologies, from ultra-secure communication to powerful quantum computing. However, before these delicate states can be used, they must be rigorously verified to ensure their quality and integrity.
Conventional verification methods, such as quantum state tomography, are resource-intensive. This is because verifying a quantum state requires a large number of individual measurements on many 'copies' of the quantum system under investigation. These are then combined for verification. The demand for copies increases exponentially: the larger the system under investigation, the greater the number of copies required. Standard techniques measure each copy of the state. However, since each measurement of the quantum state destroys it, no states remain for the actual application.
Optical switches enable reliable sampling
To overcome these limitations, the team at the University of Vienna developed a new protocol that samples only a subset of the generated quantum states. 'The key to the practical implementation of this protocol is the use of active optical switches. These switches allow us to randomly forward individual quantum states either to a verifier (for certification) or to a user (for the actual quantum task),' explains Lee Rozema from the University of Vienna, one of the lead authors of the study.
These active optical switches were used to accurately and randomly capture states from the source. This is because, if it can be ensured that the samples are random, the verifier can use statistical methods to certify the user's unmeasured quantum states. High-quality optical switches that can operate as fast as the source generates photons and do not alter the quantum state are essential for this implementation. In this process, only the measured sample is destroyed. The quality of the user's unmeasured states, on the other hand, is certified in real time in a non-destructive manner and released for subsequent quantum operations.
The new protocol also overturns the previous assumption that all states generated by the source must be identical, making it more robust for real-world scenarios. In addition, the new protocol paves the way for device-independent certification, which means that certification remains valid even if the measuring devices are not trustworthy e.g. if they are controlled by a potential attacker.
Ready for tomorrow's quantum networks
'Our experimental setup successfully implements this advanced certification protocol in real time, which is a crucial step towards practical, secure quantum technologies,' explains Michael Antesberger from the University of Vienna, co-first author of the study. Mariana Schmid from the University of Vienna, also co-first author, adds: 'This method is incredibly efficient, offers optimal scalability and significantly reduces the resource requirements for robust certification.'
'This work paves the way for more reliable quantum communication networks and advanced photonic quantum computers. This will be crucial for benchmarking the large-scale quantum networks of tomorrow,' adds Philip Walther from the University of Vienna, lead author of the publication.
Summary:
- Entangled quantum states are the fundamental building blocks of many new quantum technologies, such as quantum computers and ultra-secure communication.
- However, these quantum systems must be thoroughly tested before they can be used - a process that has been very resource-intensive to date, destroying all available quantum systems.
- Scientists at the University of Vienna have developed a new method for more efficient verification: optical switches make it possible to randomly forward individual quantum states either to a verifier (for certification) or to a user (for the actual quantum task).
About the University of Vienna:
At the University of Vienna, curiosity has been the core principle of academic life for more than 650 years. For over 650 years the University of Vienna has stood for education, research and innovation. Today, it is ranked among the top 100 and thus the top four per cent of all universities worldwide and is globally connected. With degree programmes covering over 180 disciplines, and more than 10,000 employees we are one of the largest academic institutions in Europe. Here, people from a broad spectrum of disciplines come together to carry out research at the highest level and develop solutions for current and future challenges. Its students and graduates develop reflected and sustainable solutions to complex challenges using innovative spirit and curiosity.
Original publication:
Antesberger M., Walther P., Cao H., Schmid M., Dakic B., Rozema L.: Experimental Quantum State Certification by Actively Sampling Photonic Entangled States. In Science Advances, 2026
DOI: 10.1126/sciadv.aea4144