Illustration of the tomography of a two-qubit open quantum system through the measurement of transport observables. Credit: Jeanne Bourgeois et al

A team from UNIGE shows that it is possible to determine the state of a quantum system from indirect measurements when it is coupled to its environment.

What is the state of a quantum system? Answering this question is essential for exploiting quantum properties and developing new technologies. In practice, this characterization generally relies on direct measurements, which require extremely well-controlled systems, as their sensitivity to external disturbances can distort the results. This constraint limits their applicability to specific experimental contexts.

A team from the University of Geneva (UNIGE) presents an alternative approach, tailored to open quantum systems, in which the interaction with the environment is turned into an advantage rather than an obstacle. Published in Physical Review Letters—with the "Editor’s Suggestion" label—this work brings quantum technologies a step closer to real-world conditions.

Quantum technologies—whether computers, sensors, or cryptographic systems—all rely on one essential step: the characterization of quantum states. In other words, it involves identifying all the parameters that describe a system in order to obtain a complete and usable description.

This process, known as quantum state tomography (QST), requires a large number of measurements. These protocols generally assume that the system is coupled only very weakly to its environment, as any uncontrolled interaction can alter both the results and the properties of the quantum system itself. This constraint is particularly significant for quantum computing platforms.

Making the environment an ally

Scientists at UNIGE have developed a more flexible method that departs from conventional approaches. Rather than measuring the system directly, their protocol relies on transport measurements—that is, measurements based on the flow of particles through the quantum system.

More specifically, the method applies to systems coupled to multiple environments, such as those subject to differences in potential or temperature. These imbalances induce particle flows through the quantum system. By carefully measuring these currents and their correlations, it becomes possible to access the parameters describing the quantum state without resorting to direct projective measurements on the system itself.

"Our work shows that the interaction with the environment, often considered a source of unwanted disturbances, can instead become an informational resource when properly exploited," explains Géraldine Haack, senior lecturer in the Department of Applied Physics at the UNIGE Faculty of Science, a recipient of the Sandoz Foundation Early Career Program.

She led this project in collaboration with Jeanne Bourgeois, first author, then a Master’s student at UNIGE and now affiliated with EPFL as a doctoral student, and Gianmichele Blasi, then a postdoctoral researcher at UNIGE and currently a postdoctoral researcher at IFISC, University of the Balearic Islands (Mallorca).

Devices closer to real-world applications

While this approach does not replace the protocols required for quantum computing—which rely on highly isolated systems—it does offer a major advantage for the characterization and certification of quantum states in open quantum devices, particularly quantum sensors. These sensors, capable of achieving extreme sensitivity, have applications across many fields, ranging from health care—such as advanced imaging and diagnostics—to geophysics, natural resource exploration, and autonomous navigation.

This method is also relevant for quantum neuromorphic computing, a computing paradigm inspired by brain function that relies on physical systems continuously interacting with their environment. In this context, information is processed through the collective evolution of the system rather than through isolated logical operations, making the characterization of open quantum states particularly important. The recent results from the UNIGE team therefore provide a key tool for advancing these promising quantum technologies towards real-world applications.

Publication details

Jeanne Bourgeois et al, Transport Approach to Quantum State Tomography, Physical Review Letters (2026). DOI: 10.1103/zk56-jn7t. On arXiv: DOI: 10.48550/arxiv.2501.16819

Citation: New method reveals quantum states using indirect measurements of particle flows (2026, January 20) retrieved 20 January 2026 from https://phys.org/news/2026-01-method-reveals-quantum-states-indirect.html

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