A Novel Approach to Open Quantum State Tomography

A research group at the University of Geneva (UNIGE) demonstrates that the state of a quantum system can be inferred through indirect measurements when the system interacts with its surrounding environment. Published in Physical Review Letters and highlighted as an Editor’s Suggestion, the study moves quantum technologies closer to practical, real-world operating conditions.

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What defines the state of a quantum system? Answering this question is critical for harnessing quantum effects and advancing new technologies. In practice, such characterization typically depends on direct measurements, which demand extremely precise control of the system, since even minor external disturbances can skew the results. This limitation restricts their use to specific experimental setups.

Quantum technologies, including computing, sensing, and cryptography, all depend on a fundamental requirement: quantum state characterization. In practical terms, this means determining every parameter that defines a system so that a full and functional description can be obtained.

This procedure, called quantum state tomography (QST), involves conducting a large number of measurements. Such protocols typically assume that the system interacts only minimally with its environment, because any uncontrolled coupling can modify both the measurement outcomes and the intrinsic properties of the quantum system. This limitation is especially critical for quantum computing platforms.

Making the Environment an Ally

Researchers at UNIGE have introduced a more adaptable technique that moves away from traditional methods. Instead of performing direct measurements on the system, their approach is based on transport measurements, meaning observations derived from the movement of particles through the quantum system.

More precisely, the approach is designed for systems connected to several environments, for example, those exposed to variations in potential or temperature. Such imbalances generate particle currents through the quantum system. By accurately measuring these currents and their correlations, the parameters that define the quantum state can be obtained without performing 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.

Géraldine Haack, Senior Lecturer, Department of Applied Physics, Faculty of Science, UNIGE

Géraldine Haack is a recipient of the Sandoz Foundation Early Career Program.

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

Devices Closer to Real-World Applications

Although this method does not substitute for the protocols used in quantum computing ,which depend on highly isolated systems, it provides a significant benefit for the characterization and certification of quantum states in open quantum devices, especially quantum sensors.

These sensors, known for their exceptional sensitivity, are used in a wide range of applications, including healthcare for advanced imaging and diagnostics, as well as geophysics, natural resource exploration, and autonomous navigation.

This approach is applicable to quantum neuromorphic computing, a computational framework inspired by brain function that depends on physical systems that constantly interact with their environment. In this setting, information is handled via the collective evolution of the system, rather than isolated logical operations, which makes the characterization of open quantum states especially important.

The latest findings from the UNIGE team therefore deliver a crucial tool for moving these promising quantum technologies toward real-world applications.