Quantum technologies, whether computers, sensors or cryptographic systems, all rely on one essential step: the characterisation of quantum states. This process, known as quantum state tomography, involves identifying the full set of parameters that describe a system so that it can be controlled and used reliably.
Standard quantum state tomography requires a large number of direct measurements on a system that is isolated as much as possible from its environment. Any uncontrolled interaction with the surroundings can alter both the measurement results and the underlying quantum properties, which is a major constraint for platforms such as quantum computers that must preserve delicate superpositions and entanglement.
A team at the University of Geneva has now demonstrated that, for a broad class of open quantum systems, the interaction with the environment can be turned into an advantage rather than treated purely as a source of noise. Instead of performing projective measurements directly on the system, the researchers use transport measurements based on the flow of particles through the device.
The method applies to systems coupled to multiple environments that differ in quantities such as electric potential or temperature. These differences drive particle currents through the quantum system, creating a continuous exchange of energy and matter with the surroundings.
By precisely measuring these currents and their correlations, the team shows that it is possible to reconstruct the parameters that define the quantum state without resorting to direct tomography on the system itself. In this protocol, the environment effectively acts as a probe that encodes information about the state into measurable flows.
"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 Geraldine Haack, assistant professor in the Department of Applied Physics at the UNIGE Faculty of Science and recipient of the Sandoz Foundation Early Career Program. She led the project with first author Jeanne Bourgeois, who carried out the work as a Masters student at UNIGE and is now a doctoral student at EPFL, and with postdoctoral researcher Gianmichele Blasi, who is currently at IFISC at the University of the Balearic Islands.
This transport-based approach does not replace the highly controlled protocols used in quantum computing architectures that demand extreme isolation. However, it offers a major advantage for the characterisation and certification of quantum states in open quantum devices, where coupling to the environment is unavoidable and even central to the function of the device.
Quantum sensors are a prime example of such open systems. Designed to reach extreme sensitivity by exploiting quantum coherence and correlations, they are typically operated in conditions where they interact continuously with external fields or media. Potential applications range from advanced medical imaging and diagnostics to geophysics, natural resource exploration and autonomous navigation.
In these real-world settings, techniques that embrace rather than suppress environmental coupling can simplify calibration and verification. The Geneva team's protocol shows how measurements of transport properties that are already accessible in many experimental platforms can yield detailed information about the underlying quantum states.
The work 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 neuromorphic devices, information is processed through the collective, dynamical evolution of the system rather than through isolated logical operations, making the characterisation of open quantum states particularly important.
By providing a way to infer quantum states in such continuously driven, dissipative systems, the new approach offers a key tool for advancing neuromorphic architectures that harness quantum effects. It suggests that the same environmental interactions that complicate traditional quantum computing architectures may be central resources for emerging information processing schemes.
The study, published in Physical Review Letters with the Editors Suggestion label, highlights how rethinking the role of the environment can bring quantum technologies closer to practical operation conditions. Instead of fighting decoherence at all costs, engineers may increasingly design devices where carefully engineered couplings and transport pathways encode and reveal quantum information.
The research appears under the title "Transport Approach to Quantum State Tomography" and develops a general framework for extracting state information from transport observables in multi-terminal quantum structures. The authors show how current and noise measurements can be mapped onto state parameters in realistic models, pointing the way to experimental implementations.
As quantum platforms diversify beyond isolated qubits towards complex networks and hybrid systems, methods that work with open dynamics are expected to gain importance. The Geneva results demonstrate that a transport perspective can bridge the gap between fundamental concepts in quantum information and the practical constraints of devices operating in real environments.
Research Report:Transport Approach to Quantum State Tomography
Related Links
Universite de Geneve
Understanding Time and Space
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