Characterizing the nonclassicality of quantum systems under minimal assumptions is an important challenge for quantum foundations and technology. Here we introduce a theory-independent method of process tomography and perform it on a superconducting qubit. We demonstrate its decoherence without assuming quantum theory or trusting the devices by modelling the system as a general probabilistic theory. We show that the superconducting system is initially well-described as a quantum bit, but that its realized state space contracts over time, which in quantum terminology indicates its loss of coherence. The system is initially nonclassical in the sense of generalized contextuality: it does not admit of a hidden-variable model where statistically indistinguishable preparations are represented by identical hidden-variable distributions. In finite time, the system becomes noncontextual and hence loses its nonclassicality. Moreover, we demonstrate in a theory-independent way that the system undergoes non-Markovian evolution at late times. Our results extend theory-independent tomography to time-evolving systems, and show how important dynamical physical phenomena can be experimentally monitored without assuming quantum theory. Reducing the assumptions required for certification of genuinely quantum behaviour is important both for quantum foundations and technologies. Here, the authors propose a theory-independent framework for quantum process tomography, and test it on a superconducting qubit, witnessing decoherence, loss of contextuality and non-Markovian evolution.

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Nature

Researchers developed a theory-independent method to characterize quantum system behavior without assuming quantum theory. Testing on a superconducting qubit, they demonstrated decoherence by modeling the system as a general probabilistic theory. The system initially exhibited nonclassical behavior through generalized contextuality but lost this property over time as its state space contracted. The method also revealed non-Markovian evolution at late times, extending theory-independent tomography to time-evolving systems and enabling experimental monitoring of quantum phenomena without quantum theory assumptions.

Theory-independent monitoring of the decoherence of a superconducting qubit with generalized contextuality