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

TL;DR

This paper introduces a theory-independent method to monitor decoherence in superconducting qubits, revealing the loss of nonclassicality and non-Markovian dynamics without assuming quantum mechanics.

Contribution

It develops a generalized, theory-independent process tomography technique and applies it to superconducting qubits to observe decoherence and dynamical phenomena.

Findings

Superconducting qubit initially exhibits nonclassicality via generalized contextuality.

The system's state space contracts over time, indicating decoherence.

The system undergoes non-Markovian evolution at late times.

Abstract

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.