## Section: New Results

### Deterministic hidden coordinate for a qubit under fluorescence measurement

Participants: Alain Sarlette, Pierre Rouchon

The experimentalists in the group have set up an experiment with continuous heterodyne measurement of an energy loss operator on a superconducting qubit. We have observed that in the associated mathematical model, due to the degeneracy of the diffusion operator, the resulting quantum trajectories are supported not in the entire Bloch sphere, but instead they belong to the surface of a *deterministically* evolving ellipsoid. We have entirely characterized this fact and highlighted that such behavior is not generic. A paper comparing this to the experimental data and a more general theory about deterministic evolutions in quantum stochastic differential equations are being finalized. This work has been presented at [28] .