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Quantum State Engineering in Open Systems: Harnessing Dissipation and Measurement

Subject Area Theoretical Condensed Matter Physics
Term from 2020 to 2025
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 440745404
 
Final Report Year 2025

Final Report Abstract

Weak measurement is an alternative to the dogma of von Neumann’s “collapse” of a wave function: it avoids the necessity of a complete destruction of the wave function while being capable of extracting partial information from the system measured. Concomitantly, measurement is associated with the backaction of the detector on the system state. The core idea of the proposal was to exploit the almost non-destructive nature of weak measurement and, at the same time, harness its backaction for the purpose of quantum engineering of nontrivial, highly entangled quantum states—a paradigmatic leap in addressing many-body physics. This advance was accomplished in our project, with a total of 45 publications achieved during the funding period, through three key steps organized in corresponding work packages concerning (i) weak-measurement-induced generation of states, (ii) weak-measurementinduced state stabilization, and (iii) weak-measurement-induced state manipulation. In certain limits, weak measurement protocols give rise to dynamics closely related to that of open dissipative systems, where it is also possible to achieve state generation, stabilization, and manipulation by subjecting a quantum system of interest to a combination of time-dependent driving forces and dissipation in the presence of an external environment. While drive-anddissipation protocols have been extensively studied and employed for generating exotic states before, the new element in our proposal concerned the emergence of dark spaces, marking the presence of degenerate dark states. Engineering a dark space opens the door to robust quantum state manipulation, a challenge addressed in our project for devices hosting topological zero modes. Starting with systems consisting of a few effective quasiparticles, we have targeted strongly correlated many-body (possibly topological) states. We designed ondemand few-quasiparticle states, protocols for manipulating them—resulting, e.g., in geometric phases and braiding—and for tuning the (weak measurement or driven-dissipative) protocols to affect quantum correlations in such states. We have relied on the backaction dynamics in order to engineer a host of correlated many-body states, comprising known non-trivial ground states, excited states, as well as novel nonequilibrium states. We have proposed a multitude of protocols for state generation, for stabilizing the engineered states, and for their manipulation. We expect an impact on the field of quantum information processing through our novel qubit manipulation techniques.

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