Recovering quantum correlations in classical environments without backaction: observation and interpretation

Nicolas Brunner

Jin‐Shi Xu, Kai Sun, Chuan-Feng Li +5 more

Revivals of quantum correlations in composite open quantum systems are a useful dynamical feature against detrimental effects of the environment. Their occurrence is attributed to flows of quantum information back and forth from systems to quantum environments. However, revivals also show up in models where the environment is classical, thus unable to store quantum correlations, and forbids system-environment back-action. This phenomenon opens basic issues about its interpretation involving the role of classical environments, memory effects, collective effects and system-environment correlations. Moreover, an experimental realization of back-action-free quantum revivals has applicative relevance as it leads to recover quantum resources without resorting to more demanding structured environments and correction procedures. Here we introduce a simple two-qubit model suitable to address these issues. We then report an all-optical experiment which simulates the model and permits us to recover and control, against decoherence, quantum correlations without back-action. We finally give an interpretation of the phenomenon by establishing the roles of the involved parties. In quantum systems, information can flow back and forth between the system and its environment, leading to revivals of quantum correlations. Using a simple model, Xu et al.experimentally show how revivals can occur with a classical environment despite the absence of back-action from the environment.

Patrick Bethke, Robert P. G. McNeil, Julian Ritzmann +4 more

Decoherence of a quantum system arising from its interaction with an environment is a key concept for understanding the transition between the quantum and classical world as well as performance limitations in quantum technology applications. The effects of large, weakly coupled environments are often described as a classical, fluctuating field whose dynamics is unaffected by the qubit, whereas a fully quantum description still implies some backaction from the qubit on the environment. Here we show direct experimental evidence for such a backaction for an electron-spin qubit in a GaAs quantum dot coupled to a mesoscopic environment of order 10^{6} nuclear spins. By means of a correlation measurement technique, we detect the backaction of a single qubit-environment interaction whose duration is comparable to the qubit's coherence time, even in such a large system. We repeatedly let the qubit interact with the spin bath and measure its state. Between such cycles, the qubit is reinitialized to different states. The correlations of the measurement outcomes are strongly affected by the intermediate qubit state, which reveals the action of a single electron spin on the nuclear spins.

Mauro Dorato, Laura Felline

Hideo Mabuchi

I utilize the Caves-Milburn model for continuous position measurements to formulate a broadband version of the standard quantum limit (SQL) for monitoring the position of a free mass and illustrate the use of Kalman filtering to recover the SQL for estimating a weak classical force that acts on a quantum-mechanical test particle under continuous observation. These derivations are intended to clarify the interpretation of SQL's in the context of broadband quantum measurement, with particular attention paid to the question of how it might be possible to verify that a given laboratory measurement does indeed achieve backaction-limited sensitivity. The method used to analyze force detection may be extended to the case of weak classical forces with arbitrary time dependence, including nonstationary and impulsive signals.

Michael Kästner, Philipp Uhrich