Observing different quantum trajectories in cavity QED (Vol. 42, No. 5)

Quantum systems, as isolated as they can be, always interact with their surrounding environment. This interaction can lead to correlations between system and environment and, when the states of the reservoir are inaccessible to observation, to an irreversible loss of information on the system. This deleterious decoherence effect is at the heart of the quantum theory of measurement and plays an essential role in explaining the emergence of classical behaviour in quantum systems.

However, this system-reservoir interaction can be exploited in a completely different way when the environment can be monitored. In this case, extracting information from the environment causes the state of the system to change stochastically, conditioned on the measurement record. This can then be used to manipulate the system dynamics, being an important strategy in quantum dynamical control.

While it is well known that there are infinitely many possible ways of unravelling the decoherence process in terms of stochastic trajectories, it is not always clear how to interpret these trajectories in terms of concrete physical measurements on the environment. In this paper we show how to produce, in a controllable manner, a variety of quantum trajectories in realistic cavity quantum electrodynamics setups. In the microwave regime, we show how the detection of atoms that have crossed a cavity can induce a jump in the field proportional to its quadrature. In this case, the field dynamics is quite different from the usual photodetection monitoring and can be used to produce conditional four-component cat-like states. Alternatively, in the optical domain, the detection of photons can be used to protect entangled states of atoms that have interacted with the cavity field.

This proposal for the implementation of new stochastic trajectories in terms of continuous measurements in realistic systems certainly expands the possibility of engineering quantum states of lights and atoms.