In a solid-state circuit QED system, we extend the previous study of generating and stabilizing a two-qubit Bell state [Phys. Rev. A 82, 032335 (2010)] to a three-qubit GHZ state. In a dispersive regime, we employ the homodyne joint readout for multiple qubits to infer the state for further processing, and in particular we use it to stabilize the state directly by means of an alternate-flip-interrupted Zeno (AFIZ) scheme. Moreover, the stateof-the-art feedback action based on the filtered current enables not only a deterministic generation of the pre-GHZ state in the initial stage, but also a fast recovery from occasional error in the later stabilization process. We show that the proposed scheme can maintain the state with high fidelity if the efficient quantum measurement and rapid single-qubit rotations are available.

In a solid-state circuit QED system, we demonstrate that a homodyne-current-based feedback can create and stabilize highly entangled two-qubit states in the presence of a moderate noisy environment. Particularly, we present an extended analysis for the current-based Markovian feedback, which leads to an improved feedback scheme. We show that this is essential to achieve a desirable control effect by the use of dispersive measurement.

Ab initio implementation of quantum trajectory mean-field approach and dynamical simulation of the N 2 CO photodissociation The Journal of Chemical Physics 143, 194107 (2015) Mixed-quantum-classical molecular dynamics simulation implies an effective quantum measurement on the electronic states by the classical motion of atoms. Based on this insight, we propose a quantum trajectory mean-field approach for nonadiabatic molecular dynamics simulations. The new protocol provides a natural interface between the separate quantum and classical treatments, without invoking artificial surface hopping algorithm. Moreover, it also bridges two widely adopted nonadiabatic dynamics methods, the Ehrenfest mean-field theory and the trajectory surface-hopping method.

We study transitions of a particle between two wells, separated by a reservoir, under the condition that the particle is not detected in the reservoir. Conventional quantum trajectory theory predicts that such no-result continuous measurement would not affect these transitions. We demonstrate that it holds only for Markovian reservoirs (infinite bandwidth ). In the case of finite , the probability of the particle's interwell transition is a function of the ratio /ν, where ν is the frequency of measurements. This scaling tells us that in the limit ν → ∞, the measurement freezes the initial state (the quantum Zeno effect), whereas for → ∞ it does not affect the particle's transition across the reservoir. The scaling is proved analytically by deriving a simple formula, which displays two regimes, with the Zeno effect and without the Zeno effect. It also supports a simple explanation of the Zeno effect entirely in terms of the energy-time uncertainty relation, with no explicit use of the projection postulate. Experimental tests of our predictions are discussed. It is well known that the unitary evolution of a quantum system is interrupted by measurement, so the subsequent evolution of a system depends on the measurement record. Frequent measurements with intervals t are of special interest. In the limit t → 0, they freeze the particle's motion (the quantum Zeno effect). This result is a consequence of the projection postulate applied to sequential measurements.The Zeno effect looks very surprising since it reveals the dynamical impact of the projection postulate on quantum motion. Instead, one can try to attribute the Zeno effect to the influence of the measurement devices. At first it seems as though this cannot be the case. Indeed, due to the interaction with detectors, the system acquires the energy ∼ / t, according to the energy-time uncertainty relation. As such, it is natural to expect an acceleration of the particle, instead of its freezing: the anti-Zeno effect [1]. Nevertheless, as demonstrated in this paper, the Zeno effect can be entirely attributed to the energy-time uncertainty relation, without the explicit use of the projection postulate. It would make the Zeno effect much less surprising and in fact quite expectable.The concept of continuous measurement is inherent in the quantum trajectory (QT) approach (informational evolution), which treats quantum motion based on the results of intermediate measurements [2]. It is therefore natural to investigate the Zeno-effect dynamics in this framework. A pronounced example of the informational evolution was proposed for a two-state system (qubit), coupled to a continuously monitored reservoir under the condition that no signal is registered there [3,4]. It was predicted that the qubit can change its state despite the null-result measurements. This was confirmed in experiment with a superconducting phase qubit measured via tunneling [5].In this paper we study a different arrangement, with two distant localized states, connected by a common reservoir under cont...

We demonstrate that a quantum particle, initially prepared in a quantum well, can propagate through a reservoir with a continuous spectrum and reappear in a distant well without being registered in the reservoir. It is shown that such a passage through the reservoir takes place even if the latter is continuously monitored. We discuss a possible experimental realization of such a teleportation phenomenon in mesoscopic systems.