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arXiv:quant-ph/0610179 · Physical Review A

Total quantum Zeno effect and intelligent states for a two-level system in a squeezed bath

Douglas F. Mundarain, Miguel Orszag, Jorge Stephany

Published Nov 14, 2006• 8 citations
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Abstract

In this work we show that, by frequent measurements of adequately chosen observables, a complete suppression of the decay in an exponentially decaying two-level system interacting with a squeezed bath is obtained. The observables for which the effect is observed depend on the squeezing parameters of the bath. The initial states that display total Zeno effect are intelligent states of two conjugate observables associated to the electromagnetic fluctuations of the bath.

DOI
10.1103/physreva.74.052107

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In this work we show that, by frequent measurements of adequately chosen observables, a complete suppression of the decay in an exponentially decaying two-level system interacting with a squeezed bath is obtained. The observables for which the effect is observed depend on the squeezing parameters of the bath. The initial states that display total Zeno effect are intelligent states of two conjugate observables associated to the electromagnetic fluctuations of the bath.
In this work we show that is possible to obtain Total Quantum Zeno Effect in an ustable systems for times larger than the correlation time of the bath. The effect is observed for some particular systems in which one can chose appropriate observables which frequent measurements freeze the system into the initial state. For a two level system in a squeezed bath one can show that there are two bath dependent observables displaying Total Zeno Effect when the system is initialized in some particular states. We show also that these states are intelligent states of two conjugate observables associated to the electromagnetic fluctuations of the bath.
A complete suppression of the exponential decay in a qubit (interacting with a squeezed vacuum reservoir) can be achieved by frequent measurements of adequately chosen observables. The observables and initial states (Zeno subspace) for which the effect occurs depend on the squeezing parameters of the bath. We show these_quantum Zeno dynamics_ to be substantially different for selective and non-selective measurements. In either case, the approach to the Zeno limit for a finite number of measurements is also studied numerically. The calculation is extended from one to two qubits, where we see both Zeno and anti-Zeno effects depending on the initial state. The reason for the striking differences with the situation in closed systems is discussed.
We discuss the appearance of Zeno or anti-Zeno effects in an exponentially decaying system. We consider the quantum dynamics of a continuously monitored two level system interacting with a squeezed bath. We find that the behavior of the system depends critically on the way in which the squeezed bath is prepared. For specific choices of the squeezing phase the system shows Zeno or anti-Zeno effects in conditions for which it would decay exponentially if no measurements were done. This result allows for a clear interpretation in terms of the equivalent spin system interacting with a fictitious magnetic field.
In this work we theoretically consider quantum Zeno effect for entangled (correlated) quantum states. (Our physical systems and calculations are simpler than it is case in remarkable work on entanglement protection via Zeno effect by Maniscalco et all. But Maniscalco et all and our final conclusions are the same.) Firstly, we demonstrate well-known quantum Zeno effect by frequent decay detection for single, simple, two-level quantum system, with stable, ground quantum state and one non-stable, excited quantum state which can decay, by photon emission, in the ground state. (In this case, by frequent decay detections, system stands in the excited state.) Further, we consider single photon with energy equivalent to energy difference between excited and ground energy level of mentioned system. After interaction with a half-silvered mirror, mentioned photon is in a quantum superposition of the reflected and transmitted quantum state or trajectory. Mentioned trajectories are directed toward two identical, but distant, mentioned simple two-level systems both of which are initially (before interaction with photon) in ground state. Immediately after interaction with photon (photon absorption) two two-level systems are described by entangled (correlated) quantum state ( (excited, ground) + (ground, excited) , symbolically). We prove that in this case, by frequent decay detections on both systems, mentioned two two-level systems are described by initial entangled (correlated) quantum state ( (excited, ground) +(ground, excited)).
A discussion of the quantum Zeno effect and paradox is given. The quantum Zeno paradox claims that a continuously observed system, prepared in a state which is not an eigenstate of the Hamiltonian operator, never decays. To recover the classical behavior of unstable systems we consider a two-level system interacting with a Bose field, respectively prepared in the excited state and in the Poincare invariant vacuum state. Using time-dependent perturbation theory, we evaluate for a finite time interval the probability of spontaneous decay of the two-level system. Using the standard argument to obtain the quantum Zeno paradox, we consider N measurements where N goes to infinity and we obtain that the non-decay probability law is a pure exponential, therefore recovering the classical behavior.
The quantum Zeno effect--suppression of decay by frequent measurements--was believed to occur only when the response of the detector is so quick that the initial tiny deviation from the exponential decay law is detectable. However, we show that it can occur even for exactly exponentially decaying systems, for which this condition is never satisfied, by considering a realistic case where the detector has a finite energy band of detection. The conventional theories correspond to the limit of an infinite bandwidth. This implies that the Zeno effect occurs more widely than expected thus far.