Nobel prizewinner who trapped electrons and demonstrated quantum jumps.

In the development of quantum mechanics, the evolution of a quantum system was a controversial item. The duality of unitary evolution and state reduction as proposed by John von Neumann was widely felt unsatisfactory. Among the various attempts to reconcile the two incompatible modes of dynamics, the model of decoherence has turned out rather convincing.

The origin of the quantum Zeno paradox is critically re-evaluated. It is demonstrated, that the observation of expectation values, in particular of reduced decay constants, cannot qualify as the proof of a quantum Zeno effect. Rather, the detection of the transition times of individual quantum objects provides necessary and sufficient evidence.

We present a new method to control the power of individual spectral components of a multicolor laser by mirrors with variable air gaps and by a composite resonator configuration. We demonstrate a Pr/Yb-ZBLAN fiber laser with arbitrary spectral composition of three simultaneously emitted components at 492 nm, 520 nm, and 635 nm. With 100 mW pump power at 850 nm launched into the fiber, the total laser output exceeds 10 mW.

The recoil effects of spontaneous photon emissions during optical pumping of a trapped three-level atom are exactly calculated. Without resort to the Lamb-Dicke approximation, and considering arbitrary detuning and saturation of the pump laser, the density of recoil shifts in phase space is derived. It is shown that this density is not of Gaussian shape, and that it becomes isotropic in phase space only for a branching ratio corresponding to fluorescence scattering but unfavorable for optical pumping. The dependence of its anisotropy on the laser saturation is discussed in the resonant case, and the mapping of moments of the atom's center-of-mass motion due to the pumping is presented. Moreover, it is shown how optimum parameters for protecting the center-of-mass quantum state from pump-induced disturbance depend on the specific property to be protected.

The evolution of quantum mechanics has followed the critical analysis of "gedanken" experiments. Many of these concrete speculations can become implemented today in the laboratory-thanks to now available techniques. A key experiment is concerned with the time evolution of a quantum system under repeated or continuing observation. Here, three problems overlap: (1) The microphysical measurement by a macroscopic device, (2) the system's temporal evolution, and (3) the emergence of macroscopic reality out of the microcosmos. A well-known calculation shows the evolution of a quantum system being slowed down, or even obstructed, when the system is merely observed. An experiment designed to demonstrate this "quantum Zeno effect" and performed in the late eighties on an ensemble of identical atomic ions confirmed its quantum description, but turned out inconclusive with respect to the very origin of the impediment of evolution. During the past years, experiments on individual electrodynamically stored and laser-cooled ions have been performed that unequivocally demonstrate the observed system's quantum evolution being impeded. Strategy and results exclude any physical reaction on the measured object, but reveal the effect of the gain of information as put forward by the particular correlation of the ion state with the detected signal. They shed light on the process of measurement as well as on the quantum evolution and allow an epistemological interpretation.

In the paper by Hotta and Morikawa [Phys. Rev. A 69, 052114 (2004)] the nonexistence of the quantum Zeno effect caused by indirect measurements has been claimed. It is shown here that the pertinent proof is incorrect and that the claim is unfounded.

In the Letter by Ozawa [M. Ozawa, Phys. Lett. A 356 (2006) 411–413] potentially lacking mathematical rigour in our previous comment [S. Wallentowitz, P.E. Toschek, Phys. Rev. A 72 (2005) 046101] has been insinuated. The suggested loophole for the claim to the respective measurement being impossible, however, requires mathematical modelling that includes the use of non-physical elements.

The appearance of the laser in the early sixties supplied physicists with highly excited and, hopefully, spectrally selective light fields. Obviously it represented a unique tool for preparation and probing of matter, in particular of the free atoms of a gas. The author, then at Heidelberg University, had wondered for some time why, in an act of light absorption, the excited atoms had escaped attention so far. Of course, the excitation per field mode of conventional light sources is weak. However, the novel laser seemed to be a light source capable of generating and addressing such an ephemeral population that is nowadays addressed as “saturation”.

The evolution of quantum mechanics has followed the critical analysis of "gedanken" experiments. Many of these concrete speculations can become implemented today in the laboratory - thanks to now available techniques. A key experiment is concerned with the time evolution of a quantum system under repeated or continuing observation. Here, three problems overlap: 1. The microphysical measurement by a macroscopic device, 2. the system's temporal evolution, and 3. the emergence of macroscopic reality out of the microcosmos.A well-known calculation shows the evolution of a quantum system being slowed down, or even obstructed, when the system is merely observed. An experiment designed to demonstrate this "quantum Zeno effect" and performed in the late eighties on an ensemble of identical atomic ions confirmed its quantum description, but turned out inconclusive with respect to the very origin of the impediment of evolution.During the past years, experiments on individual electrodynamically stored and laser-cooled ions have been performed that unequivocally demonstrate the observed system's quantum evolution being impeded. Strategy and results exclude any physical reaction on the measured object, but reveal the effect of the gain of information as put forward by the particular correlation of the ion state with the detected signal. They shed light on the process of measurement as well as on the quantum evolution and allow an epistemological interpretation.

The vibration of ions in the potential well of an ion trap has served for the first demonstration of laser cooling and is an essential ingredient of concepts for quantum information processing. The ion motion couples to a driven internal ion resonance such that the system obeys a Jaynes-Cummings (J-C) model that predicts coherently generated “trapping states” of the oscillatory excitation known from micro-maser dynamics. In the past, metastable states of the vibrational excitation of an individual trapped Ba ion had been observed. They were tentatively identified with the trapping states of the J-C model. Recently, an extension of this model including the spatial distribution of the light field has been shown to give rise to another type of trapping states that are robust under decoherence. The characteristics of these novel trapping states better represent the previously observed metastable vibronic states.

In a Letter by Hotta and Morikawa [M. Hotta, M. Morikawa, Phys. Lett. A 326 (2004) 32–41] the complete resolution of the quantum Zeno paradox has been claimed, invoking non-existence of the effect. It is shown here that the pertinent proof is incorrect, and the claim unfounded. We identify the logical errors made using an illustrative counterexample.

Reiterated or continuous measurement on a quantum system impedes the quantum object's evolution. This "quantum Zeno" effect (QZE), contemplated for decades, has been conventionally attributed to the reaction of the measuring device on the quantum object being measured, in the sense of the Heisenberg microscope. However, even reactionless "quantum non-demolition" measurements seem to qualify for that inhibition to take place and may constitute a "quantum Zeno paradox" (QZP). An experimental proof of QZE had been attempted in the past on clouds of ions confined in a Paul trap. In the meantime it has been shown, however, that the anticipated impediment of the quantum evolution by a sheer gain of information, i.e. by reactionless measurement cannot be, in principle, proven with an ensemble. Recently, experiments have been performed with the use of a single quantum system: an 172Yb+ ion, laser-driven on its E2 line S1/2 - D5/2, or an 171Yb+ ion, microwave-driven on its ground-state hyperfine resonance. The results of these experiments demonstrate that mere gain of information on the quantum system, while lacking dynamic action, modifies the system's evolution as it is predicted by quantum mechanics, based on the system's preparation. This seems plausible if we attribute reality to the results of the measurements. If, in contrast, reality is claimed for the prediction, two-fold discontinuity must be admitted: loss of coherence with potential measurements, and renormalization from the results of actual measurements.

The temporal evolution of a quantum system is frustrated by observing the system, even when there is no back-action on the system. This much-disputed Quantum Zeno Paradox - a clue to which is entanglement - is verified on an individual atomic ion: The evolution of the ion's spin, microwave-driven on the ground-state hyperfine resonance, alternates with probing the ion's quantum state by attempts of laser-excited resonance scattering. Enhanced chance of survival marks even the lower "dark" state correlated with detection of a null signal. - A previous conclusive purely optical experiment and related work is summarized.

The evolution of a quantum system is impeded by the system's state being observed. A test on an ensemble neither proves the causal nexus nor discloses the nature of the inhibition. Two recent experiments that make use of sequential optical or microwave-optical double resonance on an individual trapped ion disprove a dynamical effect of back action by meter or environment. They rather indicate the ionic states involved in the evolution being entangled with the potentially recorded bivalued scattered-light signal.

An ion, or atom, confined in a 'trap' and laser-irradiated on its resonance line generates a quasi-continuous flow of scattered light, and makes the line split or shifted. When the ion is probe-laser excited, on a neighbouring dipole-forbidden line to a metastable state, the flow subsides. Also, the shift or dynamic Stark splitting of the resonance and of the dipole-forbidden line vanishes. De-excitation of the ion requires the probe laser to be re-tuned to the unshifted resonance. Cyclic scanning of this laser shows a mean frequency offset of the on-off jumps from the off-on jumps. This random hysteresis may cool or heat the ion, and lock an ensemble in the metastable state.

With two ions in a spheroidal Paul trap, the harmonic trapping potential turns, by Coulomb repulsion, into a bistable well. Two Ba ions have been confined, laser-cooled, and observed, via their laser-excited resonance scattering, by a spatially resolving photomultiplier, or intensifying CCD camera. A repump laser releases the ions from a metastable state. Well-cooled ions are found localized. When the laser is detuned, the Raman cooling rate and the ions' temperature vary. A transition from the crystallized state to a toroidal gas takes place.The ions are discriminated when one (i) gets excited into a non-fluorescing metastable state, or (ii) is of another isotopic species. Hopping rates of 1 s(-1) are found at high thermal excitation. Elsewhere, the rates drop by two orders of magnitude, and vary resonantly with temperature near potential energy/kinetic energy = 15, where the bright ion's hopping upstream of the laser beam is five times more likely than downstream.Two discernible ions in a trap represent a microscopic model system for the study of reaction kinetics.

A novel robust mechanism for the generation of "trapping states" is shown to exist in the coupling of a two-level system with an oscillator, which is based on nonlinearities in the laser-induced vibronic coupling.This mechanism is exemplified with an ion confined in the potential well of a trap, where the nonlinearities are due to Franck-Condon type overlap integrals of the laser waves with the ionic centre-of-mass wavefunction.In contrast to the coherent trapping mechanism known from micro-maser theory, this mechanism works also in an incoherent regime operated by noisy lasers and is therefore much more robust against external decoherence effects.These features favour the incoherent regime, in particular for the preparation of highly excited trapping states.

This chapter contains sections titled: Introduction The hardware and basic procedure First scheme: Statistics of the sequences of equal results Second scheme: Driving the ion by fractionated π-pulses Conclusions Survey of related work References