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Transitions between atomic Rydberg states have been used for a sensitive detection of 100 GHz microwave radiation from a blackbody source. The Rydberg states have been populated using cw dye lasers. To reduce the influence of the background radiation the beam of Rydberg atoms was surrounded by a shield cooled to a temperature of 14 K. With stabilized dye lasers a NEP of 10 -19 W Hz - 1 2 can be achieved.

This paper is intended to be an introduction to the field of non-classical light. Starting from a pictorial discussion of the properties of the radiation field, examples for the generation and possible applications of squeezed states of the light field are given.

The possibility of using squeezed states for improving the shot-noise limit of the strain sensitivity of Michelson interferometers is discussed. We find that the spectrum of squeezing required depends on the method of stabilization used in the experiment. Details are given for the widely used phase-modulation technique (which also allows for recycling of the field), where we find an important application for broadband (‘two-mode’) squeezing.

It has been demonstrated that photoelectron angular distributions in resonant multiphoton ionization depend critically on the angular momentum quantum numbers of the intermediate state. This should help to interpret the spectra of atoms more complex than alkalies. In addition, information about the matrix elements for the bound-free transition can be obtained which is not available if the angle-integrated ionization cross section is measured. Reliable cross sections for photoionization out of excited atomic states are very important for astrophysics, but so far only a few laboratory data are available. The interpretation of some of these data is not straightforward, however, because of lack of knowledge about the degree of alignment of the atoms in the excited state. This problem can be solved either by optically pumping the atom into only one magnetic sublevel [24] or by measuring the photoelectron angular distribution in addition to the absolute cross section.

Resonant three-photon ionization of Ba to a structureless continuum via $6snd$ Rydberg states was performed in the range $19l~nl~30$. It is shown that state mixing in the Rydberg states strongly affects the photoion and photoelectron yields as well as the angular distributions of photoelectrons. The experimental results are explained on the basis of a three-channel quantum-defect theory for the perturbed Rydberg series.

The influence of the dynamic Stark effect on photoelectron angular distributions for two-photon-resonant three-photon ionization of sodium by way of the $4^{2}D$ fine-structure components has been investigated. Owing to the dynamic Stark shift of the atomic transition frequency, the angular distributions expected at low laser intensities were modified as different transitions were intensity tuned into and out of resonance with the laser frequency. The distributions are in good qualitative agreement with theory.

We formulate a model describing migration of population from low-l-italic to higher-l-italic states of the same principal quantum number in the highly excited state of the atom. The physical mechanism is the degenerate, nearly resonant Raman coupling. Specific calculations are performed for the hydrogen atom. The laser light is modeled as a monochromatic coherent or chaotic colored noise.

Quantum beats of the hyperfine levels of the 3 2 P 3 2 state of sodium have been observed in photoionization. The atoms are ionized stepwise with two pulsed lasers. The photoelectron angular distribution, measured as a function of the delay between the two laser pulses, exhibits a periodic variation due to the quantum beat effect. The hyperfine splitting of the intermediate state estimated from these measurements is in good agreement with literature values.

We extend a previous study of the usefulness of squeezed states of light for noise reduction in a non-ideal interferometer to the case where fringe visibility that is less than unity is caused by non-absorptive processes such as aberrations or misalignment of the beams. The main differences and similarities between this system and our earlier study of an interferometer with unbalanced losses in both arms are presented and discussed. An expression is given for the minimum detectable phase change in this system, in the presence of squeezing, as a function of fringe visibility.

We report lifetime measurements of the 1s2 3d2D, 1s24d2D, and 1s22p2P states of lithium. The results τ=14.5±0.7 ns, τ=31.0±1.0 ns, and τ=27.9±1.0 ns respectively were obtained by time resolved detection of the resonance fluorescence following pulsed laser excitation.

The Doppler-free, two-photon 5d1' 6 s ?Sll2-5d9 6s2 2D5!2 t r a n s i t i o n i n s i n g l y i o n i z e d Hg, a t t r a c t i v e a s a n o p t i c a l f r e q u e n c y s t a n d a r h a s b e e n o b s e r v e d f o r t h e f i r s t t i m e . A few ions were confined in a r a d i o f r e q u e n c y ( r f ) t r a p and the two-pho ton t r ans i t i on was d e t e c t e d by mon i to r ing t he change i n t he f l uo rescence l i gh t s c a t t e r e d by the ions f rom a l a s e r beam t u n e d t o t h e f i r s t r e s o n a n c e t r a n s i t i o n a t 194 nm. The r a d i a t i v e l i f e t i m e o f t h e s t a t e and t h e a b s o l u t e wavenumber o f t he two-pho ton t r ans i t i on were measured t o b e 0 . 0 9 0 ( 1 5 ) S and 17 757.15?(3) cm-' r e s p e c t i v e l y .

The development of the laser has led to the availability of coherent light beams of small divergence with high power and small spectral bandwidth spanning the range from VUV (vacuum ultraviolet) to FIR (far infrared). Lasers have been used in many applications in engineering, medicine, chemistry and pure research. As the whole field of laser applications is far too large to be covered in a few lectures four subjects have been selected here, namely dye laser and high resolution spectroscopy, remote sensing of the atmosphere, laser isotope separation, lasers in chemistry.

Since the pioneering experiment by Hanbury Brown and Twiss1 and the developments in the quantum description of the electromagnetic radiation field initiated by Glauber2 and Sudarshan3 the field of photon statistics and correlation has gained increasing attention. Today some applied techniques based on light correlation measurements are well established, e.g., in astronomy4 and diagnostics of particle motion.5 One of the frontiers in present research is concerned with nonclassical properties of the radiation field referred to in the literature as photon antibunching and squeezed states. In the following discussions we attempt to describe the status and address future prospects of experiments on these phenomena.

Gravitational wave detection with optical methods requires interferometers with extremely high strain sensitivity. Possibilities to increase the interferometer sensitivity using the squeezing and recycling techniques are discussed with respect to mirror losses and response time.

Publisher Summary This chapter deals with angular correlation in multi-photon ionization in atomic systems. The correlation becomes more complex, and consequently more interesting, for multi-photon processes: the correlation then involves the states of polarization of more than one photon. If electron spins in the initial state are assumed to be randomly oriented and if the spin polarization of the outbound electron is not measured, the correlation may be described uniquely by the alignment of an atom in the intermediate state and the polarization of the ionizing radiation field. The process of resonant multi-photon excitation to reach the intermediate state, which may involve absorption of one or more photons of polarized light, generally produces a non-equal distribution of populations of the magnetic substates, m, of the intermediate state. As a result, the intermediate state will have some degree of alignment. The total photo-ionization amplitude is then obtained by summing over final states for each initial orbital. With the squaring of the sum of the individual amplitudes to obtain the final expression for the differential cross section, cross-terms are generated that, in general, do not vanish in the differential cross section.

In transitions between bound states, multiphoton processes become important at higher laser intensities when the Rabi-frequency is comparable to or larger than the width of the transition line. This has been shown, e.g., in the study of resonance fluorescence of a two-level atom [1]. Bound-free transitions, however, cannot be saturated owing to the large width of the continuum. Consequently, multiphoton processes should not be important. The only effects expected at higher laser intensities are depletion of the bound state population and possibly continuum-continuum transitions, the latter resulting in multiple peaks in the photo-electron energy spectrum [2].

The investigation of atomic resonance fluorescence has always been of special interest as a means for the determination of atomic parameters. In addition, information on the interaction mechanism between atoms and radiation can be obtained. In the standard fluorescence experiment the frequency distribution of the incident photons is larger than the natural width of the respective transition; as a consequence the correlation time in the photon-atom interaction is determined by the lifetime of the atoms in the excited state. With the development of lasers and especially of tunable dye lasers in recent years it became possible to study the case where the incident radiation has a spectral distribution which is narrower than the natural width. This corresponds to a correlation time of the incoming light wave which is much longer than the excited-state lifetime. In this chapter a survey of experiments on the resonance fluorescence of atoms in monochromatic laser fields will be given.

Publisher Summary This chapter focuses on the general properties of Rydberg atoms, methods of production and detection, and spectroscopic techniques. The properties of the Rydberg atoms are very much hydrogen-like. Their energy is given by the Rydberg formula. Differences between Rydberg states of atoms and molecules arise from rotational vibrational excitations of the molecular ion. This leads to drastic effects when the frequency of the orbiting electron becomes comparable to the rotational vibrational frequencies. As a result, one finds a breakdown of the Born–Oppenheimer approximation and observes autoionization if part of the rotational vibrational energy is transferred to the Rydberg electron. For a certain class of molecules, even low-lying excited states show typical properties of Rydberg states. Because smooth changes are characteristic of classical systems, Rydberg atoms can be expected to show classical properties. In particular, according to Bohr's correspondence principle, the frequency of electromagnetic radiation emitted for transitions between neighboring states approaches the frequency at which the electron rotates around the ionic core.

Traditionally, precision frequency measurements and lifetime measurements have provided numerous and stringent tests of physical principles. Two of the important merits of trapped ions for precision frequency spectroscopy and lifetime measurements are long confinement times and a gentle, highly non-perturbative environment. Present experiments give the first glimpse of the deep reservoir of precision spectroscopic investigations possible with stored ions. Examples include the g-2 experiment that yields the highest precision yet obtained in fundamental tests of QED [1],and the measurement of the electron to proton mass ratio to an accuracy of 0.04 ppm [2]. In other experiments, trapped ions are radiatively cooled to temperatures below 0.1K [3–5]. This directly reduces Doppler effects to all orders and thereby enhances resolution and precision in the method of trapped ion spectroscopy. Furthermore, because the radiation of the laser used for cooling is usually tuned to near resonance with a highly allowed transition in order to facilitate efficient cooling, many photons per second per ion are scattered; the high rate of scattered photons provides a powerful scheme not only to detect ions but also to detect atomic transitions to metastable states by the method of double resonance [6,7]. This method can give unit detection efficiency or, equivalently, a signal-to-noise (S/N) ratio limited only by the quantum-statistical fluctuation in the number of ions that make the transition and not by detection solid angle, detector quantum efficiency, etc. [8]. This is the maximum S/N ratio possible. We further note that this highly efficient detector of an atomic transition works even if the cycling/cooling transition and the transition to the metastable state share no common level [9]. With the continual and rapid maturation of laser cooled, stored ion spectroscopy, there has come a flurry of activity. Examples include a 300 fold improvement in the limits for spatial anisotropy by using laser cooled, electromagnetically trapped 9Be+ ions [10], a laser cooled atomic frequency standard with an accuracy comparable to the best cesium beam atomic frequency standards [11], a demonstration of strong coupling in a small cloud of laser compressed and laser cooled ions [12], and a precise measurement of the electron-to-proton mass ratio [13]. In this paper we describe some of the first results obtained in our study of the Doppler-free, two-photon 5d10 6s 2S1/2−5d9 6s2 2D5/2 transition in singly ionized mercury stored in a miniature rf trap [14].

We present the high efficient generation of narrow-band heralded single photons, widely tunable in wavelength and bandwidth using resonator enhanced spontaneous down conversion in a crystalline whispering gallery mode resonator.

The reconstruction of particle trajectories makes it possible to distinguish between different types of charged particles. In the era of particle colliders and high luminosities, this was one of the key aspects for the discovery of many new particles, lately the Higgs-boson. In high-energy physics, where trajectories are rather long. large size trackers muste be used to achieve sufficient position resolution. This is not the case in low-energy particle physics experiments, where particle trajectories are very short. With current position-sensitive detection technologies it is difficult to obtain sufficient position resolution for particle identification in large sensitive volumes since all these detectors are based on the read-out of the ionization signal. This limitation is due to the diffusion of the drifting electrons. In this paper we demonstrate a "proof-of-principle" experiment for a new method for the tracking of charged particles. It takes advantage of the scintillation signal which is not affected by diffusion while propagating through the sensor. With our setup we achieved a position resolution of about 28 \textmu m in the best case The reconstruction of particle trajectories makes it possible to distinguish between different types of charged particles. In high-energy physics, where trajectories are rather long (several meters), large size trackers must be used to achieve sufficient position resolution. However, in low-background experiments like the search for neutrinoless double beta decay, tracks are rather short (some mm to several cm, depending on the detector in use) and three-dimensional trajectories could only be resolved in gaseous time-projection chambers so far. For detectors of a large volume of around one cubic meter (large in the scope of neutrinoless double beta search) and therefore large drift distances (several decimeters to 1 m), this technique is limited by diffusion and repulsion of charge carriers. In this work we present a “proof-of-principle” experiment for a new method of the three-dimensional tracking of charged particles by scintillation light: we used a setup consisting of a scintillator, mirrors, lenses, and a novel imaging device (the hybrid photon detector) in order to image two projections of electron tracks through the scintillator. We took data at the T-22 beamline at DESY with relativistic electrons with a kinetic energy of 5 GeV and from this data successfully reconstructed their three-dimensional propagation path in the scintillator. With our setup we achieved a position resolution in the range of 170–248 µm. The reconstruction of particle trajectories makes it possible to distinguish between different types of charged particles. In high-energy physics, where trajectories are rather long, large size trackers must be used to achieve sufficient position resolution. However, in low-background experiments tracks are rather short and three dimensional trajectories could only be resolved in time-projection chambers so far. For detectors of large volume and therefore large drift distances, which are inevitable for low-background experiments, this technique is limited by diffusion of charge carriers. In this work we present a "proof-of-principle" experiment for a new method for the three dimensional tracking of charged particles by scintillation light: We used a setup consisting of a scintillator, mirrors, lenses and a novel imaging device (the hybrid photo detector) in order to image two projections of electron tracks through the scintillator. We took data at the T-24 beam-line at DESY with relativistic electrons with a kinetic energy of 5 GeV and from this data successfully reconstructed their three dimensional propagetion path in the scintillator. With our setup we achieved a position resolution of about 28 mum in the best case.

Commercial optical communication is presently based on incoherent detection, while the application of more complex coherent detection schemes is still being developed. In both cases laboratory systems are approaching a limit that seems to tie the hands of the communication engineer – a purely quantum effect known as shot noise. It has been realised, however, that this is not as fundamental a limit as it may appear. New possibilities become available using light with a special property called 'squeezed noise'.

We report on a novel and efficient source of polarization squeezing using a single pass through an optical fiber. Simply passing this Kerr squeezed beam through a carefully aligned lambda/2 waveplate and splitting it on a polarization beam splitter, we find polarization squeezing of up to 5.1 +/- 0.3 dB. The experimental setup allows for the direct measurement of the squeezing angle., 4 pages, 4 figures

By using a sub-wavelength nano-particle as a field probe and a tailored detection scheme we are able to reconstruct the electric energy density in the focal plane of a high numerical aperture focusing system.

We report on a compact and stable optical parametric oscillator in a whispering gallery mode resonator, that combines very low pump thresholds with versatile tunability.

We witness experimentally the presence of macroscopic coherence in Gaussian quantum states using a recently proposed criterion (E.G. Cavalcanti and M. Reid, Phys. Rev. Lett. 97, 170405 (2006)). The macroscopic coherence stems from interference between macroscopically distinct states in phase space and we prove experimentally that even the vacuum state contains these features with a distance in phase space of $0.51\pm0.02$ shot noise units (SNU). For squeezed states we found macroscopic superpositions with a distance of up to $0.83\pm0.02$ SNU. The proof of macroscopic quantum coherence was investigated with respect to squeezing and purity of the states., 5 pages, 6 figures

The homogeneity test of glass plates in a Fizeau interferometer is hampered by the superposition of multiple interference signals coming from the surfaces of the glass plate as well as the empty Fizeau cavity. To evaluate interferograms resulting from such nested cavities, various approaches such as the use of broadband light sources have been applied. In this paper, we propose an adaptive frequency comb interferometer to accomplish the cavity selection. An adjustable Fabry–Perot resonator is used to generate a variable frequency comb that can be matched to the length of the desired cavity. Owing to its flexibility, the number of measurements needed for the homogeneity test can be reduced to four. Furthermore, compared to approaches using a two-beam interferometer as a filter for the broadband light source, the visibility of the fringe system is considerably higher if a Fabry–Perot filter is applied.

We develop a theoretical description for polarizers that goes beyond the paraxial approximation. By combining existing theories for fields with nonplanar wavefronts, we are able to derive a simple power series expansion expressing the electric field of a light beam after a polarizer as a linear function of the field and its spatial derivatives evaluated before the polarizer. The first few terms of such expansion are explicitly given, and their physical meaning is discussed.

We present an optical system covering 81 % of the solid angle into which an ion emits light. With this configuration we focus light close to the diffraction limit, while also approaching optimal light matter coupling.

We reconstruct the polarization sector of a bright polarization squeezed beam starting from a complete set of Stokes measurements. Given the symmetry that underlies the polarization structure of quantum fields, we use the unique SU(2) Wigner distribution to represent states. In the limit of localized and bright states, the Wigner function can be approximated by an inverse three-dimensional Radon transform. We compare this direct reconstruction with the results of a maximum likelihood estimation, finding an excellent agreement., Comment: 15 pages, 5 figures. Contribution to New Journal of Physics, Focus Issue on Quantum Tomography. Comments welcome

Quantum key distribution (QKD) is a two step protocol. In one step, quantum states are exchanged between two parties, usually called Alice and Bob. Afterwards, quantum correlations and classical communication are used to generate a shared secret key. In this paper, we focus on the exchange of quantum states with a prototype setup and present estimated secret bit rates. The continuous-variable QKD protocol [1–3] employs binary encoding in optical coherent states and postselection. In our experiment, the protocol is adapted to a long range fibre-based setup at the telecommunication wavelength (1550nm). The system employs heterodyne detection, using free-space optics and exploiting the polarisation degree of freedom. This allows for measuring the Qfunctions of the transmitted signal states and the estimation of the secret key rates for different fibre lengths.

We investigated modes of a birefringent whispering gallery mode resonator whose optical axis is tilted with respect to its symmetry axis. Polarization eigenstates that have not been observed in other WGM configurations will be presented.

The possibility of all-optical phase-preserving amplitude regeneration for star-8QAM is demonstrated using a modified nonlinear optical loop mirror. Experiments show a reduction in amplitude noise on both amplitude levels simultaneously, considering two different types of signal distortions: deterministic low-frequency amplitude modulation and broadband amplitude noise. Furthermore, using this amplitude regeneration, the robustness against nonlinear phase noise from fiber nonlinearity in a transmission line is increased. The scheme suppresses the conversion of amplitude noise to nonlinear phase noise. This is shown for simultaneous amplitude regeneration of the two amplitude states as well as for amplitude regeneration of the high-power states only. If the transmission is limited by nonlinear phase noise, single-level operation at the more critical higher-power state will benefit because of the wider plateau region. Numerical simulations confirm the experimental results.

We introduce and experimentally implement counter-intuitive entanglement creation by locally splitting a classical mode that is part of a larger discordant state. Possible applications are quantum advantage in information encoding and assisted dense coding.

Nowadays the complete control of fibre properties, such as dispersion, is an indispensable technology. For the nonlinearity, however, possibilities are limited particularly due to the difficulty of creating a nonlinear phase response with sign opposite to the Kerr effect. Promising candidates are interferometric devices, such as modified Mach-Zehnder interferometers proposed for nonlinearity management [1] or nonlinear amplifying loop mirrors (NALM) for nonlinear phase-shift compensation in phase-encoded optical transmission systems [2].

Non-zero fiber dispersion in a Sagnac interferometer can significantly improve multilevel phase-preserving amplitude regeneration. Soliton effects can be used to create irregularly spaced plateaus for high-power states of complex modulation formats.

We discuss the scattering of a light pulse by a single atom in free space using a purely semi-classical framework. The atom is treated as a linear elastic scatterer allowing to treat each spectral component of the incident pulse separately. For an increasing exponential pulse with a dipole radiation pattern incident from full solid angle the spectrum resulting from interference of incident and scattered components is a decreasing exponential pulse., Comment: 5 pages, one figure

In recent years there has been an increasing interest in achieving efficient coupling of a single photon to a single atom in free space. Independent of the particular application at hand, optimizing the efficiency of the coupling is tantamount to maximizing the electric field component parallel to the dipole moment of the specific atomic transition. In this contribution we present the field distributions which achieve this maximization for several optical focusing devices.

We experimentally demonstrate distillation of continuous variable entangled light that has undergone non‐Gaussian attenuation loss. The continuous variable entanglement is generated with optical fibers and sent through a lossy channel, where the transmission is varying in time. By employing simple linear optical components, a measurement induced operation and classical communication, we demonstrate that the entanglement is probabilistically increased.