Superconductivity and macroscopic quantum phenomena

It is often asserted that superconducting systems are manifestations of quantum mechanics on a macroscopic scale. In this review article it is demonstrated that this quantum assertion is true within the framework of the microscopic theory of superconductivity.     

State reduction in quantum mechanics: A calculational example

The question is posed as to the role of the state reduction hypothesis in the passage of a molecular beam through a Stern-Gerlach apparatus of the ordinary sort, and of the sort proposed by Wigner, in which the beam is rejoined at the end of the apparatus. The effect of the magnetic field is calculated, as is the effect of a “detector” atom placed asymmetrically in the apparatus, and sensitive to the passage of the molecular beam nearby. It is shown that in the absence of the detector atom the rejoined beam is represented by a pure case density matrix. However, when the detector is present in one arm of our Stern-Gerlach apparatus, the beam is found to be in a statistical mixture approximating “reduction”, even if we never “look” at the detector atom. It is also seen that the beam density matrix is reduced to diagonal form even though it is not physically affected by the detector.     

Observation of electromagnetically induced transparency in cesium molecules

We have studied electromagnetically induced transparency (EIT) in diatomic cesium molecules in a vapor cell by using tunable diode lasers. We have observed a sub-natural Λ-resonance in absorption molecular band B ¹Π _u − X ¹Σ _g ⁺ at different cesium vapor pressures. The width of the EIT resonance shows a linear dependence on a cesium vapor pressure.     

Quantum lithography with classical light

We show how to achieve subwavelength diffraction and imaging with classical light, previously thought to require quantum fields. By correlating wave vector and frequency in a narrow band, multiphoton detection process that uses Doppleron-type resonances, we show how to achieve arbitrary focal and image plane patterning with classical laser light at submultiples of the Rayleigh limit, with high efficiency, visibility, and spatial coherence. A frequency-selective measurement process thus allows one to simulate, semiclassically, the path-number correlations that distinguish a quantum entangled field.     

Quantum interferences in cooperative Dicke emission from spatial variation of the laser phase

We report the generation of a new quantum interference effect in spontaneous emission from a resonantly driven system of two identical two-level atoms due to the spatial variation of the laser phase at the positions of the atoms. This interference affects significantly the spectral features of the emitted radiation and the quantum entanglement in the system. The interference leads to dynamic coupling of the populations and coherences in a basis, determined by the laser phase and represents a kind of vacuum mediated superexchange between the symmetric and antisymmetric states.     

Improvement of spectral resolution by using the excitation dependence of dipole–dipole interaction in a dense atomic gas

We have studied the selective reflection from the interface between a dense rubidium (Rb) atomic vapor and a transparent dielectric. A remarkable narrowing of the spectrum, which can be used to improve the resolution of spectroscopy of dense media, has been demonstrated. This narrowing results from the reduction of the dipole–dipole interaction between atoms when the Rb vapor is excited by a strong pump laser. By using this technique, we have resolved the hyperfine structure of the Rb D₂ line, which is hidden by collisional broadening. PACS 32.70.Jz; 42.50.Ct; 34.80.Dp     

Temporal coherent control of superfluorescent pulses

We demonstrate that collective atomic interferences can be investigated by measuring the superfluorescence (SF) time delay. A pair of broadband (≈20 nm), ultrashort (≈80 fs), collinear pulses with a variable delay coherently excites rubidium (Rb) atoms. The generated superfluorescent pulses at 420 nm on the cascade transition are recorded by a picosecond streak camera. Both intensity and SF time delay of the 420 nm pulse are altered as the delay between input pulses varies. In particular, the SF time delay of the normalized 420 nm pulse exhibits oscillations with different periods. This can be understood in terms of atomic and quantum interferences due to two possible two-photon excitation pathways through the intermediate levels (Rb D-lines).     

Development of a laser-based fluorescence microscope with subnanosecond time resolution

This report describes the development of a fluorescence microscope based on a standard inverted optical microscope which incorporates a pulsed picosecond dye laser excitation source and a detector consisting of a gated image intensifier coupled to a CCD camera. Fluorescence images have been obtained using gate durations of 0.5 ns from this apparatus, representing a reduction in gate duration of an order of magnitude compared with similar instruments reported by others recently. Subnanosecond gated fluorescence images of V79-4 Chinese hamster lung fibroblasts stained with a phthalocyanine photosensitizer used in photodynamic therapy are presented. The results of these measurements are discussed in terms of the intracellular distribution of the sensitizer. Other potential applications and limitations of this technique are also outlined.     

The effect of pH on the photophysics and photochemistry of disulphonated aluminum phthalocyanine

The results of a study of the effect of pH on the photophysics and photochemistry of di-sulphonated aluminum phthalocyanine (AlPcS₂) in aqueous solution are presented. The pH dependence of the triplet quantum yield, fluorescence quantum yield, singlet-oxygen quantum yield, triplet lifetime, fluorescence lifetime and apparent dimerization constants is investigated and the results interpreted in terms of the pH dependence of the nature of the axial ligands. Evidence that the aluminum–axial ligand bond strength, rather than dimer binding energy that determines the extent of dimerization is provided by semi-empirical and ab initio calculations. Possible dimer structures obtained using ab initio calculations are discussed.