Phase-space formulation of quantum mechanics and quantum state reconstruction for physical systems with Lie-group symmetries

We present a detailed discussion of a general theory of phase-space distributions, introduced recently by the authors [J. Phys. A {\bf 31}, L9 (1998)]. This theory provides a unified phase-space formulation of quantum mechanics for physical systems possessing Lie-group symmetries. The concept of generalized coherent states and the method of harmonic analysis are used to construct explicitly a family of phase-space functions which are postulated to satisfy the Stratonovich-Weyl correspondence with a generalized traciality condition. The symbol calculus for the phase-space functions is given by means of the generalized twisted product. The phase-space formalism is used to study the problem of the reconstruction of quantum states. In particular, we consider the reconstruction method based on measurements of displaced projectors, which comprises a number of recently proposed quantum-optical schemes and is also related to the standard methods of signal processing. A general group-theoretic description of this method is developed using the technique of harmonic expansions on the phase space.Comment: REVTeX, 18 pages, no figure

Imaging fluorescence lifetime modulation of a ruthenium-based dye in living cells: the potential for oxygen sensing

Fluorescence lifetime measurements of long excited-state lifetime, oxygen-quenched ruthenium dyes are emerging as methods for intracellular oxygen sensing. Fluorescence lifetime imaging microscopy (FLIM) studies in cells have been reported previously. Many current FLIM systems use high repetition rate (∼107 Hz) lasers optimized for nanosecond lifetime measurements, making measurement of long, microsecond lifetime fluorophores difficult. Here, we present an experimental approach for obtaining a large temporal dynamic range in a FLIM system by using a low repetition rate (101 Hz), high output, nitrogen pumped dye laser and a wide-field, intensified CCD camera for image detection. We explore the feasibility of the approach by imaging the oxygen-sensitive dye tris(2,2′-bipyridyl)dichloro-ruthenium(II) hexahydrate (RTDP) in solution and in living cells. We demonstrate the ability of the system to resolve 60% variations in RTDP fluorescence lifetime upon oxygen cycling in solution. Furthermore, the FLIM system was able to resolve an increase in RTDP fluorescence lifetime in cultured human epithelial cells under diminished oxygen conditions. The technique may be useful in developing methods for quantifying intracellular oxygen concentrations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48916/2/d31406.pd

Some Secrets of Fluorescent Proteins: Distinct Bleaching in Various Mounting Fluids and Photoactivation of cyan fluorescent proteins at YFP-Excitation

Background
The use of spectrally distinct variants of green fluorescent protein (GFP) such as cyan or yellow mutants (CFP and YFP, respectively) is very common in all different fields of life sciences, e.g. for marking specific proteins or cells or to determine protein interactions. In the latter case, the quantum physical phenomenon of fluorescence resonance energy transfer (FRET) is exploited by specific microscopy techniques to visualize proximity of proteins.

Methodology/Principal Findings
When we applied a commonly used FRET microscopy technique - the increase in donor (CFP)-fluorescence after bleaching of acceptor fluorophores (YFP), we obtained good signals in live cells, but very weak signals for the same samples after fixation and mounting in commercial microscopy mounting fluids. This observation could be traced back to much faster bleaching of CFP in these mounting media. Strikingly, the opposite effect of the mounting fluid was observed for YFP and also for other proteins such as Cerulean, TFP or Venus. The changes in photostability of CFP and YFP were not caused by the fixation but directly dependent on the mounting fluid. Furthermore we made the interesting observation that the CFP-fluorescence intensity increases by about 10 - 15% after illumination at the YFP-excitation wavelength – a phenomenon, which was also observed for Cerulean. This photoactivation of cyan fluorescent proteins at the YFP-excitation can cause false-positive signals in the FRET-microscopy technique that is based on bleaching of a yellow FRET acceptor.

Conclusions/Significance
Our results show that photostability of fluorescent proteins differs significantly for various media and that CFP bleaches significantly faster in commercial mounting fluids, while the opposite is observed for YFP and some other proteins. Moreover, we show that the FRET microscopy technique that is based on bleaching of the YFP is prone to artifacts due to photoactivation of cyan fluorescent proteins under these conditions

New trends in studying structure and function of biological membranes

Thirty years ago Singer and Nicolson constructed the “fluid mosaic model” of the membrane, which described the structural and functional characteristics of the plasma membrane of non-polarized cells like circulating blood lymphocytes as a fluid lipid phase accommodating proteins with a relatively free mobility. It is a rare phenomenon in biology that such a model could survive 30 years and even today it has a high degree of validity. However, in the light of new data it demands some modifications. In this minireview we present a new concept, which revives the SN model, by shifting the emphasis from fluidity to mosaicism, i.e. to lipid microdomains and rafts. A concise summary of data and key methods is given, proving the existence of non-random co-distribution patterns of different receptor kinds in the microdomain system of the plasma membrane. Furthermore, we present evidence that proteins are not only accommodated by the lipid phase, but they are integral structural elements of it. Novel suggestions to the SN model help to develop a modernized version of the old paradigm in the light of new data

Pressure Measurement Systems

Measurements of the steady pressure in a fluid flow may be required to determine other thermodynamic properties, to determine forces on a body due to the pressure distribution over it, or in order to determine the dynamic head and flow velocity (for further details on the latter see Sect. 5.1. Pressure is a scalar representation of molecular activity, a measure of the nondirectional molecular motions. Thus it must, by definition, be measured by a device at rest relative to the flow. Whilst the common practice in the fluid mechanics community is to denote the pressure as static (as opposed to the coordinate-dependent total pressure, Sect. 3.1), this terminology introduces a fundamental redundancy. In practice, pressure is commonly measured both at walls and in the freestream using the types of measurement device shown in Fig. 4.1 connected to a transducer of suitable sensitivity and range. The orifice of a small wall tapping represents a simple way to obtain the pressure impressed on the wall by the external flow. So-called static pressure tubes approximate the local fluid pressure in the freestream if the disturbance presented to the flow can either be accounted for or is not large to begin with. However this can only ever be strictly true for steady laminar flow due to the normal velocity component introduced when a flow becomes turbulent. Measurement of freestream pressure is one of the hardest challenges in fluid mechanics. Fig. 4.1 This chapter addresses measurement of pressure using wall tappings (Sect. 4.1) and static pressure tubes (Sect. 4.2), and especially errors due to the intrusive flow presence of real, finite-sized devices and calibrations to correct for these. Bryer and Pankhurst [4.1] and Chue [4.2] provided seminal monographs on the general topic of pressure probes in 1971 and 1975, respectively, which give detailed descriptions of measurement devices, coverage of the background to the various corrections and a survey of older data. The topic is covered here more concisely, with a view to practical use by the engineer, and with reference to modern literature. The reader is referred to Bryer and Pankhurst [4.1] and Chue [4.2] for further details on most sections. In more recent years a further method for obtaining pressure on the surface of a wind tunnel model has been developed, based on pressure sensitive paints (PSP). The introduction of PSP provides a method to measure the pressure on the surface of a model directly without the transducers and tubing associated with conventional means. A paint, the luminescence of which is dependent on air pressure, is applied to the surface of a wind tunnel model and the pressure distribution is obtained from the images produced by proper illumination. In Sect. 4.4 the basics of PSP are discussed and further subsections address in detail different paints, paint application procedures, imaging systems and image processing. In discussing the achievable accuracy of PSP techniques, both the spatial and temporal resolution is examined. The thermal sensitivity of the paint dye is introduced and this is closely linked to temperature-sensitive paints (TSP), as discussed in Chap. 7, Sect. 7.4