Practical implementation of multilevel quantum cryptography

The physical principles of a quantum key distribution protocol using four-level optical systems are discussed. Quantum information is encoded into polarization states created by frequency-nondegenerate spontaneous parametric down-conversion in collinear geometry. In the scheme under analysis, the required nonorthogonal states are generated in a single nonlinear crystal. All states in the selected basis are measured deterministically. The results of initial experiments on transformation of the basis polarization states of a four-level optical system are discussed.     

Selective excitation in pulsed EPR of a spin-correlated triplet-radical pair

Experiments are described in which a low-amplitude microwave pulse excites only one out of three allowed transitions of the quinone radical (Q(A)(-)) in a spin-correlated triplet-radical pair 3PQ(A)(-) of the bacterial photosynthetic reaction center. A second high-amplitude pulse produces a FID whose temporal shape is strongly modulated with frequencies determined by electron-electron dipolar interaction in the pair. The FID is detected in both the in-phase and the out-of-phase channels. The out-of-phase FID is a result of switching off the magnetic dipolar interaction between 3P and Q(A)(-) due to decay of 3P during the time interval between the two pulses. Refocusing of FID by an additional non-selective pulse allows a dead-time free measurement of this modulation. The influence of the dead-time problem on the distance determination is discussed.     

Direct observation of nondiffusive motion of a Brownian particle

The thermal position fluctuations of a single micron-sized sphere immersed in a fluid were recorded by optical trapping interferometry with nanometer spatial and microsecond temporal resolution. We find, in accord with the theory of Brownian motion including hydrodynamic memory effects, that the transition from ballistic to diffusive motion is delayed to significantly longer times than predicted by the standard Langevin equation. This delay is a consequence of the inertia of the fluid. On the shortest time scales investigated, the sphere's inertia has a small, but measurable, effect.     

Electron interactions and infrared spectra of para isomers of X(CF3Y)C6H4 compounds

IR spectra are reported for, in which X represents various polar substituents and Y a bridge with multiple bonds. There is a linear relation between the frequency of the v^E _C-F(H) degenerate vibration and Taft's induction constant * for the radical attached to the CF₃ group. Frequency v^E _C-F is also correlated with the _n, which shows that the conjugated system can transmit the electron effect of X to the CF₃ group. Compounds are compared as regards this transmission, and some features of the frequencies of the multiple bonds are elucidated.See[6, 7] for brief details of the spectra of some of the compounds.     

Electron-phonon coupling and phonon generation in normal-metal microbridges

The second derivative of current—voltage characteristic, $\text{d}{}^2\text{I}\text{d}\text{V}{}^2$, of a small orifice connecting two pieces of normal metals is shown to be proportional to the function $\text{G(ω) =}\text{α}\text{?}{}^2\text{(ω)F(ω)}$ at ω = eV, where F(ω) is the phonon density of states, and α̃² (ω) the square of the electron—phonon matrix element averaged over the Fermi surface and multiplied by the additional structure factor taking into account the geometry of the orifice. The constriction is shown to work, in a current-carrying state, as a source of non-equilibrium phonons emitted in the immediate vicinity of the orifice.     

Stereochemistry of the complexation of derivatives of tricyclo[4.2.2.05,8] decadiene and related cycloalkenes with AgEu(fod)4

The stereochemical features of the complexation of derivatives of tricyclo[4.2.2.0^{5, 8}]decadiene and related cycloalkenes with AgEu (fod)₄ have been investigated by PMR spectroscopy with silver-lanthanide shift reagents. The bidentate structure of the complexes investigated has been established on the basis of the results of calculations carried out with the use of silver—lanthanide-induced shifts.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 736–739, November–December, 1985.We express our thanks to A. S. Koz'min and V. N. Kirin for supplying the samples of compounds 1 and 2.     

Weakly correlated superconductors

A concept of weak correlation in superconductors is introduced, which is similar, but not equal, to weak coupling (of the Josephson type), and some consequences of the idea are briefly considered. It is shown that the dependence of critical temperature of layered superconductors on the number of layers may be explained taking into account the correlation energy, and may not be accounted for by the Josephson model (neglecting correlation).³