E. A. Zubova

Quantum variation measurement of a force

The quantum limit for the resolution of a small force using an optical transducer of a displacement with a coherent nonmodulated pump is proved to be less than the standard quantum limit if one measures the quadrature amplitude in the output wave, squeezed by the ponderomotive nonlinearity mechanism. This squeezing has a spectral dependence and we propose a procedure showing it.

On the quantum limit for resolution in force measurement using an optical displacement transducer

Abstract On the example of the simplest optical transducer it is shown that in continuous coordinate measurement, back action does not limit the value of the minimum registered force if the radiative friction introduced by the device is not taken into account. It means that there is no standard quantum limit (SQL) for the resolution in the measurements of force. With radiative friction taken into account, the minimal uncertainty in force ΔF min ⋍ξΔF SQL (ω F /ω 0 ) 1 4 is calculated, where ω F i s the frequency of force, ω0 is optical frequency and ξ a constant of the order of unity, achievable at optimum pump power. For a mechanical resonator with intrinsic friction larger than the radiative one, the minimum error in the force measurement (at optimum pump power) is described by the quantum Nyquist formula with zero-point fluctuations energy ħΩ/2 substituted by ħΩ. In the reflected wave one has to measure not the phase but the quadrature amplitude in a modified homodyne scheme. It is also shown that the pump can be in a coherent state: no squeezed, photon number or any nonclassical states are required.

On the quantum limit for resolution in force measurement in interferometric optical transducers of displacement

Abstract The quantum limit for resolution of force using a transducer of displacement with an optical Fabry-Perot resonator is proved to be less than the standard quantum limit (SQL) if one measures not the phase but a specially chosen quadrature amplitude in the output wave, squeezed by a mechanism of ponderomotive nonlinearity. Taking into account radiative friction, introduced by the transducer into the mechanical oscillator, we show that the minimal registered force is equal to F min ⋍ ξ F SQL (ω F /ω o ) 1 4 , if the mirrors of the resonator are without losses (here ω F is the frequency of force, ω o is the optical frequency and ξ is a constant of the unity order, achievable at optimum pump power). Losses in mirrors limit the minimal registered force by value F min 1 ⋍ ζ F SQL ( R/q ) 1 4 where R is the sum of the loss coefficients of both mirrors and q is the transmission coefficient of the input mirror, ζ is about unity. Using Fabry-Perot resonators in a Michelson interferometer with full identical arms conserves the above results but it also gives the possibility to work with small power entering the detectors (dark fringe regime). Deterioration of sensitivity caused by small nonidentity of the arms is considered.

Simulation of melting in crystalline polyethylene

We carry out a molecular dynamics simulation of the first stages of constrained melting in crystalline polyethylene (PE). When heated, the crystal undergoes two structural phase transitions: from the orthorhombic (O) phase to the monoclinic (M) phase, and then to the columnar (C), quasi-hexagonal, phase. The M phase represents the tendency to the parallel packing of planes of PE zigzags, and the C phase proves to be some kind of oriented melt. We follow both the transitions O→M and M→C in real time and establish that, at their beginning, the crystal tries (and fails) to pass into the partially ordered phases similar to the RI and RII phases of linear alkanes, correspondingly. We discuss the molecular mechanisms and driving forces of the observed transitions, as well as the reasons why the M and C phases in PE crystals substitute for the rotator phases in linear alkanes.

The value of the force of radiative friction

We discuss the value of the radiative friction for a single mirror and a mirror in a Fabry-Perot resonator, and show that in the last case this friction can be experimentally registered.

New coarse-grained DNA model

A new coarse-grained model of the DNA molecule has been proposed, which was elaborated on the basis of its all-atomic model analysis. The model has been shown to rather well reproduce the DNA structure under low and room temperatures. The Young’s and torsion moduli calculated using the coarse-grained model are in close agreement with experimental data and the theoretical results of other authors. The model can be used for DNA fragments of several hundreds base pairs for rather long time scales (of the order of μs) and for simulating their interactions with other structures.

High-temperature quasi-hexagonal phase in the simplest model of a polymer crystal

ISSN 0012-5016, Doklady Physical Chemistry, 2008, Vol. 418, Part 2, pp. 15–18.

Diffusion of topological solitons and dielectric αc relaxation in a polymeric crystal

A molecular dynamics simulation was performed to estimate the effective mass and the diffusion and friction coefficients of point defects in macromolecular chains of crystalline polyethylene. The results were compared with theoretical mass and kinetic coefficient predictions for topological solitons, with which these defects were identified. The results are used to discuss the soliton model of dielectric αc relaxation in weakly oxidized polyethylene.

Dynamics of Soliton-Like Excitations in a Chain of a Polymer Crystal: Influence of Neighbouring Chains Mobility

Abstract We investigate influence of mobility of neighbouring chains on dynamics of soliton-like excitations in a chain of the simplest polymer crystal (polyethylene in the “united atoms” approximation) using molecular dynamics simulation. We present results for point-like structural defects: static and moving at low, medium and high velocities; and examine how the structure of a crystal will affect them.

Vacancy mobility in polymer crystals

A molecular-dynamics model of the behavior of a vacancy in the chain of an equilibrium polymer crystal (the “collective atom” approximation for polyethylene) is developed for the first time. It is shown that a defect of this type in a polymer crystal has a soliton mobility, as opposed to vacancies in crystals of low-molecular substances.