Accuracy of bubble velocity measurement with a four-point optical fibre probe

For the operation of high void fraction bubbly flows in bubble\ud columns, insight in primary parameters such as bubble size,\ud shape and velocity as well as gas volume fraction is essential.\ud At high gas volume fractions the flow system becomes\ud opaque, ruling out non-intrusive optical techniques. As an\ud alternative optical fibre probes can be used, which have the\ud advantage of low cost, simplicity of setup and easy\ud interpretation of the results.\ud By using four-point optical fibre probe, properties of bubbles\ud can be studied, such as bubble velocity, bubble size, etc.\ud However, the effect of bubble wobbling behaviour and\ud physical properties of liquids on the accuracy of the velocity\ud measurements has not been investigated in detail.\ud In the present study, the performance of a four-point optical\ud fibre probe was evaluated for five different liquids. The probe\ud performance and causes of inaccuracies are discuss

Saturation of Spin-Polarized Current in Nanometer Scale Aluminum Grains

We describe measurements of spin-polarized tunnelling via discrete energy levels of single Aluminum grains. In high resistance samples ($\sim G\Omega$), the spin-polarized tunnelling current rapidly saturates as a function of the bias voltage. This indicates that spin-polarized current is carried only via the ground state and the few lowest in energy excited states of the grain. At the saturation voltage, the spin-relaxation rate $T_1^{-1}$ of the highest excited states is comparable to the electron tunnelling rate: $T_1^{-1}\approx 1.5\cdot 10^6 s^{-1}$ and $10^7s^{-1}$ in two samples. The ratio of $T_1^{-1}$ to the electron-phonon relaxation rate is in agreement with the Elliot-Yafet scaling, an evidence that spin-relaxation in Al grains is governed by the spin-orbit interaction.Comment: 5 pages, 4 figure

Gamma-ray bursts: postburst evolution of fireballs

The postburst evolution of fireballs that produce $\gamma$-ray bursts is studied, assuming the expansion of fireballs to be adiabatic and relativistic. Numerical results as well as an approximate analytic solution for the evolution are presented. Due to adoption of a new relation among $t$, $R$ and $\gamma$ (see the text), our results differ markedly from the previous studies. Synchrotron radiation from the shocked interstellar medium is attentively calculated, using a convenient set of equations. The observed X-ray flux of GRB afterglows can be reproduced easily. Although the optical afterglows seem much more complicated, our results can still present a rather satisfactory approach to observations. It is also found that the expansion will no longer be highly relativistic about 4 days after the main GRB. We thus suggest that the marginally relativistic phase of the expansion should be investigated so as to check the afterglows observed a week or more later.Comment: 17 pages, 4 figures, MNRAS in pres

Conductance of a single-atom carbon chain with graphene leads

We study the conductance of an interconnect between two graphene leads formed by a single-atom carbon chain. Its dependence on the chemical potential and the number of atoms in the chain is qualitatively different from that in the case of normal metal leads. Electron transport proceeds via narrow resonant states in the wire. The latter arise due to strong reflection at the junctions between the chain and the leads, which is caused by the small density of states in the leads at low energy. The energy dependence of the transmission coefficient near resonance is asymmetric and acquires a universal form at small energies. We find that in the case of leads with the zigzag edges the dispersion of the edge states has a significant effect on the device conductance.Comment: 9 pages, 4 figure

High efficiency tomographic reconstruction of quantum states by quantum nondemolition measurements

We propose a high efficiency tomographic scheme to reconstruct an unknown quantum state of the qubits by using a series of quantum nondemolition (QND) measurements. The proposed QND measurements of the qubits are implemented by probing the the stationary transmissions of the dispersively-coupled resonator. It is shown that only one kind of QND measurements is sufficient to determine all the diagonal elements of the density matrix of the detected quantum state. The remaining non-diagonal elements of the density matrix can be determined by other spectral measurements by beforehand transferring them to the diagonal locations using a series of unitary operations. Compared with the pervious tomographic reconstructions based on the usual destructively projective (DP) measurements (wherein one kind of such measurements could only determine one diagonal element of the density matrix), the present approach exhibits significantly high efficiency for N-qubit (N > 1). Specifically, our generic proposal is demonstrated by the experimental circuit-quantumelectrodynamics (circuit-QED) systems with a few Josephson charge qubits.Comment: 9pages,4figure