Semiconductor point contacts can be a useful tool for producing spin-polarized currents in the presence of spin-orbit (SO) interaction. Neither magnetic fields nor magnetic materials are required. By numerical studies, we show that (i) the conductance is quantized in units of 2e 2 /h unless the SO interaction is too strong, (ii) the current is spin-polarized in the transverse direction, and (iii) a spin polarization of more than 50% can be realized with experimentally accessible values of the SO interaction strength. The spin-polarization ratio is determined by the adiabaticity of the transition between subbands of different spins during the transport through the point contacts.
A trade-off relation on our knowledge about two noncommuting observables of a qubit system in simultaneous measurement is formulated. The obtained inequality offers a quantitative informationtheoretic representation of Bohr's principle of complementarity, and can be interpreted as a tradeoff relation on the asymptotic accuracy of the maximum-likelihood estimation of the probability distributions of observables.Quantum mechanics features two distinct kinds of uncertainty. One is the quantum fluctuations inherent in a measured system, and the other is the noise caused by the process of measurement. Quantum fluctuations prevent us from knowing a quantum system beyond the probability distribution of the measured observable [1], but the probability distribution itself can be accurately determined by means of an appropriate projection measurement. The uncertainty relation between two noncommuting observables such as the position and the momentum originates from those fluctuations [2,3,4,5]. On the other hand, the noise places a limit on the accuracy of simultaneous measurement. It is known, for example, that simultaneous measurements of two noncommuting observables implies that at least one of them cannot be measured without incurring a measurement error [6]. Despite a long history of study [6,7,8,9], however, the fundamental limit to simultaneous measurement of two noncommuting observables has yet to be fully understood.A classic analysis of the problem was given by Arthurs et al. [10,11] who, under a special condition called unbiasedness, have shown that the lower bound of the uncertainty product of canonically conjugate observables is twice as large as the standard lower bound of /2, where is the Planck constant divided by 2π. The underlying physics behind this doubling of the lower bound is that, under the condition of unbiasedness, fluctuations of a system's observable and the noise generated in the measurement process become uncorrelated and that they simply add up. More recently, a number of studies on related problems are conducted without invoking the unbiasedness condition and various uncertainty relations are derived [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23].In this paper, we derive a trade-off relation concerning the measurement accuracy of two noncommuting observables for a qubit system by considering nonideal joint measurements. We also show that our characterization of the measurement accuracy is closely related to the Fisher information [24,25], which provides the asymptotic accuracy of the maximum-likelihood estimation of the probability distribution of an observable for a finite number of samples. In reality, only a finite number of samples are available [26], which give us only an imperfect informa-tion about the probability distribution of an observable for an unknown state. The crucial observation made in this paper is that this imperfection is further deteriorated in the case of simultaneous measurement due to noncommutability of the observables.We first formulate a simultaneous mea...
Viscous liquids often exhibit flow slippage on solid walls. The occurrence of flow slippage has a large impact on the liquid transport and the resulting energy dissipation, which are crucial for many applications. It is natural to expect that slippage takes place to reduce the dissipation. However, (i) how the density fluctuation is affected by the presence of the wall and (ii) how slippage takes place through forming a gas layer remained elusive. Here, we report possible answers to these fundamental questions: (i) Density fluctuation is intrinsically enhanced near the wall even in a quiescent state irrespective of the property of wall, and (ii) it is the density dependence of the viscosity that destabilizes the system toward gas-layer formation under shear flow. Our scenario of shear-induced gas-phase formation provides a natural physical explanation for wall slippage of liquid flow, covering the slip length ranging from a microscopic (nanometers) to macroscopic (micrometers) scale.
We propose a spin-injection method utilizing quantum point contacts (QPCs) fabricated on narrow-gap semiconductors with strong Rashba spin-orbit (SO) interaction. When the conductance through a QPC is quantized in units of 2e2 /h, the current is spin-polarized in the transverse direction to the current even in the absence of magnetic field. The spin polarization, which would be larger than 50% in InGaAs heterostructures, can be detected by connecting the QPC to a ferromagnetic lead. Then the conductance is maximal (minimal) when the magnetization in the lead is parallel or antiparallel (perpendicular) to the spin polarization of the current. The same conductance in the parallel and antiparallel alignments is explained by the Onsager relation, reflecting the fact that the SO interaction does not break the time reversal symmetry.1 Introduction Injection of spin-polarized current into semiconductors is an important issue for the development of spin-based electronics, "spintronics." To manipulate electron spins, the Rashba spin-orbit (SO) interaction is useful since its strength is controllable by applying an electric field [1-3].In our previous paper [4], we have theoretically studied the ballistic transport through a quantum point contact (QPC) in the presence of Rashba SO interaction and shown that the QPC can be a useful tool for the spin injection. No magnetic field is required. We have found that the conductance is quantized in units of 2e2 /h and that the current is spin-polarized in the transverse direction. The spin polarization of more than 50% can be realized in InGaAs heterostructures. The QPC structure is easy to fabricate compared with other devices which have been proposed for the spin injection [5][6][7][8][9].In this paper, we discuss the detection of the spin-polarized current when the QPC is connected to a ferromagnetic lead. The conductance depends on the spin polarization of the current from QPC and magnetization direction in the lead. The conductance is maximal (minimal) when the magnetization is parallel or antiparallel (perpendicular) to the spin polarization of the current. The same conductance in the parallel and antiparallel alignments can be explained by the Onsager relation.
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