We investigate coherent time-evolution of charge states (pseudo-spin qubit) in a semiconductor double quantum dot. This fully-tunable qubit is manipulated with a high-speed voltage pulse that controls the energy and decoherence of the system. Coherent oscillations of the qubit are observed for several combinations of many-body ground and excited states of the quantum dots. Possible decoherence mechanisms in the present device are also discussed.Initiated by various experiments on atomic systems, studies on coherent dynamics have been extended to small-scale quantum computers [1]. Nano-fabrication technology now allows us to design artificial atoms (quantum dots) and molecules (coupled quantum dots), in which atomic (molecular)-like electronic states can be controlled with external voltages [2,3,4]. Coherent manipulation of the electronic system in quantum dots and a clear understanding of decoherence in practical structures are crucial for future applications of quantum nanostructures to quantum information technology.In this Letter, we describe the coherent manipulation of charge states, in which an excess electron occupies the left dot or the right dot of a double quantum dot (DQD). The coherent oscillations between the two charge states are produced by applying a rectangular voltage pulse to an electrode. Although this scheme is analogous to experiments on a superconducting island [5], our qubit is effectively isolated from the electrodes during the manipulation, while it is influenced by strong decoherence during the initialization due to the coupling with the electrodes. This controlled decoherence provides an efficient initialization scheme.We consider a DQD consisting of left and right dots connected through an interdot tunneling barrier. The left (right) dot is weakly coupled to the source (drain) electrode via a tunneling barrier [see Fig. 1(a)]. The conductance through the device is strongly influenced by the onsite and interdot Coulomb interactions [6]. In the weakcoupling regime at a small source-drain voltage, V sd , a finite current is only observed at the triple points, where tunneling processes through the three tunneling barriers are allowed. Under an appropriate condition where only the interdot tunneling is allowed, Coulomb interactions effectively isolate the DQD from the source and drain electrodes. In this case, we can consider two charge states, in which an excess electron occupies the left dot (|L ) or the right dot (|R ) with electrochemical potentials E L and E R , respectively. In practice, each charge state involves (many-body) ground and excited states. When the two specific states are energetically close to each other and the excitation to other states can be neglected, the system can be approximated as a two-level system (qubit). It is characterized by the energy offset, ε ≡ E R − E L , and the interdot tunneling, which gives an anti-crossing energy, ∆ [3]. The effective Hamiltonian iswhere σ x and σ z are the Pauli matrices for pseudo-spin bases of |L and |R . When E L and E R of ...

Single-electron tunneling through a quantum dot is detected by means of a radio-frequency single-electron transistor. Poisson statistics of single-electron tunneling events are observed from frequency domain measurements, and individual tunneling events are detected in the time-domain measurements. Counting tunneling events gives an accurate current measurement in the saturated current regime, where electrons tunnel into the dot only from one electrode and tunnel out of the dot only to the other electrode.

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We investigate rf transport through an AlGaAs/GaAs single-electron transistor (SET). The presented rf–SET scheme provides a transmission coefficient proportional to the admittance of the device, which is desirable for impedance analysis as well as for high-sensitivity charge detection. The impedance of a SET, including the small tunneling capacitance, is successfully analyzed at the high frequency of 643 MHz, and is compared with a simple model. The ability to measure the impedance of a SET would expand the measurable regime of single-electron tunneling behavior.

In this paper we present results of investigation of the main optical properties of East Asian clouds with a ground-based polarization lidar placed in Daejeon, Republic of Korea. Asian dust is located in elevated layers of the atmosphere in spring, travels long distances, and causes significant damage to ecology. We present backscattering matrices of clouds obtained from polarimetric remote measurements which comprise information on the scattering and absorption properties of cloud particles, their morphology, and spatial orientation. Theory of our applied lidar polarization experiment is presented in terms of the instrumental vectors of a transmitter and a receiver. Methods of solving linear and nonlinear systems of equations comprising echo signals are considered. Some numerical and measurement results are presented to illustrate the efficiency and versatility of the method of estimating the cloud parameters.