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 ...

A bidirectional single-electron counting device is demonstrated. Individual electrons flowing in forward and reverse directions through a double quantum dot are detected with a quantum point contact acting as a charge sensor. A comprehensive statistical analysis in the frequency and time domains and of higher order moments of noise reveals antibunching correlation in single-electron transport through the device itself. The device can also be used to investigate current flow in the attoampere range, which cannot be measured by existing current meters.