Semiconductor Charge Qubit
Toshiaki Hayashi and Toshimasa Fujisawa
Physical Science Laboratory
The study of quantum computing has attracted great attention because it is more efficient at some specific calculations than classical computing. The elementary unit of quantum computing (qubit) has two quantum states which are taken as a set of basis states (|0> and |1>). Unlike a classical bit, any state of a qubit which represents quantum information can be described as a linear combination of the two basis states. Quantum information processing devices are required to perform universal unitary gate operations within the decoherence time of the qubits. Although much effort has been invested, the experimental realization of quantum computing is still challenging.
We have studied a charge qubit in a semiconductor double quantum dot (QD) because all parameters we need for unitary gate operations can be controlled electrically . The double QD consists of two QDs coupled to each other via a tunnel barrier and electrons can flip back and forth between the two QDs. For simplicity, we consider that we have one electron in the double QD. The two states of the charge qubit are the states in which the electron occupies one of the two QDs.
We employed a lateral double QD fabricated from a GaAs / AlGaAs heterostructure, as shown in Fig. 1. A voltage pulse is applied to the drain electrode to modify the electronic state of the double QD abruptly. This non-adiabatic transition allows us to perform initialization, coherent manipulation, and read-out operations of the qubit state. We have also demonstrated rotation gate operation  and phase-shift gate operation  on the charge qubit at 20 mK.
Figure 2 shows the normalized current through the device, which can roughly be taken as the averaged number of electrons in the right QD at measurement operation. This shows an oscillating behavior as a function of pulse length, which corresponds to rotation gate operation. We also showed that it was possible to change the oscillation frequency, which is related to the coupling energy between the two QDs, by changing the gate voltages.
The present result is the first step towards physical realization of quantum computing.
 T. Hayashi et al., Phys. Rev. Lett. 91 (2003) 226804.
 T. Fujisawa et al., Physica E, in press.
Fig. 1. Schematic diagram of double QD sample.
Fig. 2. Averaged number of electrons as a function of pulse length.
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