Multiple Two-qubit Operations Using Semiconductor Coupled Charge Qubits


Takeshi Ota, Gou Shinkai*, Toshiaki Hayashi, and Toshimasa Fujisawa*
Physical Science Laboratory, *Tokyo Institute of Technology

 Two-qubit unitary operations are key ingredients for performing quantum algorithms and correlating multiple qubits. Typical operations, such as controlled-rotation (CROT), which rotates the target qubit state conditionally on the control qubit state, and SWAP, which swaps quantum states of the two qubits, have been demonstrated using superconductor charge qubits and semiconductor spin qubits[1, 2]. However usually one type of operations is realized depending on the type of coupling (Ising, Heisenberg, etc.). Although other operations can in principle be designed in combination with some one-qubit operations, simple sequences for shorter operation time or a smaller number of steps have been desired to maintain the coherency of the system.
 In this work, using coupled semiconductor charge qubits consisting of two sets of coupled double quantum dots (DQD), we fabricated two-qubit device in which two spatially separated electrons in the two qubits change their locations coherently and collectively (the correlated coherent oscillations) and two-qubit operations such as CROT, SWAP can be performed in a single step[3]. Figure 1 shows a scanning electron micrograph (SEM) of the two-qubit device. The two qubits with individual source and drain electrodes are electrically isolated, and thus independent currents can be measured simultaneously. All qubit parameters can be controlled by 11 gate voltages. By applying high-frequency voltage pulse to first qubit and measuring the current, coherent oscillation demonstrating the superposition of the charge states |0> and |1> is observed. Here, |0> and |1> represent the location of the charge in the right and left dot, respectively. Due to the electrostatic coupling between the two qubits, the coherent oscillation of the first qubit is strongly influenced by the charge states of the second qubit. Figure 2 shows the demonstration of the CROT operation of the first qubit using the second qubit as a control qubit. Figure 3(a) shows the correlated coherent oscillations in which SWAP operation is performed.
 This work was partly supported by SCOPE from the Ministry of Internal Affairs and Communications of Japan.

[1] T. Yamamoto et al., Nature 425 (2003) 941.
[2] J. R. Petta et al., Science 309 (2005) 2180.
[3] G. Shinkai et al., Phys. Rev. Lett. 103 (2009) 056802.

Fig. 1. Two-qubit device using
coupled semiconductor
double quantum dots.
Fig. 2. Coherent oscillations of the first qubit
representing CROT operation. The
oscillations (a) and (b) correspond to
the case that the second qubit is |1>
and |0>, respectively.
Fig. 3. (a) Correlated coherent
oscillations of the first qubit.
The oscillations (b) correspond
to those shown in Fig. 2.

[back] [Top] [Next]