Small, but consistent, steps are being taken towards the realization of a quantum computer. The demonstration of the coupling of two quantum bits in a solid-state device moves us closer to that goal.
The technologies of Josephson-junction-based qubits have been progressing rapidly, ever since its first demonstration by a superconducting charge qubit1. A variety of systems have been implemented with remarkable progress in coherence time and read-out schemes. Although the current level of this solid-state device is still not as advanced as that of the most advanced microscopic-system-based qubits, these developments, together with the potential scalability, have renewed its position as a strong candidate as a building block for the quantum computer. Recently, coherent oscillation and microwave spectroscopy in capacitively-coupled superconducting qubits have been reported. The next challenging step toward quantum computation is a realization of logic gates. Here we demonstrate a conditional gate operation using a pair of coupled superconducting charge qubits. Using a pulse technique, we prepare different input states and show that they can be transformed by controlled-NOT (C-NOT) gate operation in the amplitude of the states. Although the phase evolution during the gate operation is still to be clarified, the present results are a major step toward the realization of a universal solid-state quantum gate.
The proposal of quantum information processing has shown us the first serious possibility, though still limited in applications, to break away from the traditional spell that has been binding integration scale and processing power for some time. The qubit (quantum bit), the basic element of quantum information processing, consists of a quantum two-level system that can be brought to a superposition by an external mean. Among the numerous possible two-level systems, various typical microscopic two-level systems such as nuclear spins and atomic states have been utilized first to demonstrate the realization of quantum calculation. However, to realize a scaled-up version of a quantum computing system consisting of hundreds of qubits, a solid state electronics version of it is considered to be indispensable. We demonstrate the first electronic control of 1-qubit achieved in a solid state device using a submicron electron device called a single-electron box, a small conductive box connected to the outside by a tunnel junction and a gate capacitor. The scalability of such a solid state qubit is considered to be a prerequisite for a practical quantum computer of the future.
The dream of quantum computers has moved a small step closer to reality with the first successful demonstrations of quantum algorithms in experiments and a novel proposal to build a quantum computer based on the silicon technology used in existing computers. Quantum computers would exploit the ability of quantum particles to be in a superposition of two states at the same time and, for certain tasks, would have the potential to outperform classical computers by many orders of magnitude (Physics World March pp47–52).