In a jointly-conducted experiment, NEC and the RIKEN Institute of Physical and Chemical Research have demonstrated a quantum logic gate operation in a solid-state device consisting of two coupled quantum bits (qubits).This represents a significant step toward the realisation of practical quantum computing.
The quantum computer, when it is finally brought to fruition, will far surpass the capabilities of even the most modern of today’s supercomputers, as it utilises a “qubit,” a quantum superposition of the “0” and “1” states, as the basic information unit. Information processing in a quantum computer is conducted by a series of quantum gate operations to the qubit, and it has been theoretically proven that only two kinds of gate operations are sufficient for quantum computation, no matter how complicated the algorithms are and no matter how many qubits they require.
The two gates are the one-qubit rotation gate which controls the state of one qubit, and the controlled-NOT (C-NOT) gate which works as a conditional gate for two qubits. In 1999, NEC was able to control the quantum state of one qubit made of small superconductors. This was claimed to be the first demonstration of the one-qubit rotation gate in solid state devices.
In February 2003, a joint NEC/RIKEN laboratory demonstrated the first quantum entanglement in a two-qubit solid-state device. No one has succeeded in realising this two-qubit conditional gate. Now NEC and RIKEN have proposed a new scheme to implement the C-NOT gate using a similar type of two-qubit circuit made of superconductors, and have experimentally demonstrated that it works properly as a logic gate for two qubits.
Having realised both the one-qubit rotation gate and the two-qubit conditional gate, the research laboratory will work toward achieving further integration of the qubit to demonstrate a quantum algorithm, an effort which is expected to contribute to the practical realisation of a quantum computer.