Two-qubit logic gate

Nature 526, 410–414 (2015)

Attempts to make quantum information processors practical to manufacture have concentrated on the use of solid-state platforms based on superconducting or semiconductor materials. However, owing to the difficulties of coupling qubits and dephasing problems, semiconductor systems trail behind superconducting systems in terms of performance. Now, Menno Veldhorst and co-workers from the University of New South Wales, Australia and Keio University, Japan have demonstrated a two-qubit logic gate based on single spins in silicon. The device has a double-quantum-dot structure fabricated on a ²⁸Si epilayer with a residual ²⁹Si concentration of 800 ppm. The quantum dot qubits are individually controlled by electrically tuning the electron spin resonance frequency using the Stark shift. The spin states of qubits are manipulated by a microwave pulse. The measurements were conducted at 50 mK. A Ramsey experiment showed the dephasing time of a single-spin qubit was around 100 μs. The two-qubit gate was realized by using controlled-phase operations combined with single-qubit operation. When the phase difference is π between two qubits with different antiparallel states, clear anticorrelation is observed in the two-spin probabilities, which demonstrates the operation of a CNOT gate. The small size of the gate and the fact that it can be made by standard CMOS technology offers the prospect of realizing a large-scale quantum processor.