One of the causes of the problem is short coherence time of a quantum bit in the host material. Researchers have engineered phosphorus-doped silicon to be composed solely of 28Si stable isotope. High-field (3.4 T), low-temperature (2.9 K) electron spin resonance of P in Si was combined with hyperpolarization of the 31P nuclear spin to obtain an initial state of sufficient purity to create a non-classical, inseparable state. The state was verified using density matrix tomography based on geometric phase gates, and entanglement operation was performed simultaneously on high density spin pairs as large as 10^10.
Researchers suggest that the present result satisfies one of the essential requirements for a silicon-based quantum information processor. A column of "Nature News" (see link below) pointed out the importance of the result and cited several challenges suggested by specialists toward quantum information processor.
