Complete Scheme for Two-Qubit Quantum Computing using Pulsed ESR of $^{15}$N@C$_{60}$

$^{15}$N@C$_{60}$ is a fullerene molecule containing an atom of nitrogen-15. Its long electron spin decoherence time makes it attractive for quantum computing. The electronic and nuclear spins of the nitrogen atom are good quantum numbers in a strong magnetic field, coupled by the hyperfine interaction. Pulsed electron nuclear double resonance (ENDOR) can be used to initialize, manipulate and measure this two-qubit system. We used dynamic nuclear polarization (DNP) to prepare an initial state in which the nuclear and electronic spins were aligned with the applied field. We measured this to be an 80\% pure state. The decoherence time of N@C$_{60}$, $T_2$, can be increased to 215 $\mu$s at 4 K. The electronic $T_1$ time is the relevant timescale for reading out the result of a computation. At 4.2 K this is 4.5 minutes, and the nuclear $T_1$ is greater than 12 hours. www.nanotech.org