Efficient Dynamic Nuclear Polarization at High Magnetic Fields

By applying a new technique for dynamic nuclear polarization involving simultaneous excitation of electronic and nuclear transitions, we have enhanced the nuclear polarization of the nitrogen nuclei in ${}^{15}\mathrm{N}@{\mathrm{C}}_{60}$ by a factor of ${10}^3$ at a fixed temperature of 3 K and a magnetic field of 8.6 T, more than twice the maximum enhancement reported to date. This methodology will allow the initialization of the nuclear qubit in schemes exploiting $\mathrm{N}@{\mathrm{C}}_{60}$ molecules as components of a quantum information processing device.

Figure 1

Energy levels and allowed transitions of ${}^{15}\mathrm{N}@{\mathrm{C}}_{60}$ in a strong magnetic field, $B$, parallel to the $z$-axis. The system is well described by the effective spin Hamiltonian $H=B(g{\mu }_B{S}_z-g_N{\mu }_N{I}_z)+hA{S}_z{I}_z$. The three terms describe the electronic Zeeman, nuclear Zeeman and isotropic hyperfine interactions. The hyperfine constant is $A=21.9\mathrm{MHz}$. The four nuclear transitions are labeled ${\nu }_{I1}-{\nu }_{I4}$. The two triply degenerate electronic transitions are labeled ${\nu }_{S1}$ and ${\nu }_{S2}$.

Figure 2

Schematic of the pulsed version of the PONSEE sequence for ${}^{15}\mathrm{N}@{\mathrm{C}}_{60}$. ${\nu }_{I4}$ radiation is shown in step 2, but ${\nu }_{I3}$ or ${\nu }_{I2}$ could be used instead.

Figure 3

240 GHz cw ESR spectrum of ${}^{15}\mathrm{N}@{\mathrm{C}}_{60}$ in thermal equilibrium at 3 K.

Figure 4

cw ESR spectrum of ${}^{15}\mathrm{N}@{\mathrm{C}}_{60}$ at 4.2 K after 2.5 h ${\nu }_{S2}$ radiation. The nuclear polarization is due to flip-flop transitions. The smooth curve is the bi-Lorentian best fit. The ratio of the areas of these peaks shows the nuclear polarization to be 21%.

Figure 5

cw ESR spectrum of ${}^{15}\mathrm{N}@{\mathrm{C}}_{60}$ at 3 K after 20 min PONSEE. The smooth curve is the bi-Lorentzian best fit with the linewidth of the two peaks constrained to be the same. The nuclear polarization is 62%, an enhancement of $\epsilon =1100$. Inset shows $\mathrm{N}@{\mathrm{C}}_{60}$ molecule.