Fig. 2: Experimental implementation.
a,b, Full experiment sequence (a) consists of the initial hyperpolarization of the nuclear spins (b) via nearby NV centres (Methods); the polarization is then flipped into the \(\hat{{\bf{x}}}\) direction via a \(\frac{\uppi }{2}\) \(\hat{{\bf{y}}}\) pulse (yellow box). This is followed by the subsequent application of n-pole sequences (pink or cyan box) with equal probability. c, An n-pole sequence is constructed from the systematic arrangement of 2n monopoles, ⊕ (red circle) and ⊖ (blue circle). d, ⊕ (⊖) is defined as a train of N = N− + N+ spin-locking pulses interrupted after N+ (N−) spin locks by \(\hat{{\bf{y}}}\) pulses of angle γy. We choose N+ = 200, N− = 100 and N = 300 throughout. e, Each spin-locking pulse (duration τ) consists of an \(\hat{{\bf{x}}}\) pulse of angle θx (yellow box), followed by a read-out window of time tfree during which only the dipole–dipole interactions (equation (1)) act.
