Credit: GETTY

Diamonds come in many shapes, but also in many colours, owing to crystallographic defects known as 'colour centres'. Several hundred kinds of luminescent defect are known in diamond, but one in particular is the best friend of many in the quantum-information community: the nitrogen–vacancy (N–V) centre, formed by a substitutional nitrogen atom adjacent to a lattice vacancy. These defects fluoresce bright red, but what makes them so attractive for quantum-information purposes is the electronic spin associated with N–V centres and the possibility of controlling that spin using optical and microwave excitation. Moreover, the electronic spin can couple, via the hyperfine interaction, to nearby nuclear spins of carbon-13 (which, in natural abundance, make up 1.1% of the diamond lattice). Such a system of individual electronic and nuclear spins has been shown to provide — if properly controlled — a promising basis for a quantum register that can be operated at room temperature (Science 316, 1312–1316; 2007).

Whereas the electronic spins can be conveniently initialized, manipulated and read out, the nuclear spins are remarkably well isolated from their surroundings and therefore can store information, in the form of quantum states, for long periods of time. Roughly speaking, the nearby electron constitutes the 'outside world' for the nuclear spin. However, when the electron is probed, it undergoes rapid transitions between ground and optically excited states; as the hyperfine interaction with the nuclear spins is different for ground and excited states, the nuclear spin sees a rapidly changing effective magnetic field — a potential killer of the information encoded in the nuclear spin.

Liang Jiang and colleagues have now taken a close look at this problem and report that, in a typical experiment, the fluctuations of the electron during optical manipulation are in fact so fast that the randomly accumulated phase of the nuclear spins averages out (Phys. Rev. Lett. 100, 073001; 2008). Similar phenomena in which reservoir fluctuations lead to long — and not, as might be expected, short — coherence times are known in different fields, most prominently as 'motional narrowing' in nuclear magnetic resonance. For diamond-based quantum-information processing, the insight gained by Jiang et al. indicates that the individual advantages of the electronic and nuclear spins can be widely exploited without one getting in the way of the other.