In order to make a quantum computer, it’s first necessary to have a good way to implement qubits, the basic unit of quantum information. Although this can be done experimentally with photons or ions at low temperatures, photons and ions are hard to control in practice. Solid state implementations, using traditional semiconductors, for example, should make control easier, but there are both theoretical and practical difficulties.
One related possibility that has received some attention involves a well-known material: diamonds. A specific type of point defect in diamond crystals – nitrogen-vacancy (NV) color centers – may be well-suited for qubit implementation. The NV center is a defect in the crystal lattice in which an included nitrogen atom occurs next to a lattice vacancy that would ordinarily contain a carbon atom. Such a defect can emit photons (photoluminescence) under stimulation by electric or magnetic fields.
NV defects also have their own quantum spin states, which can be initialized, manipulated, and read optically. The problem of using these spin states as qubits is the practical one of coupling one to another, since isolated qubits aren’t useful for quantum computing. Most of the total spin derives from the spin of nitrogen nuclei, but these aren’t close enough together in diamond crystals to couple effectively. New research presents a theoretical argument for producing indirect coupling of NV sites through electron spin interactions under the influence of an externally applied electromagnetic field.
Driving a Hard Bargain with Diamond Qubits
Since the nuclear spin at each site (bearing a qubit) is able to “see” its local electron spin partner via the hyperfine interaction, it follows that if the electron spins at neighboring sites can interact sufficiently strongly, then the two nuclei will be able to communicate indirectly through their electron brothers. While the idea is simple, it turns out to be more complex than one might imagine, in fact, Bermudez et al. start by establishing that the electron-mediated interaction between nuclei at adjacent NV sites will be far too weak to be useful! They estimate that even quite closely neighboring sites, 10 nanometers (nm) apart, will have an effective nuclear-nuclear coupling strength of only 0.1 hertz—requiring several seconds to achieve a useful exchange of information. This is impractical, since nuclear spins suffer dephasing (losing any stored qubit) in less than a second.
Happily, the Ulm team has discovered a solution. Their analysis predicts that when the spins are resonantly driven by externally applied electromagnetic fields, the effective strength of the interaction increases dramatically. In essence, they introduce a new energy scale into the problem, replacing the effect of the crystal field splitting (which acts to suppress the effective nuclear-nuclear coupling) with the Rabi frequencies of the driven spins—a parameter that is under experimental control. Remarkably, with a suitable choice for this parameter, the coupling between nuclei is enhanced a thousandfold, becoming entirely practicable as a channel to exchange quantum information. As an added bonus, the act of driving the spins serves to protect the quantum state from the decohering effects of the surroundings. The effect is equivalent to “dynamic decoupling,” in which a spin that is periodically inverted at a frequency faster than the local magnetic field fluctuations acquires an aggregate zero phase.
Electron-Mediated Nuclear-Spin Interactions between Distant Nitrogen-Vacancy Centers