Figure 1
Illustration: Alan Stonebraker

Figure 1: Protocol for improving the readout signal from a single nitrogen-vacancy (NV-) center in diamond: NV- centers arise when a substitutional nitrogen atom (N) is next to a vacancy (V) as shown in the inset. The traditional readout technique used by most researchers to date is shown schematically as Route 1, which connects the two states that form the logical qubit without involving the nuclear spin state of the 14N. However, by driving the system along route 2 and then 3, the signal-to-noise is increased by √3. This technique is universal because it works for an NV- center in any local environment. The states are labeled by |electron spin state, 14N nuclear spin state〉. Routes 1 and 3 proceed via the excited states, and route 3 requires a level anticrossing (LAC) in the excited states, reached by the application of a 50 mT magnetic field.