We report on a planar microwave resonator providing arbitrarily polarized oscillating magnetic fields that enable selective excitation of the electronic spins of nitrogen-vacancy (NV) centers in diamond. The polarization plane is parallel to the surface of diamond, which makes the resonator fully compatible with (111)-oriented diamond. The field distribution is spatially uniform in a circular area with a diameter of 4 mm, and a near-perfect circular polarization is achieved. We also demonstrate that the original resonance frequency of 2.8 GHz can be varied in the range of 2−3.2 GHz by introducing varactor diodes that serve as variable capacitors.In recent years, the nitrogen-vacancy (NV) center in diamond has emerged as a promising platform for quantum information processing and nanoscale metrology. 1-7At the heart of both technologies lies an exquisite control of the NV electronic spin.8 The ground state of the negatively charged NV center is an S = 1 spin triplet with the m S = ±1 sublevels lying D gs = 2.87 GHz above the m S = 0 sublevel under zero magnetic field. The external static magnetic field B dc applied parallel to the quantization axis n NV (along one of the 111 crystallographic axes) further splits the m S = ±1 levels by 2γB dc , with γ = 28 GHz/T being the gyromagnetic ratio of the NV electronic spin [see Fig. 1].Initialization, control, and readout of this three-level V system are accomplished by an optically detected magnetic resonance (ODMR) technique, in which opticalLin. pol.Cir. pol. pumping by a green (∼500 nm) laser serves to initialize and read out the NV spin and short pulses of oscillating magnetic fields B ac (∼2.87 GHz) drive it into an arbitrary quantum-mechanical superposition. In ideal magnetic resonance experiments, B ac and B dc ( n NV ) should be orthogonal, 9,10 whereas in reality the direction of B ac at the location of the target NV center is hard to control or even know about. This is because a metal wire or a microfabricated stripline, commonly used for experiments with NV centers, generates B ac that is highly dependent on the positions. Moreover, B ac from these sources is linearly polarized, while the NV spin has clear transition selection rules; The m S = 0 ↔ 1 (−1) transition is driven by σ + (σ − ) circularly polarized fields. To fully exploit the S = 1 nature of the NV spin for quantum information and metrology applications, 11-14 it is highly desired to have a reliable means to generate arbitrarily polarized microwave fields.A few configurations for polarization-controlled B ac have been adopted for the NV system. A popular one is a pair of crisscrossed striplines, which generates desired fields only beneath the crossing point.15,16 A pair of parallel striplines may be a simpler alternative.17 In both examples, however, it is by design difficult or impossible to make the polarization plane perpendicular to n NV and thus to B dc .In this paper, we present a simple planar resonator circuit providing spatially uniform, arbitrarily polarized, and in-plane microwave magne...