Temporal Coherence of Photons Emitted by Single Nitrogen-Vacancy Defect Centers in Diamond Using Optical Rabi-Oscillations

Photon interference among distant quantum emitters is a promising method to generate large scale quantum networks. Interference is best achieved when photons show long coherence times. For the nitrogen-vacancy defect center in diamond we measure the coherence times of photons via optically induced Rabi oscillations. Experiments reveal a close to Fourier-transform (i.e., lifetime) limited width of photons emitted even when averaged over minutes. The projected contrast of two-photon interference (0.8) is high enough to envisage applications in quantum information processing. We report 12 and 7.8 ns excited state lifetimes depending on the spin state of the defect.

Figure 1

(a) Structure of the NV center in diamond. The NV center comprises a substitutional nitrogen (N), and a neighboring vacancy (V). (b) The scheme of energy levels of the NV defect center. Thick arrows indicate spin-selective excitation and fluorescence emission pathways (transitions a,b and c).

Figure 2

Spin-selective decay curves of a single NV defect at ambient conditions. The inset shows the time sequence of the experiment. The $m_S=0$ spin states have been initialized by a $\mu \mathrm{s}$ long optical pulse followed by long ($\mu \mathrm{s}$) delay. Next, the center was excited by 150 fs laser pulse and the fluorescence decay was recorded (this sequence was used for lifetime measurements of $m_S=0$ state, squares, corresponds to transition a shown in Fig. 1). Introduction of a spin-selective microwave $\pi$ pulse after the polarization pulse initializes the system into $m_S=\pm 1$ prior to lifetime measurements (open and filled circles, transition b, c shown in Fig. 1). Solid lines are single exponential fit curves.

Figure 3

Rabi oscillations of a single NV electron spin recorded using temporal gating (solid circles). After initialization of the NV center into the $m_S=0$ state a resonant microwave pulse of variable length was applied. Consecutively a short (100 fs) pulse was used for reading out. In order to make the detection scheme sensitive to the $m_S=0$ state population, only photons with long lifetime were selected by choosing the delay of detection window to be 100 ns. Measurement based on discrimination of spin states via total fluorescence is shown for comparison (open circles).

Figure 4

Second-order fluorescence intensity autocorrelation function for the NV center at low temperature under resonant excitation of $m_S=0$ spin state (corresponds to transition a shown in Fig. 1). The Rabi frequency of the excitation laser was varied for different curves and equals to 0.44, 0.59, 0.67, and 1.00 GHz (increase from bottom to top). The fit functions (solid lines) are based on Eq. (1) (see text).