We report a study of the 3 E excited-state structure of single negatively-charged nitrogen-vacancy (NV) defects in diamond, combining resonant excitation at cryogenic temperatures and optically detected magnetic resonance. A theoretical model is developed and shows excellent agreement with experimental observations. Besides, we show that the two orbital branches associated with the 3 E excited-state are averaged when operating at room temperature. This study leads to an improved physical understanding of the NV defect electronic structure, which is invaluable for the development of diamond-based quantum information processing.PACS numbers: 78.55. Qr, 42.50.Ct, 42.50.Md, 61.72.J Coupling between flying and stationary qubits is one of the crucial requirements for scalable quantum information processing [1,2]. Among many quantum systems including single atoms [3] and semiconductor quantum dots [4], the negatively-charged nitrogen-vacancy (NV) color center in diamond is a promising solid-state candidate for realizing such interface, owing to long spin coherence time of their spin states [5] and availability of a strong optical transition [6]. Nevertheless, even though NV defects have been intensively studied during the last decades, the excited-state structure as well as the dynamics of excitation-emission cycles are surprisingly not yet fully understood. This knowledge is however of crucial importance for the realization of long-distance entanglement protocols based on coupling of spin-state to optical transitions [7,8,9].In this Letter, we report a study of the excited-state structure of single NV defects as a function of local strain, combining resonant excitation at cryogenic temperatures and optically detected magnetic resonance (ODMR). Besides, we show that the two orbital branches associated with the 3 E excited-state are averaged at room temperature. A theoretical model is developed and shows excellent correspondence with experimental observations. The NV color center in diamond consists of a substitutional nitrogen atom (N) associated with a vacancy (V) in an adjacent lattice site, giving a defect with C 3v symmetry. For the negatively-charged NV color center addressed in this study, the ground state is a spin triplet 3 A 2 [10,11,12]. Spin-spin interaction splits ground state spin sublevels by 2.88 GHz into a spin singlet S z , where z corresponds to the NV symmetry axis, and a spin doublet S x , S y (see Fig. 1(a)). The excited state 3 E is also a spin triplet, associated with a broadband photoluminescence emission with zero phonon line (ZPL) around 637 nm (1.945 eV). Besides, the 3 E excited state is an orbital doublet, which degeneracy is lifted by non-axial strain into two orbital branches, E x and E y , each orbital branch being formed by three spin states S x , S y and S z (see Fig. 1(a)) [13]. As optical transitions 3 A 2 → 3 E are spin-conserving, excitation spectra of single NV color centers might show six resonant lines, corresponding to transitions between identical spin sublevels.The order o...

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. DOI: 10.1103/PhysRevLett.100.077401 PACS numbers: 78.55.Qr, 42.50.Ct, 42.50.Md, 61.72.Jÿ Coherent control of single quantum systems and the generation of nonclassical states has attracted widespread attention because of their application in quantum physics and quantum information science. Solid state systems are often considered to be promising and also difficult because of inhomogeneities and fast dephasing. Spins in solids, for example, associated with quantum dots or single dopant atoms offer promising figures of merit for both parameters. As a particular example, the nitrogen-vacancy (NV) defect in ultrapure diamond shows a long spin phase memory time (0.35 ms) [1] even at ambient conditions due to spin-free and rigid lattice. In addition to its excellent spin properties, spin selectivity of optical transitions of the NV defect allows initialization and readout of the spin state with sensitivity routinely reaching a single atom [2].The narrow spin resonance transitions of single defects make them a sensitive magnetometer at the nanoscale. As an example, it was demonstrated that the electron spin associated with a single NV defect can be used for reading out spin states of proximal nuclear [3] and electron spins [1,4,5]. Magnetic coupling between electron spin of NV defects and neighboring nuclei was used as a resource for generating entangled states [6]. Such multispin entanglement is a crucial element for quantum computation and communications protocols [7]. However, the generation of entanglement using magnetic dipolar coupling is limited to closely spaced spins. The maximum distance where spinspin interaction can be used for controlling nonlocal quantum states depends on coherence time and strength of spinspin interaction. Although coherences associated with electron and nuclear spins in diamond are particularly long, a few nanometers distance is a realistic limit for magnetic coupling. One way to gain entanglement over larger distance is to use the coupling of spin state to optical transitions [8,9]. Such generation of entanglement over long distance via interference of photons recently attracted considerable interest [10]. Experimental demonstration of entanglement between stationary and flying qubits [11,12] followed by realization of interference of photon pairs from distant trapped ions [13] and their entanglement via photonic channels [1...

Using pulsed optically detected magnetic resonance techniques, we directly probe electron-spin resonance transitions in the excited-state of single nitrogenvacancy (NV) color centers in diamond. Unambiguous assignment of excited state fine structure is made, based on changes of NV defect photoluminescence lifetime. This study provides significant insight into the structure of the emitting 3 E excited state, which is invaluable for the development of diamond-based quantum information processing.Excited-state spectroscopy of single NV defect in diamond using optically detected magnetic resonance2Over the last decade, the negatively charged nitrogen-vacancy (NV) color center in diamond has attracted a lot of interest because it can be optically addressed as single quantum system [1] and exhibits several important properties for quantum information science applications. First, its perfect photostability at room temperature enables to realize a practical NV-based single photon source [2, 3] for quantum cryptography applications [4,5]. Second, NV color centers have a paramagnetic ground state which spin can be optically polarized, read-out and exhibits long coherence time even at room temperature [6,7]. Coherent manipulation of electron and nuclear spins of single NV color centers has been used to realize solid-state quantum physics experiments, ranging from coherent coupling of a single NV color center to other single spins in the diamond crystalline matrix [7,8,9], to the implementation of a quantum register [10] and conditional two-qubit CNOT gates [11], and very recently the generation of Bell and GHZ states with long coherence times [12].Despite these results, which make the NV color center a competitive candidate for solid-state quantum information processing, the excited-state structure of the defect is not yet fully understood [13,14]. This knowledge is however crucial for single-spin high-speed coherent optical manipulation through Λ-based transitions [15,16,17] as well as for future implementation of quantum information protocols like quantum repeaters [18,19] that can be used as a building block for a quantum network [20].Resonant optical excitation of single NV color centers at low temperature [21] and cw electron-spin resonance experiments [22] have recently provided new insights into the structure of the excited-state, showing that its fine structure is strongly affected by local strain in the diamond matrix [21,22]. Recent ensemble experiments have also studied the behaviour of an infrared emission line that gives a better understanding of the metastable state responsible for spin polarization [23].Here we develop a new approach to probe the excited-state fine structure of single NV color centers. Using pulsed optically detected magnetic resonance techniques, we directly probe electron-spin resonance transitions of the excited-state. Unambiguous assignment of excited state fine structure is made, based on changes of NV defect photoluminescence lifetime.

Important features in the spectral and temporal photoluminescence excitation of single nitrogen-vacancy (NV) centers in diamond are reported at conditions relevant for quantum applications. Bidirectional switching occurs between the neutral (NV(0)) and negatively charged (NV(-)) states. Luminescence of NV(-) is most efficiently triggered at a wavelength of 575 nm which ensures optimum excitation and recharging of NV(0). The dark state of NV(-) is identified as NV(0). A narrow resonance is observed in the excitation spectra at 521 nm, which mediates efficient conversion to NV(0).

Diamond provides unique technological platform for quantum technologies including quantum computing and communication. Controlled fabrication of optically active defects is a key element for such quantum toolkit. Here we report the production of single color centers emitting in the blue spectral region by high energy implantation of carbon ions. We demonstrate that single implanted defects show sub-poissonian statistics of the emitted photons and can be explored as single photon source in quantum cryptography. Strong zero phonon line at 470.5 nm allows unambiguous identification of this defect as interstitial-related TR12 color center.