We report the full implementation of a quantum cryptography protocol using a stream of single photon pulses generated by a stable and efficient source operating at room temperature. The single photon pulses are emitted on demand by a single nitrogen-vacancy (NV) color center in a diamond nanocrystal. The quantum bit error rate is less that 4.6% and the secure bit rate is 9500 bits/s. The overall performances of our system reaches a domain where single photons have a measurable advantage over an equivalent system based on attenuated light pulses.PACS numbers: 03.67. Dd, 42.50.Dv Since its initial proposal in 1984 [1] and first experimental demonstration in 1992 [2], Quantum Key Distribution (QKD) has reached maturity through many experimental realizations [3], and it is now commercially available [4]. However, most of the practical realizations of QKD rely on weak coherent pulses (WCP) which are only approximation of single photon pulses (SPP), that would be desirable in principle. The presence of pulses containing two photons or more in WCPs is an open door to information leakage towards an eavesdropper. In order to remain secure, the WCP schemes require to attenuate more and more the initial pulse, as the line losses become higher and higher, resulting in either a vanishingly low transmission rate -or a loss of security [5,6]. The use of an efficient source of true single photons would therefore considerably improve the performances of existing or future QKD schemes, especially as far as high-losses schemes such as satellite QKD [7] are considered.In this letter we present the first complete realization of a quantum cryptographic key distribution based on a pulsed source of true single photons. Our very reliable source of single photon has been used to send a key over a distance of 50 m in free-space at a rate of 9500 secret bits per second including error correction and privacy amplification. Using the published criteria that warrant absolute secrecy of the key against any type of individual attacks [5, 6], we will show that our set-up reaches the region where a single photon QKD scheme takes a quantitative advantage over a similar system using WCP.Single photon sources have been extensively studied in recent years and a great variety of approaches has been proposed and implemented [8,9,10,11,12,13]. Our single photon source is based on the fluorescence of a single Nitrogen-Vacancy (NV) color center [14] inside a diamond nanocrystal [15,16] at room temperature. This molecular-like system has a lifetime of 23 ns when it is contained in a 40 nm nanocrystal [15]. Its zero-phonon line lies at 637 nm and its room temperature fluorescence spectrum ranges from 637 nm to 750 nm [17]. This center is intrinsically photostable: no photobleaching has been observed over a week of continuous saturating irradia- tion of the same center. The nanocrystals are held by a 30 nm thick layer of polymer that has been spin coated on a dielectric mirror [15]. The mirror is initially slightly fluorescing, but this background light i...

Biomedicine and cell and molecular biology require powerful imaging techniques of the single molecule scale to the whole organism, either for fundamental science or diagnosis. These applications are however often limited by the optical properties of the available probes. Moreover, in cell biology, the measurement of the cell response with spatial and temporal resolution is a central instrumental problem. This has been one of the main motivations for the development of new probes and imaging techniques either for biomolecule labeling or detection of an intracellular signaling species. The weak photostability of genetically encoded probes or organic dyes has motivated the interest for different types of nanoparticles for imaging such as quantum dots, nanodiamonds, dye-doped silica particles, or metallic nanoparticles. One of the most active fields of research in the past decade has thus been the development of rare-earth based nanoparticles, whose optical properties and low cytotoxicity are promising for biological applications. Attractive properties of rare-earth based nanoparticles include high photostability, absence of blinking, extremely narrow emission lines, large Stokes shifts, long lifetimes that can be exploited for retarded detection schemes, and facile functionalization strategies. The use of specific ions in their compositions can be moreover exploited for oxidant detection or for implementing potent contrast agents for magnetic resonance imaging. In this review, we present these different applications of rare-earth nanoparticles for biomolecule detection and imaging in vitro, in living cells or in small animals. We highlight how chemical composition tuning and surface functionalization lead to specific properties, which can be used for different imaging modalities. We discuss their performances for imaging in comparison with other probes and to what extent they could constitute a central tool in the future of molecular and cell biology.

The mechanochromic and thermochromic luminescence properties of a molecular copper(I) iodide cluster formulated [Cu(4)I(4)(PPh(2)(CH(2)CH=CH(2)))(4)] are reported. Upon mechanical grinding in a mortar, its solid-state emission properties are drastically modified as well as its thermochromic behavior. This reversible phenomenon has been attributed to distortions in the crystal packing leading to modifications of the intermolecular interactions and thus of the [Cu(4)I(4)] cluster core geometry. Notably, modification of the Cu-Cu interactions seems to be involved in this phenomenon directly affecting the emissive properties of the cluster.

Concentrated colloidal solutions of well-dispersed YVO4:Eu nanoparticles are synthesized by precipitation reactions at room temperature and stabilized by sodium hexametaphosphate. X-ray diffraction and electron microscopy characterizations show that the crystalline nanoparticles exhibit an ellipsoidal form with two characteristic dimensions of around 15 and 30 nm. In comparison with the bulk, a lower luminescence efficiency as well as a higher concentration quenching are observed. These deviations are explained as the variations of some characteristics of the colloidal samples, such as the crystallinity and the surface chemistry. When these parameters are optimized, the quantum yield of the luminescence reaches 38% for the nanoparticles containing a europium concentration of 15%.

The quantum properties of the fluorescence light emitted by diamond nanocrystals containing a single nitrogen-vacancy (NV) colored center is investigated. We have observed photon antibunching with very low background light. This system is therefore a very good candidate for the production of single photon on demand. In addition, we have measured larger NV center lifetime in nanocrystals than in the bulk, in good agreement with a simple quantum electrodynamical model.Comment: 8 pages, 5 figures, revised version, to appear in PR

We report an experimental study of the longitudinal relaxation time (T1) of the electron spin associated with single nitrogen-vacancy (NV) defects hosted in nanodiamonds (ND). We first show that T1 decreases over three orders of magnitude when the ND size is reduced from 100 to 10 nm owing to the interaction of the NV electron spin with a bath of paramagnetic centers lying on the ND surface. We next tune the magnetic environment by decorating the ND surface with Gd 3+ ions and observe an efficient T1-quenching, which demonstrates magnetic noise sensing with a single electron spin. We estimate a sensitivity down to ≈ 14 electron spins detected within 10 s, using a single NV defect hosted in a 10-nm-size ND. These results pave the way towards T1-based nanoscale imaging of the spin density in biological samples. PACS numbers:The ability to detect spins is the cornerstone of magnetic resonance imaging (MRI), which is currently one of the most important tools in life science. However, the sensitivity of conventional MRI techniques is limited to large spin ensembles, which in turn restricts the spatial resolution at the micrometer scale [1, 2]. Extending MRI techniques at the nanoscale can be achieved at sub-Kelvin temperature with magnetic resonance force microscopy, through the detection of weak magnetic forces [3, 4]. Another strategy consists in directly sensing the magnetic field created by spin magnetic moments with a nanoscale magnetometer. In that context, the electron spin associated with a nitrogen-vacancy (NV) defect in diamond has been recently proposed as an ultrasensitive and atomic-sized magnetic field sensor [5]. In the last years, many schemes based on dynamical decoupling pulse sequences have been devised for sensing ac or randomly fluctuating magnetic fields with a single NV spin [6][7][8][9]. These protocols recently enabled nuclear magnetic resonance measurements on a few cubic nanometers sample volume [10,11] and the detection of a single electron spin under ambient conditions [12].An alternative approach for sensing randomly fluctuating magnetic fields -i.e. magnetic noise -is based on the measurement of the longitudinal spin relaxation time (T 1 ) of the NV defect electron spin. Using an ensemble of NV defects and a T 1 -based sensing scheme, Steinert et al. recently demonstrated magnetic noise sensing with a sensitivity down to 1000 statistically polarized electron spins, as well as imaging of spin-labeled cellular structures with a diffraction-limited spatial resolution (≈ 500 nm) [13]. Bringing the spatial resolution down to few nanometers could be achieved by using a single NV defect integrated in a scanning device, e.g. with a nanodiamond (ND) attached to the tip of an atomic force microscope (AFM) [14,15]. With this application in mind, we study here the T 1 time of single NV defects hosted in NDs, as a function of ND size and magnetic environment. We first report a decrease of T 1 over three orders of magnitude when the ND size is reduced from 100 to 10 nm. This behavior is explained by ...

Three copper(I) iodide clusters coordinated by different phosphine ligands formulated [Cu(4)I(4)(PPh(3))(4)] (1), [Cu(4)I(4)(Pcpent(3))(4)] (2), and [Cu(4)I(4)(PPh(2)Pr)(4)] (3) (PPh(3) = triphenylphosphine, Pcpent(3) = tricyclopentylphosphine, and PPh(2)Pr = diphenylpropylphosphine) have been synthesized and characterized by (1)H and (31)P NMR, elemental analysis and single crystal X-ray diffraction analysis. They crystallize in different space groups, namely, monoclinic P21/c, cubic Pa ̅3, and tetragonal I ̅42m for 1, 2, and 3, respectively. The photoluminescence properties of clusters 1 and 3 show reversible luminescence thermochromism with two highly intense emission bands whose intensities are temperature dependent. In accordance to Density Functional Theory (DFT) calculations, these two emission bands have been attributed to two different transitions, a cluster centered (CC) one and a mixed XMCT/XLCT one. Cluster 2 does not exhibit luminescence variation in temperature because of the lack of the latter transition. The absorption spectra of the three clusters have been also rationalized by time dependent DFT (TDDFT) calculations. A simplified model is suggested to represent the luminescence thermochromism attributed to the two different excited states in thermal equilibrium. In contrast with the pyridine derivatives, similar excitation profiles and low activation energy for these phosphine-based clusters reflect high coupling of the two emissive states. The effect of the Cu-Cu interactions on the emission properties of these clusters is also discussed. Especially, cluster 3 with long Cu-Cu contacts exhibits a controlled thermochromic luminescence which is to our knowledge, unknown for this family of copper iodide clusters. These phosphine-based clusters appear particularly interesting for the synthesis of original emissive materials.

We present a study of the charge state conversion of single nitrogen-vacancy (NV) defects hosted in nanodiamonds (NDs). We first show that the proportion of negatively-charged NV − defects, with respect to its neutral counterpart NV 0 , decreases with the size of the ND. We then propose a simple model based on a layer of electron traps located at the ND surface which is in good agreement with the recorded statistics. By using thermal oxidation to remove the shell of amorphous carbon around the NDs, we demonstrate a significant increase of the proportion of NV − defects in 10-nm NDs. These results are invaluable for further understanding, control and use of the unique properties of negatively-charged NV defects in diamond.