Quantum continuous variables [1] are being explored [2,3,4,5,6,7,8,9,10,11,12,13,14] as an alternative means to implement quantum key distribution, which is usually based on single photon counting [15]. The former approach is potentially advantageous because it should enable higher key distribution rates. Here we propose and experimentally demonstrate a quantum key distribution protocol based on the transmission of gaussian-modulated coherent states (consisting of laser pulses containing a few hundred photons) and shot-noise-limited homodyne detection; squeezed or entangled beams are not required [13]. Complete secret key extraction is achieved using a reverse reconciliation [14] technique followed by privacy amplification. The reverse reconciliation technique is in principle secure for any value of the line transmission, against gaussian individual attacks based on entanglement and quantum memories. Our table-top experiment yields a net key transmission rate of about 1.7 megabits per second for a loss-free line, and 75 kilobits per second for a line with losses of 3.1 dB. We anticipate that the scheme should remain effective for lines with higher losses, particularly because the present limitations are essentially technical, so that significant margin for improvement is available on both the hardware and software.

We observed photon antibunching in the fluorescent light emitted from a single nitrogen-vacancy center in diamond at room temperature. The possibility of generating triggerable single photons with such a solid-state system is discussed.

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...