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...

Inertial sensors relying on atom interferometry offer a breakthrough advance in a variety of applications, such as inertial navigation, gravimetry or ground- and space-based tests of fundamental physics. These instruments require a quiet environment to reach their performance and using them outside the laboratory remains a challenge. Here we report the first operation of an airborne matter-wave accelerometer set up aboard a 0g plane and operating during the standard gravity (1g) and microgravity (0g) phases of the flight. At 1g, the sensor can detect inertial effects more than 300 times weaker than the typical acceleration fluctuations of the aircraft. We describe the improvement of the interferometer sensitivity in 0g, which reaches 2 x 10-4 ms-2 / √Hz with our current setup. We finally discuss the extension of our method to airborne and spaceborne tests of the Universality of free fall with matter waves.

We have designed and realized a prototype that implements a continuousvariable quantum key distribution protocol based on coherent states and reverse reconciliation. The system uses time and polarization multiplexing for optimal transmission and detection of the signal and phase reference, and employs sophisticated error-correction codes for reconciliation. The security of the system is guaranteed against general coherent eavesdropping attacks. The performance of the prototype was tested over preinstalled optical fibres as part of a quantum cryptography network combining different quantum key distribution technologies. The stable and automatic operation of the prototype over 57 hours yielded an average secret key distribution rate of 8 kbit/s over a 3 dB loss optical fibre, including the key extraction process and all quantum and classical communication. This system is therefore ideal for securing communications in metropolitan size networks with high speed requirements.

Abstract. We describe the operation of a light pulse interferometer using cold 87 Rb atoms in reduced gravity. Using a series of two Raman transitions induced by light pulses, we have obtained Ramsey fringes in the low gravity environment achieved during parabolic flights. With our compact apparatus, we have operated in a regime which is not accessible on ground. In the much lower gravity environment and lower vibration level of a satellite, our cold atom interferometer could measure accelerations with a sensitivity orders of magnitude better than the best ground based accelerometers and close to proven spaced-based ones. PACS. PACS-key discribing text of that key -PACS-key discribing text of that keyAtom interferometry is one of the most promising candidates for ultra-accurate measurements of gravito-inertial forces [1], with both fundamental [2,3,4,5] and practical (navigation or geodesy) applications. Atom interferometry is most often performed by applying successive coherent beam-splitting and -recombining processes separated by an interrogation time T to a set of particles [6]. Understanding matter waves interferences phenomena follows from the analogy with optical interferometry [7,8]: the incoming wave is separated into two wavepackets by a first beam-splitter; each wave then propagates during a time T along a different path and accumulates a different phase; the two wavepackets are finally recombined by a last beam-splitter. To observe the interferences, one measures the two output-channels complementary probability amplitudes which are sine functions of the accumulated phase difference ∆φ. This phase difference increases with the paths length, i.e. with the time T between the beamsplitting pulses.When used as inertial sensors [9,10], the atoms are usually left free to evolve during the interrogation time T so that the interferometer is only sensitive to gravitoinertial effects. In particular, one avoids residual trapping fields that would induce inhomogeneities or fluctuations and would affect the atomic signal. The interrogation time T is consequently limited by, on the one hand, the free expansion of the atomic cloud, and, on the other hand, the free fall of the atomic cloud. The limitation of expansion is alleviated by the use of ultracold gases [11,12], Send offprint requests to: Fig. 1. Top: The atom interferometer assembled in the Airbus. The main rack on the left houses the laser sources and the control electronics. The rack on the front right contains the uninterruptable power-supply, the electrical panel and the high-power laser part. The rack on the back right hosts atomoptics part of the experiment. Bottom: the architecture of the atom interferometer.

We demonstrate a fast and direct calibration method for systems using a single laser for optical tweezers and particle position detection. The method takes direct advantage of back-focal-plane interferometry measuring not an absolute but a differential position, i.e. the position of the trapped particle relative to the center of the optical tweezers. Therefore, a fast step-wise motion of the optical tweezers yields the impulse response of the trapped particle. Calibration parameters such as the detector's spatial and temporal response and the spring constant of the optical tweezers then follow readily from fitting the measured impulse response.

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Analyses of the kinetics and magnitude of enhanced two-wave mixing gain under externally applied square-wave and sinusoidal electric fields are used to determine photocarrier drift mobility. These direct measurements do not require that any other photorefractive parameters be known.