A source of triggered entangled photon pairs is a key component in quantum information science; it is needed to implement functions such as linear quantum computation, entanglement swapping and quantum teleportation. Generation of polarization entangled photon pairs can be obtained through parametric conversion in nonlinear optical media or by making use of the radiative decay of two electron-hole pairs trapped in a semiconductor quantum dot. Today, these sources operate at a very low rate, below 0.01 photon pairs per excitation pulse, which strongly limits their applications. For systems based on parametric conversion, this low rate is intrinsically due to the Poissonian statistics of the source. Conversely, a quantum dot can emit a single pair of entangled photons with a probability near unity but suffers from a naturally very low extraction efficiency. Here we show that this drawback can be overcome by coupling an optical cavity in the form of a 'photonic molecule' to a single quantum dot. Two coupled identical pillars-the photonic molecule-were etched in a semiconductor planar microcavity, using an optical lithography method that ensures a deterministic coupling to the biexciton and exciton energy states of a pre-selected quantum dot. The Purcell effect ensures that most entangled photon pairs are emitted into two cavity modes, while improving the indistinguishability of the two optical recombination paths. A polarization entangled photon pair rate of 0.12 per excitation pulse (with a concurrence of 0.34) is collected in the first lens. Our results open the way towards the fabrication of solid state triggered sources of entangled photon pairs, with an overall (creation and collection) efficiency of 80%.

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...

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

Entanglement is at the heart of quantum physics, both for its conceptual foundations and for applications in quantum communication. Remarkably, entanglement can be 'swapped': if we prepare two independent entangled pairs A1-A2 and B1-B2, a joint measurement on A1 and B1 (called a 'Bellstate measurement', BSM) has the effect of projecting A2 and B2 onto an entangled state, although these two particles have never interacted nor share any common past 1,2 . Entanglement swapping with photon pairs has already been experimentally demonstrated 3-6 using pulsed sources-where the challenge was to achieve sufficiently sharp synchronization of the photons in the BSM-but never with continuous-wave sources, as originally proposed 2 . Here, we present an experiment where the coherence time of the photons exceeds the temporal resolution of the detectors. Hence, photon timing can be obtained by the detection times, and pulsed sources can be replaced by continuous-wave sources, which do not require any synchronization 6,7 . This allows for the first time the use of completely autonomous sources, an important step towards real-world quantum networks with truly independent and distant nodes.The BSM is the essential element in an entanglement-swapping experiment. Linear optics allows the realization of only a partial BSM 8 by coupling the two incoming modes on a beam splitter and observing a suitable detection pattern in the outgoing modes. Such a measurement is successful in at most 50% of the cases. Still, a successful partial BSM entangles two photons that were, up to then, independent. The physics behind this realization is the bosonic character of photons. It is therefore crucial that the two incoming photons are indistinguishable: they must be identical in their spectral, spatial, polarization and temporal modes at the beam splitter; spectral overlap is achieved by the use of similar filters, spatial overlap by the use of single-mode optical fibres and polarization is matched by a polarization controller. In addition, the temporal resolution must be unambiguous: detection at a time t ± t d , where t d is the temporal resolution of the detector, must single out a unique time mode. In previous experiments, synchronized pulsed sources created both of the photons at the same time and path lengths had to be matched to obtain the required temporal overlap. The pulse length, that is, the coherence length of the photons, was τ c t d (typically τ c < 1 ps), but two subsequent pulses were separated by more than t d (ref. 9). The drawback of such a realization is that the two sources cannot be totally autonomous, because of the indispensable synchronization. For the case where τ c > t d (ref. 10), the detectors always single out a unique time mode. As a benefit, we can give up the pulsed character of the sources and the synchronization between them. By implementing this, we realize for the first time the entanglement swapping scheme as originally proposed in ref. 2.The experimental scheme is shown in Fig. 1. Each of the two nonlinear cr...

In a coupled quantum-dot nano-cavity system, the photoluminescence from an off-resonance cavity mode exhibits strong quantum correlations with the quantum dot transitions, even though its autocorrelation function is classical. Using new pump-power dependent photon-correlation measurements, we demonstrate that this seemingly contradictory observation that has so far defied an explanation stems from cascaded cavity photon emission in transitions between excited multiexciton states. The mesoscopic nature of quantum dot confinement ensures the presence of a quasicontinuum of excitonic transitions part of which overlaps with the cavity resonance.A quantum dot (QD) coupled to a photonic crystal cavity provides a promising system for studying cavity quantum-electrodynamics (QED) in the solid state [1,2]. In contrast to their atom-based counterparts, these systems exhibit features that arise from their complex environment. A common effect that surfaced in previous experiments is strong off-resonant emission of a cavity mode (CM) containing one or multiple QDs. Photon correlation measurements revealed that the cavity-mode emission is anti-correlated with the QD excitons at the level of single quanta, proving that cavity feeding is mediated solely by a single QD [3,4]. Surprisingly, however, the photon stream emitted by the far off-resonant CM did not show any significant quantum correlations. Previous experimental [5,6,7] and theoretical [8,9] investigations have focused on explaining cavity feeding in terms of dephasing of the QD excitons mediated either by coupling to acoustic phonons or to free carriers. However, all of the attempts to describe cavity feeding using Markovian dephasing of the fundamental exciton line fail to explain the above mentioned photon correlation signatures that appear to be true for all studied QD cavity-QED systems.In this Letter, we unequivocally demonstrate that the far off-resonant excitation of the CM is solely due to the mesoscopic nature of quantum dot confinement, which in turn leads to an energetically broad cascaded emission of the QD. In this setting, cavity feeding and its photon correlation signatures can be regarded as an intrinsic feature of QD-cavity systems that arises from the complicated QD multi-exciton level structure. We carry out pump-power dependent photoluminescence (PL) as well as photon auto-and cross-correlation measurements on a nano-structure incorporating a single QD embedded in a photonic crystal (PC) defect cavity [3]. To explain our experimental observations, we develop a new theoretical model for the QD-cavity system, perform numerical calculations of its semi-classical dynamics and compare its predictions with the new experimental findings. While a quantitative comparison between numerical and experimental results is intrinsically difficult, the qualitative agreement we achieve is excellent. In particular, the unusual correlation features found experimentally are naturally reproduced by the model and the simulations.Before proceeding, we remark that acoustic...

We report on the experimental realization and characterization of an asynchronous heralded single photon source based on spontaneous parametric down conversion. Photons at 1550 nm are heralded as being inside a single-mode fiber with more than 60% probability, and the multi-photon emission probability is reduced by a factor up to more than 500 compared to Poissonian light sources. These figures of merit, together with the choice of telecom wavelength for the heralded photons are compatible with practical applications needing very efficient and robust single photon sources. PACS numbers: 42.50.Ar, 42.50.Dv, 42.65.Lm, 03.67.Hk With the present development of quantum communication and computing technologies, including quantum key distribution [1] and quantum teleportation [2] the interest for true single photon sources is rising. Many different implementations have been investigated, using single molecule or atom excitation [3,4,5,6], color centers in diamonds [7,8], quantum dots [9,10,11,12,13] or pulsed parametric down conversion sources [14,15]. All theses solutions have various advantages and tradeoffs between high purity and efficient single photon production, repetition rate, wavelength of the photons, and ease of use. The aim of this paper is to show that a spontaneous parametric down conversion (SPDC) source made of a bulk non-linear cristal at room temperature and a simple basic optical setup can be used to herald single photons at telecom wavelength in a very efficient way (see figure 1). The photons are also directly available in a standard single-mode telecom optical fiber, making this source a good choice for quantum communication applications such as scalable quantum networks. In this context, the term "heralded" means that photons are not generated on demand, but instead an electric signal announces the presence of a photon in a fiber. Indeed, as photons are created in pairs, the detection of one photon can be used to announce the presence of the complementary photon [16].The source presented in this paper is an asynchronous heralded single photon source (AHSPS) because the heralding signals are not synchronized with a periodic clock, the SPDC pump being continuous. It exhibits a very good probability of producing one photon and a very low probability of producing more than one photon per heralding signal. The latter probability depends on the pump power applied to the crystal, and several measurements are presented to characterize this dependency. * Electronic address: sylvain.fasel@physics.unige.ch SPDC Source 532 nm LPF Nd:YAG KNb0 3 Asynchronous Heralded Single Photon Source TTL 810nm 1550nm Application heralded photons heralding signal DF Si APD VA DM FIG. 1: Schematic of the asynchronous heralded single photon source (DM: dichroic mirror, DF: neutral density filter for pump attenuation, LPF: low-pass pump filters, VA: variable optical fiber attenuator)Our AHSPS is made of two main parts, as depicted in figure 1. The first consists in the SPDC photon pair creation stage which consists in a typ...

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