Quantum communication is the art of transferring quantum states, or quantum bits of information (qubits), from one place to another. On the fundamental side, this allows one to distribute entanglement and demonstrate quantum nonlocality over significant distances. On the more applied side, quantum cryptography offers, for the first time in human history, a provably secure way to establish a confidential key between distant partners. Photons represent the natural flying qubit carriers for quantum communication, and the presence of telecom optical fibres makes the wavelengths of 1310 and 1550 nm particulary suitable for distribution over long distances. However, to store and process quantum information, qubits could be encoded into alkaline atoms that absorb and emit at around 800 nm wavelength. Hence, future quantum information networks made of telecom channels and alkaline memories will demand interfaces able to achieve qubit transfers between these useful wavelengths while preserving quantum coherence and entanglement. Here we report on a qubit transfer between photons at 1310 and 710 nm via a nonlinear up-conversion process with a success probability greater than 5%. In the event of a successful qubit transfer, we observe strong two-photon interference between the 710 nm photon and a third photon at 1550 nm, initially entangled with the 1310 nm photon, although they never directly interacted. The corresponding fidelity is higher than 98%.Comment: 7 pages, 3 figure

A new kind of correlated photon-pair source based on a waveguide integrated on a periodically poled lithium niobate substrate is reported. Using a pump laser of a few μW at 657 nm, photon-pairs are generated/degenerated at 1314 nm. Detecting ~1500 coincidences per second, a conversion rate of 10-6 pairs per pump photon can be inferred, which is four orders of magnitude higher than that obtained with previous bulk sources. These results are very promising for the realisation of sources for quantum communication and quantum metrology experiments requiring a high signal-to-noise ratio or working with more than one photon-pair at a tim

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...

We report on energy-time and time-bin entangled photon-pair sources based on a periodically poled lithium niobate (PPLN) waveguide. Degenerate twin photons at 1314 nm wavelength are created by spontaneous parametric down-conversion and coupled into standard telecom fibers. Our PPLN waveguide features a very high conversion efficiency of about 10 −6 , roughly 4 orders of magnitude more than that obtained employing bulk crystals [1]. Even if using low power laser diodes, this engenders a significant probability for creating two pairs at a timean important advantage for some quantum communication protocols. We point out a simple means to characterize the pair creation probability in case of a pulsed pump. To investigate the quality of the entangled states, we perform photon-pair interference experiments, leading to visibilities of 97% for the case of energy-time entanglement and of 84% for the case of time-bin entanglement. Although the last figure must still be improved, these tests demonstrate the high potential of PPLN waveguide based sources to become a key element for future quantum communication schemes. *