Photon-pair generation in photonic crystal fibre with a 1.5GHz modelocked VECSEL

Abstract

Four-wave mixing (FWM) in optical fibre is a leading technique for generating high-quality photon pairs. We report the generation of photon pairs by spontaneous FWM in photonic crystal fibre pumped by a 1.5 GHz repetition-rate vertical-external-cavity surface-emitting laser (VECSEL).The photon pairs exhibit high count rates and a coincidence-to-accidental ratio of over 80. The VECSEL's high repetition-rate, high average power, tunability, and small footprint make this an attractive source for quantum key distribution and photonic quantum-state engineering. * Electronic address: p.mosley@bath.ac.ukThe drive towards photonic quantum-enhanced technologies is placing ever more stringent demands on the performance of nonclassical light sources [1][2][3]. However due to their size, cost, complexity, and limited generation rates it is challenging to incorporate state-of-the-art photon sources into turn-key systems. Nevertheless in recent years, huge advances have been made in producing higher-quality single photons from straightforward equipment operating at room temperature. Optical nonlinearity has been at the forefront of these efforts [4][5][6][7][8][9][10].By propagating a high-intensity laser pulse through a nonlinear medium, pairs of photons can be spontaneously generated, either through parametric downconversion (PDC), a threewave mixing process that requires the presence of χ (2) nonlinearity, or by χ (3) -mediated fourwave mixing (FWM). The highest-performance sources are usually pumped by Ti:Sapphire oscillators [11][12][13][14][15][16], placing them orders of magnitude higher in both complexity and cost than the attenuated laser sources typically used in commercially-available quantum technologies. Furthermore, the repetition rate of these lasers, usually 80 MHz or so, places a limit on the rate at which high-quality photon pairs can be delivered by a single source [17]. These factors present significant obstacles to implementing real-world photonic quantum-enhanced technologies.In this paper we demonstrate photon-pair generation through four-wave mixing in a photonic crystal fibre (PCF) driven by a 1.5 GHz modelocked tunable vertical-external-cavity surface-emitting laser (VECSEL) [18]. VECSELs have not previously been used for photonpair generation but are attractive for a number of reasons [19]. Modelocked VECSELs produce transform-limited ultrafast pulses with peak power in the correct range for photonpair generation by FWM in fibre [20]. Modelocking a VECSEL requires a compact cavity containing only three or four components providing the possibility of very small-footprint photon-pair sources that require little maintenance. The short cavity results in a high pulse repetition frequency which could allow an order of magnitude increase in the rate of pair generation relative to 80 MHz laser systems. VECSELs are intrinsically flexible: the wavelength can be continuously tuned by an intracavity etalon [18]; the repetition rate can be adjusted without interrupting modelocking by translating the output couple...

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“…(8.1), the detection rate R can be considerably increased proportionally to the repetition rate of the pump laser. In particular, high repetition rate lasers allow generating photon pairs at high rate while maintaining n as low as possible, as required for negligible multi-photon generation events , Morris et al 2014.…”