John Jeffers

High brightness, low second-order correlation function single-photon sources (SPSs) are an alternative to commonly employed weak coherent pulse (WCP) sources for discrete variable quantum key distribution (QKD) and offer potential key-rate and finite-block scaling advantages. However, the loss tolerance of SPS-based QKD is compromised by photon number splitting (PNS) attacks against non-negligible multiphoton emissions. Decoy state (DS) techniques mitigate against PNS attacks, with WCP-DS QKD over several hundred km in fibre being demonstrated. DS QKD protocols for different source photon number statistics have been proposed, such as for binomial and thermal distributions. Here, we investigate the use of generalised DS techniques assuming we do not have access to the true photon number statistics of the SPS. Thus, we bound the source distribution using the mean photon number and the second-order correlation function, which provides us with enough partial knowledge to compute our decoy SPS protocols. Hence, we provide finite-key security bounds for an SPS-based Efficient BB84 for several decoy protocols with optimised parameters, and derive required SPS characteristics to achieve a key rate enhancement over DS WCPs and match their loss tolerance.

Quantum key distribution (QKD) with solid-state single-photon emitters is gaining traction due to their rapidly improving performance and compatibility with future quantum networks. We report a bright quantum dot based source of telecom photons by frequency converting a near- infrared InGaAs quantum dot to the telecom C-band (1). We implement polarisation encoded BB84 quantum key distribution (QKD), achieving a positive asymptotic key rate over 175 km of optical fibre. We also present finite key analysis optimised for typically non-ideal single- photon sources, achieving 8 orders of magnitude improvement with finite key rates of 40 kbps over 50 km in practical acquisition times of one hour (2). To extend the distances further, we take inspiration from decoy state QKD protocols – typically used to overcome photon- splitting attacks when using weak coherent states – and demonstrate a QD excitation scheme for implementing modulation of the photon number distribution. We show experimentally that the decoy state protocol enables the distribution of a secret key over more than 200 km of optical fibre.

High brightness, low-g2 single-photon sources (SPSs) are an alternative to commonly employed weak coherent pulse (WCP) sources for discrete variable quantum key distribution (QKD) and offer potential key-rate and finite-block scaling advantages. However, the loss tolerance of SPS-based QKD is compromised by photon number splitting (PNS) attacks against non-negligible multiphoton emissions. Decoy state (DS) techniques mitigate against PNS attacks, with WCP-DS QKD over several hundred km in fibre being demonstrated. Here, we adapt the DS method to any practical SPS that can easily generate multiple photon number distributions (PND) by attenuating its original photon emissions. Hence, we provide finite-key security bounds for a Multi-PND (adapted 2-Decoy) protocol using Efficient BB84 with optimised parameters. We use a particular true quantum dot source to compare its key rate generation with a Single-PND (adapted Non-Decoy) protocol for several finite block sizes. As expected, the Multi-PND gives higher key rates than the Single-PND, except for considerably small blocks. Moreover, the Multi-PND protocol goes beyond 200 km of tolerable fibre distance for high acquisition times. In this work, we set a generalised method to employ the DS techniques with any realistic SPS and further research may be done implementing distinct SPS characteristics.

We demonstrate fibre-based quantum key distribution over 175 km using a bright frequency converted quantum dot single-photon source. The source is capable of producing count rates approaching 2 MHz at 1550 nm with second order correlations on the order of 3%. This allows for a measured key rate of 130 bps (100 kbps) at 175 km (50 km) in the asymptotic regime using static encoding and predicted positive key rate out to 188 km. This can be extended to 240 km using ultra-low loss fibre based on the measured source parameters.