Fadri Grünenfelder

Current implementations of quantum key distribution (QKD) typically rely on prepare-and-measure (P&M) schemes. Unfortunately, these implementations are not completely secure, unless security proofs fully incorporate all imperfections of real devices. So far, existing proofs have primarily focused on imperfections of either the light source or the measurement device. In this work, we establish a security proof for the loss-tolerant P&M QKD protocol that incorporates imperfections in both the source and the detectors. Specifically, we demonstrate the security of this scheme when the emitted states deviate from the ideal ones and Bob’s measurement device does not meet the basis-independent detection efficiency condition. Furthermore, we conduct an experiment to characterise the detection efficiency mismatch of commercial single-photon detectors as a function of the polarisation state of the input light, and determine the expected secret key rate in the presence of state preparation flaws when using such detectors. Our work provides a way towards guaranteeing the security of actual implementations of widely deployed P&M QKD.

Bandwidth-limited devices in the transmitter of fast QKD implementations cause pulse correlations that leak information about previous setting choices. To take them into account in the existing security proofs, a measure of their strengths is needed. This is experimentally challenging, especially for long-range correlations, which are not experimentally accessible. In this work, we propose a new characterization method that exploits a linear model of the modulation devices. We show that this model predicts an upper bound for arbitrary order correlations that makes their characterization possible. We also present experimental results using the proposed method. In doing so, we can retrieve security even in the presence of arbitrary long correlations, with similar performance to classical security proofs.

Quantum key distribution (QKD) protocols leverage quantum mechanics to achieve information theoretically secure communication, yet real-world implementations must address experimental limitations, particularly phase correlations in weak coherent laser pulses (WCPs). High-speed gain-switching lasers, commonly used in QKD, can exhibit residual photons causing phase correlations between consecutive pulses, challenging the perfect phase randomization assumption crucial for the decoy-state BB84 protocol. Theoretical work has proposed security proofs that require knowledge of how closely each phase's probability distribution approximates uniformity, which is complex to estimate experimentally. In this study we introduce an experimental method to characterise phase correlations of any length under realistic conditions by modelling the phase generation process within the laser cavity. Additionally, we experimentally benchmark this practical routine for measuring second-order correlations using a double Michelson interferometer with tunable amplitude attenuators, allowing comprehensive characterisation of the phase generation process and accurate measurement of the phase probability distribution, thus enhancing the security of QKD systems.

Quantum key distribution (QKD) has raised as an attractive alternative to classical cryptography due to its security being provided by quantum mechanics rather than relying on algorithms that could potentially be broken in the future, rendering current communications insecure. However, many of the security proofs rely on assumptions that may not agree with reality, for instance, device imperfections can open loopholes that could potentially be exploited by a malicious party in order to extract part, if not all, of the secret key.

Quantum random number generators (QRNGs) have obtained notable attention and undergone substantial development, driven by their utility across diverse fields including simulations, gambling, and cryptography. This surge in interest stems from their unique capacity to deliver inherent randomness, which can only be derived from the probabilistic nature of quantum mechanics. The key challenge lies in validating the quantum origin of the randomness produced, which usually requires either a thorough characterization of the elements in the setup or very experimentally challenging loophole-free bell tests. In this work, we present a simple, self-testing and cost-effective quantum random number generator (QRNG) designed to operate with an untrusted measurement device and a partially characterized source, yielding a high rate of random bits. We consider a prepare-and-measure scenario where the preparation device takes a binary input x and a binary output b is received from the measurement device. Depending on the input, the preparation device sends either a weak coherent state (x=1) or a vacuum state (x=0). The measurement device employs homodyne detection to distinguish between these states, and the output value is chosen when the detector current is below (b=0) or above (b=1) a certain threshold. In order to certify the quantum origin of the randomness generated by output b, we need to track the correlations between input and output and the average energy per pulse must respect an upper bound. By using a continuous wave laser to seed the pulsed laser that generates the states, we avoid the need for expensive electro-optical modulators as used in https://arxiv.org/abs/2004.08307. With this scheme we achieve an extraction rate of certified quantum randomness of around 625kHz.

The ideal Quantum random number generator (QRNG) is a black box which allows the users to test the quantum nature of the generated numbers. Producing a device which is close to this ideal is very demanding and will yield a low rate of random bits. Here we propose a simple setup which is self-testing on the detection part, meaning that only the source has to be characterized. We expect the implementation of this device to yield a random bit rate of around 10 Mpbs.

This research introduces a simplified variation of the time-based BB84 protocol, employing time-bin encoding and one decoy state. The proposed approach significantly simplifies the security analysis, enabling the identification of potential vulnerabilities by avoiding interference in the transmission of specific state combinations. This simplification reduces the reliance on finite key analysis and allows us to better characterize the source imperfections without much compromise on the secret key rate (SKR).