arXiv subjects

Nicholas Zaunders

Publications and source records attributed to Nicholas Zaunders.

7 recordsLinked to original sources

arXiv2026

Co-transmission of classical data and continuous-variable entanglement over a single quantum channel

Displacement-based simultaneous quantum-classical communications (SQCC) protocols, as originally proposed, are generally incompatible with the majority of useful quantum communication schemes, such as entanglement distribution or repeater-based quantum key distribution: direct measurement of the classical signal also measures and destroys the quantum state, leaving point-to-point Gaussian quantum key distribution as the only quantum communication scheme amenable to integration with SQCC. In this work, we apply the classically-modulated quantum communication protocol proposed by Zaunders and Ralph [arXiv:2606.03181v3] to circumvent this issue and demonstrate the distribution of continuous-variable Gaussian entanglement simultaneously with classical information. We characterise the quality of the distributed entangled state and outline how the scheme is suitable for use in repeater-based networks. Lastly, we compute the secret key generation rate of the Gaussian CMQC scheme in the point-to-point case and compare it to the equivalent point-to-point SQCC protocol.

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arXiv2026

Generalised simultaneous transmission of arbitrary quantum states and classical information

We present a protocol which allows for arbitrary optical quantum states to simultaneously carry and transmit classical data, without sacrificing the integrity of either the quantum or classical information. Our scheme encodes classical information via displacements in the phase space prior to transmission and retrieves each classical symbol via a Gaussian continuous-variable teleportation. The original quantum state is then restored by guessing the the original displacement and performing the appropriate inverse operation. In the limit of sufficiently high classical signal and high squeezing, we show that our scheme is capable of perfectly reconstructing both the input classical signal and the input quantum state without loss of coherence. An example is given in terms of the transmission of a dual-rail Bell state.

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arXiv2025

Utility of noiseless linear amplification and attenuation in single-rail discrete-variable quantum communications

Quantum communication offers many applications, with teleportation and superdense coding being two of the most fundamental. In these protocols, pre-shared entanglement enables either the faithful transfer of quantum states or the transmission of more information than is possible classically. However, channel losses degrade the shared states, reducing teleportation fidelity and the information advantage in superdense coding. Here, we investigate how to mitigate these effects by optimising the measurements applied by the communicating parties. We formulate the problem as an optimisation over general positive operator-valued measurements (POVMs) and compare the results with physically realisable noiseless attenuation (NA) and noiseless linear amplification (NLA) circuits. For teleportation, NLA/NA and optimised POVMs improve the average fidelity by up to 78% while maintaining feasible success probabilities. For superdense coding, they enhance the quantum advantage over the classical channel capacity by more than 100% in some regimes and shift the break-even point, thereby extending the tolerable range of losses. Notably, the optimal POVMs effectively reduce to NA or NLA, showing that simple, experimentally accessible operations already capture the essential performance gains.

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arXiv2025

Software-enhanced simultaneous quantum-classical communication protocol with Gaussian post-selection

Simultaneous quantum-classical communication (SQCC) protocols offer a practical approach to continuous-variable quantum key distribution (CV-QKD) by encoding quantum and classical signals onto the same optical pulse. However, like most QKD protocols, their performance is limited when experimental parameters, such as modulation variance, are optimised based on stationary channel assumptions. In fluctuating environments, such as free-space links, this can result in sub-optimal key rates and reduced transmission distances. In this work, we introduce Gaussian post-selection into the SQCC framework, enabling a software-based optimisation of the modulation variance after channel estimation. This passive approach enhances key rates in both asymptotic and finite-size regimes without requiring hardware modifications and remains effective even when receiver imperfections are taken into account. We demonstrate that our protocol improves the transmission distance and robustness of SQCC relative to the standard fixed-variance SQCC protocol, and approaches the performance of a fully pre-optimised system across both fibre and free-space channels. In particular, we show that the protocol enables full communication windows under ideal weather conditions and maintains higher duty cycles during adverse weather in satellite-to-ground scenarios. These results highlight the practicality of post-selection based SQCC for real-world quantum communication over both terrestrial fibre networks and satellite-based free-space links.

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arXiv2025

Entanglement distribution via satellite: an evaluation of competing protocols assuming realistic free-space optical channels

A key technical requirement of any future quantum network is the ability to distribute quantum-entangled resources between two spatially separated points at a high rate and high fidelity. Entanglement distribution protocols based on satellite platforms, which transmit and receive quantum resources directly via free-space optical propagation, are therefore excellent candidates for quantum networking, since the geometry and loss characteristics of satellite networks feasibly allow for up to continental-scale ($\sim10^3$ km) over-the-horizon communication without the infrastructure, cost, or losses associated with equivalent fibre-optic networks. In this work, we explore two network topologies commonly associated with quantum networks - entanglement distribution between two satellites in low-Earth orbit mediated by a third satellite and entanglement distribution between two ground stations mediated by a satellite in low-Earth orbit, and two entanglement distribution schemes - one where the central satellite is used as a relay, and the other where the central satellite is used to generate and distribute the entangled resource directly. We compute a bound on the rate of distribution of distillable entanglement achieved by each protocol in each network topology as a function of the network channels for both single-rail discrete- (DV) and continuous-variable (CV) resources and use or non-use of probabilistic noiseless linear quantum amplification (NLA). In the case of atmospheric channels we take into account the turbulent and optical properties of the free-space propagation. We determine that for the triple-satellite network configuration, the optimal strategy is to perform a distributed NLA scheme in either CV or DV, and for the ground-satellite-ground network the optimal strategy is to distribute a DV resource via the central satellite.

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arXiv2025

Enhanced Simultaneous Quantum-Classical Communications Under Composable Security

Simultaneous quantum-classical communications (SQCC) protocols are a family of continuous-variable quantum key distribution (CV-QKD) protocols which allow for quantum and classical symbols to be integrated concurrently on the same optical pulse and mode. In this work, we present a revised analysis of simultaneous quantum-classical communications in Gaussian-modulated coherent-state CV-QKD protocols. We address security concerns inherently associated with SQCC schemes and provide an updated model of the coupling between the classical and quantum channels. We provide evidence for our model via Monte Carlo simulation. We compute the performance of our revised SQCC protocol in terms of the secret-key generation rate optimised over free parameters and demonstrate improved quantum efficiency for a given classical bit-error rate. Lastly, we extend our analysis into the finite-key regime, where we propose a scheme for composably-secure SQCC under realistic operating conditions and demonstrate that our scheme retains the advantage in quantum performance over previous models.

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arXiv2024

Quantum-Amplified Simultaneous Quantum-Classical Communications

Classical free-space optical (FSO) communication promises massive data throughput rates relative to traditional wireless technologies - an attractive outcome now being pursued in the context of satellite-ground, inter-satellite and deep-space communications. The question we investigate here is: how can we minimally alter classical FSO systems, both in infrastructure and in energy input, to provide some element of quantum communication coexisting with classical communications? To address this question, we explore additional Gaussian displacements to classical FSO encoding on the satellite, determining the minimum signal requirements that will meet given specifications on the combined classical and quantum communications throughput. We then investigate whether enhanced quantum-based amplifiers embedded in receivers, which have proven advantageous in standalone quantum communication, can enhance our combined classical-quantum communication throughput. We show how this is indeed the case, but only at the cost of some additional receiver complexity, relative to standalone quantum communications. This additional complexity takes the form of an additional beamsplitter and two heterodyne detectors at the receiver. Our results illustrate a viable pathway to realising quantum communication from classical FSO systems with minimal design changes.

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