arXiv subjects

Matthew S. Winnel

Publications and source records attributed to Matthew S. Winnel.

18 recordsLinked to original sources

arXiv2026

All-photonic quantum key distribution beyond the single-repeater bound

Quantum protocols require classical signaling, and when classical signals propagate faster than quantum ones, standard rate-loss limits can be surpassed. We introduce an all-photonic measurement-device-independent quantum key distribution protocol that exceeds the single-repeater bound without error correction. When quantum signals travel at two-thirds the classical speed, the key rate scaling approaches $η^{2/5}$. We propose a single-rail, temporally multiplexed architecture that extends twin-field-type protocols to multiple nodes and surpasses their key rate without ideal quantum memories.

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arXiv2026

Capacity-Achieving Entanglement Purification Protocol for Pauli Dephasing Channel

Quantum communication enables secure information transmission and entanglement distribution, but these tasks are fundamentally limited by the capacities of quantum channels. While quantum repeaters can mitigate losses and noise, entanglement swapping via a central node is ineffective against the Pauli dephasing channel due to degradation from Bell-state measurements. This suggests that purifying distributed Bell states before entanglement swapping is necessary. Although one-way hashing codes are known to saturate the dephasing channel capacity, no explicit two-way purification protocol has previously been shown to achieve this bound. In this work, we present a two-way entanglement purification protocol with an explicit, scalable circuit that asymptotically achieves the dephasing channel capacity. With each iteration, the fidelity of Bell states increases. At the final round, the residual dephasing error is suppressed doubly-exponentially, scaling as $\mathcal{O}(p^{2^{n}})$, enabling near-perfect Bell pairs for any fixed number of purification rounds $n$. The explicit circuit we propose is versatile and applicable to any number of Bell pairs, offering a practical solution for mitigating decoherence in quantum networks and distributed.

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arXiv2025

Gaussian Atemporality: When Gaussian Quantum Correlations Imply Common Cause

Conventionally, covariances do not distinguish between spatial and temporal correlations. The same covariance matrix could equally describe temporal correlations between observations of the same system at two different times or correlations made on two spatially separated systems that arose from some common cause. Here, we demonstrate Gaussian quantum correlations that are `atemporal', such that the covariances governing their quadrature measurements are unphysical without postulating some common cause. We introduce Gaussian atemporality robustness as a measure of atemporality, illustrating its efficient computability and operational meaning as the maximum noise which can be added without removing this uniquely quantum phenomenon. We illustrate that (i) specific spatiotemporal Gaussian correlations possess an intrinsic arrow of time, such that Gaussian atemporality robustness is zero in one temporal direction and not the other and (ii) that it measures quantum correlations beyond entanglement.

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arXiv2025

An entangled photon source for the telecom C-band based on a semiconductor-confined spin

Multiphoton entangled states are a key resource for quantum networks and measurement-based quantum computation. Scalable protocols for generating such states using solid-state spin-photon interfaces have recently emerged, but practical implementations have so far relied on emitters operating at short wavelengths, incompatible with low-loss fibre transmission. Here, we take a key step towards the generation of telecom wavelength multi-qubit entangled states using an InAs/InP quantum dot. After establishing that all essential criteria for generating cluster states using a ground state spin as the entangler are satisfied, we implement a scalable protocol to entangle the resident spin with sequentially emitted photons directly in the telecom C-band. We demonstrate a two-qubit (spin-photon) entanglement fidelity of $59.5\pm 8.7\%$ and a lower bound of three-qubit (spin-photon-photon) entanglement fidelity of $52.7\pm 11.4\%$. Our results close the performance gap between short-wavelength quantum dot systems and the existing telecom infrastructure, establishing a route towards practical large photonic cluster states for fibre-based quantum network applications.

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arXiv2025

A heralded quantum amplifier of multi-photon states

Large-scale quantum networking systems will inevitably require methods to overcome photon loss. While the no-cloning theorem forbids perfect and deterministic amplification of unknown quantum states, probabilistic heralded amplification schemes offer a viable path forward. Yet, for over a decade, successful multi-photon state amplification has remained out of reach, despite the fundamental importance of such states in achieving quantum advantage in optical applications. Here, we experimentally demonstrate a high-fidelity and post-selection-free amplifier for multi-photon states. We achieve heralded amplification of states with up to two photons in a single optical mode, with over a hundredfold intensity gain, and verify the coherence-preserving operation of our scheme. Our approach is scalable to higher photon numbers and enables noiseless amplification of complex multi-photon quantum states, with applications in large-scale quantum communication systems, distributed quantum metrology, and information processing.

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arXiv2024

Coherent Quantum Communications Across National Scale Telecommunication Infrastructure

Quantum communications harness quantum phenomena like superposition and entanglement to enhance information transfer between remote nodes. Coherent quantum communications, essential for phase-based quantum internet architecture, require optical coherence among nodes and typically involve single-photon interference. Challenges like preserving optical coherence and integrating advanced single-photon detectors have impeded their deployment in existing telecommunication networks. This study introduces innovative approaches to the architecture and techniques supporting coherent quantum communications, marking their first successful integration within a commercial telecom infrastructure between Frankfurt and Kehl, Germany. Employing the Twin Field Quantum Key Distribution protocol, we achieved encryption key distribution at 110 bit/s over 254 km. This system features measurement-device-independent properties and non-cryogenically cooled detectors, and represents the first effective quantum repeater implementation on telecom infrastructure, the longest practical quantum key distribution deployment to date, and validates the feasibility of a phase-based quantum internet architecture.

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arXiv2024

Classical-Quantum Dual Encoding for Laser Communications in Space

In typical laser communications classical information is encoded by modulating the amplitude of the laser beam and measured via direct detection. We add a layer of security using quantum physics to this standard scheme, applicable to free-space channels. We consider a simultaneous classical-quantum communication scheme where the classical information is encoded in the usual way and the quantum information is encoded as fluctuations of a sub-Poissonian noise-floor. For secret key generation, we consider a continuous-variable quantum key distribution protocol (CVQKD) using a Gaussian ensemble of squeezed states and direct detection. Under the assumption of passive attacks secure key generation and classical communication can proceed simultaneously. Compared with standard CVQKD. which is secure against unrestricted attacks, our added layer of quantum security is simple to implement, robust and does not affect classical data rates. We perform detailed simulations of the performance of the protocol for a free-space atmospheric channel. We analyse security of the CVQKD protocol in the composable finite-size regime.

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arXiv2023

Deterministic preparation of optical squeezed cat and Gottesman-Kitaev-Preskill states

Large-amplitude squeezed cat and high-quality Gottesman-Kitaev-Preskill (GKP) states are powerful resources for quantum error correction. However, previous schemes in optics are limited to low success probabilities, small amplitudes, and low squeezing. We overcome these limitations and present scalable schemes in optics for the deterministic preparation of large-amplitude squeezed cat states using only Gaussian operations and photon-number measurements. These states can be bred to prepare high-quality approximate GKP states, showing that GKP error correction in optics is technically feasible in near-term experiments.

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arXiv2023

Saturating the Maximum Success Probability Bound for Noiseless Linear Amplification using Linear Optics

A noiseless linear amplifier (NLA) performs the highest quality amplification allowable under the rules of quantum physics. Unfortunately, these same rules conspire against us via the no-cloning theorem, which constrains NLA operations to the domain of probabilistic processes. Nevertheless, they are useful for a wide variety of quantum protocols, with numerous proposals assuming access to an optimal NLA device which performs with the maximum possible success probability. Here we propose the first linear optics NLA protocol which asymptotically achieves this success probability bound, by modifying the Knill-Laflamme-Milburn near-deterministic teleporter into an amplifier.

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arXiv2022

Noiseless Linear Amplification and Loss-Tolerant Quantum Relay using Coherent State Superpositions

Noiseless linear amplification (NLA) is useful for a wide variety of quantum protocols. Here we propose a fully scalable amplifier which, for asymptotically large sizes, can perform perfect fidelity NLA on any quantum state. Given finite resources however, it is designed to perform perfect fidelity NLA on coherent states and their arbitrary superpositions. Our scheme is a generalisation of the multi-photon quantum scissor teleamplifier, which we implement using a coherent state superposition resource state. Furthermore, we prove our NLA is also a loss-tolerant relay for multi-ary phase-shift keyed coherent states. Finally, we demonstrate that our NLA is also useful for continuous-variable entanglement distillation, even with realistic experimental imperfections.

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arXiv2022

Achieving the ultimate end-to-end rates of lossy quantum communication networks

The field of quantum communications promises the faithful distribution of quantum information, quantum entanglement, and absolutely secret keys, however, the highest rates of these tasks are fundamentally limited by the transmission distance between quantum repeaters. The ultimate end-to-end rates of quantum communication networks are known to be achievable by an optimal entanglement distillation protocol followed by teleportation. In this work, we give a practical design for this achievability. Our ultimate design is an iterative approach, where each purification step operates on shared entangled states and detects loss errors at the highest rates allowed by physics. As a simpler design, we show that the first round of iteration can purify completely at high rates. We propose an experimental implementation using linear optics and photon-number measurements which is robust to inefficient operations and measurements, showcasing its near-term potential for real-world practical applications.

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arXiv2022

Distributing entanglement in first generation discrete and continuous variable quantum repeaters

Quantum repeaters are used to overcome the exponential photon loss scaling that quantum states acquire as they are transmitted over long distances. While repeaters for discrete variable encodings of quantum information have existed for some time, novel approaches for continuous variable encoding quantum repeaters have only recently been proposed. In this work, we present a method of using a discrete variable repeater protocol to distribute continuous variable states and utilize it to compare the rates of continuous variable entanglement distribution between first generation continuous and discrete variable quantum repeaters. Such a comparison allows us to begin to benchmark the two quite different approaches.

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arXiv2022

Ideal Quantum Tele-amplification up to a Selected Energy Cut-off using Linear Optics

We introduce a linear optical technique that can implement ideal quantum tele-amplification up to the $n^\mathrm{th}$ Fock state, where $n$ can be any positive integer. Here tele-amplification consists of both quantum teleportation and noiseless linear amplification (NLA). This simple protocol consists of a beam-splitter and an $(n+1)$-splitter, with $n$ ancillary photons and detection of $n$ photons. For a given target fidelity, our technique improves success probability and physical resource costs by orders of magnitude over current alternative teleportation and NLA schemes. We show how this protocol can also be used as a loss-tolerant quantum relay for entanglement distribution and distillation.

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arXiv2021

Simple and loss-tolerant free-space QKD using a squeezed laser

We consider a continuous-variable (CV) quantum key distribution (QKD) protocol over free-space channels, which is simpler and more robust than typical CV QKD protocols. It uses a bright laser, squeezed and modulated in the amplitude quadrature, and self-homodyne detection. We consider a scenario, where the line of sight is classically monitored to detect active eavesdroppers, so that we can assume a passive eavesdropper. Under this assumption, we analyse security of the QKD protocol in the composable finite-size regime. Proper modulation of the squeezed laser to the shot-noise level can completely eliminate information leakage to the eavesdropper and also eliminate the turbulence-induced noise of the channel in the amplitude quadrature. Under these conditions, estimation of the eavesdropper's information is no longer required. The protocol is extremely robust to modulation noise and highly loss-tolerant, which makes it a promising candidate for satellite-based continuous-variable quantum communication.

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arXiv2021

Overcoming the repeaterless bound in continuous-variable quantum communication without quantum memories

One of the main problems in quantum communications is how to achieve high rates at long distances. Quantum repeaters, i.e., untrusted, intermediate relay stations, are necessary to overcome the repeaterless bound which sets the fundamental rate-distance limit of repeaterless communications. In this work, we introduce a continuous-variable protocol which overcomes the repeaterless bound and scales like the single-repeater bound using just one linear-optical device called a "quantum scissor", combining the entanglement distillation and entanglement swapping elements of previous repeater proposals into a single step, thus, removing the need for quantum memories. Implementing a standard continuous-variable quantum key distribution protocol using our repeater we predict key rates which surpass the repeaterless bound. Our protocol works well for non-ideal single-photon sources and non-ideal single-photon detectors, and can tolerate some level of excess noise, making our protocol implementable with existing technology. We show that our scheme can be extended to longer repeater chains using quantum memories, using less physical resources than previous schemes. Furthermore, for applications beyond key distribution, our scheme generalises to higher order and distils more entanglement at the cost of a reduced probability of success.

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arXiv2021

Minimization of information leakage in continuous-variable quantum key distribution

A communication protocol based on a Gaussian modulation of squeezed states in a single quadrature and measured via homodyne detection can completely eliminate information leakage to an eavesdropper in a pure-loss channel. However, the asymmetry of the protocol with respect to the quadratures of light presents security issues and the eavesdropper's information is not necessarily minimized for general asymmetric attacks. Here, we perform asymptotic security analysis of the asymmetric protocol against general asymmetric collective attacks and bound the eavesdropper's information via the Heisenberg uncertainty principle. The bound is not tight and therefore, we symmetrize the protocol in a heralding way, discarding the issues of asymmetry altogether. Our proposed heralding protocol asymptotically eliminates information leakage in a pure-loss channel and minimizes leakage in a noisy channel.

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arXiv2020

Generalized quantum scissors for noiseless linear amplification

We generalize the concept of optical state truncation and noiseless linear amplification to enable truncation of the Fock-state expansion of an optical state to higher order and to simultaneously amplify it using linear optics. The resulting generalized quantum scissors are more efficient for noiseless linear amplification than employing multiple scissors in parallel and are experimentally practical. As a particular example, we focus on a third-order scissor device and demonstrate advantages in terms of fidelity with the target state, probability of success, distillable entanglement, and the amount of non-Gaussianity introduced.

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arXiv2020

Quantum repeater for continuous variable entanglement distribution

Quantum repeaters have been proposed as a way of extending the reach of quantum communication. First generation approaches use entanglement swapping to connect entangled links along a long distance channel. Recently, there have been proposals for first generation quantum repeaters for continuous variables. In this paper, we present an improved continuous variable repeater scheme using optimal Gaussian entanglement swapping. Our scheme uses the noiseless linear amplifier for entanglement distillation. We show that with the simplest configuration of the noiseless linear amplifier and under the assumption of good quantum memories and perfect sources and detectors, our scheme beats the direct transmission upper limit for shorter distances and can offer advantages over previous CV repeater schemes.

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