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Leif K. Oxenløwe

Publications and source records attributed to Leif K. Oxenløwe.

At least 19 recordsLinked to original sources
arXiv2025

High-dimensional Path-Encoded Entanglement Distribution Between Photonic Chips Enabled by Multimode Phase Stabilisation

The reliable distribution of high-dimensional entangled quantum states, an important resource in quantum technologies, through optical fibre networks is challenging due to the need to maintain coherence across multiple modes. Here we demonstrate the distribution of four-dimensional path-encoded entangled quantum states between photonic chips, enabled by a novel multimode phase stabilisation algorithm. The algorithm utilises the reconfigurability of the integrated photonic circuits to complete one iteration of phase stabilisation in just two measurement rounds for an arbitrary number of modes, and requires no additional hardware to the quantum measurements it enables. As a result, we are able to perform complete quantum state tomography across two chips using the minimum number of local projective measurements to verify the fidelity of the distributed entangled state to be 86% (compared to 8.1% without the phase stabilisation) with an entanglement entropy of 0.995+/-0.002.

quant-ph↗
arXiv2023

Quantum Key Distribution using Deterministic Single-Photon Sources over a Field-Installed Fibre Link

Quantum-dot-based single-photon sources are key assets for quantum information technology, supplying on-demand scalable quantum resources for computing and communication. However, longlasting issues such as limited long-term stability and source brightness have traditionally impeded their adoption in real-world applications. Here, we realize a quantum key distribution field trial using true single photons across an 18-km-long dark fibre, located in the Copenhagen metropolitan area, using an optimized, state-of-the-art, quantum-dot single-photon source frequency-converted to the telecom wavelength. A secret key generation rate of >2 kbits/s realized over a 9.6 dB channel loss is achieved with a polarization-encoded BB84 scheme, showing remarkable stability for more than 24 hours of continuous operation. Our results highlight the maturity of deterministic single-photon source technology while paving the way for advanced single-photon-based communication protocols, including fully device-independent quantum key distribution, towards the goal of a quantum internet.

quant-ph↗
arXiv2023

Quantum Key Distribution over 100 km underwater optical fiber assisted by a Fast-Gated Single-Photon Detector

Nowadays Quantum Key Distribution represents the most mature quantum technology, and multiple countries as well as private institutions are building their quantum network. However, QKD devices are still far from representing a product within everyone's reach. Indeed, limitations in terms of compatibility with existing telecom infrastructure and limited performances in terms of secret key rate, using non-cryogenic detection systems, are still critical. In this work, we implemented a quantum key distribution link between Sicily (Italy) and Malta utilizing two different Single-Photon Avalanche Diode (SPAD) detectors. The performances of a standard commercial SPAD have been compared with the results achieved with a new prototype of fast-gated System in a Package (SiP) SPAD; the SiP detector has shown to be able to accomplish a fourteen times higher key rate compared with the commercial device over the channel showing 20 dB of losses.

quant-ph↗
arXiv2022

Advances in silicon quantum photonics

Quantum technology is poised to enable a step change in human capability for computing, communications and sensing. Photons are indispensable as carriers of quantum information - they travel at the fastest possible speed and readily protected from decoherence. However, the system requires thousands of near-transparent components with ultra-low-latency control. For quantum technology to be implemented, a new paradigm photonic system is required: one with in-built coherence, stability, the ability to define arbitrary circuits, and a path to manufacturability. Silicon photonics has unparalleled density and component performance, which, with CMOS compatible fabrication, place it in a strong position for a scalable quantum photonics platform. This paper is a progress report on silicon quantum photonics, focused on developments in the past five years. We provide an introduction on silicon quantum photonic component and the challenges in the field, summarise the current state-of-the-art and identify outstanding technical challenges, as well as promising avenues of future research. We also resolve a conflict in the definition of Hong-Ou-Mandel interference visibility in integrated quantum photonic experiments, needed for fair comparison of photon quality across different platforms. Our aim is the development of scalability on the platform, to which end we point the way to ever-closer integration, toward silicon quantum photonic systems-on-a-chip.

quant-ph↗
arXiv2022

Deploying an inter-European quantum network

Around forty years have passed since the first pioneering works have introduced the possibility of using quantum physics to strongly enhance communications safety. Nowadays Quantum Cryptography, and in particular, Quantum Key Distribution (QKD) exited the physics laboratories to become commercial technologies that increasingly trigger the attention of States, military forces, banks, and private corporations. This work takes on the challenge of bringing QKD closer to a consumer technology: optical fibers deployed and used by telecommunication companies of different States have been used to realize a quantum network, the first-ever connecting three different countries. This pushes towards the necessary coexistence of QKD and classical communications on the same infrastructure, which currently represents a main limit of this technology. Our network connects Trieste to Rijeka and Ljubljana via a trusted node in Postojna; a key rate of over 3 kbps has been achieved in the shortest link, and a 7-hour long measurement has demonstrated the system stability and reliability. Finally, the network has been used for a public demonstration of QKD at the G20 Digital Ministers' Meeting in Trieste. The reported experimental results, together with the significant interest that one of the most important events of international politics has attracted, showcase the maturity of the QKD technology bundle, placing it in the spotlight for consumer applications in the near term.

quant-ph↗
arXiv2021

Quantum randomness generation via orbital angular momentum modes crosstalk in a ring-core fiber

Genuine random numbers can be produced beyond a shadow of doubt through the intrinsic randomness provided by quantum mechanics theory. While many degrees of freedom have been investigated for randomness generation, not adequate attention has been paid to the orbital angular momentum of light. In this work, we present a quantum random number generator based on the intrinsic randomness inherited from the superposition of orbital angular momentum modes caused by the crosstalk inside a ring-core fiber. We studied two possible cases: a first one, device-dependent, where the system is trusted, and a second one, semi-device-independent, where the adversary can control the measurements. We experimentally realized the former, extracted randomness, and, after privacy amplification, we achieved a generation rate higher than 10 Mbit/s. In addition, we presented a possible realization of the semi-device-independent protocol, using a newly introduced integrated silicon photonic chip. Our work can be considered as a starting point for novel investigations of quantum random number generators based on the orbital angular momentum of light.

quant-ph↗
arXiv2021

Photonic integrated chip enabling orbital angular momentum multiplexing for quantum communication

Light carrying orbital angular momentum constitutes an important resource for both classical and quantum information technologies. Its inherently unbounded nature can be exploited to generate high-dimensional quantum states or for channel multiplexing in classical and quantum communication in order to significantly boost the data capacity and the secret key rate, respectively. While the big potentials of light owning orbital angular momentum have been widely ascertained, its technological deployment is still limited by the difficulties deriving from the fabrication of integrated and scalable photonic devices able to generate and manipulate it. Here, we present a photonic integrated chip able to excite orbital angular momentum modes in an 800 m long ring-core fiber, allowing us to perform parallel quantum key distribution using 2 and 3 different modes simultaneously. The experiment sets the first steps towards quantum orbital angular momentum division multiplexing enabled by a compact and light-weight silicon chip, and further pushes the development of integrated scalable devices supporting orbital angular momentum modes.

quant-ph↗
arXiv2021

Characterization and stability measurement of deployed multicore fibers for quantum applications

Multicore fibers are expected to be a game-changer in the coming decades thanks to their intrinsic properties, allowing a larger transmission bandwidth and a lower footprint in optical communications. In addition, multicore fibers have recently been explored for quantum communication, attesting their uniqueness in transporting high-dimensional quantum states. However, investigations and experiments reported in literature have been carried out in research laboratories, typically making use of short fiber links in controlled environments. Thus, the possibility of using long distance multicore fibers for quantum applications is still to be proven. We here characterize for the first time, in terms of phase stability, multiple strands of a 4-core multicore fiber installed underground in the city of L'Aquila, with an overall fiber length up to about 25 km. In this preliminary study, we investigate the possibility of using such an infrastructure to implement quantum-enhanced schemes, such as high-dimensional quantum key distribution, quantum-based environmental sensors, and more in general quantum communication protocols.

quant-ph↗
arXiv2021

Path-encoded high-dimensional quantum communication over a 2 km multicore fiber

Quantum key distribution (QKD) protocols based on high-dimensional quantum states have shown the route to increase the key rate generation while benefiting of enhanced error tolerance, thus overcoming the limitations of two-dimensional QKD protocols. Nonetheless, the reliable transmission through fiber links of high-dimensional quantum states remains an open challenge that must be addressed to boost their application. Here, we demonstrate the reliable transmission over a 2 km long multicore fiber of path-encoded high-dimensional quantum states. Leveraging on a phase-locked loop system, a stable interferometric detection is guaranteed, allowing for low error rates and the generation of 6.3 Mbit/s of secret key rate.

quant-ph↗
arXiv2021

All-Optical Nonlinear Pre-Compensation of Long-Reach Unrepeatered Systems

We numerically demonstrate an all-optical nonlinearity pre-compensation module for state-of-the-art long-reach Raman-amplified unrepeatered links. The compensator design is optimized in terms of propagation symmetry to maximize the performance gains under WDM transmission, achieving 4.0dB and 2.6dB of SNR improvement for 250-km and 350-km links.

physics.app-ph↗
arXiv2020

Error protected qubits in a silicon photonic chip

General purpose quantum computers can, in principle, entangle a number of noisy physical qubits to realise composite qubits protected against errors. Architectures for measurement-based quantum computing intrinsically support error-protected qubits and are the most viable approach for constructing an all-photonic quantum computer. Here we propose and demonstrate an integrated silicon photonic architecture that both entangles multiple photons, and encodes multiple physical qubits on individual photons, to produce error-protected qubits. We realise reconfigurable graph states to compare several schemes with and without error-correction encodings and implement a range of quantum information processing tasks. We observe a success rate increase from 62.5% to 95.8% when running a phase estimation algorithm without and with error protection, respectively. Finally, we realise hypergraph states, which are a generalised class of resource states that offer protection against correlated errors. Our results show how quantum error-correction encodings can be implemented with resource-efficient photonic architectures to improve the performance of quantum algorithms.

quant-ph↗
arXiv2019

Boosting the secret key rate in a shared quantum and classical fibre communication system

During the last 20 years, the advance of communication technologies has generated multiple exciting applications. However, classical cryptography, commonly adopted to secure current communication systems, can be jeopardized by the advent of quantum computers. Quantum key distribution (QKD) is a promising technology aiming to solve such a security problem. Unfortunately, current implementations of QKD systems show relatively low key rates, demand low channel noise and use ad hoc devices. In this work, we picture how to overcome the rate limitation by using a 37-core fibre to generate 2.86 Mbit/s per core that can be space multiplexed into the highest secret key rate of 105.7 Mbit/s to date. We also demonstrate, with off-the-shelf equipment, the robustness of the system by co-propagating a classical signal at 370 Gbit/s, paving the way for a shared quantum and classical communication network.

quant-ph↗
arXiv2019

Field trial of a finite-key quantum key distribution system in the Florence metropolitan area

In-field demonstrations in real-world scenarios boost the development of a rising technology towards its integration in existing infrastructures. Although quantum key distribution (QKD) devices are already adopted outside the laboratories, current field implementations still suffer from high costs and low performances, preventing this emerging technology from a large-scale deployment in telecommunication networks. Here we present a simple, practical and efficient QKD scheme with finite-key analysis, performed over a 21 dB-losses fiber link installed in the metropolitan area of Florence (Italy). Coexistence of quantum and weak classical communication is also demonstrated by transmitting an optical synchronization signal through the same fiber link.

quant-ph↗
arXiv2019

Air-core fiber distribution of hybrid vector vortex-polarization entangled states

Entanglement distribution between distant parties is one of the most important and challenging tasks in quantum communication. Distribution of photonic entangled states using optical fiber links is a fundamental building block towards quantum networks. Among the different degrees of freedom, orbital angular momentum (OAM) is one of the most promising due to its natural capability to encode high dimensional quantum states. In this article, we experimentally demonstrate fiber distribution of hybrid polarization-vector vortex entangled photon pairs. To this end, we exploit a recently developed air-core fiber which supports OAM modes. High fidelity distribution of the entangled states is demonstrated by performing quantum state tomography in the polarization-OAM Hilbert space after fiber propagation, and by violations of Bell inequalities and multipartite entanglement tests. The present results open new scenarios for quantum applications where correlated complex states can be transmitted by exploiting the vectorial nature of light.

quant-ph↗
arXiv2018

Generation and sampling of quantum states of light in a silicon chip

Implementing large instances of quantum algorithms requires the processing of many quantum information carriers in a hardware platform that supports the integration of different components. While established semiconductor fabrication processes can integrate many photonic components, the generation and algorithmic processing of many photons has been a bottleneck in integrated photonics. Here we report the on-chip generation and processing of quantum states of light with up to eight photons in quantum sampling algorithms. Switching between different optical pumping regimes, we implement the Scattershot, Gaussian and standard boson sampling protocols in the same silicon chip, which integrates linear and nonlinear photonic circuitry. We use these results to benchmark a quantum algorithm for calculating molecular vibronic spectra. Our techniques can be readily scaled for the on-chip implementation of specialised quantum algorithms with tens of photons, pointing the way to efficiency advantages over conventional computers.

quant-ph↗
arXiv2018

Perturbation-based FEC-assisted Iterative Nonlinearity Compensation for WDM Systems

A perturbation-based nonlinear compensation scheme assisted by a feedback from the forward error correction (FEC) decoder is numerically and experimentally investigated. It is shown by numerical simulations and transmission experiments that a feedback from the FEC decoder enables improved compensation performance, allowing the receiver to operate very close to the full data-aided performance bounds. The experimental analysis considers the dispersion uncompensated transmission of a 5 x 32 GBd WDM system with DP-16QAM and DP-64QAM after 4200 km and 1120 km, respectively. The experimental results show that the proposed scheme outperforms single-channel digital backpropagation. A perturbation-based nonlinear compensation scheme assisted by a feedback from the forward error correction (FEC) decoder is numerically and experimentally investigated. It is shown by numerical simulations and transmission experiments that a feedback from the FEC decoder enables improved compensation performance, allowing the receiver to operate very close to the full data-aided performance bounds. The experimental analysis considers the dispersion uncompensated transmission of a 5 x 32 GBd WDM system with DP-16QAM and DP-64QAM after 4200 km and 1120 km, respectively. The experimental results show that the proposed scheme outperforms single-channel digital backpropagation.

eess.SP↗
arXiv2018

Optical wavelength conversion of high bandwidth phase-encoded signals in a high FOM 50cm CMOS compatible waveguide

We demonstrate wavelength conversion of QAM signals including 32GBd QPSK and 10GBd 16QAM in a 50cm long high index doped glass spiral waveguide. The quality of the generated idlers over a 10nm bandwidth is sufficient to achieve a BER performance below the HD FEC threshold (less than 3.8 x 10-3), with an OSNR penalty of less than 0.3 dB compared to the original signal. Our results confirm that this is a promising platform for nonlinear optical signal processing, a result of both very low linear propagation loss (less than 0.07 dB/cm) and the large material bandgap that ensures negligible nonlinear loss at telecom wavelengths.

physics.app-ph↗