This manuscript presents the numerical study, manufacture processes, and experimental characterization of an integrated optical circuit intended to be at the heart of a quantum relay particularly designed for qubits encoded on photos as telecom wavelengths. In spite of progress in photonics and telecommunications technologies, losses in optical fibres and especially darkcounts in the detectors limit the maximum achievable distance for quantum communication to about a hundred kms. The quantum relay, based on quantum teleportation, makes it possible to increase the communication signal-to-noise ratio and thus to push this maximum distance forward. In this context, we show that integrated optics offers compact and stable components suitable for insertion of a “relay-ship” in fibre-based quantum communication systems. We adapted a quantum relay model to our particular case showing the relevance of such a structure with realistic parameters. Subsequently we determined the properties of all the necessary optical functions using BPM simulation. Actually, a photon-pair source and two electro-optically controlled, 50 / 50 beam splitters have to be merged on a single chip. In addition, tapered-waveguide sections have been designed at each input / output of the chip in order to improve coupling between fibres and integrated structures. All the required optical functions on the chip were tested and validated by performing characterizations in both classical and single-photon regimes. As a conclusion, we showed that this “quantum relay-chip” could increase the maximal achievable distance of a quantum communication link by a factor of 1,7.
This manuscript presents the numerical study, manufacture processes, and experimental characterization of an integrated optical circuit intended to be at the heart of a quantum relay particularly designed for qubits encoded on photos as telecom wavelengths. In spite of progress in photonics and telecommunications technologies, losses in optical fibres and especially darkcounts in the detectors limit the maximum achievable distance for quantum communication to about a hundred kms. The quantum relay, based on quantum teleportation, makes it possible to increase the communication signal-to-noise ratio and thus to push this maximum distance forward. In this context, we show that integrated optics offers compact and stable components suitable for insertion of a “relay-ship” in fibre-based quantum communication systems. We adapted a quantum relay model to our particular case showing the relevance of such a structure with realistic parameters. Subsequently we determined the properties of all the necessary optical functions using BPM simulation. Actually, a photon-pair source and two electro-optically controlled, 50 / 50 beam splitters have to be merged on a single chip. In addition, tapered-waveguide sections have been designed at each input / output of the chip in order to improve coupling between fibres and integrated structures. All the required optical functions on the chip were tested and validated by performing characterizations in both classical and single-photon regimes. As a conclusion, we showed that this “quantum relay-chip” could increase the maximal achievable distance of a quantum communication link by a factor of 1,7.
We investigate an integrated optical circuit on lithium niobate designed to implement a teleportation-based quantum relay scheme for one-way quantum communication at a telecom wavelength. Such an advanced quantum circuit merges for the first time both optical–optical and electro-optical nonlinear functions necessary for implementing the desired on-chip single-qubit teleportation. On the one hand, spontaneous parametric down-conversion is used to produce entangled photon pairs. On the other, we take advantage of two photon routers, consisting of electro-optically controllable couplers, to separate the paired photons and to carry out a Bell state measurement, respectively. After having validated all the individual functions in the classical regime, we performed a Hong–Ou–Mandel experiment to mimic a one-way quantum communication link. Such a quantum effect, seen as a prerequisite towards achieving teleportation, has been obtained at one of the routers when the chip was coupled to an external single-photon source. The two-photon interference pattern shows a net visibility of 80%, which validates the proof of principle of a 'quantum relay circuit' for qubits carried by telecom photons. In the case of optimized losses, such a chip could increase the maximal achievable distance of one-way quantum key distribution links by a factor of 1.8. Our approach and results emphasize the potential of integrated optics on lithium niobate as a key technology for future reconfigurable quantum information manipulation.
Quantum teleportation is a crucial function in quantum networks. The implementation of photonic quantum teleportation could be highly simplified by quantum photonic circuits. To extend chip-to-chip teleportation distance, more effort is needed on both chip design and system implementation. In this work, we demonstrate a time-bin-based chip-to-chip photonic quantum teleportation over optical fibers under the scenario of a star-topology quantum network. Three quantum photonic circuits are designed and fabricated on a single chip, each serving specific functions: heralded single-photon generation at the user node, entangled photon pair generation and BSM at the relay node, and projective measurement of the teleported photons at the central node. The unbalanced Mach-Zehnder interferometers (UMZI) for time-bin encoding in these quantum photonic circuits are optimized to reduce insertion losses and suppress noise photons generated on the chip. Besides, an active feedback system is employed to suppress the impact of fiber length fluctuation between the circuits, achieving a stable quantum interference for the BSM in the relay node. As a result, a photonic quantum teleportation over optical fibers of 12.3 km is achieved based on these quantum photonic circuits, showing the potential of chip integration for the development of quantum networks.
Periodically poled lithium niobate waveguides (PPLN/W) are nowadays considered to be one of the most useful toolboxes for quantum communication experiments. Thanks to the high optical confinement over longer lengths than in bulk configurations (a few cm in our case), such structures provide highly efficient non-linear interactions, both in down- and up-conversion regimes. Within the framework of long-distance quantum communication at telecom wavelength, PPLN/Ws have therefore been proved to be a key ingredient for building ultra-bright sources of energy-time, time-bin, as well as polarization entangled pairs of photons, up-conversion based detectors for telecom photon detection and photonic quantum interfaces for coherent wavelength conversion, as well as quantum relays in the entanglement swapping configuration. During the presentation, we will discuss some recent experimental advances regarding two quantum relay scenarios employed to extend the maximum achievable distance of quantum communication. Based on picosecond regime operation, the first relies on the entanglement swapping configuration, and the other on single qubit teleportation using a telecom oriented chip dedicated to one-way quantum communication. In the first realization, we demonstrate that the utilization of PPLN/Ws and standard components available from the telecommunications industry, associated with the picosecond operation regime enables reaching near-perfect two-photon interference (or coalescence) while maintaining high effective photon pair rates. Using pairs of photons emitted around 1550 nm and appropriate filtering stages, we obtain the highest coalescence net visibility, i.e., of 99%, ever reported. As coalescence is seen a major step towards entanglement swapping, we believe that our picosecond guided-wave scheme provides an efficient trade-off between CW and femtosecond regimes, and emphasize why it should lead to realistic quantum relay and repeater schemes, namely by offering an reduced-constraint solution to the synchronization issue when two completely independent lasers are employed. We shall focus on our experimental demonstration and detail a comparative study of performance with similar reported experiments. As a second realization, we will give further directions towards the integration of several functions on a single optical chip, such as a photon pair generator and electro-optically controllable couplers. The idea is to realize a telecom-like quantum relay chip on lithium niobate through which teleportation of single qubits can be done so as to provide the necessary relay function in a compact, stable, efficient, and user-friendly fashion. We will discuss preliminary results obtained with this device for fully fiber-based one-way quantum communication systems. We will conclude by emphasizing the large potential of what we call "guided-wave quantum communication" developed in our laboratory.
Quantum cryptography is on the verge of commercial application. One of its greatest limitations is over long distance—secret key rates are low and the longest fibre over which any key has been exchanged is currently 100km. We investigate the quantum relay, which can increase the maximum distance at which quantum cryptography is possible. The relay splits the channel into sections and sends a different photon across each section, increasing the signal-to-noise ratio. The photons are linked as in teleportation, with entangled photon pairs and Bell measurements. We show that such a scheme could allow cryptography over hundreds of kilometres with today's detectors. It could not, however, improve the rate of key exchange over distances where the standard single section scheme already works. We also show that reverse key reconciliation, previously used in continuous variable quantum cryptography, gives a secure key over longer distances than forward key reconciliation.
Quantum networking enables the distribution of quantum information across distant nodes, unlocking capabilities such as secure communication, distributed quantum computing, enhanced metrology, and tests of fundamental physics. Photons serve as the natural carriers of quantum information over long distances due to their low loss in optical fibers and robustness to environmental decoherence. However, the practical realization of quantum networks remains limited by the lack of photon sources that are simultaneously compatible with diverse quantum systems, stable over long timescales, and suitable for deployment in real-world fiber infrastructure. This dissertation addresses these challenges through the co-development of photon-pair sources and a field-deployed quantum networking testbed. We present two complementary chip-scale sources of correlated photon pairs tailored for distinct bottlenecks in quantum networking. The first is a hybrid visible–telecom photon-pair source designed to address a fundamental spectral mismatch between quantum nodes and communication channels. While most quantum memories and processors operate at visible to near- nfrared wavelengths, optical fibers exhibit minimal loss in the telecommunications band. By integrating periodically poled nonlinear waveguides with photonic integrated circuits, we demonstrate a platform capable of directly generating visible–telecom photon pairs, enabling efficient interfacing between heterogeneous quantum systems and existing fiber networks. The second source is a fully packaged, broadband telecom photon-pair source based on silicon waveguide spirals, designed for stable, long-duration operation. This stability enables extended measurements necessary to overcome channel losses and environmental drifts in deployed fiber networks. These sources are evaluated within RoQNET (Rochester Quantum Network), a ∼20 km fiber link between the Rochester Institute of Technology and the University of Rochester. Using this platform, we demonstrate the transmission and on-chip modulation of heralded telecom single photons, verify their nonclassical statistics, and characterize long-term channel behavior. Additionally, we explore programmable time-bin entanglement distribution using spectral waveshapers, providing a flexible approach to quantum communication. Together, this work establishes a co-designed source–network framework for scalable, heterogeneous quantum networking.