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Memory-assisted quantum key distribution resilient against multiple-excitation effects
摘要： Memory-assisted measurement-device-independent quantum key distribution (MA-MDI-QKD) has recently been proposed as a technique to improve the rate-versus-distance behavior of QKD systems by using existing, or nearly-achievable, quantum technologies. The promise is that MA-MDI-QKD would require less demanding quantum memories than the ones needed for probabilistic quantum repeaters. Nevertheless, early investigations suggest that, in order to beat the conventional memory-less QKD schemes, the quantum memories used in the MA-MDI-QKD protocols must have high bandwidth-storage products and short interaction times. Among different types of quantum memories, ensemble-based memories offer some of the required specifications, but they typically suffer from multiple excitation effects. To avoid the latter issue, in this paper, we propose two new variants of MA-MDI-QKD both relying on single-photon sources for entangling purposes. One is based on known techniques for entanglement distribution in quantum repeaters. This scheme turns out to offer no advantage even if one uses ideal single-photon sources. By finding the root cause of the problem, we then propose another setup, which can outperform single memory-less setups even if we allow for some imperfections in our single-photon sources. For such a scheme, we compare the key rate for different types of ensemble-based memories and show that certain classes of atomic ensembles can improve the rate-versus-distance behavior.
关键词： quantum cryptography,quantum networks,quantum key distribution (QKD),quantum memory,single-photon source,quantum communications
更新于2025-09-23 15:22:29
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[Springer Theses] Quantum Confined Excitons in 2-Dimensional Materials || Introduction: 2d-Based Quantum Technologies
摘要： Framed within the growing giant of quantum technologies, with billions in expenditure world-wide and rapidly growing, we harness the exciting physics and technological promise of 2-dimensional materials to create atomically-thin quantum devices capable of emitting single photons and capturing single spins. In this thesis we present the alliance of 2d and quantum information technology - the first steps towards hybrid light-matter quantum networks set in a low power, scalable on-chip platform.
关键词： quantum technologies,single spins,single photons,quantum networks,2-dimensional materials
更新于2025-09-23 15:21:21
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Versatile relative entropy bounds for quantum networks
摘要： We provide a versatile upper bound on the number of maximally entangled qubits, or private bits, shared by two parties via a generic adaptive communication protocol over a quantum network when the use of classical communication is not restricted. Although our result follows the idea of Azuma et al (2016 Nat. Commun. 7 13523) of splitting the network into two parts, our approach relaxes their strong restriction, consisting of the use of a single entanglement measure in the quanti?cation of the maximum amount of entanglement generated by the channels. In particular, in our bound the measure can be chosen on a channel-by-channel basis, in order to make it as tight as possible. This enables us to apply the relative entropy of entanglement, which often gives a state-of-the-art upper bound, on every Choi-simulable channel in the network, even when the other channels do not satisfy this property. We also develop tools to compute, or bound, the max-relative entropy of entanglement for channels that are invariant under phase rotations. In particular, we present an analytical formula for the max-relative entropy of entanglement of the qubit amplitude damping channel.
关键词： quantum networks,quantum communication,quantum information
更新于2025-09-23 15:21:01
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Quantum teleportation using highly coherent emission from telecom C-band quantum dots
摘要： A practical way to link separate nodes in quantum networks is to send photons over the standard telecom fibre network. This requires sub-Poissonian photon sources in the wavelength band around 1550 nm, with photon coherence times sufficient to enable the many interference-based technologies at the heart of quantum networks. Here, we show that droplet epitaxy InAs/InP quantum dots emitting in the telecom C-band can provide photons with coherence times exceeding 1 ns under low power non-resonant excitation, and demonstrate that these coherence times enable near-optimal interference with a C-band polarisation-encoded laser qubit, with visibilities only limited by the quantum dot multiphoton emission. Using entangled photons, we further show teleportation of such qubits in six different bases with average postselected fidelity reaching 88.3 ± 4.0%. Beyond direct applications in long-distance quantum communication, the high degree of coherence in these quantum dots is promising for future spin-based telecom quantum network applications.
关键词： Quantum dots,Telecom C-band,Quantum networks,Quantum teleportation
更新于2025-09-19 17:13:59
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Entanglement Swapping with Photons Generated on Demand by a Quantum Dot
摘要： Photonic entanglement swapping, the procedure of entangling photons without any direct interaction, is a fundamental test of quantum mechanics and an essential resource to the realization of quantum networks. Probabilistic sources of nonclassical light were used for seminal demonstration of entanglement swapping, but applications in quantum technologies demand push-button operation requiring single quantum emitters. This, however, turned out to be an extraordinary challenge due to the stringent prerequisites on the efficiency and purity of the generation of entangled states. Here we show a proof-of-concept demonstration of all-photonic entanglement swapping with pairs of polarization-entangled photons generated on demand by a GaAs quantum dot without spectral and temporal filtering. Moreover, we develop a theoretical model that quantitatively reproduces the experimental data and provides insights on the critical figures of merit for the performance of the swapping operation. Our theoretical analysis also indicates how to improve state-of-the-art entangled-photon sources to meet the requirements needed for implementation of quantum dots in long-distance quantum communication protocols.
关键词： quantum communication,quantum dot,entanglement swapping,quantum networks
更新于2025-09-19 17:13:59
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Efficient fiber in-line single photon source based on colloidal single quantum dots on an optical nanofiber
摘要： We demonstrate a fiber in-line single photon source based on a hybrid system of colloidal single quantum dots deposited on an optical nanofiber and cooled down to cryogenic temperature (3.7 K). We show that a charged state (trion) of the single quantum dot exhibits a photo-stable emission of single photons with high quantum efficiency, narrow linewidth (3 meV FWHM) and fast decay time ( 10.0 ± 0.5 ns). The single photons are efficiently coupled to the guided modes of the nanofiber and eventually to a single mode optical fiber. The brightness (efficiency) of the single photon source is estimated to be 16 ± 2% with a maximum photon count rate of 1.6 ± 0.2 MHz and a high single photon purity ( g2(0) = 0.11 ± 0.02 ). The device can be easily integrated to the fiber networks paving the way for potential applications in quantum networks.
关键词： quantum dots,single photon source,quantum networks,optical nanofiber,cryogenic temperature
更新于2025-09-19 17:13:59
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Strong Coupling of Two Individually Controlled Atoms via a Nanophotonic Cavity
摘要： We demonstrate photon-mediated interactions between two individually trapped atoms coupled to a nanophotonic cavity. Specifically, we observe collective enhancement when the atoms are resonant with the cavity and level repulsion when the cavity is coupled to the atoms in the dispersive regime. Our approach makes use of individual control over the internal states of the atoms and their position with respect to the cavity mode, as well as the light shifts to tune atomic transitions individually, allowing us to directly observe the anticrossing of the bright and dark two-atom states. These observations open the door for realizing quantum networks and studying quantum many-body physics based on atom arrays coupled to nanophotonic devices.
关键词： collective enhancement,quantum networks,photon-mediated interactions,level repulsion,quantum many-body physics,nanophotonic cavity
更新于2025-09-19 17:13:59
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Slow and fast single photons from a quantum dot interacting with the excited state hyperfine structure of the Cesium D1-line
摘要： Hybrid interfaces between distinct quantum systems play a major role in the implementation of quantum networks. Quantum states have to be stored in memories to synchronize the photon arrival times for entanglement swapping by projective measurements in quantum repeaters or for entanglement purification. Here, we analyze the distortion of a single-photon wave packet propagating through a dispersive and absorptive medium with high spectral resolution. Single photons are generated from a single in(Ga)As quantum dot with its excitonic transition precisely set relative to the cesium D1 transition. the delay of spectral components of the single-photon wave packet with almost Fourier-limited width is investigated in detail with a 200 MHz narrow-band monolithic Fabry-Pérot resonator. Reflecting the excited state hyperfine structure of Cesium, “slow light” and “fast light” behavior is observed. As a step towards room-temperature alkali vapor memories, quantum dot photons are delayed for 5 ns by strong dispersion between the two 1.17 GHz hyperfine-split excited state transitions. Based on optical pumping on the hyperfine-split ground states, we propose a simple, all-optically controllable delay for synchronization of heralded narrow-band photons in a quantum network.
关键词： slow light,optical pumping,single-photon wave packet,quantum networks,quantum dot,cesium D1 transition,fast light,Fabry-Pérot resonator
更新于2025-09-16 10:30:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Quantum Storage of Frequency-Multiplexed Heralded Single Photons
摘要： Quantum memories for light are important devices in quantum information science, in particular for applications such as quantum networks and quantum repeaters [1]. It has been predicted that multimode quantum memories able to store independently multiple modes would greatly help the scaling of quantum networks by decreasing the entanglement generation time between remote quantum nodes [1]. Current research focuses mostly on time multiplexing in rare-earth doped crystals and in spatial multiplexing in atomic gases. Beyond these demonstrations, rare-earth doped crystals, thanks to their large inhomogeneous broadening, represent a unique quantum system which could also add another degree of freedom for multiplexing, the storage of multiple frequency modes [2]. In this contribution, we report on the first demonstration of quantum storage of a frequency multiplexed single photon into a laser-written waveguide integrated in a Pr3+:Y2SiO5 crystal. The great advantage of using a confined waveguide for this task is that the power required to prepare the quantum memory is strongly reduced due to the increased light-matter interaction in the waveguide. This enables simultaneous preparation of several memories at different frequencies, with a moderate laser power.
关键词： quantum repeaters,Quantum memory,single photons,quantum networks,frequency multiplexing
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Fast Photon Storage in a Fiber-Based Atom-Cavity System
摘要： Miniaturized ?ber-based optical resonators are emerging as a robust and scalable technology for realizing ?ber-based quantum networks. First demonstrated in [1], these cavities have been integrated with a variety of emitters, e.g. atoms, ions and solid-state systems, to build single-photon quantum interfaces. Our system consists of a single 87Rb atom coupled to a high bandwidth (κ = 2π×40 MHz (cid:2) γ), single-sided ?ber Fabry-Perot cavity (FFPC), Fig. 1(a). The unique system of four in-vacuum lenses, with their focal points coinciding with the ?ber-cavity center, provides the necessary optical access for 3D-cooling and trapping, addressing and high resolution ?uorescence imaging of the atoms, Fig. 1(b). For the D2-transition of 87Rb, the strongly coupled atom-cavity system shows a sixfold Purcell broadening and 90% emission into the cavity mode [2]. Here we present deterministic generation and storage of fast photon pulses. For single-photon generation the atomic state is prepared in F = 1 ground state. A π?polarized classical control laser pulse creates a single photon in the cavity mode with an ef?ciency of ～80%, Fig. 1(c). The pulse shape of the emitted single photon can be tailored by the temporal shape of the control pulse on a time scale lower than the atomic excited state lifetime of ～26 ns, Fig. 1(d). For the storage of fast (short) coherent light pulses, the atomic system is prepared in the |F = 2, m f = ?2(cid:4) state by optical pumping. The temporally shaped input coherent laser pulse at the single-photon level incidents on the cavity mirror while a π?polarized control classical laser pulse with an optimized temporal pro?le, dresses the atom-cavity system such that the storage ef?ciency of the input pulse is maximised. We store photon pulses with a temporal width down to 10 ns with an overall ef?ciency of ～5%.
关键词： 87Rb atom,single-photon quantum interfaces,photon storage,fiber Fabry-Perot cavity,fiber-based optical resonators,quantum networks
更新于2025-09-11 14:15:04