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arXiv:2407.07991 · npj Quantum Information

Entanglement of photonic modes from a continuously driven two-level system

Jiaying Yang, Ingrid Strandberg, Alejandro Vivas-Viaña, Akshay Gaikwad, Claudia Castillo-Moreno, Anton Frisk Kockum, Muhammad Asad Ullah, Carlos Sánchez Muñoz, Axel Martin Eriksson, Simone Gasparinetti

Published Apr 28, 2025• 5 citations
arXivHTMLPDF

Abstract

The ability to generate entangled states of light is a key primitive for quantum communication and distributed quantum computation. Continuously driven sources, including those based on spontaneous parametric downconversion, are usually probabilistic, whereas deterministic sources require accurate timing of the control fields. Here, we experimentally generate entangled photonic modes by continuously exciting a quantum emitter - a superconducting qubit - with a coherent drive, taking advantage of mode matching in the time and frequency domain. Using joint quantum state tomography and logarithmic negativity, we show that entanglement is generated between modes extracted from the two sidebands of the resonance fluorescence spectrum. Because the entangled photonic modes are perfectly orthogonal, they can be transferred into distinct quantum memories. Our approach can be utilized to distribute entanglement at a high rate in various physical platforms, with applications in waveguide quantum electrodynamics, distributed quantum computing, and quantum networks.

DOI
10.1038/s41534-025-00995-1

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The ability to generate entangled states of light is a key primitive for quantum communication and distributed quantum computation. Continuously driven sources, including those based on spontaneous parametric downconversion, are usually probabilistic, whereas deterministic sources require accurate timing of the control fields. Here, we experimentally generate entangled photonic modes by continuously exciting a quantum emitter - a superconducting qubit - with a coherent drive, taking advantage of mode matching in the time and frequency domain. Using joint quantum state tomography and logarithmic negativity, we show that entanglement is generated between modes extracted from the two sidebands of the resonance fluorescence spectrum. Because the entangled photonic modes are perfectly orthogonal, they can be transferred into distinct quantum memories. Our approach can be utilized to distribute entanglement at a high rate in various physical platforms, with applications in waveguide quantum electrodynamics, distributed quantum computing, and quantum networks.
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We propose a scheme for optical high-order sideband generation and efficient sideband information transfer from one optical mode to the other in a hybrid system consisting of a quantum dot coupled to both modes of a lossy bimodal photonic crystal cavity. Here one mode of the cavity is coherently driven by a two-tone continuous-wave laser field and the two cavity modes are not coupled to each other due to their orthogonal polarizations. The influences of the system parameters including the cavity-waveguide coupling rate and all kinds of relative detunings on optical high-order sideband generation and transfer efficiency are discussed. In addition to numerical simulations demonstrating this effect, a physical explanation of the underlying mechanism and an experimental feasibility of the proposed bimodal cavity scheme are also presented. Due to an intrinsic highly multimode sideband structure in the proposed scheme, the ability to engineer and convert photons between different frequencies in a solid-state approach has extensive technological implications not only for classical communication systems, but also future integrated quantum networks.
Realizing a fully connected network of quantum processors requires the ability to distribute quantum entanglement. For distant processing nodes, this can be achieved by generating, routing, and capturing spatially entangled itinerant photons. In this work, we demonstrate the deterministic generation of such photons using superconducting transmon qubits that are directly coupled to a waveguide. In particular, we generate two-photon N00N states and show that the state and spatial entanglement of the emitted photons are tunable via the qubit frequencies. Using quadrature amplitude detection, we reconstruct the moments and correlations of the photonic modes and demonstrate state preparation fidelities of 84%. Our results provide a path toward realizing quantum communication and teleportation protocols using itinerant photons generated by quantum interference within a waveguide quantum electrodynamics architecture.
Abstract In this work, we propose a scheme for the generation of atom--photon entangled states in a hybrid system comprising a (χ(2))(\chi^{(2)})(χ(2)) nonlinear medium and a multilevel atom, both embedded inside a cavity. Under appropriate tuning of the pump, cavity modes, and control-pulse frequencies, a pump photon is coherently down-converted into a single cavity photon while simultaneously exciting the atom, thereby generating a joint atom--photon excitation. In this setting, it is possible to generate an entangled state between the photon and the atom. By generating the excitation directly within the atomic system, importantly, this approach avoids the fundamental inefficiencies that arise when interfacing single photons with quantum memories. The nonlinear medium is driven by a pump pulse, while a control pulse transfers the atomic excitation to long-lived memory states. The properties of the emitted photon, particularly its central frequency, are determined by the interplay between the pump, control field, and cavity modes, providing the flexibility to tailor the emitted photon to existing quantum communication channels. Thus, the proposed scheme offers a promising route toward the realization of quantum repeater nodes in quantum networks.
We use a three-level artificial atom in the ladder configuration as a source of correlated, single microwave photons of different frequency. The artificial atom, a transmon-type superconducting circuit, is driven at the two-photon transition between ground and second-excited state, and embedded into an on-chip switch that selectively routes different-frequency photons into different spatial modes. Under continuous driving, we measure power cross-correlations between the two modes and observe a crossover between strong antibunching and superbunching, typical of cascade decay, and an oscillatory pattern as the drive strength becomes comparable to the radiative decay rate. By preparing the source in a superposition state using an excitation pulse, we achieve deterministic generation of entangled photon pairs, as demonstrated by nonvanishing phase correlations and more generally by joint quantum state tomography of the two itinerant photonic modes.
We investigate the deterministic generation and distribution of entanglement in large quantum networks by driving distant qubits with the output fields of a nondegenerate parametric amplifier. In this setting, the amplifier produces a continuous Gaussian two-mode squeezed state, which acts as a quantum-correlated reservoir for the qubits and relaxes them into a highly entangled steady state. Here we are interested in the maximal amount of entanglement and the optimal entanglement generation rates that can be achieved with this scheme under realistic conditions taking, in particular, the finite amplifier bandwidth, waveguide losses, and propagation delays into account. By combining exact numerical simulations of the full network with approximate analytic results, we predict the optimal working point for the amplifier and the corresponding qubit-qubit entanglement under various conditions. Our findings show that this passive conversion of Gaussian into discrete-variable entanglement offers a robust and experimentally very attractive approach for operating large optical, microwave, or hybrid quantum networks, for which efficient parametric amplifiers are currently developed.