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A Hybrid Source for Deterministic Generation of Atom-Photon Entangled States

Priyanshu Tiwari, Aleksa Krstić, Frank Setzpfandt, Robert Loew, Sina Saravi

Published Jul 16, 2026• 0 citations
Publisher

Abstract

Abstract In this work, we propose a scheme for the generation of atom--photon entangled states in a hybrid system comprising a \((\chi^{(2)})\) 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.

DOI
10.21203/rs.3.rs-10319323/v1

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Abstract In this work, we propose a scheme for the generation of atom--photon entangled states in a hybrid system comprising a ((\chi^{(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 propose a scheme for quantum-light generation in a nonlinear cavity hybridized with a two-level system and theoretically show that, when excited by a series of controlled pump pulses, the hybrid source generates Fock states with high probabilities, e.g., one- and two-photon states can be generated near on demand, and Fock states with up to seven photons with a probability above 50%. The tailorable nature of the nonlinear cavity allows for generating Fock states with arbitrary frequencies, even with a fixed two-level system, creating fundamentally new opportunities in all areas of quantum technologies.
Quantum repeaters promise to enable quantum networks over global distances by circumventing the exponential decrease in success probability inherent in direct photon transmission. We propose a realistic, functionally integrated quantum-repeater implementation based on single atoms in optical cavities. Entanglement is directly generated between the single-atom quantum memory and a photon at telecom wavelength. The latter is collected with high efficiency and adjustable temporal and spectral properties into a spatially well-defined cavity mode. It is heralded by a near-infrared photon emitted from a second, orthogonal cavity. Entanglement between two remote quantum memories can be generated via an optical Bell-state measurement, while we propose entanglement swapping based on a highly efficient, cavity-assisted atom-atom gate. Our quantum-repeater scheme eliminates any requirement for wavelength conversion such that only a single system is needed at each node. We investigate a particular implementation with rubidium and realistic parameters for Fabry–Perot cavities based on CO2\hbox {CO}_2CO2​ laser-machined optical fibers. We show that the scheme enables the implementation of a rather simple quantum repeater that outperforms direct entanglement generation over large distances and does not require any improvements in technology beyond the state of the art.
Promising access to high-speed quantum networks relies on the creation of high-dimensional entangled memories that provide quantum communication with higher capacity of noisy quantum channels, thereby reducing the transmission time of information. Yet, the distribution of multidimensional entanglement between remote memory nodes is still faintly investigated. We propose an experimentally feasible protocol of deterministic generation of high-dimensional entanglement between distant multilevel atoms confined in high-finesse optical cavities and driven by laser pulses. Three-dimensional entanglement is generated deterministically between remote atoms by triggering a two-photon wave packet, which mediates a superposition of states chosen by the laser pulse parameters. The efficient transfer of atomic states between remote nodes allows the construction of a three-dimensional quantum repeater, where the successful creation of entanglement can be verified by a simple method for reliable measurement of atomic ground Zeeman states.
Quantum repeaters enable the generation of reliable entanglement across long distances despite the underlying channel noise. Nevertheless, realizing quantum repeaters poses a difficult engineering challenge due to various device constraints and design tradeoffs. Herein we propose and analyze an efficient hybrid quantum repeater design that integrates atom-based quantum processing units, spontaneous parametric down-conversion photon sources, and atomic frequency comb quantum memories. Our design leverages the strong spectrotemporal multiplexing capability of the quantum memory to enable high-rate elementary-link entanglement generation between repeater nodes. Transferring the photonic entanglement into matter-qubit entanglement, together with deterministic quantum operations, further enables reliable long-distance entanglement distribution. We analyze photon-loss channels in the hybrid architecture and propose suitable error-suppression strategies that are natively incorporated into our repeater protocol. Using numerical simulations, we demonstrate the advantages of our hybrid design for end-to-end secret key rates in a linear repeater-chain model. With continued advances in relevant hardware technologies, we envision that the proposed hybrid design is well suited for large-scale quantum networks.
Quantum repeaters are vital ingredients for realization of long-distance quantum networks [1]. They can be realized by preparing nodes that can generate an entangled light-matter state, namely, a single photon entangled to a quantum memory. This allows for entangling two distant quantum memories, by propagating the single photons from each node to an intermediate place for Bell state measurements. A common approach for generating such entangled light-matter states is generating an entangled photon pair in a nonlinear source, and coupling one of the photons in the pair to a quantum memory for storage [2]. However, such an approach faces two main challenges: Photon-pair generation in nonlinear sources is a probabilistic process. Moreover, due to modal and spectral mismatches, coupling single photons from a nonlinear source to a quantum memory is generally not an efficient process. These make the successful generation of an entangled light-matter state highly probabilistic.
Summary form only given. Hybrid quantum technologies seek to combine the advantages of two individual quantum architectures by transferring the information between the two systems. We want to benefit from the high mobility and ease of transmission of photons for quantum communication and exploit the excellent readout and storage capabilities of atomic qubits as a quantum memory, which is essential to build up quantum repeater networks for quantum data processing. Efficient interaction of photons with atoms requires a match of the photons spectral properties to those of the resonances of the atomic species. The atomic transition usually has a much narrower bandwidth than single photons generated by spontaneous parametric down-conversion (SPDC), the current gold standard of producing high-purity heralded single photons at flexible wavelengths. To develop hybrid quantum technologies therefore requires significantly reducing the single photon emission spectra to fulfill these requirements. The way we try to achieve this is by using an optical cavity to enhance the probability of creating the photons in the spectral and spatial resonator mode.We report 100% duty cycle generation of sub-MHz single photon pairs at the Rubidium D1 line using cavityenhanced spontaneous parametric down-conversion [3]. The double exponential decay of the temporal intensity cross-correlation function exhibits a bandwidth of 666 ± 16 kHz for the single photons, an order of magnitude below the natural linewidth of the target transition. This is, to our knowledge, the narrowest bandwidth of single photons from SPDC to date. A new method of placing a half-wave plate inside the cavity helps to achieve triple resonance between pump, signal and idler photon, reducing the bandwidth and simplifying the locking scheme. Additionally, stabilisation of the cavity to the pump frequency enables the 100% duty cycle. The quantum nature of the source is confirmed by the idler-triggered second-order autocorrelation function at τ = 0 to be g(2) s,s (0) = 0.016 ± 0.002 for a heralding rate of 5 kHz and antibunching below 0.5 is observed up to heralding rates of 250 kHz [3]. The high purity of the source is matched by high indistinguishability of the photons demonstrated in a Hong-Ou-Mandel (HOM) interference experiment with a visibility V = 96.5 ± 0.5 %. Additionally, the modelocked two-photon state of the generated pairs leads to revivals of the HOM dip. We observe these revivals up to 100 m path difference between signal and idler photon, independently proofing the long coherence length of our photons.