Efficient and compact quantum network node based on a parabolic mirror on an optical chip

TL;DR

This work demonstrates a cavity-free neutral-atom quantum networking node that leverages a parabolic mirror to trap and collect photons from a single Rubidium atom, achieving a measured photon collection/detection efficiency of 3.66% per excitation and a raw atom–photon Bell fidelity of 0.93 (inferred 0.98 after readout correction). The device is millimeter-scale, monolithically integrated on a vacuum chip with fiber interfaces, enabling robust, scalable, plug-and-play network nodes. The approach highlights near-optimal light–matter coupling limited primarily by the optics NA, and it lays the groundwork for modular neutral-atom quantum repeaters and distributed quantum information processing with potential extension to atomic arrays and telecom-compatible links. Overall, this cavity-free, fiber-integrated node provides a practical, high-fidelity building block for scalable quantum networks.

Abstract

We demonstrate a neutral atom networking node that combines high photon collection efficiency with high atom photon entanglement fidelity in a compact, fiber integrated platform. A parabolic mirror is used both to form the trap and to collect fluorescence from a single rubidium atom, intrinsically mode matching $σ$ polarized emitted photons to the fiber and rendering the system largely insensitive to small imperfections or drifts. The core optics consist of millimeter scale components that are pre aligned, rigidly bonded on a monolithic invacuum assembly, and interfaced entirely via optical fibers. With this design, we measure an overall photon collection and detection efficiency of $3.66\%$, from which we infer an overall collection efficiency of $6.6\%$ after the single--mode fiber coupling. We generate atom photon entangled states with a raw Bell state fidelity of 0.93 and an inferred fidelity of 0.98 after correcting for atom readout errors. The same node design has been realized in two independent setups with comparable performance and is compatible with adding high NA objective lenses to create and control atomic arrays at each node. Our results establish a robust, cavity free neutral atom interface that operates near the limit set by the collection optics numerical aperture and provides a practical building block for scalable quantum network nodes and repeaters.

Figures (8)

• Figure 1: Optical setup and experimental sequence. (a) Setup used for single-atom and single-photon characterization as well as atom-photon entanglement verification. For the $g^{(2)}$ measurement, the beamsplitter is a 50:50 non-polarizing beamsplitter. For atom-photon entanglement we replaced the beamsplitter with a polarizing beamsplitter (PBS). (b) Image of the in-vacuum components which include all the optics required at the core of the experiment, except two external MOT beams. (c) Experimental sequence for atom-photon entanglement generation and verification. (d) The atom is initially prepared in $\ket{f=1,m_f=0}$ by optical pumping, then excited to $\ket{f=0,m_f=0}$ by a Gaussian pulse. Emitted photons with $\sigma_{\pm}$ polarization are collected and coupled to the SM fiber. (e) Histogram of the single-photon detection time. The fit is a convolution of a Gaussian with FWHM of $18.7~\mathrm{ns}$ with an exponential with a decay time of $26.4~\mathrm{ns}$ in agreement with the lifetime of the excited state.
• Figure 1: Design of the full vacuum system (a) Pancake chamber with the optical chip mounted inside. (b) and (c) Valve for turbo pump connection. (d) and (e) Ion pumps for pumping of both upper and lower chamber. (f) Rb ampoule sitting inside the bellows. (g) eight fiber feedthroughs.
• Figure 2: (a) After detection of a photon, a microwave $\pi$ pulse of length $5.3~\mu\mathrm{s}$ transfers the population: $\ket{\uparrow} \rightarrow \ket{\uparrow'}$. The new qubit basis $(\ket{\downarrow}, \ket{\uparrow'})$ has a long coherence time at the magic bias field of $\sim3.23~\mathrm{G}$. (b) a two-photon microwave-RF pulse is used to rotate the atomic basis on the Bloch sphere for entanglement verification. (c) Rabi oscillation of the clock transition at rate $118~\mathrm{kHz}$, indicating optical pumping fidelity larger than $0.987(12)$, after correcting for errors in atom measurement. (d) Rabi oscillation for the $\ket{\downarrow} \leftrightarrow \ket{\uparrow'}$ transition driven with a two-photon microwave-RF pulse. The single-photon detuning from $\ket{f=2,m_f=0}$ is $-250~\mathrm{kHz}$ which results in Rabi oscillation at $2.87~\mathrm{kHz}$. The $\pi/2$ pulse length is $87~\mu \mathrm{s}$. (e) Ramsey data on the clock transition $\ket{f=1,m_f=0} \rightarrow \ket{f=2,m_f=0}$ showing a coherence time of $T_2^*=3.32(9)~\mathrm{ms}$. (f) Ramsey results for the transition $\ket{\uparrow} \rightarrow \ket{\uparrow'}$ with a coherence time of $T_2^* = 0.11(1)~\mathrm{ms}$ obtained from the fit. (g) Ramsey fringes of the two-photon transition, indicating $T_2^* = 3.2~\mathrm{ms}$ at $B=3.23~\mathrm{G}$.
• Figure 2: Pancake chamber and on-chip modules (a) Top view of the pancake chamber showing the optical modules mounted on the Macor chip. (b) Parabolic mirror. (c) MOT beam module. (d) GRIN module for optical pumping and excitation. (e) Eight fiber feedthroughs. (f) Coil assembly designed to mount around the pancake chamber.
• Figure 3: (a) and (c) parity oscillations in $z$- and $x$-basis, respectively. Even parity: $P(\uparrow',H) + P(\downarrow,V)$, odd parity: $P(\uparrow',V) + P(\downarrow,H)$. The solid curves are fit to sinusoidal functions. (b) and (d) measured joint probabilities in $z$- and $x$-basis, respectively, at HWP angles giving the maximum visibility in the corresponding parity oscillation.