Non-classical photon sources are essential for distributed quantum networks. Photons generated from matter systems with memory are particularly promising for on-demand network integration. Room-temperature atomic vapors offer robustness and scalability advantages over cryogenic solid-state sources and complex ultracold atom systems. While previous room-temperature photon sources had limitations in memory time or photonic state purity, this research aims to overcome these challenges by developing a single-photon source based on room-temperature atomic vapor memory capable of generating a photon on demand with a storage time comparable to photon time-of-flight between distant locations, ideally in the millisecond regime. The main challenges in achieving this are thermal atomic motion and collisional decoherence, limiting storage time in previous DLCZ-type sources to a few microseconds. Quantum readout noise further compromises performance. While alternative approaches involve generating a photon in one system and storing it in another, this approach has only demonstrated microsecond to millisecond storage for weak classical pulses. This paper aims to create a deterministic, ensemble-based, room-temperature single-photon source with significantly improved antibunching and non-classical memory using motional averaging, spin-protecting wall coatings, and a Raman transition at the "magic detuning".

Deterministic solid-state single-photon sources have made significant progress but require cryogenic temperatures for efficient photon interference. Ultracold atoms offer excellent performance but are experimentally complex. Room-temperature atomic systems are attractive for their scalability and robustness, particularly those based on the DLCZ proposal, which combines single-photon generation and storage. However, limitations in room-temperature DLCZ-type sources include short on-demand retrieval times (few microseconds) due to thermal motion and collisional decoherence, as well as quantum readout noise. Alternative approaches using separate generation and storage systems have shown some success, but their application to single-photon communication needs further demonstration. Prior work demonstrated storage of weak classical pulses ranging from microseconds to milliseconds in room-temperature gases, and recent advances showed storage of external classical light pulses for up to a second, yet single-photon communication remains unproven within this context.

The experiment uses a thermal cesium vapor cell with a small cross-section (300 µm × 300 µm × 10 mm) to enable fast motional averaging and high laser pulse intensity. Atoms are initially optically pumped into a coherent spin state. A collective excitation is written into the ensemble with low probability using a far-detuned, π-polarized write pulse. Detection of a scattered heralding single photon projects the ensemble onto a long-lived, symmetric Dicke state. Narrowband filter cavities are used to achieve motional averaging, extending the heralding photon detection beyond the atomic transit time and equalizing atomic contributions. A spin-protecting coating on the cell walls preserves the spin state, extending the excitation lifetime. Four-wave mixing (FWM) noise, a common limitation, is suppressed by using a magic detuning for the read process, exploiting destructive interference of Raman amplitudes via coupling to different excited states. An asymmetric linear cavity enhances light-atom interaction. Polarization and spectral filtering separate the heralding photon from excitation photons. After a variable delay, a σ-polarized read pulse coherently retrieves the stored excitation as a deterministic single photon. Two superconducting nanowire single-photon detectors (SNSPD) detect the heralding and retrieval photons. The experimental sequence includes optical pumping, write pulse, variable delay, and read pulse. Photon correlations are characterized by measuring the 2nd-order cross-correlation (gWR) and auto-correlation (gRR) functions. A two-mode squeezed state model, incorporating experimentally determined atomic noise, is used to fit the data and extract parameters such as intrinsic retrieval efficiency. The quantum memory capabilities are assessed by varying the delay between write and read pulses, analyzing the cross-correlation function gWR(τ), and Cauchy-Schwarz parameter R(τ) to determine non-classical correlation and Bell inequality violation limits. The 1/e lifetime of the collective excitation is also measured to evaluate the effectiveness of the spin-protecting coating.

The researchers successfully demonstrated a room-temperature single-photon source with clear antibunching (g⁽²⁾_RR|W=1 = 0.20 ± 0.07), indicating a strong single-photon character. Non-classical correlations between heralding and retrieved photons were maintained for up to τ_NC = (0.68 ± 0.08) ms, two orders of magnitude longer than previously achieved with room-temperature systems. Correlations sufficient for violating Bell inequalities were observed for up to τ_B = (0.15 ± 0.03) ms. The intrinsic retrieval efficiency was estimated to be (70 ± 8)%. The experimental results closely matched theoretical model predictions. The long temporal shape of the retrieved light (tens of microseconds) allowed the SNSPDs to function as photon-number-resolving detectors, enabling accurate multiphoton event accounting. The overall write-read efficiency was approximately 5 × 10⁻⁵, while the probability of a write event followed by double retrieval was around 2 × 10⁻⁷. Analysis of the read noise confirmed that FWM noise was suppressed below other noise sources. The non-classical memory time was limited by atomic decay due to wall collisions but demonstrated the effectiveness of the spin-protecting coating with a collective excitation lifetime of τ_M = 0.89−0.49+0.23 ms. The coherence time of the ground-state Zeeman levels (T2) was approximately 2 ms.

The results demonstrate the ability to herald, store, and read out a long-lived single collective atomic excitation at room temperature. The near-millisecond storage time and high cross-correlation enable applications in quantum networks, particularly for entanglement generation over long distances (up to 200 km). The simplicity of the system allows for scalability and the creation of multiple parallel systems for quantum repeater and simulator applications. Further improvements in storage time (hundreds of milliseconds) are projected by increasing cell size and using advanced coatings, along with improved excitation light rejection. The use of a top-hat optical mode could also relax spectral filtering requirements.

This study successfully demonstrated a room-temperature single-photon source with a significantly improved non-classical memory time. This advance opens new possibilities for long-distance quantum communication and other quantum information processing applications by providing a scalable and robust platform for building quantum networks and repeaters. Future research could focus on further extending the storage time, exploring larger phase spaces by accumulating excitations, and investigating interfacing with other quantum platforms.

The primary limitation of the study is the atomic decay due to wall collisions, which limits the non-classical memory time. While the spin-protecting coating significantly extends the lifetime, further improvements are possible. Another limitation is the overall write-read efficiency, which could be enhanced through further optimization of the experimental parameters and reduction of optical losses. The relatively small size of the vapor cell might also limit the scalability in certain contexts.