Room-Temperature Single-Er-Qudit System

• The paper presents a breakthrough using a single Er ion as a five-level quantum emitter with record optical coherence (T₂ >500 μs) at ambient conditions.
• It employs CMOS-compatible nanofabrication with hollow nanopillar structures and ladder-type upconversion to enable deterministic, background-free single-photon readout.
• The system opens avenues for scalable quantum photonic circuits and telecom interfaces, promising applications in quantum communications and integrated photonic networks.

A room-temperature single-Er-qudit system constitutes an individually addressable, spatially isolated, multilevel (d=5) quantum emitter based on a single erbium (Er³⁺) ion, realized in a nanostructured semiconductor matrix and operating across both visible and telecommunication (C-band) optical frequencies. Such a system achieves record-long quantum coherence (optical T₂ > 500 μs at ambient conditions), enables high-contrast, background-free single-photon readout mediated by ladder-type upconversion, and is fabricated using CMOS-foundry-compatible nanotechnologies. The combination of scalable nanofabrication, optimized photonic engineering, and advanced readout protocols overcomes the traditional cryogenic requirements for quantum emitters in the telecom band, providing a practical route to integrated quantum photonic circuits and sources (Kaloyeros et al., 17 Jan 2026).

1. Physical Structure and Quantum States

The core of the system consists of a single Er³⁺ ion embedded in an amorphous silicon carbide–on–insulator nanophotonic device, specifically a hollow nanopillar (HNP) structure with critical lateral dimensions below 5 nm. The Er³⁺ ion functions as a five-level qudit, with its 4f-manifold energy eigenstates utilized for optical transitions. The typical configuration exploits the ⁴I₁₅/₂ ground state, the telecom-band ⁴I₁₃/₂ excited state, and higher levels such as ²H₁₁/₂ (518 nm), ⁴F₉/₂ (660 nm), and ⁴I₁₁/₂ (980 nm) accessed via excited-state absorption. The device geometry ensures spatial isolation to reduce dipolar dephasing and enables deterministic single-ion addressing through advanced ion implantation protocols (Kaloyeros et al., 17 Jan 2026).

2. Upconversion-Enabled Readout: Mechanisms and Models

Single-photon readout is achieved by resonant optical excitation at λₚ ≈ 1533.9 nm (⁴I₁₅/₂ → ⁴I₁₃/₂), followed by a secondary pump-induced or spontaneous excited-state absorption into higher-lying manifolds (e.g., ²H₁₁/₂), culminating in visible (518 nm), red (660 nm), or NIR (980 nm) emission. The upconversion process is governed by sequential absorption of pump photons, rigorously described by coupled rate equations:

$$\begin{aligned} \frac{dN_{1}}{dt} &= I_p\,\sigma_{01}\,N_{0} - \frac{N_{1}}{\tau_{1}} - I_p\,\sigma_{u}\,N_{1}, \ \frac{dN_{2}}{dt} &= I_p\,\sigma_{u}\,N_{1} - \frac{N_{2}}{\tau_{2}}, \ N_{0} + N_{1} + N_{2} &= 1, \end{aligned}$$

where $N_{0}$, $N_{1}$, and $N_{2}$ represent the populations of the ground, intermediate, and upconverted manifolds, respectively, $I_p$ is the pump intensity, $\sigma_{01}$ and $\sigma_u$ are the ground-state and upconversion cross-sections, and $\tau_{1,2}$ are the respective radiative lifetimes. The background-free single-photon regime is confirmed by measured second-order correlation $g^{(2)}(0) < 0.2$ at upconversion wavelengths, indicating the absence of multiphoton or ensemble fluorescence (Kaloyeros et al., 17 Jan 2026).

3. Nanofabrication and Photonic Engineering

The platform employs conformal CVD of amorphous a-SiC:O to define HNP sidewalls, followed by reactive-ion etching and deterministic ion implantation, yielding single Er³⁺ occupancy per $\sim12$ HNP array. After thermal annealing to activate Si–C–O sensitizer centers, the HNPs are integrated into 400 nm a-SiCOI waveguide layers, promoting waveguided pump absorption and emission extraction. Devices exhibit sub-2 dB/facet coupling losses with further optimization, and readily support integration into nanophotonic circuits. This technological approach allows scaling to large arrays and seamless compatibility with CMOS fabrication flows (Kaloyeros et al., 17 Jan 2026).

4. Coherence, Photon Statistics, and Performance Metrics

Empirical measurements at room temperature reveal a photon-echo T₂ (coherence time) of 568 ± 61 μs and Ramsey dephasing time T₂* ≈ 32 μs for the telecom transition (¹⁵³⁴ nm) in the single qudit. Rabi oscillations with Ω_R/2π ≈ 660 kHz and >96% contrast are observed. Upconverted visible emissions exhibit lifetimes τ₂ in the range of 164–700 μs, supporting photon emission rates suitable for quantum networking. Single-photon purity is evidenced by $g^{(2)}(0)$ values of 0.12–0.26 across telecom and upconversion channels. Upconversion saturation and power broadening set limits on excitation flux and coherence; pump intensities above 1 W/cm² lead to Δν_L ≈ 37 MHz and moderate T₂* reduction (Kaloyeros et al., 17 Jan 2026).

Metric	Value
${\sigma}_{01}$ (ground→telecom)	$\sim5 \times 10^{-17}$ cm²
Φ_sat (saturation photon flux)	$2 \times 10^{19}$ cm⁻²·s⁻¹
τ₁ (telecom excited-state)	1.2 ms
τ₂ (upconversion, 518/660/980 nm)	164/310/700 μs
$\Omega_R$/2π (Rabi frequency)	660 kHz, >96% contrast
T₂ (optical echo, telecom)	568 ± 61 μs
$g^{(2)}(0)$ (telecom, 518, 980 nm)	0.26, 0.12, 0.18

5. Comparison with Alternative Upconversion Approaches

Single-Er-qudit upconversion differs fundamentally from four-wave mixing Bragg scattering (FWM-BS) methods and non-coherent multi-ion upconversion. In the FWM-BS approach, energy exchange between signal and idler via dual-pump processes in dispersion-engineered Si₃N₄ waveguides achieves upconversion with internal conversion efficiencies exceeding 25% at peak pump powers of ∼10 W (Agha et al., 2013). However, FWM-BS is an external photonic interface, not an intrinsic single-emitter process, and typically requires complex phase-matching and high-peak pulsed sources. In contrast, the single-Er-qudit architecture leverages the direct ladder-type upconversion of an individual quantum system, enabling background-free, deterministic photon emission and visibility in single-photon statistics (Kaloyeros et al., 17 Jan 2026). Mechanistically, it is more closely related to incoherent single-photon upconversion between pairs of ions mediated by decoherence, as described by Shishkov et al., where quantum yields η > 50% are possible under suitable regime of decoherence and dipole-dipole coupling (Shishkov et al., 2017).

6. Applications and Prospects for Quantum Technologies

The demonstration of microsecond-scale coherence and upconversion-enabled single-photon readout at room temperature removes a critical barrier for functional integration into quantum information and communication systems. The single-Er-qudit platform is suitable for on-chip quantum repeater nodes, quantum key distribution transmitters, and visible–telecom photonic interface elements, all scalable by standard semiconductor manufacturing. A plausible implication is the realization of room-temperature quantum photonic integrated circuits (qPICs) with entanglement distribution, on-chip frequency translation, and spectrally multiplexed quantum memory elements based on single Er³⁺ qudits. The observed photon purity and coherence metrics are sufficient for basic quantum networking primitives, and further integration with electro-optic modulators and low-loss waveguides is anticipated (Kaloyeros et al., 17 Jan 2026).

7. Technical Challenges, Trade-offs, and Future Directions

Performance trade-offs are evident: increases in upconversion pump intensity elevate photon rates but can reduce coherence via power broadening and dynamical Stark shifts. Upconversion channels provide faster emission but exhibit shorter excited-state lifetimes and somewhat lower optical T₂. Efficient integration into low-loss photonic structures, minimization of inhomogeneous broadening, and thermal management are ongoing engineering challenges. The unique regime, combining background-free single-photon upconversion, high room-temperature coherence, and full CMOS compatibility, positions the single-Er-qudit system as a primary candidate for scalable cryogen-free telecom quantum emitters (Kaloyeros et al., 17 Jan 2026). Further extensions may explore hybridization with circuit QED, Purcell enhancement, and multi-qudit entanglement.

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For further details and data on specific implementations, refer to (Kaloyeros et al., 17 Jan 2026, Shishkov et al., 2017), and (Agha et al., 2013).