---
title: Room-Temperature Single-Er-Qudit System
url: https://www.emergentmind.com/topics/room-temperature-single-er-qudit-system
type: topic
---

# Room-Temperature Single-Er-Qudit System

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 [2601.11879].

## 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 [2601.11879].

## 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 [2601.11879].

## 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 [2601.11879].

## 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 [2601.11879].

| 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 [1304.5754]. 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 [2601.11879]. 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 [1706.08981].

## 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 [2601.11879].

## 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 [2601.11879]. 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 [2601.11879], [1706.08981], and [1304.5754].

Source: https://www.emergentmind.com/topics/room-temperature-single-er-qudit-system