🔬 physics

Telecom-band quantum memory with chlorine defects in silicon carbide

This paper demonstrates that chlorine-based defects in 4H-SiC serve as a promising wafer-scalable platform for chip-scale quantum memories by exhibiting room-temperature spin coherence, optically detected magnetic resonance, and zero-phonon line emission across the entire telecommunication range.

Original authors: Georgy Astakhov, Andrei Anisimov, Kalliopi Mavridou, Ashin Mathews, Manfred Helm

Published 2026-06-26

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CC BY 4.0

Original authors: Georgy Astakhov, Andrei Anisimov, Kalliopi Mavridou, Ashin Mathews, Manfred Helm

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the internet as a massive, high-speed highway made of glass fibers. Currently, the "cars" (quantum information) that travel on this highway are often built for a different type of road (visible light). To get them onto the fiber-optic highway, we usually have to build expensive, complex "translators" to change their color. This paper introduces a new type of car that is born ready to drive on the fiber-optic highway, and it's built inside a very common material: Silicon Carbide (the same stuff used in tough ceramics and high-power electronics).

Here is the story of what the researchers found, broken down into simple concepts:

1. The "Magic Glitch" in the Crystal

Think of a Silicon Carbide crystal as a perfectly organized city grid made of silicon and carbon atoms. The researchers intentionally dropped a "misfit" atom into this grid: a Chlorine atom. When a chlorine atom sits next to a missing spot (a vacancy) in the grid, it creates a tiny, unique defect.

The paper claims this specific defect acts like a quantum memory. It can store information (like a tiny switch that is "on" or "off") and, crucially, it speaks the language of the fiber-optic internet right from the start.

2. Speaking the Language of the Internet (Telecom Bands)

Most quantum systems we know (like diamonds with nitrogen defects) shout in "visible light" (like a bright flashlight). To send that message down a fiber-optic cable, you have to use a machine to translate it into "infrared light" (the telecom bands), which is the standard language for long-distance internet.

The chlorine defects in this study are special because they naturally "speak" in the O-band and C-band. These are the specific colors of light that travel best through glass fibers with the least amount of signal loss. It's like finding a car that comes out of the factory already equipped with the exact tires needed for the highway, rather than needing to swap them out later.

3. The "Blink" and the "Spin"

To be a useful memory, the defect needs two things:

It needs to blink efficiently: The researchers found that when they shined a laser on these defects, they glowed brightly. Even better, a huge chunk of that light (39%) was the pure "signal" color, rather than being scattered as noise. This is a very high score compared to other known quantum defects.
It needs to spin: Inside the defect, there is a tiny magnetic "spin" (like a microscopic compass needle). The researchers showed they could control this spin using radio waves, even at room temperature. This is the "memory" part—storing the information in the direction the spin is pointing.

4. The "Ghost" and the "Real" Signal

The researchers played detective to figure out exactly what they were looking at. They used a special isotope of chlorine (Chlorine-35) which has a tiny "nuclear spin" of its own, like a smaller compass inside the main one.

By listening to the radio waves interacting with the defect, they heard a complex "chatter." This chatter revealed that the main spin was talking to the chlorine nucleus. They mapped out this conversation and found it matched their computer simulations of a specific type of chlorine defect.

However, they also found some "ghost" signals. In some samples, they saw other radio wave patterns that didn't match the chlorine. They concluded these were likely caused by other, unknown defects in the material that also happen to glow in the telecom range. This is like hearing a choir where the main singers are the chlorine, but there are also some background singers (unidentified defects) making noise.

5. The "Short Attention Span" Problem

The researchers tried to hold the quantum information for a while to see how long the memory lasts. They found that the "spin" stays coherent (holds the information) for a very short time—less than a millionth of a second (sub-microsecond).

Why is it so short? The paper suggests it's not because the spin is naturally unstable, but because the defect is "fickle" with its electrical charge. It's like a lightbulb that keeps flickering between being "on" and "off" (changing charge states) too quickly. This flickering kills the memory signal before it can last long. The researchers note that if they can stop this flickering, the memory could last much longer.

6. The "Second Story" of the Defect

The paper also discovered that these defects have a "second floor." When hit with more laser power, they emit light at different, shorter wavelengths. The researchers believe this comes from a higher energy state of the same defect, acting like a second room in the house that lights up differently. This adds a layer of complexity to how the defect works, suggesting it's more intricate than previously thought.

Summary

In short, this paper introduces a new "quantum memory" made from chlorine atoms trapped in silicon carbide.

The Good News: It naturally speaks the language of fiber-optic cables, it's bright, and it can be controlled with radio waves.
The Catch: The memory is currently short-lived because the defect gets confused about its electrical charge, causing the signal to fade quickly.
The Verdict: It is a very promising candidate for building future quantum networks, provided scientists can figure out how to stabilize its electrical charge so the memory lasts longer.

Technical Summary: Telecom-band Quantum Memory with Chlorine Defects in Silicon Carbide

Problem Statement
The realization of long-distance quantum networks requires quantum memories with photonic interfaces operating in the low-loss telecommunication bands. While solid-state spin qubits (e.g., in diamond or silicon) offer long coherence times, their native optical transitions often lie in the visible spectrum, necessitating complex frequency conversion. Conversely, existing telecom-band emitters in silicon or silicon carbide (SiC) often suffer from low photon emission rates or lack demonstrated coherent spin control. There is a critical need for a wafer-scalable material platform that intrinsically combines bright telecom emission with robust, coherent spin functionality.

Methodology
The authors investigated chlorine-related defects in 4H-SiC, created via standard ion implantation of $^{35}$ Cl followed by thermal annealing in commercially available wafers. The study employed a multi-faceted experimental approach:

Optical Spectroscopy: Photoluminescence (PL) measurements were conducted under 976 nm excitation to identify Zero-Phonon Lines (ZPLs) and phonon sidebands (PSB) across the O- and C-bands. Time-resolved PL was used to measure excited-state lifetimes.
Optically Detected Magnetic Resonance (ODMR): ODMR spectra were acquired at room temperature and low temperature (10 K) in zero and external magnetic fields to identify spin transitions.
RF-Assisted Spectroscopy: Tunable bandpass filters were used to correlate specific ODMR resonances with distinct spectral regions of the PL emission.
Coherent Control: Pulsed radiofrequency (RF) sequences were applied to measure spin relaxation ( $T_1$ ) and coherence ( $T_2^*$ ) times using Rabi oscillations and Ramsey interferometry.
Simulation: Spin Hamiltonian simulations (using the EasySpin package) were performed to model electron-nuclear spin interactions and reproduce experimental ODMR spectra under magnetic fields.

Key Contributions and Results

Telecom-Band Emission: The study reports chlorine-based defects in 4H-SiC emitting in the O-band (1348.7 nm) and C-band. The defects exhibit a Debye-Waller factor of up to 39%, significantly higher than nitrogen-vacancy (NV) centers in diamond or intrinsic vacancies in SiC, indicating efficient zero-phonon emission.
Excited-State Dynamics: Time-resolved measurements reveal a short excited-state lifetime of 450 ps. The authors attribute this to non-radiative channels potentially arising from incomplete lattice recovery, noting it is shorter than theoretical predictions for radiative recombination.
Spin-Photon Interface: The defects are demonstrated to be spin-active at room temperature, exhibiting ODMR resonances in the sub-GHz range (500–700 MHz). The authors identify a ground-state spin-photon interface with zero-field splitting parameters yielding resonances distinct from known SiC defects (like divacancies).
Hyperfine Structure and Nuclear Spin: Using ODMR and Ramsey interferometry, the authors resolve hyperfine structure arising from the interaction with $^{35}$ Cl nuclear spins ( $I=3/2$ ). The hyperfine coupling constant is determined to be $|A| = 34$ MHz.
Coherent Spin Control: The study demonstrates coherent manipulation of the defect spin. Ramsey fringes reveal an intrinsic inhomogeneous spin coherence time ( $T_2^*$ ) of approximately $2 \pm 1$ $\mu$ s. However, the observed spin relaxation time ( $T_1 \approx 660$ ns) and coherence are limited by rapid quenching of the ODMR contrast, which the authors attribute to charge-state metastability (dynamics between neutral and positively charged states) rather than intrinsic spin-lattice relaxation.
Defect Identification: While the data qualitatively aligns with the theoretical model of a positively charged chlorine-vacancy ( $ClV^+$ ) with $S=1$ spin, the authors note discrepancies. Specifically, the magnetic field dependence suggests an off-axis configuration for the $ClV^+$ defect, whereas PL data previously suggested an on-axis configuration. Additionally, the study identifies a separate family of resonances ( $\nu_3, \nu_4$ ) in the 100–400 MHz range associated with unidentified intrinsic defects, distinct from the Cl-related defects.

Significance and Claims
The paper claims that chlorine-related defects in SiC represent a promising platform for chip-scale quantum memories operating in fiber-optic telecommunication windows. The work establishes the first demonstration of coherent spin control and hyperfine resolution for these defects. The authors emphasize that the combination of telecom emission, wafer-scale compatibility, and demonstrated spin-photon interfaces addresses a key bottleneck for scalable quantum networks.

However, the authors maintain a modest tone regarding the current state of the technology. They acknowledge that the spin coherence times are currently limited by charge-state dynamics rather than intrinsic spin properties, and that the exact electronic structure and optical spin polarization cycle require further theoretical refinement. The paper concludes that while the system is a compelling candidate for integrated quantum technologies, challenges regarding charge-state control, spectral stability, and the isolation of single emitters must be addressed to fully realize its potential for long-distance quantum networks.