⚛️ quantum physics

Pulsed coherent spectroscopy of a quantum emitter in hexagonal Boron Nitride

This study demonstrates that individual B centers in hexagonal Boron Nitride exhibit optical coherent control via power-dependent Rabi oscillations, high single-photon purity, and a measurable inhomogeneous coherence time, establishing them as viable candidates for triggered quantum emitters in photonic platforms.

Original authors: Jake Horder, Hugo Quard, Kenji Watanabe, Takashi Taniguchi, Nathan Coste, Igor Aharonovich

Published 2026-02-23

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CC BY 4.0

Original authors: Jake Horder, Hugo Quard, Kenji Watanabe, Takashi Taniguchi, Nathan Coste, Igor Aharonovich

Original paper licensed under CC BY 4.0 (http://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 you are trying to build a super-advanced computer, but instead of using tiny silicon chips, you want to use single particles of light (photons) to carry information. To do this, you need a reliable "light switch" that can be turned on and off with perfect precision, emitting exactly one photon at a time. This is the holy grail of quantum technology.

This paper is about finding and mastering one of the best candidates for this switch: a tiny, atomic-sized defect inside a material called hexagonal Boron Nitride (hBN). Think of hBN as a very thin, flat sheet of atomic Lego bricks. Inside this sheet, the researchers found a specific "glitch" or missing piece in the pattern, called the B center.

Here is the story of what they did, explained simply:

1. The Problem: Finding the Right Switch

For years, scientists have been looking for materials that act like perfect quantum light switches. Some materials are too messy, some are hard to control, and some are hard to fit onto a computer chip. The B center in hBN is special because it's like a high-quality, factory-made switch that is easy to create and fits perfectly into future nano-chips.

2. The Experiment: Teaching the Atom to Dance

The researchers wanted to see if they could control this B center with laser light. They treated the atom like a tiny spinning top (a quantum bit).

The Setup: They used a very fast, pulsed laser (like a camera flash that fires billions of times a second) tuned to a specific color (436 nm, which is a deep blue-violet).
The Goal: They wanted to make the atom jump between two states: "Off" (ground state) and "On" (excited state).

3. The Rabi Oscillations: The "Push and Pull"

Imagine you are pushing a child on a swing.

If you give a tiny push, they go a little high.
If you push harder, they go higher.
If you push at just the right moment and with just the right strength, you can make them go all the way to the top and flip over.

The researchers did this with light. By changing the power of their laser pulses, they could control exactly how much the atom "swings."

They pushed the atom all the way to the top (a "π-pulse"), flipping it from Off to On.
They even pushed it so hard it swung past the top and started coming back down (up to 5π).
The Result: They proved they have total control. When they stopped the swing exactly at the top (the π-pulse), the atom emitted a photon with 93% purity. This means almost every time they asked for a light particle, they got exactly one, and no extra noise.

4. The Ramsey Interferometry: The "Echo Test"

Controlling the switch is great, but for a quantum computer, the switch also needs to hold a "memory" of its state for a moment. This is called coherence.

Imagine you are in a large, empty hall and you clap your hands. You hear an echo. If the hall is full of people talking (noise), the echo gets messy and fades away quickly. If the hall is silent, the echo is clear and lasts longer.

The Test: The researchers used a special laser trick (Ramsey interferometry) to create a "superposition" state. This is like spinning a coin on a table so it's neither Heads nor Tails, but both at the same time.
The Measurement: They let this "spinning coin" spin for a tiny fraction of a second, then checked if it was still spinning smoothly.
The Result: They found that the B center kept its "spin" (coherence) for 0.60 nanoseconds.
- Why is this impressive? In the world of quantum physics, this is a very long time! It means the atom is very stable and isn't getting confused by its surroundings. It's almost as stable as it theoretically could be, even without using any fancy "noise-canceling" equipment.

5. Why This Matters

Think of this discovery as finding a perfectly tuned violin string in a noisy room.

Before: We knew the string existed, but we weren't sure if we could play a clear note on it without it going out of tune.
Now: We know we can play the note perfectly (Rabi oscillations), we know it produces a pure sound (single photons), and we know the note rings out clearly for a long time (coherence).

The Big Picture

This paper proves that the B center in Boron Nitride is a top-tier candidate for building the future of quantum photonic computers. Because this material is a "van der Waals" material (basically, a stack of atomic sheets), it can be easily glued onto different chips and integrated into tiny circuits.

In short: The researchers found a tiny, atomic light switch that is easy to control, very pure, and surprisingly stable. This is a massive step toward building real-world quantum computers that use light instead of electricity.

1. Problem Statement

Quantum emitters are essential for quantum communication, computing, and metrology. While solid-state platforms like semiconductor quantum dots, single molecules, and color centers in wide-bandgap materials (e.g., diamond NV centers) are promising, they face significant scalability challenges. These include spectral inhomogeneity, difficulty in deterministic defect creation, and limitations in material processing and integration.

Hexagonal Boron Nitride (hBN) has emerged as a versatile nanophotonic platform due to its van der Waals structure and compatibility with on-chip integration. Specifically, the B center (a carbon split-interstitial defect) in hBN, characterized by a Zero Phonon Line (ZPL) at 436 nm, shows promise due to its reproducible optical linewidth and precise creation via electron-beam irradiation. However, a critical gap remains in understanding the coherent optical control of individual B centers. Previous work has demonstrated two-photon interference and structural observation, but a comprehensive demonstration of deterministic population control (Rabi oscillations) and phase coherence (Ramsey interferometry) under pulsed resonant excitation was lacking.

2. Methodology

The researchers employed a combination of cryogenic optical spectroscopy and ultrafast pulse shaping to characterize a single B center in a high-purity hBN crystal.

Sample Preparation: A thin hBN flake was exfoliated onto a SiO₂/Si substrate. Individual B centers were created using electron-beam irradiation.
Excitation Setup:
- A frequency-doubled femtosecond Ti:Sapphire laser (centered at 436 nm) provided resonant pulses.
- A 4f pulse-shaping stage with a slit at the Fourier plane was used to spectrally narrow the laser bandwidth from 2 nm to 300 GHz (≈0.19 nm) to match the emitter's linewidth.
- Excitation was performed either off-resonantly (405 nm pump) or resonantly (436 nm) to probe the ZPL and the Phonon Sideband (PSB).
Detection Strategy: Due to the difficulty of isolating the resonant ZPL from scattered pump light, the researchers utilized the Phonon Sideband (PSB) emission (centered around 460 nm) as the detection channel. This allowed for robust isolation of the signal using simple spectral filtering (440 nm long-pass filter).
Experimental Techniques:
1. Time-Resolved Photoluminescence (TRPL): To measure the radiative lifetime ( $T_1$ ).
2. Photoluminescence Excitation (PLE): To map the absorption linewidth and observe power broadening.
3. Rabi Oscillations: By varying the power of fixed-duration resonant pulses, the team drove the population between the ground ( $|g\rangle$ ) and excited ( $|e\rangle$ ) states to observe coherent control.
4. Ramsey Interferometry: A sequence of two $\pi/2$ pulses separated by a variable delay ( $\tau$ ) was implemented using a free-space Michelson interferometer to probe the phase coherence of the superposition state.

3. Key Contributions

Demonstration of Deterministic Optical Control: The paper provides the first clear evidence of Rabi oscillations up to 5 $\pi$ in a single B center, confirming its behavior as a robust two-level system under pulsed resonant excitation.
High-Purity Single-Photon Generation: The authors achieved a single-photon purity of 93% ( $g^{(2)}(0) = 0.07$ ) at a $\pi$ -pulse, validating the emitter's suitability for triggered quantum applications.
Direct Measurement of Coherence: Using Ramsey interferometry, the team directly measured the inhomogeneous coherence time ( $T_2^*$ ) of the optical transition, a metric previously unquantified for this specific emitter under these conditions.
Validation of hBN as a Quantum Platform: The work establishes B centers in hBN as viable candidates for scalable, coherent quantum photonics, comparable to established systems like NV centers and quantum dots.

4. Key Results

Radiative Lifetime and Linewidth:
- The radiative lifetime was measured as $T_1 = 1.95$ ns, corresponding to a Fourier-limited homogeneous linewidth of $\approx 82$ MHz.
- PLE spectroscopy revealed an inhomogeneously broadened linewidth of 970 MHz at saturation power, indicating spectral diffusion typical of solid-state defects but small enough to allow coherent control.
Rabi Oscillations:
- Clear oscillations were observed as a function of the square root of the pulse power.
- The first $\pi$ -pulse (full population inversion) occurred at 11.63 $\mu$ W.
- Oscillations were sustained up to 5 $\pi$ at 250 $\mu$ W. The minima did not reach zero due to phonon-assisted excitation and slow spectral wandering.
Single-Photon Purity:
- Under $\pi$ -pulse excitation, the second-order autocorrelation function yielded $g^{(2)}(0) = 0.07$ (raw), confirming high-purity single-photon emission without background correction.
Coherence Time ( $T_2^*$ ):
- Ramsey interferometry revealed a Gaussian decay of interference visibility.
- The extracted inhomogeneous coherence time was $T_2^* = 0.60$ ns.
- This value is remarkably close to the lifetime-limited upper bound ( $2T_1 = 3.90$ ns), suggesting that the B center's coherence is intrinsically strong and only moderately perturbed by environmental noise (spectral diffusion), even without active stabilization schemes.

5. Significance and Outlook

This work represents a critical step toward integrating hBN-based quantum emitters into scalable quantum photonic platforms.

Robustness: The ability to achieve coherent control and high-purity emission without electrical or laser-based stabilization highlights the robustness of the B center's optical transition.
Scalability: The compatibility of hBN with nanophotonic structures (e.g., photonic crystals, Bragg gratings) suggests that future integration could provide Purcell enhancement. This would shorten the radiative lifetime, thereby increasing the homogeneous coherence limit and further mitigating the relative impact of spectral diffusion.
Future Applications: The demonstrated control over the Bloch sphere rotations positions B centers as a promising building block for quantum networks, quantum repeaters, and on-chip quantum computing based on two-dimensional materials.

In conclusion, the paper establishes the B center in hBN as a fully addressable, coherent, and deterministic quantum emitter, bridging the gap between material discovery and practical quantum device implementation.