🔬 mesoscale physics

A photonic source with half-a-GHz single-photon flux

This paper reports a deterministic single-photon source that achieves an in-fibre single-photon flux exceeding 500 MHz with over 100 pW of optical power by combining high excitation rates with high system efficiency.

Original authors: P. Zahalka, S. Huijser, A. Pancaldi, S. Kruger, X. Zhao, Z. Liu, I. Suleiman, R. Jensen, L. Stefan, A. Ludwig, V. Remesh, J. C. Loredo, P. Lodahl

Published 2026-09-07

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CC BY 4.0

Original authors: P. Zahalka, S. Huijser, A. Pancaldi, S. Kruger, X. Zhao, Z. Liu, I. Suleiman, R. Jensen, L. Stefan, A. Ludwig, V. Remesh, J. C. Loredo, P. Lodahl

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

In the realm of quantum technology, the ability to generate light one particle at a time is a fundamental requirement for building secure communication networks and powerful new computers. These individual particles of light, known as photons, act as carriers of information that cannot be copied or intercepted without detection. However, for these systems to work effectively, the source of these photons must be both pure and fast. Purity ensures that the device emits exactly one photon at a time, rather than a random burst of many, while speed determines how much information can be sent in a given moment. The faster the source can produce these single particles, the more practical the technology becomes for real-world use, moving from slow laboratory experiments to high-speed data transmission.

A team of researchers has now pushed the boundaries of this speed, creating a device that generates a stream of single photons at a rate exceeding 500 million per second. This achievement represents the highest flux of single photons ever reported from a solid-state source. The researchers achieved this by taking a continuous beam of laser light and slicing it into extremely short, precise pulses. They then used these pulses to excite a tiny semiconductor structure, known as a quantum dot, which acts as an artificial atom. When struck by the laser pulse, this quantum dot absorbs the energy and immediately re-emits it as a single photon. By repeating this process half a billion times every second, the team created a steady, high-speed stream of light that is powerful enough to be measured directly with standard equipment.

The key to this high speed lies in how the researchers prepared the laser light. Instead of using a laser that naturally pulses, they started with a steady, continuous beam and used electronic switches to carve out short bursts of light. These bursts were timed with extreme precision, allowing the researchers to control exactly when each pulse arrived. They tested this system at different speeds, eventually driving the quantum dot at a rate of one billion pulses per second. At this speed, the quantum dot is excited again before it has fully finished emitting the previous photon, creating a situation where the signals begin to overlap. Despite this crowding, the system maintained a high level of quality, successfully delivering over 500 million single photons per second into an optical fiber.

To verify the performance of their source, the team measured the light output using a standard power meter, a device commonly found in laboratories for measuring light intensity. This approach provided a direct and simple way to calculate the efficiency of their system without needing complex, specialized detectors. They found that at their highest operating speed, the source delivered an optical power of over 100 picowatts. This might sound like a tiny amount of energy, but for a stream of single photons, it is a significant amount that confirms the device is working with high efficiency. The measurements showed that the system retained an efficiency of over 50 percent, meaning that more than half of the photons generated by the quantum dot successfully made it into the fiber cable for use.

The researchers also examined the quality of the photons to ensure they were suitable for advanced quantum applications. They checked that the light was indeed coming in single packets and that the photons were indistinguishable from one another, a property required for quantum computers to perform calculations. While the quality did decrease slightly as the speed increased due to the overlapping of signals, the results remained robust. The team demonstrated that even at these extreme rates, the source could produce photons with the necessary purity and indistinguishability for many practical tasks. This work suggests that by simply increasing the speed of the laser pulses, it is possible to generate massive amounts of quantum light without sacrificing the fundamental properties that make it useful.

This breakthrough offers a new path for developing quantum technologies that require high data rates. Because the source produces enough light to be measured with off-the-shelf equipment, it simplifies the process of characterizing and calibrating these devices. The ability to generate such a high flux of single photons opens the door for more complex experiments in quantum simulation and could serve as a reliable standard for measuring light in metrology. By proving that a solid-state source can operate at gigahertz speeds while maintaining high efficiency, the researchers have provided a crucial component for the next generation of quantum information systems, bringing the promise of ultra-fast quantum communication closer to reality.

Technical Summary: A Photonic Source with Half-a-GHz Single-Photon Flux

Problem and Motivation
Advanced optical quantum technologies, including quantum information processing, quantum key distribution (QKD), and quantum-enhanced machine learning, fundamentally rely on deterministic single-photon sources. The throughput of these protocols scales linearly with the source clock rate but decreases exponentially with source efficiency. Furthermore, in secure communication via optical fibers, signal losses and decoherence necessitate high repetition rates to avoid prohibitively long integration times. Consequently, there is a critical demand for photonic sources that deliver pure, indistinguishable photons at rates directly useful to end-users, specifically targeting the highest possible single-photon flux.

Methodology
The authors developed a deterministic single-photon source based on an InAs/GaAs quantum dot (QD) embedded in a photonic crystal waveguide (PCW). The experimental approach involved three primary components:

Pulse Carving: To achieve tunable, high-frequency excitation, the team generated laser pulses by carving a continuous-wave (CW) laser (linewidth < 1 kHz) using two cascaded, temperature-controlled lithium-niobate intensity electro-optic modulators (EOMs). An FPGA controlled the repetition rate and relative timing, while a fast pulse generator defined the electrical pulse duration (adjustable between 40 ps and 100 ps). This setup produced coherent optical pulses with a 40 ps full-width at half-maximum (FWHM) and over 40 dB extinction contrast at driving rates up to 1 GHz.
Quantum Dot Excitation: The carved pulses were used to resonantly excite a neutral exciton state in the QD, which has an emission wavelength of 933.1 nm and a decay lifetime ( $\tau$ ) of 220 ps. Notably, the collection optics were deliberately designed without spectral filters to report the intrinsic performance limits of the source, including phonon sideband emission.
Characterization: The source was characterized using time-trace measurements, second-order autocorrelation ( $g^{(2)}(\tau)$ ), and Hong-Ou-Mandel (HOM) interference experiments. Crucially, the optical power of the single-photon stream was measured directly using a standard, off-the-shelf optical powermeter to determine system efficiency.

Key Results
The study successfully demonstrated a single-photon source operating at excitation rates up to 1 GHz, achieving the following metrics:

Single-Photon Flux: At a 1 GHz excitation rate, the source delivered an in-fibre single-photon flux of 515 MHz. This represents the highest single-photon flux reported to date.
System Efficiency: The system efficiency was measured directly via optical power. At 1 GHz excitation, the source produced an optical power of 110 pW (at $\pi$ -pulse drive), corresponding to a system efficiency of 51.5%. Efficiencies of 56.2% and 53.8% were recorded at 80 MHz and 500 MHz, respectively.
Photon Quality:
- Purity: The second-order autocorrelation at zero delay, $g^{(2)}(0)$ , was measured at (3.98±0.01)% for 80 MHz, (3.63±0.02)% for 500 MHz, and (11.13±0.03)% for 1 GHz. The increase at 1 GHz is attributed to the overlap of subsequent single-photon pulses given the 220 ps lifetime relative to the 1 ns period.
- Indistinguishability: Two-photon interference (HOM) measurements yielded visibilities ( $V_{HOM}$ ) of (81.83±0.03)% at 80 MHz, (80.87±0.03)% at 500 MHz, and (55.14±0.03)% at 1 GHz. The authors note that these values are obtained without spectral filtering; adding filters improved $V_{HOM}$ to 92.30% at 80 MHz but incurred a ~12% loss in transmission.
Pulse Dynamics: The 40 ps excitation pulses are significantly longer than the 220 ps emitter lifetime. While such conditions typically increase multi-photon emission, the PCW geometry facilitates destructive interference between laser leakage and multi-photon fields, maintaining high single-photon purity.

Significance and Claims
The paper claims that by combining GHz excitation rates with system efficiencies exceeding 50%, the authors have produced a single-photon stream with over 500 MHz flux, setting a new benchmark for solid-state sources.

A distinct contribution of this work is the methodology for efficiency characterization. By delivering sufficient optical power (>100 pW) to be detected by standard powermeters, the authors provide a "simple and direct way" of determining fiber efficiency. This approach avoids the complex calibration requirements of superconducting nanowire single-photon detectors (SNSPDs), which often rely on assumptions regarding dead-times, bias currents, and photon-number dependent efficiencies.

The authors suggest immediate applications for this source as a power standard in metrology or as a de-multiplexed source for many-photon quantum simulation experiments operating in the 10–20 photon range. They anticipate that this advancement will unlock and enable various experimental photonic quantum information protocols that were previously limited by source flux.

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