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On-Sky Single-photon Time resolution of 35 ps with White Rabbit synchronization: towards the measurement of the size of a White Dwarf star

The IC4Stars project reports achieving an on-sky single-photon time resolution of 35 ps RMS by synchronizing two TDCs via the White Rabbit protocol over a 30 m fiber, marking a significant step toward measuring the diameter of the white dwarf Sirius B using intensity interferometry.

Original authors: F. Izraelevitch-Patitucci, S. Tolilla, I. Ellafi, J. -P. Rivet, M. Hugbart, G. Labeyrie, O. Lai, A. Zmija, W. Guerin, R. Kaiser

Published 2026-06-25

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Original authors: F. Izraelevitch-Patitucci, S. Tolilla, I. Ellafi, J. -P. Rivet, M. Hugbart, G. Labeyrie, O. Lai, A. Zmija, W. Guerin, R. Kaiser

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 trying to measure the size of a tiny, distant star that is so small it looks like a single point of light, even through the world's biggest telescopes. It's like trying to measure the width of a human hair from the top of a skyscraper. This is the challenge the IC4Stars team took on. Their goal? To measure the size of Sirius B, a "White Dwarf" star (the burnt-out core of a star) that is roughly the size of Earth but weighs as much as our Sun.

To do this, they didn't use a giant camera. Instead, they used a technique called Intensity Interferometry. Think of it like this: instead of trying to take a picture of the star, they are listening to the "rhythm" of the starlight. When light from a star hits two different detectors, the photons (particles of light) tend to arrive in little "bunches" or pairs. By measuring exactly how close together these bunches arrive, scientists can figure out how big the star is.

Here is the simple breakdown of their recent success, using everyday analogies:

1. The Problem: Timing is Everything

To catch these tiny "bunches" of light, the team needs two telescopes working together, separated by about a kilometer. They need to know exactly when a photon hits Telescope A and when it hits Telescope B. The difference in time must be measured in picoseconds (one-trillionth of a second).

If their clocks are even slightly off, the "rhythm" gets lost, and the measurement fails. It's like trying to clap in perfect unison with a friend who is a mile away; if your watches aren't perfectly synced, you'll never clap at the exact same moment.

2. The Solution: The "White Rabbit" Synchronization

The team used a special protocol called White Rabbit to sync their clocks.

The Analogy: Imagine two runners in different stadiums. To make sure they start at the exact same time, they don't just look at their own watches. Instead, they are connected by a super-fast fiber-optic cable (like a 30-meter long "telephone wire" made of light). This cable acts as a master conductor, telling both runners, "Go!" at the exact same nanosecond.
The Result: They successfully synced two separate computers (Time-to-Digital Converters, or TDCs) using this fiber cable. The "jitter" (or wobble) in their timing was incredibly small—only about 18 picoseconds. This is like two people clapping with a delay so tiny it's practically instantaneous.

3. The Detectors: Catching the Light

The team used special sensors called PhotonPix detectors to catch the starlight.

The Challenge: These detectors can get "clogged" if too much light hits a tiny spot all at once, much like a highway getting jammed if too many cars try to enter a single lane.
The Fix: They figured out that if they spread the starlight out over a wider area (like widening the highway to 10 lanes), the detectors could handle the traffic smoothly without getting confused or slowing down. They also adjusted the "voltage" (the energy pushing the electrons) to make the detectors react as fast as possible.

4. The Big Test: On-Sky Results

In March 2026, they set up their equipment on the Epsilon Telescope in Nice, France. They pointed it at bright stars like Regulus and Vega to test their system.

The Achievement: They successfully measured the "bunching" of light from the stars.
The Resolution: Their entire system (detectors + clocks + cables) achieved a time resolution of about 35 picoseconds.
The Analogy: This is a massive improvement. If their previous setup was like trying to time a race with a stopwatch that had a slow, sticky button, this new setup is like using a high-speed camera that can freeze a bullet in mid-air. They improved their timing precision by a factor of 10.

5. What This Means for the Future

The paper confirms that their "rhythm-measuring" machine works perfectly in the real world, not just in the lab.

The Goal: Now that they know their system is accurate, they are ready to tackle the main target: Sirius B.
The Next Step: They plan to set up two telescopes far apart (about 1 kilometer) and link them with this same fiber-optic "White Rabbit" cable. By measuring the light from Sirius B with this super-precise timing, they will finally be able to calculate its exact size.

In summary: The team built a super-precise "light stopwatch" that can sync two separate telescopes down to a trillionth of a second. They proved it works by catching the rhythm of starlight, and now they are ready to use it to measure the size of a tiny, dead star that has been a mystery for a long time.

Technical Summary: On-Sky Single-photon Time Resolution of 35 ps with White Rabbit Synchronization

Problem Statement
The IC4Stars (Intensity Correlation for Stars) project aims to measure the angular diameter of the white dwarf Sirius B using intensity interferometry. Theoretical predictions suggest Sirius B has an Earth-sized radius and a solar mass, resulting in an angular diameter of approximately 30 micro-arcseconds (µas). Resolving this object in the blue spectral range requires a telescope separation of roughly one kilometer. Achieving a Signal-to-Noise Ratio (SNR) of ~10 over a few nights at 8-meter-class telescopes necessitates overcoming significant technical hurdles, specifically:

Achieving single-photon detection with time resolution in the tens of picoseconds scale.
Synchronizing two instruments separated by ~1 km with precision exceeding the detector's time resolution.
Managing raw time-tag data for online monitoring and analysis.
Developing efficient algorithms to compute the second-order correlation function, $g^{(2)}$ , in real-time and post-analysis.

Methodology
The authors present a prototype instrument and a series of characterization tests leading to an on-sky observation campaign conducted in March 2026 at the 1-meter Epsilon Telescope (C2PU, Observatoire de la Côte d'Azur).

Detector Characterization: The system utilizes PhotonPix single-photon detectors (Photonscore) featuring a photocathode and Microchannel Plate (MCP). Laboratory tests characterized the detector's jitter as a function of Bias Voltage and beam diameter. The team identified a "local saturation" effect where focusing a photon stream into a small diameter (<1 mm) degrades jitter performance and reduces count rates. To mitigate this, the experimental setup employed a 6-mm beam diameter to ensure uniform illumination of the sensitive area.
Synchronization and DAQ: The Data Acquisition System (DAQ) consists of two Time Tagger Ultra (Swabian Instruments) units, each disciplined by a White Rabbit (WR) synchronization unit (Safran WR-LEN). The WR units are linked via a 30-meter telecom optical fiber, providing a common 10 MHz clock and 1 PPS signal. A Pulse Generator provided reference signals to characterize the system's intrinsic jitter in the laboratory.
On-Sky Setup: The optical module, mounted on the telescope's Cassegrain focus, included a divergent lens (6 mm beam), a dichroic mirror, a 417-nm bandpass filter (1-nm bandwidth), and a 50:50 beam splitter directing light to the two detectors. The DAQ stations were mounted directly on the telescope's back face to minimize cable lengths.
Observation Strategy: The campaign observed bright, unresolved stars (Regulus and Vega) to measure the temporal bunching peak at zero baseline. Data acquisition involved recording raw time tags from both TDCs, which were then processed to compute the second-order correlation function.

Key Results

Laboratory Characterization:
- Detector jitter decreased with increasing bias voltage; the system operated at maximum bias.
- Jitter increased significantly when the beam diameter was reduced below 3 mm due to local saturation.
- The Time Tagger Ultra demonstrated an intrinsic jitter of $\sigma_{TDC} \approx 11.0$ ps.
- The White Rabbit synchronization units demonstrated a synchronization jitter of $\sigma_{WR} \approx 8.8$ ps.
- The complete laboratory DAQ system (two TDCs + two WRs + 30m fiber) achieved a total jitter of $\sigma_{DAQ}^{(lab)} = 18.9$ ps RMS.
On-Sky Performance:
- DAQ Jitter: Field measurements of the DAQ system (using the Pulse Generator reference) yielded a jitter of $\sigma_{DAQ}^{(field)} = 18.0$ ps RMS, consistent with laboratory benchmarks.
- Total Time Resolution: The analysis of the temporal second-order correlation function, $g^{(2)}(t) - 1$ , resulted in a total system time resolution of $\sigma_{Tot}^{(field)} = (34.7 \pm 4.4)$ ps RMS. This value is consistent with the theoretical sum of the DAQ and detector contributions ( $\sigma_{Tot}^{(lab)} = 31.7$ ps).
- Coherence Time and SNR: The measured coherence time was $(0.426 \pm 0.051)$ ps, consistent with the expected 0.492 ps derived from filter transmission data. The observed SNR was 8.19, matching the expected 8.28.
- Count Rate Discrepancy: The observed count rate was approximately 40% lower than theoretical predictions based on stellar luminosity, atmospheric extinction, and detector efficiency. The authors note this deficit is under investigation.

Significance and Claims
The paper claims a significant milestone in the development of intensity interferometry for stellar diameter measurement. The primary achievement is the demonstration of an on-sky time resolution of approximately 35 ps, representing a factor of 10 improvement over previous setups. The authors assert that:

The system successfully synchronized two independent TDCs over a 30-meter fiber link using the White Rabbit protocol with 18.0 ps RMS precision.
The total system resolution (detectors + electronics + synchronization) is consistent with laboratory characterizations, validating the scalability of the approach.
The successful measurement of the bunching peak with an SNR consistent with predictions confirms the viability of the data acquisition and correlation algorithms for future, larger-scale campaigns.

The authors conclude that this resolution level implies that future telescope baselines must be known with millimeter-scale precision to fully exploit the system's capabilities. They plan to deploy the system across two separated telescopes to perform spatial intensity interferometry, a necessary step toward the ultimate goal of measuring the diameter of Sirius B.

1. The Problem: Timing is Everything

2. The Solution: The "White Rabbit" Synchronization

3. The Detectors: Catching the Light

4. The Big Test: On-Sky Results

5. What This Means for the Future

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