| 日期 07 November 2025
Lennart Jehle, a PhD researcher in Prof. Philip Walther’s group at the University of Vienna, works on experiments in quantum optics and photonics. The Walther Group focuses on controlling single photons for applications in quantum computing and quantum communication.
In his research, Lennart studies how to build a reliable single-photon source that emits exactly one photon at a time with high efficiency. For this, he uses Swabian Instruments’ Time Tagger series, which provides the timing precision needed to observe photon events that happen within only a few picoseconds and a flexible way of analyzing the data. The instruments have become an important part of his work, helping him measure and analyze effects that were previously difficult to detect.
In his recent project, Lennart investigated how re-excitation occurs in quantum dots (QD) under phonon-assisted excitation, analyzing the temporal and spectral behavior of the emitted photons and showing how this newly found understanding can help to reduce multiphoton noise while maintaining high single-photon efficiency. Even when a QD is excited by a laser pulse that lasts only a few picoseconds, there is a chance that a photon is emitted while the QD is still interacting with the laser pulse, such that it can be excited a second time. These two photons can be separated by as little as a few trillionths of a second, which makes them extremely difficult to detect as distinct events.
For Lennart’s research, resolving the two closely spaced photons was essential. These two photons are detected by two different detectors connected to separate channels of the Time Tagger so that both events can be seen individually. However, if the timing resolution is too low, it becomes impossible to determine which photon arrived first. Using Swabian Instruments’ Time Tagger, Lennart was able to resolve this timing precisely and study which of the two happened first 1. The setup also allowed him to store all raw time-tag data and later test new analysis ideas on the same measurements without repeating long experiments.
“… [the user] can just take the same data and process it in a different way, without repeating the measurement. That really made life a lot easier, you can try out new things even after the experiment is done. That’s why I became a big fan of storing all our data.” - Lennart explained.
From the beginning of his PhD, Lennart’s lab integrated Swabian Instruments’ Time Taggers into their setup. Starting with Time Tagger 20 and later moving to the Time Tagger Ultra and Time Tagger X, the team benefited from the instruments’ mix of powerful hardware and easy-to-use software. The GUI allowed for quick checks in the lab, while the Python API offered deep control for custom logic and data processing. Together, they created a workflow that encouraged curiosity and experimentation.
As Lennart explained, “When the ‘one-hour task’ becomes a ‘three-minute test,’ you try things you otherwise wouldn’t.” That flexibility made a real difference in how the team worked day to day. They could capture raw time tags, test different gating and correlation methods, and analyze the data in real time or replay the stored data later to test different ideas.
In his setup, he uses a Hanbury-Brown and Twiss configuration. The detectors and an electronic clock fed signals into the Time Tagger, allowing him to define gated and delayed virtual channels to isolate and analyze specific events. By using these, he could reduce multiphoton noise, study re-excitation dynamics, and refine the experiment’s signal-to-noise ratio without physically changing the setup, and measure the coincidences and correlations of photon events.
Upgrading to the Time Tagger X made a clear difference in the experiment. With the capability of RMS timing jitter of 3 ps, the Time Tagger no longer contributed noticeably to the overall timing uncertainty, allowing Lennart to directly distinguish the first and second photon events that occur in quick succession.
With the improved timing resolution, Lennart’s group was able to directly observe the first and second photon events that occur in quick succession during re-excitation. Previous studies had only inferred this effect from theory or indirect measurements, but their experiment made it visible. This result provided clearer experimental evidence of how re-excitation happens in quantum dots and helped the team better understand how to reduce unwanted multiphoton events when developing reliable single-photon sources for quantum communication.
Beyond the physics, the Time Tagger changed the team’s research rhythm. The intuitive software and reliable hardware made it easier to test ideas, store data, and analyze new hypotheses later, saving both time and resources. The group’s workflow became more streamlined, and their confidence in the collected data increased. Lennart has since co-authored several publications that relied on these measurements and plans to continue exploring theoretical collaborations that build on the same time-resolved data.
Throughout the project, Lennart highlighted how simple it was to get started with the Time Tagger. The interface, Python API, and documentation helped him learn quickly and focus on physics rather than code. When specific questions arose, the Swabian Instruments support team responded promptly, often with ready-to-use solutions. This reliability and partnership mindset were key factors in maintaining momentum during critical phases of the research.
He also mentioned a few new ideas for future development, such as extending the virtual-channel logic for live measurements. This feature is already under development, and feedback like Lennart’s helps guide these improvements to make the tools even more versatile for the academic community.
Lennart’s work shows how precise timing and practical tools can make demanding experiments easier to manage. The Time Tagger gave him the resolution needed to observe closely spaced photon events and the flexibility to analyze data in various ways without repeating measurements. What started as curiosity about a theoretical effect became a concrete result that helped him and his colleagues better understand re-excitation and improve the performance of single-photon sources used in quantum optics research; as he said, “For the real experiment, timing resolution is crucial…”
The Time Tagger’s picosecond resolution and flexible software environment provide the foundation for accurate time-correlated measurements and deeper scientific understanding. We’re looking forward to exploring new projects and expanding what’s measurable.
L. Jehle, L. M. Hansen, P. I. Sund, T. W. Sandø, R. Joos, M. Jetter, S. L. Portalupi, M. Bozzio, P. Michler, and P. Walther, “Asymmetric two-photon response of an incoherently driven quantum emitter,” arXiv preprint arXiv:2507.07082, 2025. [Online]. Available: https://arxiv.org/abs/2507.07082 ↩︎
Discover the range of applications that use our products
Read more
Time‐correlated single photon counting (TCSPC) is based on the detection of single photons and the measurement of their arrival me with high me resolution. In this application note we demonstrate a commercially available TCSPC setup with a record breaking low jitter using a Single Quantum Eos SNSPD system and a Swabian Instruments Time Tagger X.
Read more