Time-Correlated Single Photon Counting (TCSPC)

Time-Correlated Single Photon Counting (TCSPC) is a technique that measures the precise arrival times of individual photons emitted by a sample following an excitation with a pulsed light source. By referencing the timing of each detected photon to a corresponding excitation pulse, TCSPC effectively addresses many challenges inherent to photon-based measurements, such as weak signals, fast decay rates, and the need for high temporal resolution.

The intrinsic high sensitivity and precision of TCSPC are integral to applications in quantum optics and quantum communication, such as single-photon source characterization, Hanbury Brown and Twiss (HBT) measurements, second-order correlation ( $g^2$) analysis, antibunching measurements, and coincidence experiments. TCSPC is also widely used in life sciences for optical tomography and rapid fluorescence lifetime imaging microscopy (FLIM), in materials science for fluorescence spectroscopy, and in metrology for intensity interferometry, satellite laser ranging (SLR), and light detection and ranging (LIDAR).

To facilitate this level of precision, advanced single-photon detectors (SPDs) record the arrival times of incident photons with high sensitivity. Timing electronics, or “time tagging” devices, stamp and record the time delay between the excitation pulse and photon detection event as “start” and “stop” signals, respectively.

These detected time delays (or “time tags”) enable a range of measurements, including auto-correlation and cross-correlation, multi-channel photon counting, fluorescence lifetime imaging (FLIM), and real-time accumulation of time delay data into histograms. The potential for TCSPC relies on the accuracy and precision of timing electronics to acquire and analyze signals, which is dependent on such devices’ hardware and software capabilities, respectively.

Cutting-edge timing electronics provide endless capabilities for TCSPC applications. Swabian Instruments Time Tagger Series combines powerful and adaptable hardware with intuitive software to address the diverse demands of photon-based research, making it one of the most versatile solutions available for event timing.

• From a hardware perspective, Swabian Instruments’ Time Tagger offer ultra-low timing jitter, multiple input channels, and fast-speed acquisitions, combined with high transfer rates to analyze your data real-time.
• Swabian Instruments TCSPC analysis software sets them apart from competitors. One can run multiple real-time measurements simultaneously either in the TimeTaggerLab GUI or in the intuitive API (supporting Python, MATLAB, LabView, C#, C++, etc.). This unique approach to data analysis enables researchers to improve work efficiency and expedite research timelines.

Timing Electronics are crucial in Time-Correlated Single Photon Counting (TCSPC) experiments as they can record photon arrival times with high accuracy and precision. The key hardware specifications that increase the reliability of TCSPC experiments are:

Timing jitter

• Timing jitter refers to the uncertainty in determining the exact time at which a photon detection event occurs. Timing electronics with low timing jitter result in improved resolution and precision, enabling measurements to more accurately represent the actual timing of events. Acquiring timing electronics with insufficient timing resolution would result in low-quality and inaccurate time-correlated data.

  Note 1: When determining the most suitable timing electronics device for your application, one should consider the different conventions with which detectors and electronics manufacturers list timing jitters. Generally speaking, detector manufacturers provide their specifications in FWHM jitter (“Full Width Half Max”), whilst timing electronics manufacturers provide their specifications in RMS jitter (“Root Mean Squared”). The relationship between these two conventions is given by the following equation: FWHM=2.35RMS.

• The Swabian Instruments Time Tagger Series accommodates a wide range of time jitter requirements, ensuring flexibility for various experimental setups. TCSPC measurements with timing jitter as low as 1.5 ps RMS have been achieved using the Time Tagger X in HighRes mode, demonstrating its capability to support highly precise applications.

  Note 2: Timing jitter should not be confused with digital resolution/bin width. Timing jitter reflects the intrinsic precision of a device in acquiring the timing of photon detection events, whereas digital resolution defines the smallest bin width in a histogram X-axis that can be selected when analyzing the acquired data, which is key to identifying the optimal smoothing of a curve/amount of detail to best represent a piece of data. While both contribute to system performance, they serve different roles in TCSPC experiments.

Input Channels

• Input channels refer to the channels that capture signals from the connected devices, which are then precisely digitized and time-stamped. The higher the number of channels and flexibility in signal inputs, the improved versatility of a time-tagging device.

  • Swabian Instruments’ Time Taggers feature fully independent and synchronized input channels. This allows users to define Start/Stop/Sync signals to best tailor to their experimental needs.

  • The Time Taggers support multiple signal types (TTL, NIM, sinusoidal, etc.), seamlessly integrating with a variety of equipment, including single-photon detectors, lasers, oscillators, external clocks, signal generators, and positional markers such as pixel and line triggers or piezo and XYZ stages. This versatility ensures compatibility with a wide range of experimental setups.

  • While a single Time Tagger may accommodate up to 20 inputs, applications requiring scalability can leverage up to 160 channels by synchronizing multiple units seamlessly with Swabian Instruments’ Synchronizer. Synchronizing more than 8 devices can be possible by feeding an external reference generating a frequency and 1PPS signals. The limit of the number of synchronized devices depends on the scalability of the synchronization technology. The ability to scale input channels is critical for experiments or industrial applications involving multiple detectors or signal sources.

• By providing flexible channel configurations, seamless integration with diverse equipment, and high scalability, the Time Tagger Series supports current and future experimental needs and offers users long-term, adaptable solutions.

Dead time

• Dead time refers to the period after a signal has been detected during which the same input channel cannot register a subsequent signal. Low dead time is necessary to ensure that all data “picked up” by the detectors is successfully acquired, minimizing missed photon counts and preserving the integrity of the measurement.

  Note 3: For Swabian Instruments’ Time Taggers, the minimum channel dead time is inversely proportional to the maximum input frequency of such channel. To date, the dead time of Swabian Instruments Time Taggers is shorter than that of any single photon counting detector in the market, ensuring that the Time Tagger does not become a bottleneck in high-speed experiments. In situations of afterpulsing, or when detector dark counts may be undesirably tagged, one can continuously adjust the synthetic dead time per channel to maximize signal-to-noise ratio (SNR).

Data transfer rate

• The data transfer rate corresponds to the total amount of data streamed from all input channels to the analysis device, generally a PC. A high transfer rate allows for the capture and transmission of larger photon counts within shorter time windows, improving the experiment speed and statistical significance of measurements. Insufficient transfer rates can result in overflows and data losses.

• When leveraging low data transfer rate interfaces (e.g., USB 2.0 for the Time Tagger 20), the interface may be the limiting factor. The USB 2.0 interface supports transfer rates of 9 MTags/s, offering a balance between speed and simplicity. For faster data rate requirements, USB 3.0 can be leveraged for 90+ MTags/s with the Time Tagger Ultra and TIme Tagger X. In this case, the bottleneck to achieve the maximum possible data rates may be associated with the single-thread performance of the CPU. More information on overall PC requirements and CPU performance can be found in our FAQ section.

  • Note 4: Swabian Instruments offers multiple solutions to mitigate the risk of overflow during data-intensive experiments. Data Filtering options such as the ConditionalFilter() and EventDivider() allow users to maintain data integrity while maximizing the efficiency of high-speed acquisitions.

  • Note 5: For fast-feedback, low-latency outputs, the FPGA link interface can be leveraged for greater throughput (300 MTags/s and 1200 MTags/s via SFP+ and QSFP+, respectively), although with increased computational demands. With this approach, time tags are output to a secondary external FPGA processing. The measurement classes available with this approach are currently limited to CountBetweenMarkers, Histogram, and Coincidences/Combinations. A reference guide can be found on Swabian Instruments’ Github page.

Beyond the key hardware specifications (jitter, dead time, data transfer rate, number of channels), powerful TCSPC software is necessary to harness the full potential of experimental data.

Swabian Instruments’ unique approach to data analysis features a library of built-in analysis functions to carry out multiple real-time measurements simultaneously with flexibility. One can either run measurements in just a few lines of code (API available in most common programming languages such as Python, Matlab, LabVIEW, C#/C++, Mathematica, and .NET…) or with the TimeTaggerLabGUI, for even faster implementation.

When evaluating the potential of the software-defined measurements architecture characteristic of Swabian Instruments Time Taggers, one can highlight:

• Real-time acquisition and processing of incoming photon events (“time tags”) is crucial for TCSPC, as it allows for users to ensure they are achieving their desired results while their measurement is running. Swabian Instruments’ Time Taggers acquire data from multiple channels and stream it to the PC in real-time, where the software engine takes over.

• Simultaneous analysis of measurements is possible due to the software-defined architecture through which the data stream is processed and made accessible for concurrent measurements. From a single experiment, one can extract a multitude of information by performing parallel processing of the raw timing data.

Swabian Instruments’ Time Tagger Series software solution features both real-time processing and simultaneous analysis, for advanced applications within the realm of TCSPC.

With this data acquisition method and software-defined approach to data analysis, Swabian Instruments is committed to supporting you in significantly optimizing the efficiency at which you conduct your TCSPC experiments, resulting in expedited research timelines. If you have any questions, we are here to help!