Fast Time-To-Digital Converters As Ultra-Precise Stopwatches For Quantum Technologies

Quantum technologies enable versatile novel applications in modern engineering topics such as information processing, communication or sensing. In particular, photonic quantum technologies are an innovative field of development which, based on the quantization of light, implements a qubit for example in the polarization or phase of a single photon, or in other degrees of freedom of the electromagnetic field. The optical manipulation of single-photon states is ultimately followed by single-photon detections and the registration of detection times in so-called time-taggers. Time-to-digital converters (TDCs), circuits that detect electrical pulses and provide a digital representation of their time of arrival, are the core building block of every time-tagger. Therefore, TDCs play a major role in quantum optics, and their accuracy and performance directly impact the quantum photonics setup.

Quantum photonics puts into practice general quantum principles such as state superposition (in analogy to Schrödinger’s cat state) or entanglement (strong beyond-classical correlation of degrees of freedom) and makes them available in engineering applications: photonic quantum gates constitute a building block for a quantum computer, believed to reach a complexity sufficient for outperforming classical computers in domains like materials science, pharmacy, physics or cryptography. Quantum communication shows a way of utilizing single photons for an unconditionally secure transmission of information, invulnerable to any kind of attack. Several classes of quantum sensing approaches, such as nitrogen-vacancy centers in diamond, employ photons for reading out a quantum state and thereby enable a high-sensitivity measurement of magnetic or electric fields, of mechanical strain or temperature.

Furthermore, non-quantum single-photon analysis is employed in fluorescence lifetime measurements and fluorescence lifetime imaging for an investigation of molecular structures and of fast dynamical processes at molecular level in medical and biological applications. Lidar enables distance measurements and 3d scanning based on the time-of-flight analysis of single photons or weak-intensity light with applications in autonomous driving, robotics, geodesy and others.

All these techniques rely on performant TDCs as the interface between the optical setup and the subsequent electronic analysis. Their accuracy and precision are limiting factors for the achievable rate of data transmission or data processing, with timing jitter requirements ranging from the nanosecond regime to well below 10ps. Distributed setups involving more than a single TDC-unit require furthermore an adequate synchronization option.

There are numerous ways to implement a TDC for an integrated circuit. Typically, TDCs perform a fine and a coarse time measurement. The latter can be implemented by a synchronous counter. Its resolution is limited by the clock frequency, and the long-term accuracy strongly depends on a stable clock source. The fine counter extends the resolution far below the clock period. For ASIC designs, a resolution in the sub-picosecond range is possible, whereas FPGA implementations achieve single-picosecond resolutions.

Copyright: Fraunhofer IIS/EAS, Foto: BLEND3 Frank Grätz

A complete time tagger requires additional components, such as housing, connectors for input signals, circuitry to support a specific input voltage range, and a suitable data and configuration interface (e.g., USB 3.0 or Ethernet). Depending on the application, additional features such as built-in calibration or integrated postprocessing of timetags (i.e. compressed sensing) may also be required. Field-Programmable Gate Arrays (FPGAs) have proven to be an effective platform for such requirements. There are off-the shelf modules that readily provide communication modules and on-board memory to capture burst events. Another option are SoCs that combine an FPGA with a multicore processor system. These devices are capable of running embedded operating systems and are very flexible. Since the silicon is fixed, performance cannot be increased indefinitely, but with techniques such as wave union, channel averaging, or multiphase designs, jitter values below 10ps are feasible.

For now and the near future, flexible and cost-efficient FPGAs dominate the market. Advances in the quantum-technological setups, especially in detector technology, will further increase performance requirements for time-taggers and necessitate new implementation approaches. Even more important may be a grow in demand. For high-volume products we definitely expect to see a shift towards ASIC designs or chiplet based concepts. These developments will go hand in hand with a miniaturization of time-taggers to well below 19” form factors, presently dominating the market, eventually down to a co-packaging together with photonic integrated circuits and further microelectronic processing circuitry to provide entire chips for the relevant quantum applications in the future.