Photon counting instruments
Initial work focused on the electronics for QKD systems developed at the University of Bristol. In particular, the “Alice” and “Bob” electronics were developed for both the BB84 and reference frame independent systems, the latter funded by Nokia. The Nokia system included considerable mixed signal/RF design for the Lithium Niobate Polarisation controllers.
The “Bob” electronics for the BB84 system primarily consisted of a time tagger, a device which produces a timestamp for every received photon relative to the experiment’s start. This original device transferred timestamps via gigabit ethernet.
To enable wide spread adoption of “Bob’s” time tagging electronics, a generic USB 2.0 instrument was developed (shown below right), based around a Spartan 6 FPGA. The device included adjustable 12-bit input discriminators and configurable outputs to trigger pulsed lasers etc.
The time-tagger achieved a precision of 26ps RMS (shown below left) across two channels or 18.4ps single channel precision. In addition to time-tagging, hardware-based Time Correlated Single Photon Counting (TCSPC) was demonstrated to enable higher detection/excitation rates for TCSPC LiDAR as measurable click/excitation rates were limited by USB 2.0 throughput. A hardware-based coincidence counter was also demonstrated, which enabled detection of all 28-1 combinations. Below right are the measured coincidence windows prior to offset calibration.
The USB 2.0 instrument was re-developed into a 16 input channel USB 3.0 based instrument using the new FX3 Cypress USB IC, with the hardware shown below left. The timing precision was reduced to 18.55ps RMS across two channels (shown below right) or 12.95ps RMS single channel precision.
USB 3 allows for higher tag transfer rates of approximately 40 megatags/s, which gives greater options for post-processing of time-stamps. This was developed during the “Low cost multichannel time tag technology” project.
This instrument was expanded upon in the Attract funded project NXGTDC, to push the number of channels to 32 in addition to 8 configurable outputs. In addition to optimising the logic behind the time to digital converters (TDCs) to meet FPGA timing constraints, effort was placed into optimising the tag format to optimise throughput to approximately 80 megatags/s.
LiDAR systems and electronics
The work undertaken by my (now graduated) PhD student Dr. Mala Sadik as part of the SPLICE (Single Photon Lidar Imaging of Carbon Emissions) project, focused on exploiting photon number resolving detectors (in particular, silicon photomultipliers) to create single photon sensitive LiDAR systems which utilise the received number of photons to build up the correlation faster. This can be thought of a as a combination of a TCSPC instrument in addition to custom designed fast flash-ADC which measures the amplitude of the detector’s output to determine the number of photons to add to a particular bin in the histogram. Below left is a photograph of the hardware developed and below right is a typical amplitude spectrum for 1 to 6 photon events for the silicon multiplier used in the work.
My most recent work has been on the PASCAL (Photon Absorption Spectroscopy Camera for Leaks) project, creating the electronics and signal processing to perform TCSPC imaging across a 5×5 detector array in real-time. To the left is a photograph of the visible demonstrator I built and further below is a YouTube video demonstrating count rate and TCSPC imaging.