Precise time measurement and event correlation have gained a prominent role in a large variety of quantum photonics applications, including quantum communication, quantum simulation, and quantum computing. The increasing complexity and scale of quantum photonic systems, particularly those capable of generating, manipulating, and detecting single photons, necessitate advanced electronic timing systems that can accurately capture coincidence events across multiple channels. A highly effective approach for measuring the time of arrival of a photon, detected by single-photon detectors such as single-photon avalanche diodes (SPADs) or superconducting nanowires, is the cascading of a time-to-amplitude converter (TAC) with an analog-to-digital converter (ADC), a configuration distinguished by its high precision and linearity [1].
Precise time measurement and event correlation have gained a prominent role in a large variety of quantum photonics applications, including quantum communication, quantum simulation, and quantum computing. The increasing complexity and scale of quantum photonic systems, particularly those capable of generating, manipulating, and detecting single photons, necessitate advanced electronic timing systems that can accurately capture coincidence events across multiple channels. A highly effective approach for measuring the time of arrival of a photon, detected by single-photon detectors such as single-photon avalanche diodes (SPADs) or superconducting nanowires, is the cascading of a time-to-amplitude converter (TAC) with an analog-to-digital converter (ADC), a configuration distinguished by its high precision and linearity [1].
In the field of quantum optics, especially in various experiments involving characterizing entangled photon pairs or quantum correlation effects, coincidence systems are primarily employed to detect and record temporal correlations between multiple photon events. Certain experiments propose higher requirements for the time precision of coincidence electronics, aiming to achieve high-precision time measurements with single-channel time resolution of ~10 ps RMS. To support diverse quantum optics experiments, a 32-channel high-precision coincidence module has been designed and fabricated. A series of tests have been conducted, and the results show that the time precision of each channel of the electronics is better than 10 ps RMS. Furthermore, preliminary experimental applications in quantum optics experiments have been successfully conducted.
Entangled photon pairs are created by a system consisting of a 1064 nm pump diode laser that is fiber coupled to a high generation rate photon pair source. The source is a dual element periodically poled potassium titanyl phosphate (KTP) waveguide that up-converts 1064 nm photons to single 532 nm photons in the first stage. In the second stage, the green photons are down converted to energy entangled photon pairs at 800 nm and 1600 nm. The output photon pairs are guided by fiber to sorting optics where they are separated and sent into high-efficiency photon detectors. In particular, the 1600 nm photons are detected by a superconducting nanowire with efficiency over 60% and dead time less than 50 ns. Detector output electrical signals are sent to a time tagger with bin resolution as narrow as 25 ps for coincidence counting. The ultimate goal of this setup is to demonstrate a singlesource, high efficiency, high data rate, quantum communication system to enable Earth-space quantum networks. Of particular interest is a source of entangled photons that is amenable to utilization in aircraft and spacecraft under rigorous flight and environmental conditions. Test results that characterize the entangled photon pair creation and detection capabilities of our system will be presented.
The ability to generate and handle quantum correlations in twin-beam states is the foundation to overcome the imaging limitations imposed by classical light [1], relying on efficient detectors able to select temporal correlations among photons with maximized signal-to-noise ratio. Indeed, quantum information is contained in the interaction with the sample of just two photons, thus losing even one of them or mixing them up with background can completely compromise the experiment. To this end, assessing and evaluating the expected probability of measurement errors is of utmost importance to optimize the detector design. A typical experimental setup of a quantum imaging system based on Spontaneous Parametric Down Conversion (SPDC), the most effective way of producing temporally and spatially correlated entangled photons [2], involves a pump laser hitting a non-linear birefringent crystal producing a down-converted signal, some passive optical components to generate polarization-entangled photons and a pixelated photon detector that eventually reveals the position of coincident entangled photons. The overall detection efficiency of the measurement setup (i.e. heralding efficiency ηH) gathers all its possible losses and non-idealities of the mentioned setup components, namely the crystal conversion efficiency, the optical losses, the detector Photon Detection Efficiency (PDE) and fill-factor. In this study, we consider a detection based on a Single Photon Avalanche Diode (SPAD) array, able to detect coincidences on-chip and with event-driven architecture, which provides fast readout, low power consumption and optimized data throughput. However, related to every on-chip coincidence detection and event-driven based architecture, some operations require a certain amount of time to be performed that, combined with the generation of undesired single photons (both due to limited heralding efficiency and SPAD Dark Count Rate, DCR), yield to three measurement errors, here described with their related probabilities. Firstly, due to the finite duration of the coincidence window ∆tCOINC, there exist a probability Pfalseof wrongly detecting as coincident photons two single photons, and can be expressed as:\begin{equation*}{P_{false}} = \frac{{\left[ {2{\mu _{SOURCE{\text{ }}}}{\eta _H}\left( {1 - {\eta _H}} \right) + {\text{DCR}}} \right]\left( {1 - {e^{ - \left[ {2{\mu _{SOURCE}}{\eta _H}\left( {1 - {\eta _H}} \right) + {\text{DCR}}} \right]\Delta {t_{COINC}}}}} \right)}}{{{\mu _{SOURCE}}\eta _H^2 + \left[ {2{\mu _{SOURCE}}{\eta _H}\left( {1 - {\eta _H}} \right) + DCR} \right]\left( {1 - {e^{ - \left[ {2{\mu _{SOURCE}}{\eta _H}\left( {1 - {\eta _H}} \right) + {\text{DCR}}} \right]\Delta {t_{COINC}}}}} \right)}}\tag{1}\end{equation*}where µSOURCEis the number of photon pairs generated by an ideal non-linear crystal. Secondly, in the time needed for the array to "freeze" the pixels state after a coincidence event (∆tFREEZE) so that they can be readout, there is the probability Pspuriousof detecting a third spurious event after a coincidence has been detected, which is expressed as:\begin{equation*}{P_{spurious}} = 1 - {e^{ - \left[ {2{\mu _{SOURCE\eta }}_H\left( {1 - {\eta _H}} \right) + DCR} \right]\Delta {t_{FREEZE}}}}\tag{2}\end{equation*}
This dissertation is the result of research which, although electrical and computer engineering in nature, also aims to improve the performance of many systems in the field of quantum information. For example, random number generators are used in almost all areas of science, and the initial portion of this work details the theory, design, and characterization of two photon-arrival-time quantum random number generators (QRNGs). After the QRNGs were completed, it was realized that their performance was severely limited both by the maximum detection rate of the single-photon detectors used, and the precision at which the arrival times could be resolved. The single-photon detectors used for both QRNGs are single-photon avalanche photodiodes (SPADs), devices which when operated below their breakdown voltage can create a macroscopic amount of current (an avalanche) in response to a single incident photon. Some of this charge can become trapped in defects or impurities; if this trapped charge is released when the SPAD is active, a secondary ‘false’ detection event, or ‘afterpulse’ can occur. To lower the afterpulse probability to reasonable levels (< 1%), we attempted to reduce the amount of avalanche charge by halting its growth promptly with high-speed electronics, so that defects have a lower probability of becoming populated in the first place. Initial results show reductions in afterpulse probability by up to a factor of 12, corresponding to a ~20% decrease in dead time, a value that could be improved further. We developed an FPGA-based time-to-digital converter system for use specifically with SPADs, achieving a time-bin resolution of 100 ps, with lower dead time and higher maximum detection rate than all currently available detection systems. This further allowed for the creation of a new higher-order SPAD characterization technique, which was identified previously unknown subtleties to SPAD operation. Finally, we developed an ultra-low-latency QRNG, which was used in one of the recent loophole-free demonstrations of quantum nonlocality. The final latency was below 2.5 ns, to our knowledge the lowest latency QRNG to date. Of special interest, however, is our subsequent exploration into the characterization of its bit-probability drift using atomic clock stability techniques. By employing the Allan deviation and implementing precision feedback, the additional frequency drift caused by environmental fluctuations is reduced such that the resulting bit stream can pass cryptographic random number tests for sample sizes up to 5 Gb. This system is currently intended for the NIST random-number beacon, a world-wide trusted source of random bits.
Coincidence counters play the role of gating events from the background noise in almost every quantum photonics setup. Precise multi-channel and high count-rate coincidence counting tools have become desirable due to an increase in the complexity of quantum photonics experiments. However, timing analyzers are struggling to meet these needs. We are proposing a Field Programmable Gate Array (FPGA) based coincidence counting system which provides 8 operational channels with 8.9 ps root mean square (RMS) resolution (with a bin width of 7.7 ps) and a count-rate of 320 million counts per second (MCPS) (with 40 MCPS per channel). We have successfully tested our design in different quantum photonics scenarios such as the detection of two-photon interference and pseudo-photon-number resolving detection of a coherent state of light where it has shown its capability of working beyond the saturation of detectors. Also, we have introduced a Dual Data Rate Registration TDC, which improved the linearity of the time tagging operation by using both clock edges without increasing the dead-time or using the space excessively. 1.2 LSB in max DNL error, 1.8 LSB in max INL error, 10 ps in FWHM and 3.2 ps in RMS resolution improvements were achieved.