Summary & Reflection

This thesis comprises work on four major projects of somewhat disparate motivations and origins. The low dark count free space coupling project and the PPM project were partially motivated by planned improvements to deep space optical communication systems. The time-walk correction project grew naturally out of issues with SNSPD tagging that were observed in other projects. Finally, the high rate entanglement project resulted from an interest in using the differential single pixel SNSPDs to their full potential in a quantum application.

One prevailing observation from this body of work is that lessons and insights gained from each project have utility beyond the motivations that led to the project itself. These insights can be oddly general and specific at the same time. They do not necessarily align with the delineations usually used to categorize research for one goal versus another. This is how, for example, observations from a project on deep space optical communication can be used to improve the performance of a quantum entanglement source.

Here we state predominant insights gained from the four major projects of this thesis:

From the low dark count rate free space coupling project, we learn that the use of free space optics inside a cryogenic environment is not necessarily as unwieldy as one might expect. Optics inside a cryostat cannot be directly aligned without expensive actuators that may or may not function and very low temperatures. But there are ways around this, like by moving all alignment controls outside the fridge and couping in though windows. Such designs may even have dark count rates comparable or superior to fiber coupled SNSPD systems, as we demonstrate.
The time-walk correction project demonstrated how effective SNDPDs continue to be even at high count rates where their RF pulses overlap. Researchers may initially assume that the high count rate behavior of SNSPDs is ‘complicated’ or ‘corrupted,’ and that the detectors fail to operate well in this regime. But we have found, with the calibration and correction methodology, that they continue to respond predictably and with good jitter up to rates where the bias current has insufficient time to recover, thereby reducing the detector efficiency. Depending on the types of amplifiers used, time-walk effects can dramatically reduce jitter before the detector has lost even a few percent of its efficiency due to incomplete bias-current reset. Therefor, time-walk correction can easily lead to 2-3 times higher usable count rates for single detectors, and up to an order of magnitude higher rates in quantum applications that rely on multi-detector coincidences.
The PPM project demonstrated the complexity and potential utility of the photon number response in the differential single pixel SNSPDs. For better or worse, the ideal application of these detectors from quantum or classical communication will require nuanced digital processing methods. When not planned for, the PNR effects can be a nuisance. But when properly managed, it makes these detectors into highly effective photon number resolving devices — equivalent to a large array of binary-type SNSPDs. This will be useful for various photon heralding applications, and for commissioning of complex multi-photon states of light. We introduced the GMM method for PPM decoding, based on a general sense that working in a high dimensional space for event attribution will continue to be a fruitful approach for future work. SNSPD readout will only becomes more complex with the emergence of multiplexed arrays and ulta-high count rate versions. Effects like time-walk the phon number dependent response may be easier to understand and manage it they are thought of as simply modifying how detector data appears when graphed in a high dimensional space.
Work on the high rate entanglement system highlighted how important it is to intelligently manage a complex experiment. By transitioning a time-bin entanglement source to a high repetition rate compatible with the differential single pixel detectors, some aspects became simpler (relative to a low-repetition rate system), and others became more complicated. The promise of this project then hinged on managing the complex aspects reliably. One major advantage of the high repetition rate was it allowed us to use small and highly stable interferometers. With lower repetition rate time-bin entanglement sources, precise temperature control of interferometers is needed when their path length delay exceeds multiple nanoseconds ¹. Here we were able to use interferometers with a path length delay of just 80 ps, which varied in phase by less than a few degrees over multiple hours. On the other hand, the high rate system required time-walk correction for the SNSPDs in order to operate the system at state-of-the art rates. Enabling this correction method in a full system was a new challenge, and it brought about the much more elegant in-situ calibration method. The improved and mostly automated procedure for cancelling SNSPD time walk added minimal complexity for the system user, and further supported the move towards high repetition rate sources for emerging quantum networks.

Valivarthi, R., Davis, S. I., Pe textasciitilde na, C., Xie, S., Lauk, N., Narváez, L., Allmaras, J. P., et al. (2020). Teleportation systems toward a quantum internet. PRX Quantum, 1(2), 020317. doi:10.1103/PRXQuantum.1.020317 ↩