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1. Dynamic control of photon lifetime for quantum random number generation

   https://doi.org/10.1364/OPTICA.433102  

   Okawachi, Yoshitomo ; Kim, Bok Young ; Zhao, Yun ; Ji, Xingchen ; Lipson, Michal ; Gaeta, Alexander L. ( November 2021 , Optica)

   In a passive cavity geometry, there exists a trade-off between resonant enhancement and response time, which is inherently limited by the cavity photon lifetime. We demonstrate frequency-selective, dynamic control of the photon lifetime using a silicon-nitride coupled-ring resonator. The photon lifetime is tuned by controlling an avoided mode crossing using thermo-optic tuning of the cavity resonance with integrated heaters. Using this effect, we achieve fast turn-on/off of a${\mathrm{\chi <\#comment/>}}^{(3)}$degenerate optical parametric oscillator (DOPO) and on-chip true random number generation. Our approach allows us to overcome the$Q$-limited generation rate of a single-ring-based DOPO and offers a path toward the development of a scalable integrated high-quality entropy source for modern cryptographic systems.
2. Single-Photon Emitters in SiN Integrated Quantum Photonics

   https://doi.org/10.1364/QUANTUM.2022.QW4B.5  

   Senichev, Alexander ; Peana, Samuel ; Martin, Zachariah O. ; Yesilyurt, Omer ; Sychev, Demid ; Lagutchev, Alexei S. ; Boltasseva, Alexandra ; Shalaev, Vladimir M. ( January 2022 , Quantum 2.0 Conference 2022 Optica Publishing Group 2022)

   We report on the generation of single-photon emitters in silicon nitride. We demonstrate monolithic integration of these quantum emitters with silicon nitride waveguides showing a room-temperature off-chip count-rate of ~10⁴counts/s and clear antibunching behavior.
3. Quantum dot single-photon sources with ultra-low multi-photon probability

   https://doi.org/10.1038/s41534-018-0092-0  

   Hanschke, Lukas ; Fischer, Kevin A. ; Appel, Stefan ; Lukin, Daniil ; Wierzbowski, Jakob ; Sun, Shuo ; Trivedi, Rahul ; Vučković, Jelena ; Finley, Jonathan J. ; Müller, Kai ( September 2018 , npj Quantum Information)

   High-quality sources of single photons are of paramount importance for quantum communication, sensing, and metrology. To these ends, resonantly excited two-level systems based on self-assembled quantum dots have recently generated widespread interest. Nevertheless, we have recently shown that for resonantly excited two-level systems, emission of a photon during the presence of the excitation laser pulse and subsequent re-excitation results in a degradation of the obtainable single-photon purity. Here, we demonstrate that generating single photons from self-assembled quantum dots with a scheme based on two-photon excitation of the biexciton strongly suppresses the re-excitation. Specifically, the pulse-length dependence of the multi-photon error rate reveals a quadratic dependence in contrast to the linear dependence of resonantly excited two-level systems, improving the obtainable multi-photon error rate by several orders of magnitude for short pulses. We support our experiments with a new theoretical framework and simulation methodology to understand few-photon sources.
4. Photon-photon interactions in Rydberg-atom arrays

   https://doi.org/10.22331/q-2022-03-30-674  

   Zhang, Lida ; Walther, Valentin ; Mølmer, Klaus ; Pohl, Thomas ( March 2022 , Quantum)

   We investigate the interaction of weak light fields with two-dimensional lattices of atoms with high lying atomic Rydberg states. This system features different interactions that act on disparate length scales, from zero-range defect scattering of atomic excitations and finite-range dipole exchange processes to long-range Rydberg-state interactions, which span the entire array and can block multiple Rydberg excitations. Analyzing their interplay, we identify conditions that yield a nonlinear quantum mirror which coherently splits incident fields into correlated photon-pairs in a single transverse mode, while transmitting single photons unaffected. In particular, we find strong anti-bunching of the transmitted light with equal-time pair correlations that decrease exponentially with an increasing range of the Rydberg blockade. Such strong photon-photon interactions in the absence of photon losses open up promising avenues for the generation and manipulation of quantum light, and the exploration of many-body phenomena with interacting photons.
5. Early photon optical tomography signal-to-noise ratio improved by almost two orders-of-magnitude by running in the “deadtime” regime

   Sinha, L. ; Zhou, W. ; Brankov, J.G. ; Tichauer, K.M ( January 2017 , World Molecular Imaging Congress 2017)

   Optical projection tomography (OPT) is a powerful imaging modality for attaining high resolution absorption and fluorescence imaging in tissue samples and embryos with a diameter of roughly 1 mm. Moving past this 1 mm limit, scattered light becomes the dominant fraction detected, adding significant “blur” to OPT. Time-domain OPT has been used to select out early-arriving photons that have taken a more direct route through the tissue to reduce detection of scattered photons in these larger samples, which are the cause of image domain blur1. In addition, it was recently demonstrated by our group that detection of scattered photons could be further depressed by running in a “deadtime” regime where laser repetition rates are selected such that the deadtime incurred by early-arriving photons acts as a shutter to later-arriving scattered photons2. By running in this deadtime regime, far greater early photon count rates are achievable than with standard early photon OPT. In this work, another advantage of this enhanced early photon collection approach is demonstrated: specifically, a significant improvement in signal-to-noise ratio. In single photon counting detectors, the main source of noise is “afterpulsing,” which is essentially leftover charge from a detected photon that spuriously results in a second photonmore »count. When the arrival of the photons are time-stamped by the time correlated single photon counting (TCSPC) module , the rate constant governing afterpusling is slow compared to the time-scale of the light pulse detected so it is observed as a background signal with very little time-correlation. This signal is present in all time-gates and so adds noise to the detection of early photons. However, since the afterpusling signal is proportional to the total rate of photon detection, our enhanced early photon approach is uniquely able to have increased early photon counts with no appreciable increase in the afterpulsing since overall count-rate does not change. This is because as the rate of early photon detection goes up, the rate of late-photon detection reduces commensurately, yielding no net change in the overall rate of photons detected. This hypothesis was tested on a 4 mm diameter tissue-mimicking phantom (μa = 0.02 mm-1, μs’ = 1 mm-1) by ranging the power of a 10 MHz pulse 780-nm laser with pulse spread of < 100 fs (Calmar, USA) and an avalanche photodiode (MPD, Picoquant, Germany) and TCSPC module (HydraHarp, Picoquant, Germany) for light detection. Details of the results are in Fig. 1a, but of note is that we observed more than a 60-times improvement in SNR compared to conventional early photon detection that would have taken 1000-times longer to achieve the same early photon count. A demonstration of the type of resolution possible is in Fig 1b with an image of a 4-mm-thick human breast cancer biopsy where tumor spiculations of less than 100 μm diameter are observable. 1Fieramonti, L. et al. PloS one (2012). 2Sinha, L., et al. Optics letters (2016).« less