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1. Entangled photon pair generation in an integrated SiC platform

   https://doi.org/10.1038/s41377-024-01443-z  

   Rahmouni, Anouar; Wang, Ruixuan; Li, Jingwei; Tang, Xiao; Gerrits, Thomas; Slattery, Oliver; Li, Qing; Ma, Lijun (December 2024, Light: Science & Applications)

   Abstract Entanglement plays a vital role in quantum information processing. Owing to its unique material properties, silicon carbide recently emerged as a promising candidate for the scalable implementation of advanced quantum information processing capabilities. To date, however, only entanglement of nuclear spins has been reported in silicon carbide, while an entangled photon source, whether it is based on bulk or chip-scale technologies, has remained elusive. Here, we report the demonstration of an entangled photon source in an integrated silicon carbide platform for the first time. Specifically, strongly correlated photon pairs are efficiently generated at the telecom C-band wavelength through implementing spontaneous four-wave mixing in a compact microring resonator in the 4H-silicon-carbide-on-insulator platform. The maximum coincidence-to-accidental ratio exceeds 600 at a pump power of 0.17 mW, corresponding to a pair generation rate of (9 ± 1) × 10³pairs/s. Energy-time entanglement is created and verified for such signal-idler photon pairs, with the two-photon interference fringes exhibiting a visibility larger than 99%. The heralded single-photon properties are also measured, with the heraldedg⁽²⁾(0) on the order of 10⁻³, demonstrating the SiC platform as a prospective fully integrated, complementary metal-oxide-semiconductor compatible single-photon source for quantum applications. 

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2. Proof-of-Principle Laboratory Demonstration of Long-Baseline Interferometric Imaging Using Distributed Single-Photons

   https://doi.org/10.1364/QUANTUM.2022.QM3C.1  

   Brown, Matthew; Thiel, Valerian; Allgaier, Markus; Raymer, Michael; Smith, Brian; Kwiat, Paul; Monnier, John (January 2022, Quantum 2.0 Conference and Exhibition)

   We report results of very-long-baseline interferometric imaging using distributed single photons. We demonstrate source autocorrelation reconstruction, and increased signal-to-noise ratio per detected coincidence compared to using classical states as phase reference. 

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3. 648 Hilbert-space dimensionality in a biphoton frequency comb: entanglement of formation and Schmidt mode decomposition

   https://doi.org/10.1038/s41534-021-00388-0  

   Chang, Kai-Chi; Cheng, Xiang; Sarihan, Murat Can; Vinod, Abhinav Kumar; Lee, Yoo Seung; Zhong, Tian; Gong, Yan-Xiao; Xie, Zhenda; Shapiro, Jeffrey H.; Wong, Franco N. C.; et al (March 2021, npj Quantum Information)

   Abstract Qudit entanglement is an indispensable resource for quantum information processing since increasing dimensionality provides a pathway to higher capacity and increased noise resilience in quantum communications, and cluster-state quantum computations. In continuous-variable time–frequency entanglement, encoding multiple qubits per photon is only limited by the frequency correlation bandwidth and detection timing jitter. Here, we focus on the discrete-variable time–frequency entanglement in a biphoton frequency comb (BFC), generating by filtering the signal and idler outputs with a fiber Fabry–Pérot cavity with 45.32 GHz free-spectral range (FSR) and 1.56 GHz full-width-at-half-maximum (FWHM) from a continuous-wave (cw)-pumped type-II spontaneous parametric downconverter (SPDC). We generate a BFC whose time-binned/frequency-binned Hilbert space dimensionality is at least 324, based on the assumption of a pure state. Such BFC’s dimensionality doubles up to 648, after combining with its post-selected polarization entanglement, indicating a potential 6.28 bits/photon classical-information capacity. The BFC exhibits recurring Hong–Ou–Mandel (HOM) dips over 61 time bins with a maximum visibility of 98.4% without correction for accidental coincidences. In a post-selected measurement, it violates the Clauser–Horne–Shimony–Holt (CHSH) inequality for polarization entanglement by up to 18.5 standard deviations with anS-parameter of up to 2.771. It has Franson interference recurrences in 16 time bins with a maximum visibility of 96.1% without correction for accidental coincidences. From the zeroth- to the third-order Franson interference, we infer an entanglement of formation (E_of) up to 1.89 ± 0.03 ebits—where 2 ebits is the maximal entanglement for a 4 × 4 dimensional biphoton—as a lower bound on the 61 time-bin BFC’s high-dimensional entanglement. To further characterize time-binned/frequency-binned BFCs we obtain Schmidt mode decompositions of BFCs generated using cavities with 45.32, 15.15, and 5.03 GHz FSRs. These decompositions confirm the time–frequency scaling from Fourier-transform duality. Moreover, we present the theory of conjugate Franson interferometry—because it is characterized by the state’s joint-temporal intensity (JTI)—which can further help to distinguish between pure-state BFC and mixed state entangled frequency pairs, although the experimental implementation is challenging and not yet available. In summary, our BFC serves as a platform for high-dimensional quantum information processing and high-dimensional quantum key distribution (QKD). 

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4. Nonlocal subpicosecond delay metrology using spectral quantum interference

   https://doi.org/10.1364/OPTICA.458565  

   Seshadri, Suparna; Lingaraju, Navin; Lu, Hsuan-Hao; Imany, Poolad; Leaird, Daniel_E; Weiner, Andrew_M (November 2022, Optica)

   Precise knowledge of position and timing information is critical to support elementary protocols such as entanglement swapping on quantum networks. While approaches have been devised to use quantum light for such metrology, they largely rely on time-of-flight (ToF) measurements with single-photon detectors and, therefore, are limited to picosecond-scale resolution owing to detector jitter. In this work, we demonstrate an approach to distributed sensing that leverages phase modulation to map changes in the spectral phase to coincidence probability, thereby overcoming the limits imposed by single-photon detection. By extracting information about the joint biphoton phase, we measure a generalized delay—the difference in signal–idler arrival, relative to local radio frequency (RF) phase modulation. For nonlocal ranging measurements, we achieve ( $2\mathrm{\sigma <\#comment/>}$) precision of $\pm <\#comment/>0.04\mathrm{p}\mathrm{s}$and for measurements of the relative RF phase, ( $2\mathrm{\sigma <\#comment/>}$) precision of $\pm <\#comment/>{0.7}^{\circ <\#comment/>}$. We complement this fine timing information with ToF data from single-photon time-tagging to demonstrate absolute measurement of time delay. By relying on off-the-shelf telecommunications equipment and standard quantum resources, this approach has the potential to reduce overhead in practical quantum networks. 

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5. Hong-Ou-Mandel Effect Between Single-Photon Source and Thermal Light

   https://doi.org/10.1364/LS.2018.LM1B.1  

   Matekole, Elisha S.; Byrnes, Tim; Dowling, Jonathan P. (January 2018, Frontiers in Optics)

   We describe the theory of Hong-Ou-Mandel (HOM) interference, between a single photon source and thermal light and demonstrate the maximum visibility attainable for non-ideal single photon resources. 

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