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1. Probing quantum walks through coherent control of high-dimensionally entangled photons

   https://doi.org/10.1126/sciadv.aba8066  

   Imany, Poolad; Lingaraju, Navin B.; Alshaykh, Mohammed S.; Leaird, Daniel E.; Weiner, Andrew M. (July 2020, Science Advances)

   null (Ed.)

   Control over the duration of a quantum walk is critical to unlocking its full potential for quantum search and the simulation of many-body physics. Here we report quantum walks of biphoton frequency combs where the duration of the walk, or circuit depth, is tunable over a continuous range without any change to the physical footprint of the system—a feature absent from previous photonic implementations. In our platform, entangled photon pairs hop between discrete frequency modes with the coupling between these modes mediated by electro-optic modulation of the waveguide refractive index. Through control of the phase across different modes, we demonstrate a rich variety of behavior: from walks exhibiting enhanced ballistic transport or strong energy confinement, to subspaces featuring scattering centers or local traps. We also explore the role of entanglement dimensionality in the creation of energy bound states, which illustrates the potential for these walks to quantify high-dimensional entanglement. 

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2. Positive operator-valued measure for two-photon detection via sum-frequency generation

   https://doi.org/doi.org/10.1103/PhysRevA.103.043711  

   Merkouche, Sofiane; Thiel, Valérian; and Smith, Brian J. (January 2021, Physical review and Physical review letters index)

   null (Ed.)

   Spontaneous parametric down conversion (PDC), in the perturbative limit, can be considered as a probabilistic splitting of one input photon into two output photons. Conversely, sum-frequency generation (SFG) implements the reverse process of combining two input photons into one. Here we show that a single-photon projective measurement in the temporal-mode basis of the output photon of a two-photon SFG process affects a generalized measurement on the input two-photon state. We describe the positive operator-valued measure (POVM) associated with such a measurement and show that its elements are proportional to the two-photon states produced by the time-reversed PDC process. Such a detection acts as a joint measurement on two photons and is thus an important component of many quantum information processing protocols relying on photonic entanglement. Using the retrodictive approach, we analyze the properties of the two-photon POVM that are relevant for quantum protocols exploiting two-photon states and measurements. 

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3. 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. 

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4. Parity-dependent photon-subtracted two-mode squeezed vacuum states

   https://doi.org/10.1103/4x83-y4bg  

   Datta, Ananga Mohan; Blanco-Redondo, Andrea; Busch, Kurt; Perez-Leija, Armando (August 2025, Physical Review A)

   The removal of photons from certain quantum light sources produces so-called photon-subtracted states with enhanced mean photon numbers and intricate quantum correlations. Here, we propose an integrated photon-subtraction scheme that, contrary to previous approaches, is not heralded by photon correlation (coincidence) measurements. In this way, our technique exploits the “Welcher Weg” (“which way”) information problem, as it does not provide information about the modes from which the subtracted photons emanated. We show that this lack of information allows the generation of multiphoton states endowed with rich quantum correlations that strongly depend on the parity, evenness or oddness, of the number of subtracted photons. 

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5. Indistinguishable photons from an artificial atom in silicon photonics

   https://doi.org/10.1038/s41467-024-51265-1  

   Komza, Lukasz; Samutpraphoot, Polnop; Odeh, Mutasem; Tang, Yu-Lung; Mathew, Milena; Chang, Jiu; Song, Hanbin; Kim, Myung-Ki; Xiong, Yihuang; Hautier, Geoffroy; et al (August 2024, Nature Communications)

   Abstract Silicon is the ideal material for building electronic and photonic circuits at scale. Integrated photonic quantum technologies in silicon offer a promising path to scaling by leveraging advanced semiconductor manufacturing and integration capabilities. However, the lack of deterministic quantum light sources and strong photon-photon interactions in silicon poses a challenge to scalability. In this work, we demonstrate an indistinguishable photon source in silicon photonics based on an artificial atom. We show that a G center in a silicon waveguide can generate high-purity telecom-band single photons. We perform high-resolution spectroscopy and time-delayed two-photon interference to demonstrate the indistinguishability of single photons emitted from a G center in a silicon waveguide. Our results show that artificial atoms in silicon photonics can source single photons suitable for photonic quantum networks and processors. 

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