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1. Conditional π-Phase Shift of Single-Photon-Level Pulses at Room Temperature

   https://doi.org/10.1103/PhysRevLett.125.243601  

   Steven Sagona-Stophel, Reihaneh Shahrokhshahi (December 2020, Physical review letters)

   The development of useful photon-photon interactions can trigger numerous breakthroughs in quantum information science, however, this has remained a considerable challenge spanning several decades. Here, we demonstrate the first room-temperature implementation of large phase shifts (≈π) on a single-photon level probe pulse (1.5μs) triggered by a simultaneously propagating few-photon-level signal field. This process is mediated by Rb87 vapor in a double-Λ atomic configuration. We use homodyne tomography to obtain the quadrature statistics of the phase-shifted quantum fields and perform maximum-likelihood estimation to reconstruct their quantum state in the Fock state basis. For the probe field, we have observed input-output fidelities higher than 90% for phase-shifted output states, and high overlap (over 90%) with a theoretically perfect coherent state. Our noise-free, four-wave-mixing-mediated photon-photon interface is a key milestone toward developing quantum logic and nondemolition photon detection using schemes such as coherent photon conversion. 

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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. Quantum phase-sensitive diffraction and imaging using entangled photons

   https://doi.org/10.1073/pnas.1904839116  

   Asban, Shahaf; Dorfman, Konstantin E.; Mukamel, Shaul (June 2019, Proceedings of the National Academy of Sciences)

   We propose a quantum diffraction imaging technique whereby one photon of an entangled pair is diffracted off a sample and detected in coincidence with its twin. The image is obtained by scanning the photon that did not interact with matter. We show that when a dynamical quantum system interacts with an external field, the phase information is imprinted in the state of the field in a detectable way. The contribution to the signal from photons that interact with the sample scales as ∝ I p 1 / 2 , where I p is the source intensity, compared with ∝ I p of classical diffraction. This makes imaging with weak fields possible, providing high signal-to-noise ratio, avoiding damage to delicate samples. A Schmidt decomposition of the state of the field can be used for image enhancement by reweighting the Schmidt modes contributions. 

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4. Non-local Mueller polarimetry

   https://doi.org/10.1117/12.3004822  

   You, Chan Ju; Rodriguez-Fajardo, Valeria; Francis, Leia; Galvez, Enrique J (March 2024, SPIE)

   Ramella-Roman, Jessica C; Ma, Hui; Vitkin, I Alex; Elson, Daniel S; Novikova, Tatiana (Ed.)

   We present a method to determine the Mueller matrix of a sample using polarization-entangled photon pairs. One of the photons of a pair goes through a sample and is then subject to a polarization projection measurement. The other photon, which does not go through the sample, is also subject to a polarization projection. The measured quantum correlations are equivalent to polarimetry measurements, where the initial state of the photon going through the sample is determined by the polarization projection on the entangled partner that does not go through the sample. The correspondence with the classical system is acausal because quantum measurements apply to distinct Hilbert spaces. We tested this method with standard optical elements finding excellent agreement with the expectations. Thus it can be used as an alternative to classical Mueller polarimetry for conditions that would be challenging to do otherwise. 

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5. Phase‐Modulated Interferometry, Spectroscopy, and Refractometry using Entangled Photon Pairs

   https://doi.org/10.1002/qute.201900114  

   Lavoie, Jonathan; Landes, Tiemo; Tamimi, Amr; Smith, Brian J.; Marcus, Andrew H.; Raymer, Michael G. (January 2020, Advanced Quantum Technologies)

   Abstract The authors demonstrate a form of two‐photon‐counting interferometry by measuring the coincidence counts between single‐photon‐counting detectors at an output port of a Mach–Zehnder Interferometer (MZI) following injection of broad‐band time‐frequency‐entangled photon pairs (EPP) generated from collinear spontaneous parametric down conversion into a single input port. Spectroscopy and refractometry are performed on a sample inserted in one internal path of the MZI by scanning the other path in length, which acquires phase and amplitude information about the sample's linear response. Phase modulation and lock‐in detection are introduced to increase detection signal‐to‐noise ratio and implement a “down‐sampling” technique for scanning the interferometer delay, which reduces the sampling requirements needed to reproduce fully the temporal interference pattern. The phase‐modulation technique also allows the contributions of various quantum‐state pathways leading to the final detection outcomes to be extracted individually. Feynman diagrams frequently used in the context of molecular spectroscopy are used to describe the interferences resulting from the coherence properties of time‐frequency EPPs passing through the MZI. These results are an important step toward the implementation of a proposed method for molecular spectroscopy—quantum‐light‐enhanced 2D spectroscopy. 

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