
Direct characterization of ultrafast energytime entangled photon pairs
JeanPhilippe W. MacLean, John M. Donohue and Kevin J. Resch
Energytime entangled photons are critical in many quantum optical phenomena and have emerged as important elements in quantum information protocols. Entanglement in this degree of freedom often manifests itself on ultrafast timescales making it very difficult to detect, whether one employs direct or interferometric techniques, as photoncounting detectors have insufficient time resolution. Here, we implement ultrafast photon counters based on nonlinear interactions and strong femtosec ond laser pulses to probe energytime entanglement in this important regime. Using this technique and singlephoton spectrometers, we characterize all the spectral and temporal correlations of two entangled photons with femtosecond resolution. This enables the witnessing of energytime entan glement using uncertainty relations and the direct observation of nonlocal dispersion cancellation on ultrafast timescales. These techniques are essential to understand and control the energytime degree of freedom of light for ultrafast quantum optics.
How can a multipartite singlephoton pathentangled state be certified efficiently by means of local measurements? We address this question by constructing an entanglement witness based on local photon detections preceded by displacement operations to reveal genuine multipartite entanglement. Our witness is defined as a sum of two observables that can be measured locally and assessed with two measurement settings for any number of parties N. For any bipartition, the maximum mean value of the witness observable over biseparable states is bounded from the maximal eigenvalue of an N × N matrix, which can be computed efficiently. We demonstrate the applicability of our scheme by experimentally testing the witness for heralded 4 and 8partite singlephoton pathentangled states. Our implementation shows the scalability of our witness and opens the door for distributing photonic multipartite entanglement in quantum networks at high rates.
We present several entanglement criteria in terms of the quantum Fisher information that help to relate various forms of multipartite entanglement to the sensitivity of phase estimation. We show that genuine multipartite entanglement is necessary to reach the maximum sensitivity in some very general metrological tasks using a twoarm linear interferometer. We also show that it is needed to reach the maximum average sensitivity in a certain combination of such metrological tasks.
 Resolving starlight: a quantum perspective, Mankei Tsang, Contemporary Physics, 60:4, 279298, DOI: 10.1080/00107514.2020.1736375 (2019) https://arxiv.org/abs/1906.02064
The waveparticle duality of light introduces two fundamental problems to imaging, namely, the diffraction limit and the photon shot noise. Quantum information theory can tackle them both in one holistic formalism: model the light as a quantum object, consider any quantum measurement, and pick the one that gives the best statistics. While Helstrom pioneered the theory half a century ago and first applied it to incoherent imaging, it was not until recently that the approach offered a genuine surprise on the ageold topic by predicting a new class of superior imaging methods. For the resolution of two subRayleigh sources, the new methods have been shown theoretically and experimentally to outperform direct imaging and approach the true quantum limits. Recent efforts to generalise the theory for an arbitrary number of sources suggest that, despite the existence of harsh quantum limits, the quantuminspired methods can still offer significant improvements over direct imaging for subdiffraction objects, potentially benefiting many applications in astronomy as well as fluorescence microscopy.

Quantum Nonlocality in WeakThermalLight Interferometry, Mankei Tsang Phys. Rev. Lett. 107, 270402 (2011) DOI:https://doi.org/10.1103/PhysRevLett.107.270402, https://arxiv.org/abs/1108.1829
In astronomy, interferometry of light collected by separate telescopes is often performed by physically bringing the optical paths together in the form of Young's doubleslit experiment. Optical loss severely limits the efficiency of this socalled direct detection method, motivating the fundamental question of whether one can achieve a comparable performance using separate optical measurements at the two telescopes before combining the measurement results. Using quantum mechanics and estimation theory, here I show that any such spatially local measurement scheme, such as heterodyne detection, is fundamentally inferior to coherently nonlocal measurements, such as direct detection, for estimating the mutual coherence of bipartite thermal light when the average photon flux is low. This surprising result reveals an overlooked signature of quantum nonlocality in a classic optics experiment.

Accelerating quantum optics experiments with statistical learning, Cristian L. Cortes, Sushovit Adhikari, Xuedan Ma, and Stephen K. Gray, Appl. Phys. Lett. 116, 184003 (2020); https://doi.org/10.1063/1.5143786; https://arxiv.org/pdf/1911.05935
Quantum optics experiments, involving the measurement of lowprobability photon events, are known to be extremely timeconsuming. We present a methodology for accelerating such experiments using physically motivated ansatzes together with simple statistical learning techniques such as Bayesian maximum a posteriori estimation based on fewshot data. We show that it is possible to reconstruct timedependent data using a small number of detected photons, allowing for fast estimates in under a minute and providing a onetotwo order of magnitude speedup in data acquisition time. We test our approach using real experimental data to retrieve the second order intensity correlation function, G(2)(τ), as a function of time delay τ between detector counts, for thermal light as well as antibunched light emitted by a quantum dot driven by periodic laser pulses. The proposed methodology has a wide range of applicability and has the potential to impact the scientific discovery process across a multitude of domains.

Quantum theory of fourthorder interference, Z. Y. Ou, Phys. Rev. A 37, 1607 (1988) DOI:https://doi.org/10.1103/PhysRevA.37.1607
The quantum theory of fourthorder interference of light is presented in a general format and compared with classical wave theory. The conditions under which nonclassical phenomena occur are discussed. In particular, the interference between the quantum field and classical field may give rise to a nonclassical effect. For some special states of light, the interference pattern does not disappear even though one field is much stronger than the other, for which no classical analog exists. Fourthorder effects in the interference between two independent fields are analyzed in detail. It is pointed out that the fourthorder interference between independent fields will not disappear when the integration time of detection is of the order of the reciprocal bandwidth of the two light fields as long as the spectra of the two fields are symmetric around the same center frequency, and for some correlated fields, the interference does not vanish even if the detection time is much larger than the reciprocal bandwidth of the fields. A new type of fourthorder interference experiment involving a beam splitter is proposed in which local realism of the EinsteinPodolskyRosen form is violated for quantum mechanics. This general argument is then applied to the interference between two photons generated in the parametric downconversion process. The possibility of violations of Bell’s inequalities in interference experiments is investigated.
 Entangled photons, Y. Shih, IEEE Journal of Selected Topics in Quantum Electronics, vol. 9, no. 6, pp. 14551467, Nov.Dec. 2003, doi: 10.1109/JSTQE.2003.820927.
Based on the quantum behavior of entangled photon pairs generated via spontaneous parametric down conversion, this paper reviews the concept of effective twophoton wavefunction or biphoton wavepacket and emphasize the very different physics associated with the entangled twophoton system and with its "individual" subsystems. Experimental approaches for Bell state preparation, pumped by continuous wave and ultrashort pulse, and the propagation of biphoton wavepacket in dispersive media are discussed.
 The colored Hanbury Brown–Twiss effect. Silva, B., Sánchez Muñoz, C., Ballarini, D. et al. Sci Rep 6, 37980 (2016). https://doi.org/10.1038/srep37980
The Hanbury Brown–Twiss effect is one of the celebrated phenomenologies of modern physics that accommodates equally well classical (interferences of waves) and quantum (correlations between indistinguishable particles) interpretations. The effect was discovered in the late thirties with a basic observation of Hanbury Brown that radiopulses from two distinct antennas generate signals on the oscilloscope that wiggle similarly to the naked eye. When Hanbury Brown and his mathematician colleague Twiss took the obvious step to propose bringing the effect in the optical range, they met with considerable opposition as singlephoton interferences were deemed impossible. The Hanbury Brown–Twiss effect is nowadays universally accepted and, being so fundamental, embodies many subtleties of our understanding of the wave/particle dual nature of light. Thanks to a novel experimental technique, we report here a generalized version of the Hanbury Brown–Twiss effect to include the frequency of the detected light, or, from the particle point of view, the energy of the detected photons. In addition to the known tendencies of indistinguishable photons to arrive together on the detector, we find that photons of different colors present the opposite characteristic of avoiding each others. We postulate that fermions can be similarly brought to exhibit positive (bosonlike) correlations by frequency filtering.