Here are my quantum optics and photonics reading notes from June 2018: including a linear interferometer with source and detector on chip and theoretical advances on discriminating distinguishability and on linear bosonic channels.
Huang, H.-L., Luo, Y.-H., Bai, B., Deng, Y.-H., Wang, H., Zhong, H.-S., Nie, Y.-Q., Jiang, W.-H., Wang, X.-L., Zhang, J., Li, L., Liu, N., Byrnes, T., Dowling, J. P., Lu, C., & Pan, J. arXiv:1806.00156. A loophole-free Wheeler-delayed-choice experiment. and Polino, E., Agresti, I., Poderini, D., Carvacho, G., Milani, G., Lemos, G. B., Chaves, R., & Sciarrino, F. arXiv:1806.00211. Device independent certification of a quantum delayed choice experiment.
Experimental verification of loophole-free modified delayed choice experiment. The delayed choice thought experiment highlights a tension between quantum physics and hidden variable models that respect causality. The actual experiment has been performed earlier and quantum physics emerged unscathed to the surprise of no one. That said, it is good to be careful, and doubly so in light of this earlier work by Chaves Lemos and Pienaar, which showed that the original delayed choice experiment does admit a simple causal hidden variable model. Fortunately the work also provided a modified version in which quantum physics is provably incompatible with causal hidden variables. These two experiments, from Shanghai/Hefei and from Rome have verified that quantum predictions indeed hold even in this modified setting.
Stanisic, S., & Turner, P. S. arXiv:1806.01236. Discriminating distinguishability.
Construction of HOM-like distinguishability-discriminating interferometers. This article constructs the theory to answer the question: how to find interferometers that have zero coincidence probability from indistinguishable input states but the highest probability of seeing a coincidence for a distinguishable input state? One cool idea to take back home: Schur-Weyl duality in second quantisation is closely connected to Schmidt decomposition in first quantisation (Equations 17, 42, 47, 53). Once the states are converted to first quantisation, then earlier results from the discrimination of two mixed states can be used, subject to linear optics and photo-detection constraints.
Kupchak, C., Erskine, J., England, D. G., & Sussman, B. J. arXiv:1806.01245. THz-bandwidth all-optical switching of heralded single photons.
Kerr effect for fast, low-noise switching that is compatible with quantum light. A strong pump pulse can induce birefringence in a Kerr medium such as a single-mode fiber. This birefringent medium, if sandwiched between crossed polarizers, acts as a fast optically-triggered switch with an incredible 97\% switching efficiency, 80 MHz duty cycles (with 1.7 ps switching time) and 800 SNR (signal/noisy counts).
Banchi, L., Kolthammer, W. S., & Kim, M. S. arXiv:1806.02436. Multiphoton Tomography with Linear Optics and Photon Counting.
Linear optics and photon counting allow tomography of arbitrary photonic states. The task is to perform photon-counting detection on many copies of a given state after it has passed through a linear interferometer whose settings can be changed. The state-to-probabilities transformation (Equation 2) can be inverted if enough interferometer settings are measured similar to earlier work on angular momentum tomography. How many settings is enough can be determined using ideas from unitary design. Ancillary linear optics modes and detectors can reduce the number of settings required, even down to a single setting. This article brings together many ideas that are new and interesting to me.
Adcock, J. C., Morley-Short, S., Silverstone, J. W., & Thompson, M. G. arXiv:1806.03263. Hard limits on the postselectability of optical graph states.
Which graph states can and cannot be obtained from photon sources and post-selection? The article starts with a well-motivated definition: a circuit is post-selectable iff all gate failure combinations can be detected, and ignored, i.e., whenever the success signal is observed, all gates have performed their desired operations. Then it presents rules about which circuits can and cannot be post-selected. The first rule, that circuits with cycles of post-selecting gates are not post-selectable, arises from junk terms produced by the first gate creeping into the post-selected basis by the subsequent gates. The other two rules deal with states generated from post-selection on entangled photon pair sources. The algorithm that decides which entanglement classes are accessible can also be used to design interferometers that construct desired graph states.
Schwartz, M., Schmidt, E., Rengstl, U., Hornung, F., Hepp, S., Portalupi, S. L., Ilin, K., Jetter, M., Siegel, M., & Michler, P. arXiv:1806.04099. Fully on-chip single-photon Hanbury-Brown and Twiss experiment on a monolithic semiconductor-superconductor platform.
Fully on-chip Hanbury-Brown-Twiss experiment. First experiment with all three, source, interferometer and detector integrated on the chip. The components comprise a single-photon source via quantum dot (QD) in InGaAs/GaAs, a beamsplitter, and two nanowire detectors on one chip. As expected, the integrated detectors on-chip necessitate operation at 4K. The result shows respectable specifications for a fully on-chip device: QD emission to background ratio of 40:1 and g2(0) of around 0.24 for cw and 0.41 for pulsed scheme, with both the g2 values happily below the 0.5 threshold for dominance of QD single-photon signal.
Aragoneses, A., Islam, N. T., Eggleston, M., Lezama, A., Kim, J., & Gauthier, D. J. arXiv:1806.05012. Bounding the outcome of a two-photon interference measurement using weak coherent states.
Experiments on weak coherent states give information about events in which HOM interference occurred. By sending in weak coherent pulses and measuring three different coincidence probabilities, the HOM coincidence probability (i.e., one photon emerging from each output port of the beam splitter conditioned on the presence of two incident single-photon wavepackets) can be bonded from above. The results are based on ideas from this earlier work on single-photon simulation via classical light.
Sun, K., Gao, J., Cao, M., Jiao, Z., Liu, Y., Li, Z., Poem, E., Eckstein, A., Ren, R., Pang, X., Tang, H., Walmsley, I. A., & Jin, X. arXiv:1806.09569. Mapping and Measuring Large-scale Photonic Correlation with Single-photon Imaging.
Gratings convert momentum correlations to spatial correlations, which can be measured directly with ICCDs. Short gate times allow extracting correlations between two modes, i.e., joint spectrum, and suppressing noise.
Lami, L., Sabapathy, K. K., & Winter, A. arXiv:1806.11042. All phase-space linear bosonic channels are approximately Gaussian dilatable.
The set of phase-space linear bosonic channels coincides with the closure of Gaussian dilatable channels. The proof relies on showing (i) that the set of linear bosonic channels is strong-operator closed, (ii) that the closure of the set of Gaussian dilatable channels contains it, (iii) earlier result and explicit derivation that every Gaussian dilatable channel is linear bosonic. Taking the closure is necessary as proven from an example of a linear bosonic channel (binary displacement channel) that has no Gaussian dilation even if the ancillary state is allowed to have infinite energy. Theorem 13 provides the main result and Lemma 11 a simplified example. Figure 2 depicts the hierarchy of bosonic channels. This self-contained article provides rigorous proofs yet it reads like a breeze!