Here are the quantum optics gifts that Arxiv brought us in May 2018. I’ve added more papers this month than ever to my to-read-later list. (Sidenote: It would be cool to check if this bump in interesting quant-ph papers is a result of a bump in submitted quant-ph papers that happens every May when teaching recedes. Old arXiv stats seem to show that Cond-matt submissions go up in May but not HEP. Funnily enough, the numbers of downloaded papers show no such bump!) The papers that I did read are:
Paulisch, V., Perarnau-Llobet, M., González-Tudela, A., & Cirac, J. I. arXiv:1805.00712. Quantum metrology with one-dimensional superradiant photonic states.
Quantum metrology with multimode photonic states. Current results about metrological scaling beyond the shot-noise limit focus on two single modes of light probing a phase. Single mode states with a fixed and high photon number are hard to generate experimentally, but it can be easier to generate such multi-mode states. For example, Dicke states emitted from a bunch of fully exciting quantum emitters coupled identically to a common photon reservoir. The metrological resource scaling of two such states after they have interfered on a beamsplitter is a main result of this work. Dicke superradiant states, although are highly entangled internally, can achieve the Heisenberg scaling limit. What other multimode states are feasible to generate experimentally and can give a quantum metrological advantage?
Muraleedharan, G., Miyake, A., & Deutsch, I. H. arXiv:1805.01858. Quantum computational supremacy in the sampling of bosonic random walkers on a one-dimensional lattice.
Noninteracting bosonic quantum walkers are likely hard to simulate. This work determines the probabilities of detecting noninteracting bosonic quantum walkers in terms of permanents of doubly banded matrices, and these quantities are expected to be hard to compute classically. For me, it was very interesting to see that time-dependent hopping and optical control can yield arbitrary unitary evolutions (Supplementary material Section I).
Donohue, J. M., Ansari, V., Řeháček, J., Hradil, Z., Stoklasa, B., Paúr, M., Sánchez-Soto, L. L., & Silberhorn, C. arXiv:1805.02491. Quantum-limited time-frequency estimation through mode-selective photon measurement.
Quantum enhancement in the measurement of temporal and spectral separation of light pulses. This work translates ideas from super-resolution imaging (i.e, quantum-enhanced resolution of two incoherence point sources and estimating their separation) into the time-frequency domain. The time or frequency offset between two light pulses is determined by projectively measuring Hermite-Gauss time-frequency mode. This measurement is performed using sum-frequency generation with shaped ultrafast pulses in specifically engineered nonlinear waveguides. The final result is an impressive ten-fold improved resolution (as compared to the Cramer-Rao bound) of separations that ten times smaller than the spectral bandwidths.
Pecoraro, A., Cardano, F., Marrucci, L., & Porzio, A. arXiv:1805.05105. Continuous-Variable Entangled States of Light carrying Orbital Angular Momentum.
Generation and verification of CV entanglement between different OAM modes of the same beam. In this work, continuous variable entanglement is generated between two OAM modes of light using an OPO followed by a q-plate. Homodyne tomography is then performed on the emitted beam of light using another beam with complementary OAM and a single detector based on an earlier proposal. The reported homodyne visibitity is an impressive 0.97, and the overall detection efficiency (after accounting for imperfections in photodiodes, qplates and fiber-couplings) is an equally impressive 0.52.
Tiedau, J., Meyer-Scott, E., Nitsche, T., Barkhofen, S., Bartley, T. J., & Silberhorn, C. arXiv:1805.05881. A high dynamic range optical detector for measuring single photons and bright light.
Massively time-multiplexed detection allows for 123 dB intensity range detection. This work is an elegant solution for estimating the photon number of high and low photon number states using standard SNSPD detectors without frying the detectors. It involves modifying earlier loop-detector proposals, which relied on fixed coupling to the loop, to instead use an adaptive coupling realized via a Pockels cell. The to-be-detected light pulse is fed entirely into the loop, and it is allowed to bleed off slowly out of the loop and on to the detector. The detector statistics over many round trips of the light in the loop allows inferring the photon number for a wide range of light intensities. Yet another cool application of an in-loop Pockels cell.
Quesada, N., & Brańczyk, A. M. arXiv:1805.06868. Gaussian functions are optimal for waveguided nonlinear-quantum-optical processes.
Necessary conditions for a factorizable joint spectral amplitude (JSA) and heuristics for maximizing separability when the conditions are not met. I really like the novel method used here to study the JSA: map the two functions that define the JSA (i.e., the pump field and phase-matching functions) to position wavefunctions of two modes; under this mapping, the final JSA is identical to the position wavefunction of the modes after they are squeezed, rotated and skewed. This mapping allows using an earlier result that Gaussian states are necessary for obtaining an unentangled output from a linear optical interferometer. This means that a JSA can be factorized only if the incident pump field and phase-matching functions are Gaussian. The preprint ends with a conjecture that if either of these functions is non-Gaussian, then maximizing the JSA requires that the other one is Gaussian. The conjecture is supported by analytical calculations in a specific parameter regime and strong numerics.
Averchenko, V. A., Schunk, G., Förtsch, M., Fischer, M., Strekalov, D. V., Leuchs, G., & Marquardt, C. arXiv:1805.08467. Off-resonant emission of photon pairs in nonlinear optical cavities.
Theory and experiment on the effect of frequency mismatch on cavity assisted photon generation. This work is a thorough analysis of photon-pair generation in cavities if the cavity frequency does not match that of the generated photons. The mismatch reduces photon-generation rate, and it distorts the spectrum and the auto-correlation function of the generated light. The results are experimentally validated using a whispering-gallery-mode resonator similar to that demonstrated earlier (in these works).
Scheel, S., & Szameit, A. arXiv:1805.10876. PT-symmetric photonic quantum systems with gain and loss do not exist.
PT-symmetric quantum optics is not possible with gain and loss. Combined parity-time (PT) symmetric Hamiltonians possess real eigenvalues and generate unitary evolutions. One potential approach to realizing PT symmetric Hamiltonians is quantum optics, where complex gain and loss can provide the desired analog for the complex quantum potential. This work shows that this analogy is incomplete because the simple fact that applying gain and loss on a quantum state adds unavoidable noise to the light state, and so the system is not PT symmetric. For me, this was a much-needed reminder that gain and loss in quantum optics is subject to the usual irreversibility of open quantum dynamics.
Perczel, J., Kómar, P., & Lukin, M. D. arXiv:1805.11739. Quantum Optics in Maxwell’s Fish Eye Lens with Single Atoms and Photons.
A quantum-optical analysis of two atoms in Maxwell’s fish eye lens. A Maxwell’s fish eye lens is an infinite sized dielectric with a radial refractive index variation such that light rays propagate in perfect circles, and moreover all rays that are emitted from any point inside the lens meet at its antipodal point. Enclosing the dielectric in a spherical mirror of a specific radius leaves the lens behaviour unchanged. This work is a full quantum optical analysis of the two atoms in a 2D lens interacting via photons in the dielectric. The EM field is quantized rigorously using a set of single-frequency solutions of the Maxwell equations using the framework of Glauber and Lewenstein. The effective dipole-dipole coupling between the two atoms is independent of the lens radius and thus independent of their distance, which could be useful in quantum optics.
Tan, S.-H., & Rohde, P. P. arXiv:1805.11827. The resurgence of the linear optics quantum interferometer — recent advances & applications.
Up-to-date review on linear optics with a focus on theoretical advances. This review systematically introduces linear optical interferometers and highlights recent advances in encoding and preparing desired quantum states (Sections III, IV, and also in VII, VIII), and state and process tomography (Sections V, VI). I enjoyed the brief and zoomed-out presentation of photodetection and optical switching (Sections IX, X). A comprehensive list of application of linear optics is in Section XI. This review complements very well the other great recent review, which is more experiment oriented. A must read for someone entering the field!
Wu, J., de Guise, H., & Sanders, B. C. arXiv:1805.12023. Coincidence Landscapes for Polarized Bosons.
Group theoretic analysis of tritter and related experiments. This is a rigorous study of coincidences and symmetries in linear interferometry on m channels including polarization using SU(2m) group theory. Expressions for coincidence rates are presented in terms of rate matrices of SU(2m), whose well-oiled machinery suffices for describing the extra polarization degree of freedom. Section IV ‘Triad phase demystefied’ explains the triad phase of earlier tritter experiments in terms of pairwise photon-photon distinguishability in an SU(2m) analysis.
Earlier reading lists: