Harald G. L. Schwefel

Efficient microwave to optical photon conversion: an electro-optical realization

Alfredo Rueda, Florian Sedlmeir1,2,3,+,∗, Michele C. Collodo, Ulrich Vogl, Birgit Stiller, Gerhard Schunk, Dmitry V. Strekalov, Christoph Marquardt, Johannes M. Fink, Oskar Painter, Gerd Leuchs, and Harald G. L. Schwefel8,∗ Max Planck Institute for the Science of Light, Günther-Scharowsky-Straße 1/Building 24, 90158 Erlangen, Germany Institute for Optics, Information and Photonics, University Erlangen-Nürnberg, Staudtstr. 7/B2, 91058 Erlangen, Germany SAOT, School in Advanced Optical Technologies, Paul-Gordan-Str. 6, 91052 Erlangen, Germany Institute for Quantum Information and Matter and Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA currently at: Department of Physics, ETH Zürich, CH-8093 Zurich, Switzerland currently at: Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS), School of Physics, University of Sydney, New South Wales 2006, Australia currently at: Institute of Science and Technology Austria, 3400 Klosterneuburg, Austria Department of Physics, University of Otago, Dunedin, New Zealand ∗Corresponding authors: Florian.Sedlmeir@mpl.mpg.de, Harald.Schwefel@otago.ac.nz and these authors contributed equally to this workLinking classical microwave electrical circuits to the optical telecommunication band is at the core of modern communication. Future quantum information networks will require coherent microwave-to-optical conversion to link electronic quantum processors and memories via low-loss optical telecommunication networks. Efficient conversion can be achieved with electro-optical modulators operating at the single microwave photon level. In the standard electro-optic modulation scheme, this is impossible because both up- and down-converted sidebands are necessarily present. Here, we demonstrate true single-sideband up- or down-conversion in a triply resonant whispering gallery mode resonator by explicitly addressing modes with asymmetric free spectral range. Compared to previous experiments, we show a 3 orders of magnitude improvement of the electro-optical conversion efficiency, reaching 0.1% photon number conversion for a 10 GHz microwave tone at 0.42 mW of optical pump power. The presented scheme is fully compatible with existing superconducting 3D circuit quantum electrodynamics technology and can be used for nonclassical state conversion and communication. Our conversion bandwidth is larger than 1 MHz and is not fundamentally limited.

Polarization-Selective Out-Coupling of Whispering-Gallery Modes

Whispering gallery mode (WGM) resonators are an important building block for linear, nonlinear and quantum optical experiments. In such experiments, independent control of coupling rates to different modes can lead to improved conversion efficiencies and greater flexibility in generation of non-classical states based on parametric down conversion. In this work, we introduce a scheme which enables selective out-coupling of WGMs belonging to a specific polarization family, while the orthogonally polarized modes remain largely unperturbed. Our technique utilizes material birefringence in both the resonator and coupler such that a negative (positive) birefringence allows selective coupling to TE (TM) polarized WGMs. We formulate a new coupling condition suitable for describing the case where the refractive indices of the resonator and the coupler are almost the same, from which we derive the criterion for polarization-selective coupling. We experimentally demonstrate our proposed method using a lithium niobate disk resonator coupled to a lithium niobate prism, where we show a 22dB suppression of coupling to TM modes relative to TE modes.

Frequency tuning of single photons from a whispering-gallery mode resonator to MHz-wide transitions

Quantum repeaters rely on an interfacing of flying qubits with quantum memories. The most common implementations include a narrowband single photon matched in bandwidth and central frequency to an atomic system. Previously, we demonstrated the compatibility of our versatile source of heralded single photons, which is based on parametric down-conversion in a triply-resonant whispering-gallery mode resonator, with alkaline transitions [Schunk et al., Optica 2, 773 (2015)]. In this paper, we analyze our source in terms of phase matching, available wavelength-tuning mechanisms, and applications to narrow-band atomic systems. We resonantly address the D1 transitions of cesium and rubidium with this optical parametric oscillator pumped above its oscillation threshold. Below threshold, the efficient coupling of single photons to atomic transitions heralded by single telecom-band photons is demonstrated. Finally, we present an accurate analytical description of our observations. Providing the demonstrated flexibility in connecting various atomic transitions with telecom wavelengths, we show a promising approach to realize an essential building block for quantum repeaters.Quantum repeaters rely on interfacing flying qubits with quantum memories. The most common implementations include a narrowband single photon matched in bandwidth and central frequency to an atomic system. Previously, we demonstrated the compatibility of our versatile source of heralded single photons, which is based on parametric down-conversion in a triply resonant whispering-gallery mode resonator, with alkaline transitions [Schunk et al., Optica 2015, 2, 773]. In this paper, we analyse our source in terms of phase matching, available wavelength-tuning mechanisms and applications to narrowband atomic systems. We resonantly address the D1 transitions of caesium and rubidium with this optical parametric oscillator pumped above its oscillation threshold. Below threshold, the efficient coupling of single photons to atomic transitions heralded by single telecom-band photons is demonstrated. Finally, we present an accurate analytical description of our observations. Providing the demonstrated flexibility in connecting various atomic transitions with telecom wavelengths, we show a promising approach to realize an essential building block for quantum repeaters.

Dielectric tuning and coupling of whispering gallery modes using an anisotropic prism

Optical whispering gallery mode (WGM) resonators are a powerful and versatile tool in many branches of science. Fine-tuning of the central frequency and linewidth of individual resonances is, however, desirable in a number of applications, including frequency conversion, optical communications, and efficient light–matter coupling. To this end we present a detailed theoretical analysis of dielectric tuning of WGMs supported in axisymmetric resonators. Using the Bethe–Schwinger equation and adopting an angular spectrum field representation, we study the resonance shift and mode broadening of high-Q WGMs when a planar dielectric substrate is brought close to the resonator. Particular focus is given to use of a uniaxial substrate with an arbitrarily aligned optic axis. Competing red and blue resonance shifts (∼30u2009u2009MHz), deriving from generation of a near-field material polarization and back action from the radiation continuum, respectively, are found. Anomalous resonance shifts can hence be observed depending on the substrate material, whereas mode broadening on the order of ∼50u2009u2009MHz can also be realized. Furthermore, polarization-selective coupling with extinction ratios of >104 can be achieved when the resonator and substrate are of the same composition and their optic axes are chosen correctly. Double refraction and properties of outcoupled beams are also discussed.

Towards terahertz detection and calibration through spontaneous parametric down-conversion in the terahertz idler-frequency range generated by a 795 nm diode laser system

We study a calibration scheme for terahertz wave nonlinear-optical detectors based on spontaneous parametric down-conversion. Contrary to the usual low wavelength pump in the green, we report here on the observation of spontaneous parametric down-conversion originating from an in-growth poled lithium niobate crystal pumped with a continuous wave 50 mW, 795 nm diode laser system, phase-matched to a terahertz frequency idler wave. Such a system is more compact and allows for longer poling periods as well as lower losses in the crystal. Filtering the pump radiation by a rubidium-87 vapor cell allowed the frequency-angular spectra to be obtained down to ∼0.5 THz or ∼1 nm shift from the pump radiation line. The presence of an amplified spontaneous emission “pedestal” in the diode laser radiation spectrum significantly hampers the observation of spontaneous parametric down-conversion spectra, in contrast to conventional narrowband gas lasers. Benefits of switching to longer pump wavelengths are pointed out, suc...

Maximization of the optical intra-cavity power of whispering-gallery mode resonators via coupling prism

In this paper, a detailed description of the optical coupling into a Whispering Gallery Mode (WGM) resonator through a prism via frustrated total internal reflection (FTIR) is presented. The problem is modeled as three media with planar interfaces and closed expressions for FTIR are given. Then, the curvature of the resonator is taken into account and the mode overlap is theoretically studied. A new analytical expression giving the optimal geometry of a disc-shaped or ring-shaped resonator for maximizing the intra-cavity circulating power is presented. Such expression takes into consideration the spatial distribution of the WGM at the surface of the resonator, thus being more accurate than the currently used expressions. It also takes into account the geometry of the prism. It is shown an improvement in the geometry values used with the current expressions of about 30%. The reason why the pump laser signal can be seen in experiments under critical coupling is explained on this basis. Then, the conditions required for exciting the highest possible optical power inside the resonator are obtained. The aim is to achieve a highly-efficient up-conversion of a THz signal into the optical domain via the second-order nonlinearity of the resonator material.

A chaotic approach clears up imaging

A laser that emits bright, incoherent light provides an ideal light source for imaging Lasers appear to be ideal light sources for a variety of projection and imaging systems because of their spectral brightness and their ability to produce a beam of light that can be tightly collimated to travel long distances. Lasers owe these extraordinary properties to a quality called coherence. Yet, lasers are not widely used in imaging and projection applications, because the coherence of laser light is just too extreme. Spatiotemporal coherence of the imaging source leads to artifacts such as speckle, caused by the uncontrolled scattering of laser light and multipath interference that degrade the image considerably. Redding et al. (1) now report how a semiconductor laser based on a chaotic cavity can offer a “compact” solution to this problem. The availability of such low-cost, on-chip semiconductor lasers and the possibility to electrically modulate them make such lasers attractive light sources for a variety of applications, ranging from compact projectors to optical coherence tomography.

Optical microcavities: new understandings and developments: publisher’s note

This publisher’s note reports correcting Ref. [12] and adding a reference in [Photon. Res.5, OM1 (2017)2327912510.1364/PRJ.5.000OM1].

Using the lasing threshold in whispering gallery mode resonators for refractive index sensing

Lasing whispering gallery mode resonators, such as dye doped microspheres and microcapillaries, have shown tremendous potential for refractive index sensing and biosensing applications owning to the narrower resonances achieved upon lasing. This has enabled higher resolution on the determination of the resonance wavelength shift induced by a variation of the surrounding refractive index and as a consequence to reach lower detection limit compared with their fluorescent counterparts. The sensing procedure in both cases relies on tracking the wavelength shift of individual modes, therefore requiring high resolution spectral analysis. This stringent requirement not only prevents any viable commercial prospects due to high equipment cost but more importantly imposes a technological limit, related to the equipment spectral resolution, on the achievable detection limit. In this paper, we show for the first time that the lasing threshold and eventually the resonances intensity can be used for inferring changes of refractive index around a 15 μm dye doped polystyrene instead of the mode tracking procedure. The sensing mechanism relies on the spoiling of the resonator Q factor upon change of refractive index which eventually increases the lasing threshold. In addition to allow free space excitation and collection, alleviating the need for phase matched prism or fiber taper, this novel approach promises to reach lower detection limit by suppressing the need of high resolution spectral analysis of the whispering gallery mode spectra but instead relying on cost effective and highly sensitive intensity measurements.

Study of free-space coupling into mm-wave whispering-gallery mode resonators for a radioastronomy receiver

In this paper, the coupling mechanism of a free-space Gaussian beam into a whispering-gallery mode resonator through a dielectric lens is mathematically modeled and numerically solved by means of the Schelkunoff-Waterman method (the so called T-matrix method). This approach allows in principle, to quickly analyze the performance of different near-field coupling mechanisms with arbitrary excitations. The aim is to efficiently excite a WGM into a nonlinear dielectric resonator in order to detect the weak mm-wave radiation from the cosmic microwave background (CMB) by up-converting the signal into the optical domain via the nonlinearity of the medium.