We report, for the first time, the observation of spontaneous parametric down-conversion (SPDC) free of phase matching (momentum conservation).We alleviate the need to conserve momentum by exploiting the<br>position-momentum uncertainty relation and using a planar geometry source, a 6 μm thick layer of lithium niobate. Nonphase-matched SPDC opens up a new platform on which to investigate fundamental quantum<br>effects but it also has practical applications. The ultrasmall thickness leads to a frequency spectrum an order of magnitude broader than that of phase-matched SPDC. The strong two-photon correlations are still<br>preserved due to energy conservation. This results in ultrashort temporal correlation widths and huge frequency entanglement. The studies we make here can be considered as the initial steps into the emerging field of nonlinear quantum optics on the microscale and nanoscale.
Fading channel estimation for free-space continuous-variable secure quantum communication
László Ruppert,
Christian Peuntinger,
Bettina Heim,
Kevin Günthner,
Vladyslav C. Usenko,
Dominique Elser,
Gerd Leuchs,
Radim Filip,
Christoph Marquardt
We investigate estimation of fluctuating channels and its effect on security of continuous-variable quantum key distribution. We propose a novel estimation scheme which is based on the clusterization of the estimated transmittance data. We show that uncertainty about whether the transmittance is fixed or not results in a lower key rate. However, if the total number of measurements is large, one can obtain using our method a key rate similar to the non-fluctuating channel even for highly fluctuating channels. We also verify our theoretical assumptions using experimental data from an atmospheric quantum channel. Our method is therefore promising for secure quantum communication over strongly fluctuating turbulent atmospheric channels.
Quasiprobability currents on the sphere
I. Valtierra,
A. B. Klimov,
Gerd Leuchs,
Luis Sanchez-Soto
We present analytic expressions for the s-parametrized currents on the sphere for both unitary and dissipative evolutions. We examine the spatial distribution of the flow generated by these currents for quadratic Hamiltonians. The results are applied for the study of the quantum dissipative dynamics of the time-honored Kerr and Lipkin models, exploring the appearance of the semiclassical limit in stable, unstable and tunnelling regimes.
Spin-orbit coupling affecting the evolution of transverse spin
Jörg Eismann,
Peter Banzer,
Martin Neugebauer
Physical Review Research
1
(3)
033143-1-033143-4
(2019)
| Preprint
| Journal
| PDF
We investigate the evolution of transverse spin in tightly focused circularly polarized beams of light, where spin-orbit coupling causes a local rotation of the polarization ellipses upon propagation through the focal volume. The effect can be explained as a relative Gouy-phase shift between the circularly polarized transverse field and the longitudinal field carrying orbital angular momentum. The corresponding rotation of the electric transverse spin density is observed experimentally by utilizing a recently developed reconstruction scheme, which relies on transverse-spin-dependent directional scattering of a nano-probe.
Single-shot reconstruction of a subpicosecond pulse from a fiber laser system via processing strongly self-phase modulated spectra
E. A. Anashkina,
A. Andrianov V,
Gerd Leuchs
RESULTS IN PHYSICS
16
102848
(2019)
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| PDF
The single-shot reconstruction of an ultrashort pulse from a fiber laser system by the method based on recording and numerical processing of two self-phase modulated spectra after a nonlinear fiber is demonstrated experimentally. The 0.7-ps asymmetrical signal with an energy of similar to 1 mu J retrieved by this method without time direction ambiguity is in good agreement with independent measurements by second harmonic generation frequency-resolved optical gating (SHG FROG) technique.
Numerical simulation of multi-color laser generation in TM-doped tellurite microsphere at 1.9, 1.5 and 2.3 microns
E. A. Anashkina,
Gerd Leuchs,
A. V. Andrianov
RESULTS IN PHYSICS
16
102811
(2019)
| Journal
| PDF
We present the first detailed theoretical analysis of multi-color continuous wave lasing in Tm-doped tellurite spherical microresonators with whispering gallery modes pumped at a wavelength of 792 nm. The numerical model is based on solving a system of equations for intracavity field amplitudes and rate equations using the parameters of Tm-doped tellurite glass measured in the previous experiments. All fundamental whispering gallery modes in the gain bands are taken into account. We demonstrate diagrams of generation regimes depending on Q-factors and pump power, which show a possibility of single-color lasing at a wavelength of similar to 1.9 mu m, two-color lasing at wavelengths of similar to 1.9&1.5 mu m and at similar to 1.9&2.3 mu m, and three-color lasing at wavelengths of similar to 1.9&1.5&2.3 mu m. Such microlasers can play a significant role in sensing applications.
Measuring the temperature and heating rate of a single ion by imaging
Bharath Srivathsan,
Martin Fischer,
Lucas Alber,
Markus Weber,
Markus Sondermann,
Gerd Leuchs
New Journal of Physics
21
113014
(2019)
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We present a technique based on high resolution imaging to measure the absolute temperature and the heating rate of a single ion trapped at the focus of a deep parabolic mirror. We collect the fluorescence light scattered by the ion during laser cooling and image it onto a camera. Accounting for the size of the point-spread function and the magnification of the imaging system, we determine the spatial extent of the ion, from which we infer the mean phonon occupation number in the trap. Repeating such measurements and varying the power or the detuning of the cooling laser, we determine the heating rate induced by any kind of effect other than photon scattering. In contrast to other established schemes for measuring the heating rate, the ion is always maintained in a state of thermal equilibrium at temperatures close to the Doppler limit.
R&D advances for quantum communication systems
Gerd Leuchs,
Christoph Marquardt,
Luis Sanchez-Soto,
Dmitry V. Strekalov,
Alan E. Willner
Optical Fiber Telecommunications VII
Chapter 12
495-563
(2019)
| Journal
Understanding the nature of light leads to the question of how the principles of quantum physics can be harnessed in practical optical communication. A deeper understanding of fundamental physics has always advanced technology. However, the quantum principles certainly have a distinctly limiting character when looked upon from the engineering point of view. A particle cannot have well-defined momentum and position at the same time. An informative measurement will unpredictably alter the state of a quantum object. One cannot reliably clone an arbitrary quantum state. These and a number of other similar principles give rise to what is commonly known as the quantum “no-go theorems”—a disconcerting term when it comes to building something practical. And yet a search for novel principles of communication enabled by quantum physics began already in its early days and has only intensified since. On this path physicists are faced with a remarkable challenge: to turn a series of negative statements into new technological recipes.
Squeezed vacuum states from a whispering gallery mode resonator
Alexander Otterpohl,
Florian Sedlmeir,
Ulrich Vogl,
Thomas Dirmeier,
Golnoush Shafiee,
Gerhard Schunk,
Dmitry Strekalov,
Harald G. L. Schwefel,
Tobias Gehring, et al.
Squeezed vacuum states enable optical measurements below the quantum limit and hence are a valuable resource for applications in quantum metrology and also quantum communication. However, most available sources require high pump powers in the milliwatt range and large setups that hinder real-world applications. Furthermore, degenerate operation of such systems presents a challenge. Here, we use a compact crystalline whispering gallery mode resonator made of lithium niobate as a degenerate parametric oscillator. We demonstrate about 1.4 dB noise reduction below the shot-noise level for only 300 μW of pump power in degenerate single-mode operation. Furthermore, we report a record pump threshold as low as 1.35 μW. Our results show that the whispering gallery-based approach presents a promising platform for a compact and efficient source for nonclassical light.
Development of infrared fiber lasers at 1555 nm and at Er-doped zinc-tellurite glass fiber
E. A. Anashkina,
A. V. Andrianov,
V. V. Dorofeev,
A. V. Kim,
V. V. Koltashev,
Gerd Leuchs,
S. E. Motorin,
S. V. Muravyev,
A. D. Plekhovich
JOURNAL OF NON-CRYSTALLINE SOLIDS
525
119667
(2019)
| Journal
We manufactured and characterized a low-loss gain fiber based on high-purity TeO2 -ZnO-La2O3-Na2O undoped glass for 100-mu m cladding and TeO2 -ZnO-La2O3-Na2O glass for 10-mu m core doped with 0.24 mol% Er2O3. To confirm that the produced fiber is a promising active element for the infrared range, we demonstrated experimentally broadband laser amplification and CW generation at the I-4(13/2) -> I-4(15/2) transition at 1555 nm with a single-mode diode pump at 975 nm at the I-4(15/2) -> I-4(11/2) transition. We developed a numerical model calibrated to the experimental data for prediction and optimization of laser characteristics in schemes with different parameters. The model describes single- wavelength lasing as well as dual-wavelength cascade lasing at 1555 nm and 2800 nm. It was shown numerically that for the optimized parameters, the maximum slope efficiency at 2800 nm at the I-4(11/2) -> I-4(13/2) transition can reach similar to 20%. The maximum calculated efficiency at 1555 nm exceeds 30%.
The standard quantum limit of coherent beam combining
Christian Müller,
Florian Sedlmeir,
Vitaliy O. Martynov,
Christoph Marquardt,
Alexey V. Andrianov ,
Gerd Leuchs
New Journal of Physics
21
(9)
093047
(2019)
| Journal
Coherent beam combining refers to the process of generating a bright output beam by merging independent input beams of individually diffusing relative phases by locking them to each other. We report the first quantum mechanical noise limit calculations for coherent beam combining and compare our results to quantum-limited amplification. Our coherent beam combining scheme is based on an optical Fourier transformation which renders the scheme compatible with integrated optics combined with feed-back stabilization of the relative phases. The scheme can be layed out for an arbitrary number of input beams and approaches the shot noise limit for a large number of inputs.
Indefinite-Mean Pareto Photon Distribution from Amplified Quantum Noise
Mathieu Manceau,
Kirill Spasibko,
Gerd Leuchs,
Radim Filip,
Maria Chekhova
Extreme events appear in many physics phenomena, whenever the probability distribution has a "heavy tail" differing very much from the equilibrium one. Most unusual are the cases of power-law (Pareto) probability distributions. Among their many manifestations in physics, from "rogue waves" in the ocean to Levy flights in random walks, Pareto dependences can follow very different power laws. For some outstanding cases, the power exponents are less than 2, leading to indefinite values not only for higher moments but also for the mean. Here we present the first evidence of indefinite-mean Pareto distribution of photon numbers at the output of nonlinear effects pumped by parametrically amplified vacuum noise, known as bright squeezed vacuum (BSV). We observe a Pareto distribution with power exponent 1.31 when BSV is used as a pump for supercontinuum generation, and other heavy-tailed distributions (however, with definite moments) when it pumps optical harmonics generation. Unlike in other fields, we can flexibly control the Pareto exponent by changing the experimental parameters. This extremely fluctuating light is interesting for ghost imaging and for quantum thermodynamics as a resource to produce more efficiently nonequilibrium states by single-photon subtraction, the latter of which we demonstrate experimentally.
Characterization of an underwater channel for quantum communications in the Ottawa River
Felix Hufnagel,
Alicia Sit,
Florence Grenapin,
Frederic Bouchard,
Khabat Heshami,
Duncan England,
Yingwen Zhang,
Benjamin J. Sussman,
Robert W. Boyd, et al.
OPTICS EXPRESS
27
(19)
26346-26354
(2019)
| Journal
| PDF
We examine the propagation of optical beams possessing different polarization states and spatial modes through the Ottawa River in Canada. A Shack-Hartmann wavefront sensor is used to record the distorted beam's wavefront. The turbulence in the underwater channel is analysed, and associated Zernike coefficients are obtained in real-time. Finally, we explore the feasibility of transmitting polarization states as well as spatial modes through the underwater channel for applications in quantum cryptography. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Interaction of light carrying orbital angular momentum with a chiral dipolar scatterer
Pawel Wozniak,
Israel De León,
Katja Höflich,
Gerd Leuchs,
Peter Banzer
The capability to distinguish the handedness of circularly polarized light is a well-known intrinsic property of a chiral nanostructure. It is a long-standing controversial debate, however, whether a chiral object can also sense the vorticity, or the orbital angular momentum (OAM), of a light field. Since OAM is a non-local property, it seems rather counter-intuitive that a point-like chiral object could be able to distinguish the sense of the wave-front of light carrying OAM. Here, we show that a dipolar chiral nanostructure is indeed capable of distinguishing the sign of the phase vortex of the incoming light beam. To this end, we take advantage of the conversion of the sign of OAM, carried by a linearly polarized Laguerre-Gaussian beam, into the sign of optical chirality upon tight focusing. Our study provides for a deeper insight into the discussion of chiral light-matter interactions and the respective role of OAM.
Adaptive compressive tomography: A numerical study
D. Ahn,
Y. S. Teo,
H. Jeong,
D. Koutny,
J. Rehacek,
Z. Hradil,
Gerd Leuchs,
Luis Sanchez-Soto
We perform several numerical studies for our recently published adaptive compressive tomography scheme [D. Ahn et al., Phys. Rev. Lett. 122, 100404 (2019)], which significantly reduces the number of measurement settings to unambiguously reconstruct any rank-deficient state without any a priori knowledge besides its dimension. We show that both entangled and product bases chosen by our adaptive scheme perform comparably well with recently known compressed-sensing element-probing measurements, and also beat random measurement bases for low-rank quantum states. We also numerically conjecture asymptotic scaling behaviors for this number as a function of the state rank for our adaptive schemes. These scaling formulas appear to be independent of the Hilbert-space dimension. As a natural development, we establish a faster hybrid compressive scheme that first chooses random bases, and later adaptive bases as the scheme progresses. As an epilogue, we reiterate important elements of informational completeness for our adaptive scheme.
Nonlinear optics with full three-dimensional illumination
Rojiar Penjweini,
Markus Weber,
Markus Sondermann,
Robert W. Boyd,
Gerd Leuchs
Nonlinear optical interactions play a crucial role in modern technology and lead to important applications such as optical switching, optical harmonic generation, and the characterization of ultrafast material processes. Nonlinear interactions are enhanced by using a tightly focused laser beam, but nonetheless they are typically excited by a loosely focused (that is, paraxial) laser beam. Here we investigate a specific process, third-harmonic generation, excited by a highly nonparaxial beam that illuminates an interaction region from a nearly full solid angle. We elucidate the influence of the focal volume and the pump intensity on the number of frequency-tripled photons by varying the solid angle from which the pump light is focused, and we find good agreement between the experiments and numerical calculations. As the pump light is focused to a spot size much smaller than the laser wavelength, the Gouy phase does not limit the yield of frequency-converted photons, in stark contrast to the paraxial regime. We believe that our findings are generic and apply to many other nonlinear optical processes when the pump light is focused from a full solid angle.
Emission of circularly polarized light by a linear dipole
Controlling the polarization state and the propagation direction of photons is a fundamental prerequisite for many nanophotonic devices and a precursor for future on-chip communication, where the emission properties of individual emitters are particularly relevant. Here, we report on the emission of partially circularly polarized photons by a linear dipole. The underlying effect is linked to the near-field part of the angular spectrum of the dipole, and it occurs in any type of linear dipole emitter, ranging from atoms and quantum dots to molecules and dipole-like antennas. We experimentally observe it by near-field to far-field transformation at a planar dielectric interface and numerically demonstrate the utility of this phenomenon by coupling the circularly polarized light to the individual paths of crossing waveguides.
Reconstruction of Optical Pulse Intensity and Phase Based on SPM Spectra Measurements in Microstructured Tellurite Fiber in Telecommunication Range
Elena Anashkina,
Maxim Koptev,
Alexey Andrianov,
Vitaly Dorofeev,
Surinder Singh,
Gerd Leuchs,
Arkady Kim
JOURNAL OF LIGHTWAVE TECHNOLOGY
37
(17)
4375-4381
(2019)
| Journal
We report novel functionalities of a very simple, robust, and low-cost optical metrology method for the reconstruction of an intensity profile and phase of pulses in the telecommunication range based on measuring the fundamental spectrum and two self-phase modulated spectra after a nonlinear element with Kerr nonlinearity. We demonstrate, for the first time, to the best of our knowledge, the possibility of implementing this method using a microstructured tellurite fiber (with a zero dispersion wavelength of 1.55 mu m and nonlinear coefficient >500 (W km)(-1)) as a nonlinear element for characterizing optical pulses with a wide range of parameters: wavelengths from O-band to L-band, durations from <100 fs to similar to 100 ps, low energies (similar to 100 pJ), and/or low peak powers (similar to 1 W). The intensity and phase of 80-fs ultrashort pulses with an energy of similar to 100 pJ are retrieved experimentally. We give a detailed theoretical analysis of the possibilities of reconstructing telecommunication pulses, including the important problem of measuring two coherent neighboring pulses whose wings may overlap.
Vectorial vortex generation and phase singularities upon Brewster reflection
Rene Barczyk,
Sergey Nechayev,
Muhammad Abdullah Butt,
Gerd Leuchs,
Peter Banzer
Physical Review A
99
(6)
063820
063820-1- 063820-8
(2019)
| Preprint
| Journal
| PDF
We experimentally demonstrate the emergence of a purely azimuthally polarized vectorial vortex beam with a phase singularity upon Brewster reflection of focused circularly polarized light from a dielectric substrate. The effect originates from the polarizing properties of the Fresnel reflection coefficients described in Brewster’s law. An astonishing consequence of this effect is that the reflected<br>field’s Cartesian components acquire local phase singularities at Brewster’s angle. Our observations are crucial for polarization microscopy and open new avenues for the generation of exotic states of light based on spin-to-orbit coupling, without the need for sophisticated optical elements.
Reading out Fisher information from the zeros of the point spread function
M. Paúr,
B. Stoklasa,
D. Koutný,
J. Řeháček,
Z. Hradil,
J. Grover,
A. Krzic,
Luis Sanchez-Soto
We show that, for optical systems whose point spread functions exhibit isolated zeros, the information one can gain about the separation between two incoherent point light sources does not scale quadratically with the separation (which is the distinctive dependence causing Rayleigh’s curse) but only linearly. Moreover, the dominant contribution to the separation information comes from regions in the vicinity of these zeros. We experimentally confirm this idea, demonstrating significant superresolution using natural or artificially created spectral doublets.
Compressed sensing of twisted photons
F. Bouchard,
D. Koutny,
F. Hufnagel,
Z. Hradil,
J. Rehacek,
Y. S. Teo,
D. Ahn,
H. Jeong,
Luis Sanchez-Soto, et al.
The ability to completely characterize the state of a quantum system is an essential element for the emerging quantum technologies. Here, we present a<br>compressed-sensing inspired method to ascertain any rank-deficient qudit state, which we experimentally encode in photonic orbital angular momentum. We efficiently reconstruct these qudit states from a few scans with an intensified CCD camera. Since it requires only a few intensity measurements, our technique<br>would provide an easy and accurate way to identify quantum sources, channels, and systems.
Experimental demonstration of linear and spinning Janus dipoles for polarisation and wavelength selective near-field coupling
M. F. Picardi,
Martin Neugebauer,
Jörg Eismann,
Gerd Leuchs,
Peter Banzer,
F. J. Rodriguez-Fortuno,
A. V. Zayats
The electromagnetic field scattered by nano-objects contains a broad range of wave vectors and can be efficiently coupled to waveguided modes. The dominant ontribution to scattering from subwavelength dielectric and plasmonic nanoparticles is determined by electric and magnetic dipolar<br>responses. Here, we experimentally demonstrate spectral and phase selective excitation of Janus dipoles, sources with electric and magnetic dipoles oscillating out of phase, in order to control near-field interference and directional coupling to waveguides. We show that by controlling the<br>polarisation state of the dipolar excitations and the excitation wavelength to adjust their relative contributions, directionality and coupling strength can be fully tuned. Furthermore, we introduce a novel spinning Janus dipole featuring cylindrical symmetry in the near and far field, which results in either omnidirectional coupling or noncoupling. Controlling the propagation of guided light waves via fast and robust near-field <br> interference between polarisation components of a source is required in many applications in nanophotonics and quantum optics.
Mimicking Chiral Light-Matter Interaction
Sergey Nechayev,
Peter Banzer
Physical Review B
99
(24)
241101(R)
241101-1- 241101-6
(2019)
| Preprint
| Journal
| PDF
We demonstrate that electric-dipole scatterers can mimic chiral light-matter interaction by generating far-field circular polarization upon scattering, even though the optical chirality of the incident field as well as that of the scattered light is zero. The presented effect originates from the fact that electric-dipole scatterers respond selectively only to the incident electric field, which eventually results in depolarization of the transmitted beam and in generation of far-field circular polarization. To experimentally demonstrate this effect we utilize a cylindrical vector beam with spiral polarization and a spherical gold nanoparticle positioned on the optical axis -- the axis of rotational symmetry of the system. Our experiment and a simple theoretical model address the fundamentals of duality symmetry in optics and chiral light-matter interactions, accentuating their richness and ubiquity yet in highly symmetric configurations.
Large-Area 3D Plasmonic Crescents with Tunable Chirality
Eric S. A. Goerlitzer,
Reza Mohammadi,
Sergey Nechayev,
Peter Banzer,
Nicolas Vogel
Abstract Chiral plasmonic nanostructures hold promise for enhanced chiral sensing and circular dichroism spectroscopy of chiral molecules. It is therefore of interest to fabricate chiral plasmonic nanostructures with tailored chiroptical properties over large areas with reasonable effort. Here, a colloidal lithography approach is used to produce macroscopic arrays of sub-micrometer 3D chiral plasmonic crescent structures over areas >1 cm2. The chirality originates from symmetry breaking by the introduction of a step within the crescent structure. This step is produced by an intermediate layer of silicon dioxide onto which the metal crescent structure is deposited. It is experimentally demonstrated that the chiroptical properties in such structures can be tailored by changing the position of the step within the crescent. These experiments are complemented by finite element simulations and the application of a multipole expansion to elucidate the physical origin of the circular dichroism of the crescent structures.
Multi-twist polarization ribbon topologies in highly-confined optical fields
Thomas Bauer,
Peter Banzer,
Frédéric Bouchard,
Sergej Orlov,
Lorenzo Marrucci,
Enrico Santamato,
Robert W Boyd,
Ebrahim Karimi,
Gerd Leuchs
Electromagnetic plane waves, solutions to Maxwell's equations, are said to be 'transverse' in vacuum. Namely, the waves' oscillatory electric and magnetic fields are confined within a plane transverse to the waves' propagation direction. Under tight-focusing conditions however, the field can exhibit longitudinal electric or magnetic components, transverse spin angular momentum, or non-trivial topologies such as Möbius strips. Here, we show that when a suitably spatially structured beam is tightly focused, a 3-dimensional polarization topology in the form of a ribbon with two full twists appears in the focal volume. We study experimentally the stability and dynamics of the observed polarization ribbon by exploring its topological structure for various radii upon focusing and for different propagation planes.
Substrate-Induced Chirality in an Individual Nanostructure
Sergey Nechayev,
Rene Barczyk,
Uwe Mick,
Peter Banzer
We experimentally investigate the chiral optical response of an individual nanostructure consisting of three equally sized spherical nanoparticles made of different materials and arranged in \ang{90} bent geometry. Placing the nanostructure on a substrate converts its morphology from achiral to chiral. Chirality leads to pronounced differential extinction, i.e., circular dichroism and optical rotation, or equivalently, circular birefringence, which would be strictly forbidden in the absence of a substrate or heterogeneity. This first experimental observation of the substrate-induced break of symmetry in an individual heterogeneous nanostructure sheds new light on chiral light-matter interactions at substrate-nanostructure interfaces.
Investigating the Optical Properties of a Laser Induced 3D Self‐Assembled Carbon–Metal Hybrid Structure
Muhammad Abdullah Butt,
Antonino Calà Lesina,
Martin Neugebauer,
Thomas Bauer,
Lora Ramunno,
Alessandro Vaccari,
Pierre Berini,
Yuriy Petrov,
Denis Danilov, et al.
Small
15
(18)
1900512
1900512-1-1900512-9
(2019)
| Journal
Carbon‐based and carbon–metal hybrid materials hold great potential for applications in optics and electronics. Here, a novel material made of carbon and gold–silver nanoparticles is discussed, fabricated using a laser‐induced self‐assembly process. This self‐assembled metamaterial manifests itself in the form of cuboids with lateral dimensions on the order of several micrometers and a height of tens to hundreds of nanometers. The carbon atoms are arranged following an orthorhombic unit cell, with alloy nanoparticles intercalated in the crystalline carbon matrix. The optical properties of this metamaterial are analyzed experimentally using a microscopic Müller matrix measurement approach and reveal a high linear birefringence across the visible spectral range. Theoretical modeling based on local‐field theory applied to the carbon matrix links the birefringence to the orthorhombic unit cell, while finite‐difference time‐domain simulations of the metamaterial relates the observed optical response to the distribution of the alloy nanoparticles and the optical density of the carbon matrix.
Resonant electro-optic frequency comb
Alfredo Rueda,
Florian Sedlmeir,
Madhuri Kumari,
Gerd Leuchs,
Harald G. L. Schwefel
High-speed optical telecommunication is enabled by wavelength-division multiplexing, whereby hundreds of individually stabilized lasers encode information within a single-mode optical fibre. Higher bandwidths require higher total optical power, but the power sent into the fibre is limited by optical nonlinearities within the fibre, and energy consumption by the light sources starts to become a substantial cost factor1. Optical frequency combs have been suggested to remedy this problem by generating numerous discrete, equidistant laser lines within a monolithic device; however, at present their stability and coherence allow them to operate only within small parameter ranges2,3,4. Here we show that a broadband frequency comb realized through the electro-optic effect within a high-quality whispering-gallery-mode resonator can operate at low microwave and optical powers. Unlike the usual third-order Kerr nonlinear optical frequency combs, our combs rely on the second-order nonlinear effect, which is much more efficient. Our result uses a fixed microwave signal that is mixed with an optical-pump signal to generate a coherent frequency comb with a precisely determined carrier separation. The resonant enhancement enables us to work with microwave powers that are three orders of magnitude lower than those in commercially available devices. We emphasize the practical relevance of our results to high rates of data communication. To circumvent the limitations imposed by nonlinear effects in optical communication fibres, one has to solve two problems: to provide a compact and fully integrated, yet high-quality and coherent, frequency comb generator; and to calculate nonlinear signal propagation in real time5. We report a solution to the first problem.
Huygens' Dipole for Polarization-Controlled Nanoscale Light Routing
Sergey Nechayev,
Jörg Eismann,
Martin Neugebauer,
Pawel Wozniak,
Ankan Bag,
Gerd Leuchs,
Peter Banzer
Structured illumination allows for satisfying the first Kerker condition of in-phase perpendicular electric and magnetic dipole moments in any isotropic scatterer that supports electric and magnetic dipolar resonances. The induced Huygens' dipole may be utilized for unidirectional coupling to waveguide modes that propagate transverse to the excitation beam. We study two <br> configurations of a Huygens' dipole -- longitudinal electric and transverse magnetic dipole moments or vice versa. We experimentally show that only the radially polarized emission of the first and azimuthally polarized emission of the second configuration are directional in the far-field. This polarization selectivity implies that directional excitation of either TM or TE waveguide modes is possible. Applying this concept to a single nanoantenna excited with structured light, we are able to experimentally achieve scattering directivities of around 23 dB and 18 dB in TM and TE modes, respectively. This strong directivity paves the way for tunable polarization-controlled nanoscale light routing and applications in optical metrology, <br> ocalization microscopy and on-chip optical devices.
Seeded and unseeded high-order parametric down-conversion
Cameron Okoth,
Andrea Cavanna,
Nicolas Joly,
Maria Chekhova
Spontaneous parametric down-conversion (SPDC) has been one of the foremost tools in quantum optics for over five decades. Over that time, it has been used to demonstrate some of the curious features that arise from quantum mechanics. Despite the success of SPDC, its higher-order analogs have never been observed, even though it has been suggested that they generate far more unique and exotic states than SPDC. An example of this is the emergence of non-Gaussian states without the need for postselection. Here we calculate the expected rate of emission for nth-order SPDC with and without external stimulation (seeding). Focusing primarily on third-order parametric down-conversion, we estimate the photon detection rates in a rutile crystal for both the unseeded and seeded regimes.
Quantum correlations in separable multi-mode states and in classically entangled light
N. Korolkova,
Gerd Leuchs
REPORTS ON PROGRESS IN PHYSICS
82
(5)
056001
(2019)
| Journal
In this review we discuss intriguing properties of apparently classical optical fields, that go beyond purely classical context and allow us to speak about quantum characteristics of such fields and about their applications in quantum technologies. We briefly define the genuinely quantum concepts of entanglement and steering. We then move to the boarder line between classical and quantum world introducing quantum discord, a more general concept of quantum coherence, and finally a controversial notion of classical entanglement. To unveil the quantum aspects of often classically perceived systems, we focus more in detail on quantum discordant correlations between the light modes and on nonseparability properties of optical vector fields leading to entanglement between different degrees of freedom of a single beam. To illustrate the aptitude of different types of correlated systems to act as quantum or quantum-like resource, entanglement activation from discord, high-precision measurements with classical entanglement and quantum information tasks using intra-system correlations are discussed. The common themes behind the versatile quantum properties of seemingly classical light are coherence, polarization and inter and intra-mode quantum correlations.
Adaptive Compressive Tomography with No a priori Information
Daekun Ahn,
Yong Siah Teo,
Hyunseok Jeong,
Frédéric Bouchard,
Felix Hufnagel,
Ebrahim Karimi,
D Koutny,
Jarda Rehacek,
Zdenek Hradil, et al.
Physical Review Letters
122
(2019)
| Journal
| PDF
Quantum state tomography is both a crucial component in the field of quantum information and computation and a formidable task that requires an incogitable number of measurement configurations as the system dimension grows. We propose and experimentally carry out an intuitive adaptive compressive tomography scheme, inspired by the traditional compressed-sensing protocol in signal recovery, that tremendously reduces the number of configurations needed to uniquely reconstruct any given quantum state without any additional a priori assumption whatsoever (such as rank information, purity, etc.) about the state, apart from its dimension.
Plasmonic carbon nanohybrids from laser-induced deposition: controlled synthesis and SERS properties
Anastasia Povolotckaia,
Dmitrii Pankin,
Yuriy Petrov,
Anna Vasileva,
Ilya Kolesnikov,
George Sarau,
Silke Christiansen,
Gerd Leuchs,
Alina Manshina
JOURNAL OF MATERIALS SCIENCE
54
(11)
8177-8186
(2019)
| Journal
A novel single-step, laser-induced and solution-based process is presented for synthesizing complex hybrid metal/carbon nanostructures. The process relies on simply illuminating the interface between a substrate and a liquid solution of the supramolecular complex [Au13Ag12(C2Ph)(20)(PPh2(C6H4)(3)PPh2)(3)][PF6](5) (hereinafter abbreviated as SMC) with an unfocussed He-Cd laser having a wavelength of 325nm and an intensity of I=0.5W/cm(2). The process results in hybrid nanostructures of well-controlled morphology: nanoparticles (NP) and 2D flakes, which may also grow jointly to form 3D morphologically complex multipetal flower-like' structures. At the atomic scale, the obtained metamaterials are complex in composition and structure, i.e., they contain bimetallic Au-Ag nanoclusters of diameter 3-5nm incorporated inside a carbonaceous matrix. This matrix can be amorphous or crystalline, and the details of the compositional outcome can be controlled and steered by the laser deposition parameters. Au-Ag nanoclusters show plasmonic behavior including the enhancement of electromagnetic fields of visible light. This leads to the enhancement of Raman scattering by the Au-Ag nanoparticle ensemble within the carbonaceous matrix. This enables a 3D architecture for stimulating surface-enhanced Raman scattering (SERS).
Structured quantum projectiles
Hugo Larocque,
Robert Fickler,
Eliahu Cohen,
Vincenzo Grillo,
Rafal E. Dunin-Borkowski,
Gerd Leuchs,
Ebrahim Karimi
Matter wave interferometry is becoming an increasingly important technique in quantum metrology. However, unlike its photonic counterpart, this technique relies on the interference of particles possessing a nonzero rest mass and an electric charge. Matter waves can therefore experience alterations in their wavelike features while propagating through uniform fields to which a linear potential can be attributed, e.g., the Newtonian gravitational potential. Here, we derive the propagation kernel attributed to matter waves within such a potential. This kernel thereafter allows us to provide analytical formulations for structured matter waves subjected to a linear potential. Our formulations are in agreement with both the classical dynamics attributed to such waves and with previous interferometry experiments. Eigenbasis representations of structured matter waves are also introduced along with their application to enhanced interferometric measurements. Our results are not only relevant to matter wave interferometry, but also emphasize its fundamental differences with respect to photonic interferometry.
Orbital-to-Spin Angular Momentum Conversion Employing Local Helicity
Sergey Nechayev,
Jörg Eismann,
Gerd Leuchs,
Peter Banzer
Spin-orbit interactions in optics traditionally describe an influence of the polarization degree of freedom of light on its spatial properties. The most prominent example is the generation of a spin-dependent optical vortex upon focusing or scattering of a circularly polarized plane-wave by a nanoparticle, converting spin to orbital angular momentum of light. Here, we present a mechanism of conversion of orbital-to-spin angular momentum of light upon scattering of a linearly polarized vortex beam by a spherical silicon nanoparticle. We show that focused linearly polarized Laguerre-Gaussian beams of first order (ℓ=±1) exhibit an ℓ-dependent spatial distribution of helicity density in the focal volume. By using a dipolar scatterer the helicity density can be manipulated locally, while influencing globally the spin and orbital angular momentum of the beam. Specifically, the scattered light can be purely circularly polarized with the handedness depending on the orbital angular momentum of the incident beam. We corroborate our findings with theoretical calculations and an experimental demonstration. Our work sheds new light on the global and local properties of helicity conservation laws in electromagnetism.
The Wigner flow on the sphere
P. Yang,
I. F. Valtierra,
A. B. Klimov,
S. -T. Wu,
R. -K. Lee,
Luis Sanchez-Soto,
Gerd Leuchs
Physica Scripta
94
(4)
044001
(2019)
| Journal
| PDF
We derive a continuity equation for the evolution of the SU(2) Wigner function under nonlinear Kerr evolution. We give explicit expressions for the resulting quantum Wigner current, and discuss the appearance of the classical limit. We show that the global structure of the quantum current significantly differs from the classical one, which is clearly reflected in the form of the corresponding stagnation lines.
Weak measurement enhanced spin Hall effect of light for particle displacement sensing
Martin Neugebauer,
Sergey Nechayev,
Martin Vorndran,
Gerd Leuchs,
Peter Banzer
A spherical nanoparticle can scatter tightly focused optical beams in a spin-segmented manner, meaning that the far field of the scattered light exhibits laterally separated left- and right-handed circularly polarized components. This effect, commonly referred to as giant spin Hall effect of light, strongly depends on the position of the scatterer in the focal volume. Here, a scheme that utilizes an optical weak measurement in a cylindrical polarization basis is put forward to drastically enhance the spin-segmentation and, therefore, the sensitivity to small displacements of a scatterer. In particular, we experimentally achieve a change of the spin-splitting signal of 5% per nanometer displacement.
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