Interferometric Scattering (iSCAT) Microscopy & Related Techniques
Richard W. Taylor, Vahid Sandoghdar
To appear as a book chapter in Labelfree SuperResolution Microscopy, V. Astratov (ed.) by Springer, ISBN 9783030217228
arXiv: 1812.10765
(2018)
Preprint

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Interferometric scattering (iSCAT) microscopy is a powerful tool for
labelfree sensitive detection and imaging of nanoparticles to high
spatiotemporal resolution. As it was born out of detection principles central
to conventional microscopy, we begin by surveying the historical development of
the microscope to examine how the exciting possibility for interferometric
scattering microscopy with sensitivities sufficient to observe single molecules
has become a reality. We discuss the theory of interferometric detection and
also issues relevant to achieving a high detection sensitivity and speed. A
showcase of numerous applications and avenues of novel research across various
disciplines that iSCAT microscopy has opened up is also presented.
Weak measurement of elliptical dipole moments by C point splitting
Sergey Nechayev, Martin Neugebauer, Martin Vorndran, Gerd Leuchs, Peter Banzer
Physical Review Letters
121(24)
243903
(2018)

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We investigate points of circular polarization in the far field of elliptically polarized dipoles and establish a relation between the angular position and helicity of these C points and the dipole moment. In the case of highly eccentric dipoles, the C points of opposite handedness exhibit only a small angular separation and occur in the low intensity region of the emission pattern. In this regard, we introduce an optical weak measurement approach that utilizes the transverse electric (azimuthal) and transverse magnetic (radial) fareld polarization basis. Projecting the far field
onto a spatially varying postselected polarization state reveals the angular separation and the helicity of the C points. We demonstrate the applicability of this approach and determine the elliptical dipole moment of a particle sitting on an interface by measuring the C points in its far field.
LongLived RefractiveIndex Changes Induced by Femtosecond Ionization in GasFilled SingleRing PhotonicCrystal Fibers
Johannes R. Koehler, Felix Köttig, Barbara M. Trabold, Francesco Tani, Philip St. J. Russell
We investigate refractiveindex changes caused by femtosecond photoionization in a gasfilled hollowcore photoniccrystal fiber. Using spatiallyresolved interferometric sideprobing, we find that these changes live for tens of microseconds after the photoionization event — eight orders of magnitude longer than the pulse duration. Oscillations in the megahertz frequency range are simultaneously observed, caused by mechanical vibrations of the thinwalled capillaries surrounding the hollow core. These two nonlocal effects can affect the propagation of a second pulse that arrives within their lifetime, which works out to repetition rates of tens of kilohertz. Filling the fiber with an atomically lighter gas significantly reduces ionization, lessening the strength of the refractiveindex changes. The results will be important for understanding the dynamics of gasbased fiber systems operating at high intensities and high repetition rates, when temporally nonlocal interactions between successive laser pulses become relevant.
Cavity optomagnonics with magnetic textures: coupling a magnetic vortex to light
Jasmin Graf, Hannes Pfeifer, Florian Marquardt, Silvia ViolaKusminskiy
Optomagnonic systems, where light couples coherently to collective excitations in magnetically ordered solids, are currently of high interest due to their potential for quantum information processing platforms at the nanoscale. Efforts so far, both at the experimental and theoretical level, have focused on systems with a homogeneous magnetic background. A unique feature in optomagnonics is however the possibility of coupling light to spin excitations on top of magnetic textures. We propose a cavityoptomagnonic system with a non homogeneous magnetic ground state, namely a vortex in a magnetic microdisk. In particular we study the coupling between optical whispering gallery modes to magnon modes localized at the vortex. We show that the optomagnonic coupling has a rich spatial structure and that it can be tuned by an externally applied magnetic field. Our results predict cooperativities at maximum photon density of the order of C≈10−2 by proper engineering of these structures.
Nonexponential decay of a giant artificial atom
Gustav Andersson, Baladitya Suri, Lingzhen Guo, Thomas Aref, Per Delsing
In quantum optics, lightmatter interaction has conventionally been studied using small atoms interacting with electromagnetic fields with wavelength several orders of magnitude larger than the atomic dimensions. In contrast, here we experimentally demonstrate the vastly different giant atom regime, where an artificial atom interacts with acoustic fields with wavelength several orders of magnitude smaller than the atomic dimensions. This is achieved by coupling a superconducting qubit to surface acoustic waves at two points with separation on the order of 100 wavelengths. This approach is comparable to controlling the radiation of an atom by attaching it to an antenna. The slow velocity of sound leads to a significant internal timedelay for the field to propagate across the giant atom, giving rise to nonMarkovian dynamics. We demonstrate the nonMarkovian character of the giant atom in the frequency spectrum as well as nonexponential relaxation in the time domain.
Experimental investigation of highdimensional quantum key distribution protocols with twisted photons
Frédéric Bouchard, Khabat Heshami, Duncan England, Robert Fickler, Robert W. Boyd, BertholdGeorg Englert, Luis L. SánchezSoto, Ebrahim Karimi
Quantum key distribution is on the verge of real world applications, where perfectly secure information can be distributed among multiple parties. Several quantum cryptographic protocols have been theoretically proposed and independently realized in different experimental conditions. Here, we develop an experimental platform based on highdimensional orbital angular momentum states of single photons that enables implementation of multiple quantum key distribution protocols with a single experimental apparatus. Our versatile approach allows us to experimentally survey different classes of quantum key distribution techniques, such as the 1984 Bennett & Brassard (BB84), tomographic protocols including the sixstate and the Singapore protocol, and to investigate, for the first time, a recently introduced differential phase shift (Chau15) protocol using twisted photons. This enables us to experimentally compare the performance of these techniques and discuss their benefits and deficiencies in terms of noise tolerance in different dimensions.
Stabilization of transmittance fluctuations caused by beam wandering in continuousvariable quantum communication over freespace atmospheric channels
Vladyslav C. Usenko, Christian Peuntinger, Bettina Heim, Kevin Günthner, Ivan Derkach, Dominique Elser, Christoph Marquardt, Radim Filip, Gerd Leuchs
Transmittance fluctuations in turbulent atmospheric channels result in quadrature excess noise which limits applicability of continuousvariable quantum communication. Such fluctuations are commonly caused by beam wandering around the receiving aperture. We study the possibility to stabilize the fluctuations by expanding the beam, and test this channel stabilization in regard of continuousvariable entanglement sharing and quantum key distribution. We perform transmittance measurements of a real freespace atmospheric channel for different beam widths and show that the beam expansion reduces the fluctuations of the channel transmittance by the cost of an increased overall loss. We also theoretically study the possibility to share an entangled state or to establish secure quantum key distribution over the turbulent atmospheric channels with varying beam widths. We show the positive effect of channel stabilization by beam expansion on continuousvariable quantum communication as well as the necessity to optimize the method in order to maximize the secret key rate or the amount of shared entanglement. Being autonomous and not requiring adaptive control of the source and detectors based on characterization of beam wandering, the method of beam expansion can be also combined with other methods aiming at stabilizing the fluctuating freespace atmospheric channels.
LabelFree Imaging of Single Proteins Secreted from Living Cells via ISCAT Microscopy
André Gemeinhardt, Matthew Paul McDonald, Katharina König, Michael Aigner, Andreas Mackensen, Vahid Sandoghdar
Journal of Visualized Experiments (141)
(2018)

Journal
We demonstrate interferometric scattering (iSCAT) microscopy, a method capable of detecting single unlabeled proteins secreted from individual living cells in real time. In this protocol, we cover the fundamental steps to realize an iSCAT microscope and complement it with additional imaging channels to monitor the viability of a cell under study. Following this, we use the method for realtime detection of single proteins as they are secreted from a living cell which we demonstrate with an immortalized Bcell line (Laz388). Necessary steps concerning the preparation of microscope and sample as well as the analysis of the recorded data are discussed. The video protocol demonstrates that iSCAT microscopy offers a straightforward method to study secretion at the singlemolecule level.
Excitation of higherorder modes in optofluidic photonic crystal fiber
Andrei Ruskuc, Philipp Koehler, Marius A. Weber, Ana AndresArroyo, Michael H. Frosz, Philip St. J. Russell, Tijmen G. Euser
Optics Express
26(23)
3024530254
(2018)

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Higherorder modes up to LP33 are controllably excited in waterfilled kagomé and bandgapstyle hollowcore photonic crystal fibers (HCPCF). A spatial light modulator is used to create amplitude and phase distributions that closely match those of the fiber modes, resulting in typical launch efficiencies of 10–20% into the liquidfilled core. Modes, excited across the visible wavelength range, closely resemble those observed in airfilled kagomé HCPCF and match numerical simulations. Mode indices are obtained by launching planewaves at specific angles onto the fiber inputface and comparing the resulting intensity pattern to that of a particular mode. These results provide a framework for spatiallyresolved sensing in HCPCF microreactors and fiberbased optical manipulation.
Transverse Kerker Scattering for Ångström Localization of Nanoparticles
Ankan Bag, Martin Neugebauer, Pawel Wozniak, Gerd Leuchs, Peter Banzer
PHYSICAL REVIEW LETTERS
121(19)
193902
(2018)

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Angstrom precision localization of a single nanoantenna is a crucial step towards advanced nanometrology, medicine and biophysics. Here, we show that single nanoantenna displacements
down to few Angstroms can be resolved with subAngstrom precision using an alloptical method.
We utilize the tranverse Kerker scattering scheme where a carefully structured light beam excites a combination of multipolar modes inside a dielectric nanoantenna, which then upon interference, scatters directionally into the farfield. We spectrally tune our scheme such that it is most sensitive
to the change in directional scattering per nanoantenna displacement. Finally, we experimentally show that antenna displacement down to 3 A˚ is resolvable with a localization precision of 0.6 A˚.
Attosecond Control of Restoration of Electronic Structure Symmetry
Chun Mei Liu, Jörn Manz, Kenji Ohmori, Christian Sommer, Nobuyuki Takei, Jean Christophe Tremblay, Yichi Zhang
Laser pulses can break the electronic structure symmetry of atoms and molecules by preparing a superposition of states with different irreducible representations. Here, we discover the reverse process, symmetry restoration, by means of two circularly polarized laser pulses. The laser pulse for symmetry restoration is designed as a copy of the pulse for symmetry breaking. Symmetry restoration is achieved if the time delay is chosen such that the superposed states have the same phases at the temporal center. This condition must be satisfied with a precision of a few attoseconds. Numerical simulations are presented for the C6H6 molecule and Rb87 atom. The experimental feasibility of symmetry restoration is demonstrated by means of highcontrast timedependent Ramsey interferometry of the Rb87 atom.
Effective wind and temperature retrieval from Doppler asymmetric spatial heterodyne spectrometer interferograms
Jilin Liu, Daikang Wei, Yajun Zhu, Martin Kaufmann, Friedhelm Olschewski, Klaus Mantel, Jiyao Xu, Martin Riese
This paper presents a method for wind velocity and Doppler temperature retrieval from interferograms of a Doppler asymmetric spatial heterodyne spectrometer. This method is based on the analytic representation of the signal and the subsequent algorithms. It turns out to be more robust than the conventional Fourier transform method at low SNR. The influence of optical dispersion on the accuracy of the retrieved parameters is also characterized. The effective optical path difference is suggested for use in wind and temperature retrieval routines. Computer simulations are used to characterize the accuracy of the proposed method, in particular regarding the influence of optical dispersion. (C) 2018 Optical Society of America
Strong circular dichroism for the HE11 mode in twisted singlering hollowcore photonic crystal fiber
Paul Roth, Yang Chen, Mehmet Can Günendi, Ramin Beravat, Nitin N. Edavalath, Michael H. Frosz, Goran Ahmed, Gordon K. L. Wong, Philip St. J. Russell
We report a series of experimental, analytical, and numerical studies demonstrating strong circular dichroism for the HE11like core mode in helically twisted hollowcore singlering photonic crystal fiber (SRPCF), formed by spinning the preform during fiber drawing. In the SRPCFs studied, the hollow core is surrounded by a single ring of nontouching capillaries. Coupling between these capillaries results in the formation of helical Bloch modes carrying orbital angular momentum. When twisted, strong circular birefringence appears in the ring, so that coupling to the core mode is possible for only one circular polarization state. The result is a SRPCF that acts as a circular polarizer, offering 1.4 dB/m for the lowloss polarization state and 9.7 dB/m for the highloss state over a 25 nm band centered at 1593 nm wavelength. In addition, we report for the first time that the vector fields of the helical Bloch modes are perfectly periodic when evaluated in cylindrical coordinates. Such fibers have many potential applications, such as generating circularly polarized light in gasfilled SRPCF and realizing polarizing elements in the deep and vacuum ultraviolet.
Probing the TavisCummings level splitting with intermediatescale superconducting circuits
Ping Yang, Jan David Brehm, Juha Leppäkangas, Lingzhen Guo, Michael Marthaler, Isabella Boventer, Alexander Stehli, Tim Wolz, Alexey V. Ustinov, Martin Weides, et al.
We demonstrate the local control of up to eight twolevel systems interacting strongly with a microwave cavity. Following calibration, the frequency of each individual twolevel system (qubit) is tunable without influencing the others. Bringing the qubits one by one on resonance with the cavity, we observe the collective coupling strength of the qubit ensemble. The splitting scales up with the square root of the number of the qubits, being the hallmark of the TavisCummings model. The local control circuitry causes a bypass shunting the resonator, and a Fano interference in the microwave readout, whose contribution can be calibrated away to recover the pure cavity spectrum. The simulator's attainable size of dressed states is limited by reduced signal visibility, and if uncalibrated by offresonance shifts of subcomponents. Our work demonstrates control and readout of quantum coherent mesoscopic multiqubit system of intermediate scale under conditions of noise.
Nonregularity of threedimensional polarization states
José J. Gil, Andreas Norrman, Ari T. Friberg, Tero Setälä
Regular states of polarization are defined as those that can be decomposed into a pure state (fully polarized), a twodimensional (2D) unpolarized state (a state whose polarization ellipse evolves fully randomly in a fixed plane), and a threedimensional (3D) unpolarized state (a state whose polarization ellipse evolves fully randomly in the 3D space) \[Phys. Rev. A95, 053856 (2017)PLRAAN1050294710.1103/PhysRevA.95.053856\]. For nonregular states, the middle component can be considered as an equiprobable mixture of two pure states, whose polarization ellipses lie in different planes. In this work, we identify a perfect nonregular state and introduce the degree of nonregularity as a measure of the proximity of a nonregular state to regularity. We also analyze and interpret the notion of polarizationstate regularity in terms of polarimetric parameters. Our results bring new insights into the polarimetric structure of 3D light fields.
Tempering Rayleigh’s curse with PSF shaping
Martin Paúr, Bohumil Stoklasa, Jai Grover, Andrej Krzic, Luis L. SánchezSoto, Zdeněk Hradil, Jaroslav Řeháček
It has been argued that, for a spatially invariant imaging system, the information one can gain about the separation of two incoherent point sources decays quadratically to zero with decreasing separation. The effect is termed Rayleighx2019;s curse. Contrary to this belief, we identify a class of pointspread functions (PSFs) with a linear information decrease. Moreover, we show that any wellbehaved symmetric PSF can be converted into such a form with a simple nonabsorbing signum filter. We experimentally demonstrate significant superresolution capabilities based on this idea.
Reinforcement Learning with Neural Networks for Quantum Feedback
Thomas Fösel, Petru Tighineanu, Talitha Weiss, Florian Marquardt
Artificial neural networks are revolutionizing science. While the most prevalent technique involves supervised training on queries with a known correct answer, more advanced challenges often require discovering answers autonomously. In reinforcement learning, control strategies are improved according to a reward function. The power of this approach has been highlighted by spectactular recent successes, such as playing Go. So far, it has remained an open question whether neuralnetworkbased reinforcement learning can be successfully applied in physics. Here, we show how to use this method for finding quantum feedback schemes, where a networkbased "agent" interacts with and occasionally decides to measure a quantum system. We illustrate the utility by finding gate sequences that preserve the quantum information stored in a small collection of qubits against noise. This specific application will help to find hardwareadapted feedback schemes for small quantum modules while demonstrating more generally the promise of neuralnetwork based reinforcement learning in physics.
Spatially Adiabatic Frequency Conversion in Optoelectromechanical Arrays
Ondrej Černotík, Sahand Mahmoodian, Klemens Hammerer
Faithful conversion of quantum signals between microwave and optical frequency domains is crucial for building quantum networks based on superconducting circuits. Optoelectromechanical systems, in which microwave and optical cavity modes are coupled to a common mechanical oscillator, are a promising route towards this goal. In these systems, efficient, lownoise conversion is possible using a mechanically dark mode of the fields, but the conversion bandwidth is limited to a fraction of the cavity linewidth. Here, we show that an array of optoelectromechanical transducers can overcome this limitation and reach a bandwidth that is larger than the cavity linewidth. The coupling rates are varied in space throughout the array so that the mechanically dark mode of the propagating fields adiabatically changes from microwave to optical or vice versa. This strategy also leads to significantly reduced thermal noise with the collective optomechanical cooperativity being the relevant figure of merit. Finally, we demonstrate that the bandwidth enhancement is, surprisingly, largest for small arrays; this feature makes our scheme particularly attractive for stateoftheart experimental setups.
Perturbation theory of optical resonances of deformed dielectric spheres
Andrea Aiello, Jack G. E. Harris, Florian Marquardt
We analyze the optical resonances of a dielectric sphere whose surface has been slightly deformed in an arbitrary way. Setting up a perturbation series up to second order, we derive both the frequency shifts and modified linewidths. Our theory is applicable, for example, to freely levitated liquid drops or solid spheres, which are deformed by thermal surface vibrations, centrifugal forces or arbitrary surface waves. A dielectric sphere is effectively an open system whose description requires the introduction of nonHermitian operators characterized by complex eigenvalues and not normalizable eigenfunctions. We avoid these difficulties using the KapurPeierls formalism which enables us to extend the popular RayleighSchrödinger perturbation theory to the case of electromagnetic Debye's potentials describing the light fields inside and outside the nearspherical dielectric object. We find analytical formulas, valid within certain limits, for the deformationinduced first and secondorder corrections to the central frequency and bandwidth of a resonance. As an application of our method, we compare our results with preexisting ones finding full agreement.
Broadband and tunable timeresolved THz system using argonfilled hollowcore photonic crystal fiber
Wei Cui, Aidan W. SchiffKearn, Emily Zhang, Nicolas Couture, Francesco Tani, David Novoa, Philip St. J. Russell, JeanMichel Ménard
We demonstrate broadband, frequencytunable, phaselocked terahertz (THz) generation and detection based on difference frequency mixing of temporally and spectrally structured nearinfrared (NIR) pulses. The pulses are prepared in a gasfilled hollowcore
photonic crystal fiber (HCPCF), whose linear and nonlinear optical properties can be adjusted by tuning the gas pressure. This permits optimization of both the spectral broadening of the pulses due to selfphase modulation (SPM) and the generated THz spectrum. The properties of the prepared pulses, measured at several different argon gas pressures, agree well with the results of numerical modeling. Using these pulses, we perform difference frequency generation in a standard timeresolved THz scheme. As the argon pressure is gradually increased from 0 to 10 bar, the NIR pulses spectrally broaden from 3.5 to 8.7 THz, while the measured THz bandwidth increases correspondingly from 2.3 to 4.5 THz. At 10 bar, the THz spectrum extends to 6 THz, limited only by the spectral bandwidth of our timeresolved detection scheme. Interestingly, SPM in the HCPCF produces asymmetric spectral broadening that may be used to enhance the generation of selected THz frequencies. This scheme, based on a HCPCF pulse shaper, holds great promise for broadband timedomain spectroscopy in the THz, enabling the use of compact and stable ultrafast laser sources with relatively narrow linewidths (<4 THz).
Quantum nondemolition measurement of mechanical motion quanta
Luca Dellantonio, Oleksandr Kyriienko, Florian Marquardt, Anders S. Sørensen
Nature Communications
9
3621
(2018)

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The fields of optomechanics and electromechanics have facilitated numerous advances in the areas of precision measurement and sensing, ultimately driving the studies of mechanical systems into the quantum regime. To date, however, the quantization of the mechanical motion and the associated quantum jumps between phonon states remains elusive. For optomechanical systems, the coupling to the environment was shown to make the detection of the mechanical mode occupation difficult, typically requiring the singlephoton strongcoupling regime. Here, we propose and analyse an electromechanical setup, which allows us to overcome this limitation and resolve the energy levels of a mechanical oscillator. We found that the heating of the membrane, caused by the interaction with the environment and unwanted couplings, can be suppressed for carefully designed electromechanical systems. The results suggest that phonon number measurement is within reach for modern electromechanical setups.
Tomography from collective measurements
A. Muñoz, A. B. Klimov, M. Grassl, L. L. SánchezSoto
Quantum Information Processing
17(10)
(2018)

Journal
We discuss the tomography of Nqubit states using collective measurements. The method is exact for symmetric states, whereas for not completely symmetric states the information accessible can be arranged as a mixture of irreducible SU(2) blocks. For the fully symmetric sector, the reconstruction protocol can be reduced to projections onto a canonically chosen set of pure states.
Quantum cryptography with twisted photons through an outdoor underwater channel
Frédéric Bouchard, Alicia Sit, Felix Hufnagel, Aazad Abbas, Yingwen Zhang, Khabat Heshami, Robert Fickler, Christoph Marquardt, Gerd Leuchs, Robert w. Boyd, et al.
Quantum communication has been successfully implemented in optical fibres and through freespace. Fibre systems, though capable of fast key and low error rates, are impractical in communicating with destinations without an established fibre link. Freespace quantum channels can overcome such limitations and reach long distances with the advent of satellitetoground links. However, turbulence, resulting from local fluctuations in refractive index, becomes a major challenge by adding errors and losses. Recently, an interest in investigating the possibility of underwater quantum channels has arisen. Here, we investigate the effect of turbulence on an underwater quantum channel using twisted photons in outdoor conditions. We study the effect of turbulence on transmitted error rates, and compare different quantum cryptographic protocols in an underwater quantum channel, showing the feasibility of highdimensional encoding schemes. Our work may open the way for secure highdimensional quantum communication between submersibles, and provides important input for potential submersiblestosatellite quantum communication.
Exciting a chiral dipole moment in an achiral nanostructure
Controlling the electric and magnetic dipole moments of optical nanostructures is a fundamental prerequisite for light routing and polarization multiplexing at the nanoscale. A versatile approach for inducing tailored dipole moments is structured illumination. Here, we discuss the excitation of a chiral dipole moment in an achiral silicon nanoparticle. In particular, we make use of the electric and magnetic polarizabilities of the silicon nanoparticle to coherently excite a superposition of parallel electric and magnetic dipole moments phaseshifted by +/pi/2, which resembles the fundamental mode of a threedimensional chiral nanostructure. We demonstrate the wavelength dependence of the excitation scheme and measure the spin and orbital angular momenta in the emission of the induced chiral dipole moments. Our results highlight the capabilities of such tunable chiral dipole emittersnot limited by structural propertiesas flexible sources of spinpolarized light for nanoscopic devices. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
QuantumLimited TimeFrequency Estimation through ModeSelective Photon Measurement
J. M. Donohue, V. Ansari, J. Řeháček, Z. Hradil, B. Stoklasa, M. Paúr, L. L. SánchezSoto, C. Silberhorn
By projecting onto complex optical mode profiles, it is possible to estimate arbitrarily small separations between objects with quantumlimited precision, free of uncertainty arising from overlapping intensity profiles. Here we extend these techniques to the timefrequency domain using modeselective sumfrequency generation with shaped ultrafast pulses. We experimentally resolve temporal and spectral separations between incoherent mixtures of singlephoton level signals ten times smaller than their optical bandwidths with a tenfold improvement in precision over the intensityonly CramérRao bound.
HighSpeed Microscopy of Diffusion in PoreSpanning Lipid Membranes
Porespanning membranes (PSMs) provide a highly attractive model system for investigating fundamental processes in lipid bilayers. We measure and compare lipid diffusion in the supported and suspended regions of PSMs prepared on a microfabricated porous substrate. Although some properties of the suspended regions in PSMs have been characterized using fluorescence studies, it has not been possible to examine the mobility of membrane components on the supported membrane parts. Here, we resolve this issue by employing interferometric scattering microscopy (iSCAT). We study the locationdependent diffusion of DOPE 1,2dioleoylsnglycero3phosphoethanolamine) lipids (DOPE) labeled with gold nanoparticles in (l,2dioleoylsnglycero3phosphocholine) (DOPC) bilayers prepared on holey silicon nitride substrates that were either (i) oxygenplasmatreated or (ii) functionalized with gold and 6mercaptolhexanol. For both substrate treatments, diffusion in regions suspended on pores with diameters of 5 mu m is found to be free. In the case of functionalization with gold and 6mercaptolhexanol, similar diffusion coefficients are obtained for both the suspended and the supported regions, whereas for oxygenplasmatreated surfaces, diffusion is almost 4 times slower in the supported parts of the membranes. We attribute this reduced diffusion on the supported parts in the case of oxygenplasmatreated surfaces to larger membranesubstrate interactions, which lead to a higher membrane tension in the freestanding membrane parts. Furthermore, we find clear indications for a decrease of the diffusion constant in the freestanding regions away from the pore center. We provide a detailed characterization of the diffusion behavior in these membrane systems and discuss future directions.
Indefinitemean Pareto photon distribution from amplified quantum noise
Mathieu Manceau, Kirill Spasibko, Gerd Leuchs, Radim Filip, Maria Chekhova
The existence of extreme events is a fascinating phenomenon in natural and social sciences. They appear whenever the probability distribution has a `heavy tail', differing very much from the equilibrium one. Examples are `rogue waves' in the ocean and their analogues in nonlinear optics, Levy flights, and other numerous examples in physics, biology, Earth science, etc. Most famous are the statistics of income and wealth with a powerlaw (Pareto) probability distribution describing social inequality and responsible for the renowned `80/20 rule'. The power laws can be however very different; for some outstanding cases the power exponents are less than 2 leading to indefinite mean values, to say nothing of higher moments. Here we present the first evidence of such probability distributions of photon numbers using nonlinear effects pumped by parametrically amplified vacuum noise, known as bright squeezed vacuum (BSV). We observe a Pareto distribution with power exponent 1.3 when BSV pumps supercontinuum generation, and other heavytailed distributions for the optical harmonics generated from BSV. Unlike in other fields, we can flexibly control the Pareto exponent by changing the experimental parameters. Besides photonic applications such as ghost imaging, this extremely fluctuating light is also interesting for quantum thermodynamics as a resource to produce more efficiently nonequlibrium states by singlephoton subtraction, which we demonstrate in experiment.
Chirality of Symmetric Resonant Heterostructures
Sergey Nechayev, Pawel Wozniak, Martin Neugebauer, Rene Barczyk, Peter Banzer
Laser & Photonics Reviews
12(9)
(2018)

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Chiroptical effects arising in mirror‐symmetric geometrically achiral resonant heterostructures are investigated. It is shown that coalescence of extrinsic chirality, heterogeneous morphology, and substrate‐induced break of symmetry leads to pronounced circular dichroism and circular birefringence. The physics of the involved phenomena is elucidated by studying spin‐splitting in scattering and hybridized dipolar modes of a heterodimer made of gold and silicon nanoparticles of the same shape and size. The work sheds new light on the optical properties of heterogeneous nanostructures and paves the way for designing polarization‐controlled tunable heterogeneous optical elements.
Chiroptical response of a single plasmonic nanohelix
Pawel Wozniak, Israel De Leon, Katja Hoeflich, Caspar Haverkamp, Silke Christiansen, Gerd Leuchs, Peter Banzer
OPTICS EXPRESS
26(15)
1927519293
(2018)

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We investigate the chiroptical response of a single plasmonic nanohelix interacting with a weakly focused circularly polarized Gaussian beam. The optical scattering at the fundamental resonance is characterized experimentally and numerically. The angularly resolved scattering of the excited nanohelix is verified experimentally and it validates the numerical results. We employ a multipole decomposition analysis to study the fundamental and first higherorder resonance of the nanohelix, explaining their chiral properties in terms of the formation of chiral dipoles. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Multiphoton nonclassical light from clusters of singlephoton emitters
Luo Qi, Mathieu Manceau, Andrea Cavanna, Fabian Gumpert, Luigi Carbone, Massimo de Vittorio, Alberto Bramati, Elisabeth Giacobino, Lukas Lachman, Radim Filip, et al.
New Journal of Physics
20
073013
(2018)

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We study nonclassical features of multiphoton light emitted by clusters of singlephoton emitters. As signatures of nonclassicality, we use violation of inequalities for normalized correlation functions of different orders or the probabilities of multiphoton detection. In particular, for clusters of 2–14 colloidal CdSe/CdS dotinrods we observe antibunching and nonclassicality of up to the fourthorder. Surprisingly, violation of certain classical inequalities gets even more pronounced for larger clusters.
A highly miniaturized satellite payload based on a spatial heterodyne spectrometer for atmospheric temperature measurements in the mesosphere and lower thermosphere
Martin Kaufmann, Friedhelm Olschewski, Klaus Mantel, Brian Solheim, Gordon Shepherd, Michael Deiml, Jilin Liu, Rui Song, Qiuyu Chen, Oliver Wroblowski, et al.
A highly miniaturized limb sounder for the observation of the O2 Aband to derive temperatures in the mesosphere and lower thermosphere is presented. The instrument consists of a monolithic spatial heterodyne spectrometer (SHS), which is able to resolve the rotational structure of the Rbranch of that band. The relative intensities of the emission lines follow a Boltzmann distribution and the ratio of the lines can be used to derive the kinetic temperature. The SHS operates at a Littrow wavelength of 761.8 nm and heterodynes a wavelength regime between 761.9 and 765.3 nm with a resolving power of about 8000 considering apodization effects. The size of the SHS is 38 x 38 x 27 mm(3) and its acceptance angle is +/ 5 degrees. It has an etendue of 0.01 cm(2) sr. Complemented by front optics with an acceptance angle of +/ 0.65 degrees and detector optics, the entire optical system fits into a volume of about 1.5 L. This allows us to fly this instrument on a 3 or 6unit CubeSat. The vertical field of view of the instrument is about 60 km at the Earth's limb when operated in a typical low Earth orbit. Integration times to obtain an entire altitude profile of nighttime temperatures are on the order of 1 min for a vertical resolution of 1.5 km and a random noise level of about 1.5 K. Daytime integration times are 1 order of magnitude shorter. This work presents the design parameters of the optics and a radiometric assessment of the instrument. Furthermore, it gives an overview of the required characterization and calibration steps. This includes the characterization of image distortions in the different parts of the optics, visibility, and phase determination as well as flat fielding.
Dynamically Generated Synthetic Electric Fields for Photons
Static synthetic magnetic fields give rise to phenomena including the Lorentz force and the quantum Hall effect even for neutral particles, and they have by now been implemented in a variety of physical systems. Moving towards fully dynamical synthetic gauge fields allows, in addition, for backaction of the particles' motion onto the field. If this results in a timedependent vector potential, conventional electromagnetism predicts the generation of an electric field. Here, we show how synthetic electric fields for photons arise selfconsistently due to the nonlinear dynamics in a driven system. Our analysis is based on optomechanical arrays, where dynamical gauge fields arise naturally from phononassisted photon tunneling. We study open, onedimensional arrays, where synthetic magnetic fields are absent. However, we show that synthetic electric fields can be generated dynamically, which, importantly, suppress photon transport in the array. The generation of these fields depends on the direction of photon propagation, leading to a novel mechanism for a photon diode, inducing nonlinear nonreciprocal transport via dynamical synthetic gauge fields.
Longrange optical trapping and binding of microparticles in hollowcore photonic crystal fibre
Dmitry S. Bykov, Shangran Xie, Richard Zeltner, Andrey Machnev, Gordon K. L. Wong, Tijmen G. Euser, Philip St. J. Russell
Optically levitated micro and nanoparticles offer an ideal playground for investigating photon–phonon interactions over macroscopic distances. Here we report the observation of longrange optical binding of multiple levitated microparticles, mediated by intermodal scattering and interference inside the evacuated core of a hollowcore photonic crystal fibre (HCPCF). Three polystyrene particles with a diameter of 1 μm are stably bound together with an interparticle distance of ~40 μm, or 50 times longer than the wavelength of the trapping laser. The levitated boundparticle array can be translated toandfro over centimetre distances along the fibre. When evacuated to a gas pressure of 6 mbar, the collective mechanical modes of the boundparticle array are able to be observed. The measured interparticle distance at equilibrium and mechanical eigenfrequencies are supported by a novel analytical formalism modelling the dynamics of the binding process. The HCPCF system offers a unique platform for investigating the rich optomechanical dynamics of arrays of levitated particles in a wellisolated and protected environment.
Light polarization measurements in tests of macrorealism
Eugenio Roldan, Johannes Kofler, Carlos NavarreteBenlloch
Physical Review A
97(062117)
(2018)

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According to the world view of macrorealism, the properties of a given system exist prior to and independent of measurement, which is incompatible with quantum mechanics. Leggett and Garg put forward a practical criterion capable of identifying violations of macrorealism, and so far experiments performed on microscopic and mesoscopic systems have always agreed with quantum mechanics. However, a macrorealist can always assign the cause of such violations to the perturbation that measurements effect on such small systems, and hence a definitive test would require using noninvasive measurements, preferably on macroscopic objects, where such measurements seem more plausible. However, the generation of truly macroscopic quantum superposition states capable of violating macrorealism remains a big challenge. In this work we propose a setup that makes use of measurements on the polarization of light, a property that has been extensively manipulated both in classical and quantum contexts, hence establishing the perfect link between the microscopic and macroscopic worlds. In particular, we use LeggettGarg inequalities and the criterion of no signaling in time to study the macrorealistic character of light polarization for different kinds of measurements, in particular with different degrees of coarse graining. Our proposal is noninvasive for coherent input states by construction. We show for states with welldefined photon number in two orthogonal polarization modes, that there always exists a way of making the measurement sufficiently coarse grained so that a violation of macrorealism becomes arbitrarily small, while sufficiently sharp measurements can always lead to a significant violation.
Ramsey interferometry of Rydberg ensembles inside microwave cavities
Christian Sommer, Claudiu Genes
JOURNAL OF PHYSICS BATOMIC MOLECULAR AND OPTICAL PHYSICS
51(11)
115502
(2018)

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We study ensembles of Rydberg atoms in a confined electromagnetic environment such as is provided by a microwave cavity. The competition between standard free space Ising type and cavitymediated interactions leads to the emergence of different regimes where the particle particle couplings range from the typical van der Waals r(6) behavior to r(3) and to rindependence. We apply a Ramsey spectroscopic technique to map the twobody interactions into a characteristic signal such as intensity and contrast decay curves. As opposed to previous treatments requiring highdensities for considerable contrast and phase decay (Takei et al 2016 Nat. Comms. 7 13449; Sommer et al 2016 Phys. Rev. A 94 053607), the cavity scenario can exhibit similar behavior at much lower densities.
Energy transfer and correlations in cavityembedded donoracceptor configurations
The rate of energy transfer in donoracceptor systems can be manipulated via the common interaction with the confined electromagnetic modes of a microcavity. We analyze the competition between the nearfield short range dipoledipole energy exchange processes and the cavity mediated longrange interactions in a simplified model consisting of effective twolevel quantum emitters that could be relevant for molecules in experiments under cryogenic conditions. We find that freespace collective incoherent interactions, typically associated with suband superradiance, can modify the traditional resonant energy transfer scaling with distance. The same holds true for cavitymediated collective incoherent interactions in a weakcoupling but strongcooperativity regime. In the strong coupling regime, we elucidate the effect of pumping into cavity polaritons and analytically identify an optimal energy flow regime characterized by equal donor/acceptor Hopfield coefficients in the middle polariton. Finally we quantify the buildup of quantum correlations in the donoracceptor system via the twoqubit concurrence as a measure of entanglement.
Quantum ErrorCorrecting Codes for Qudit Amplitude Damping
M. Grassl, L. Kong, Z. Wei, Z. Yin, B. Zeng
IEEE Transactions on Information Theory
64(6)
46744685
(2018)

Journal
Traditional quantum errorcorrecting codes are designed for the depolarizing channel modeled by generalized Pauli errors occurring with equal probability. Amplitude damping channels model, in general, the decay process of a multilevel atom or energy dissipation of a bosonic system with Markovian bath at zero temperature. We discuss quantum errorcorrecting codes adapted to amplitude damping channels for higher dimensional systems (qudits). For multilevel atoms, we consider a natural kind of decay process, and for bosonic systems, we consider the qudit amplitude damping channel obtained by truncating the Fock basis of the bosonic modes (e.g., the number of photons) to a certain maximum occupation number. We construct families of singleerrorcorrecting quantum codes that can be used for both cases. Our codes have larger code dimensions than the previously known singleerrorcorrecting codes of the same lengths. In addition, we present families of multierror correcting codes for these two channels, as well as generalizations of our construction technique to errorcorrecting codes for the qutrit V and ? channels.
Alberto Lombardini, Vasyl Mytskaniuk, Siddharth Sivankutty, Esben Ravn Andresen, Xueqin Chen, Jérôme Wenger, Marc Fabert, Nicolas Y. Joly, Frédéric Louradour, Alexandre Kudlinski, et al.
Coherent Raman scattering microscopy is a fast, labelfree, and chemically specific imaging technique that shows high potential for future in vivo optical histology. However, the imaging depth in tissues is limited to the submillimeter range because of absorption and scattering. Realization of coherent Raman imaging using a fiber endoscope system is a crucial step towards imaging deep inside living tissues and providing information that is inaccessible with current microscopy tools. Until now, the development of coherent Raman endoscopy has been hampered by several issues, mainly related to the fiber delivery of the excitation pulses and signal collection. Here, we present a flexible, compact, coherent Raman, and multimodal nonlinear endoscope (4.2 mm outer diameter, 71 mm rigid length) based on a resonantly scanned hollowcore Kagomé lattice doubleclad fiber. The fiber design enables distortionless, backgroundfree delivery of femtosecond excitation pulses and backcollection of nonlinear signals through the same fiber. Submicrometer spatial resolution over a large field of view is obtained by combination of a miniature objective lens with a silica microsphere lens inserted into the fiber core. We demonstrate highresolution, highcontrast coherent antiStokes Raman scattering, and second harmonic generation endoscopic imaging of biological tissues over a field of view of 320 µm at a rate of 0.8 frames per second. These results pave the way for intraoperative labelfree imaging applied to realtime histopathology diagnosis and surgery guidance.
Bright squeezed vacuum in a nonlinear interferometer: Frequency and temporal Schmidtmode description
P.R. Sharapova, O.V. Tikhonova, S. Lemieux, R.W. Boyd, Maria Chekhova
Control over the spectral properties of the bright squeezed vacuum (BSV), a highly multimode nonclassical macroscopic state of light that can be generated through highgain parametric down conversion, is crucial for many applications. In particular, in several recent experiments BSV is generated in a strongly pumped SU(1,1) interferometer to achieve phase supersensitivity, perform broadband homodyne detection, or tailor the frequency spectrum of squeezed light. In this work, we present an analytical approach to the theoretical description of BSV in the frequency domain based on the BlochMessiah reduction and the Schmidtmode formalism. As a special case we consider a strongly pumped SU(1,1) interferometer. We show that different moments of the radiation at its output depend on the phase, dispersion, and the parametric gain in a nontrivial way, thereby providing additional insights on the capabilities of nonlinear interferometers. In particular, a dramatic change in the spectrum occurs as the parametric gain increases.
Tailoring multipolar Mie scattering with helicity and angular momentum
Xavier ZambranaPuyalto, Xavier Vidal, Pawel Wozniak, Peter Banzer, Gabriel MolinaTerriza
ACS Photonics
5(7 SI)
29362944
(2018)

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Linear scattering processes are usually described as a function of the parameters of the incident beam. The wavelength, the intensity distribution, the polarization or the phase are among them. Here, we discuss and experimentally demonstrate how the angular momentum and the helicity of light influence the light scattering of spherical particles. We measure the backscattering of a 4 μm diameter TiO2 single particle deposited on a glass substrate. The particle is probed at different wavelengths by different beams with total angular momenta ranging from −8 to +8 units. It is observed that the spectral behavior of the particle is highly dependent on the angular momentum and helicity of the incoming beam. While some of the properties of the scattered field can be described with a simple resonator model, the scattering of high angular momentum beams requires a deeper understanding of the multipolar modes induced in the sphere. We observe that tailoring these induced multipolar modes can cause a shift and a spectral narrowing of the peaks of the scattering spectrum. Furthermore, specific combinations of helicity and angular momentum for the excitation lead to differences in the conservation of helicity by the system, which has clear consequences on the scattering pattern.
Ultrafast Coherent Control of Condensed Matter with Attosecond Precision
Hiroyuki Katsuki, Nobuyuki Takei, Christian Sommer, Kenji Ohmori
Accounts of Chemical Research
51(5)
11741184
(2018)

Journal
Coherent control is a technique to manipulate wave functions of matter with light. Coherent control of isolated atoms and molecules in the gas phase is wellunderstood and developed since the 1990s, whereas its application to condensed matter is more difficult because its coherence lifetime is shorter. We have recently applied this technique to condensed matter samples, one of which is solid parahydrogen (pH2). Intramolecular vibrational excitation of solid pH2 gives an excited vibrational wave function called a “vibron”, which is delocalized over many hydrogen molecules in a manner similar to a Frenkel exciton. It has a long coherence lifetime, so we have chosen solid pH2 as our first target in the condensed phase. We shine a timedelayed pair of femtosecond laser pulses on pH2 to generate vibrons. Their interference results in modulation of the amplitude of their superposition. Scanning the interpulse delay on the attosecond time scale gives a high interferometric contrast, which demonstrates the possibility of using solid pH2 as a carrier of information encoded in the vibrons.
In the second example, we have controlled the terahertz collective phonon motion, called a “coherent phonon”, of a single crystal of bismuth. We employ an intensitymodulated laser pulse, whose temporal envelope is modulated with terahertz frequency by overlap of two positively chirped laser pulses with their adjustable time delay. This modulated laser pulse is shined on the bismuth crystal to excite its two orthogonal phonon modes. Their relative amplitudes are controlled by tuning the delay between the two chirped pulses on the attosecond time scale. Twodimensional atomic motion in the crystal is thus controlled arbitrarily. The method is based on the simple, robust, and universal concept that in any physical system, twodimensional particle motion is decomposed into two orthogonal onedimensional motions, and thus, it is applicable to a variety of condensed matter systems.
In the third example, the doublepulse interferometry used for solid pH2 has been applied to manybody electronic wave functions of an ensemble of ultracold rubidium Rydberg atoms, hereafter called a “strongly correlated ultracold Rydberg gas”. This has allowed the observation and control of manybody electron dynamics of more than 40 Rydberg atoms interacting with each other. This new combination of ultrafast coherent control and ultracold atoms offers a versatile platform to precisely observe and manipulate nonequilibrium dynamics of quantum manybody systems on the ultrashort time scale.
These three examples are digested in this Account.
Cavityassisted mesoscopic transport of fermions: Coherent and dissipative dynamics
David Haggenmüller, Stefan Schütz, Johannes Schachenmayer, Claudiu Genes, Guido Pupillo
We study the interplay between charge transport and lightmatter interactions in a confined geometry by considering an open, mesoscopic chain of twoorbital systems resonantly coupled to a single bosonic mode close to its vacuum state. We introduce and benchmark different methods based on selfconsistent solutions of nonequilibrium Green's functions and numerical simulations of the quantum master equation, and derive both analytical and numerical results. It is shown that in the dissipative regime where the cavity photon decay rate is the largest parameter, the lightmatter coupling is responsible for a steadystate current enhancement scaling with the cooperativity parameter. We further identify different regimes of interest depending on the ratio between the cavity decay rate and the electronic bandwidth. Considering the situation where the lower band has a vanishing bandwidth, we show that for a highfinesse cavity, the properties of the resonant Bloch state in the upper band are transferred to the lower one, giving rise to a delocalized state along the chain. Conversely, in the dissipative regime with lowcavity quality factors, we find that the current enhancement is due to a collective decay of populations from the upper to the lower band.
Magnetic and Electric Transverse Spin Density of Spatially Confined Light
Martin Neugebauer, Jörg Eismann, Thomas Bauer, Peter Banzer
When a beam of light is laterally confined, its field distribution can exhibit points where the local magnetic and electric field vectors spin in a plane containing the propagation direction of the electromagnetic wave. The phenomenon indicates the presence of a nonzero transverse spin density. Here, we experimentally investigate this transverse spin density of both magnetic and electric fields, occurring in highly confined structured fields of light. Our scheme relies on the utilization of a highre fractivindcx enoperticlc as a lecal field probe, exhibiting magnetic and electric dipole resonances in the visible spectral range. Because of the directional emission of dipole moments that spin around an axis parallel to a nearby dielectric interface, such a probe particle is capable of locally sensing the magnetic and electric transverse spin density of a tightly focused beam impinging under normal incidence with respect to said interface. We exploit the achieved experimental results to emphasize the difference between magnetic and electric transverse spin densities.
Dispersion tuning in submicron tapers for thirdharmonic and photon triplet generation
Jonas Hammer, Andrea Cavanna, Riccardo Pennetta, Maria Chekhova, Philip St. J. Russell, Nicolas Joly
Precise control of the dispersion landscape is of crucial importance if optical fibers are to be successfully used for the generation of threephoton states of light—the inverse of thirdharmonic generation (THG). Here we report gastuning of intermodal phasematched THG in submicrondiameter tapered optical fiber. By adjusting the pressure of the surrounding argon gas up to 50 bars, intermodally phasematched thirdharmonic light can be generated for pump wavelengths within a 15 nm range around 1.38 μm. We also measure the infrared fluorescence generated in the fiber when pumped in the visible and estimate that the accidental coincidence rate in this signal is lower than the predicted detection rate of photon triplets
Dominance of backward stimulated Raman scattering in gasfilled hollowcore photonic crystal fibers
Backward stimulated Raman scattering in gases provides a promising route to the compression and amplification of a Stokes seed pulse by counterpropagating against a pump pulse, as has been demonstrated already in various platforms, mainly in free space. However, the dynamics governing this process when seeded by noise has not yet been investigated in a fully controllable collinear environment. Here we report, to the best of our knowledge, the first unambiguous observation of efficient noiseseeded backward stimulated Raman scattering in a hydrogenfilled hollowcore photonic crystal fiber. At high gas pressures, when the backward Raman gain is comparable to, but lower than, the forward gain, we report quantum conversion efficiencies exceeding 40% to the backward Stokes at 683 nm from a narrowband 532 nm pump. Efficiency increases to 65% when the backward process is seeded by a small amount of backreflected forwardgenerated Stokes light. At high pump powers, the backward Stokes signal, emitted in a clean fundamental mode and spectrally pure, is unexpectedly always stronger than its forwardpropagating counterpart. We attribute this striking observation to the unique temporal dynamics of the interacting fields, which cause the Raman coherence (which takes the form of a moving fineperiod Bragg grating) to grow in strength toward the input end of the fiber. A good understanding of this process, together with the rapid development of novel antiresonantguiding hollowcore fibers, may lead to improved designs of efficient gasbased Raman lasers and amplifiers operating at wavelengths from the ultraviolet to the midinfrared.
UV Soliton Dynamics and RamanEnhanced Supercontinuum
Generation in Photonic Crystal Fiber
Pooria Hosseini, Alexey Ermolov, Francesco Tani, David Novoa, Philip Russell
Ultrafast broadband ultraviolet radiation is of importance in spectroscopy and photochemistry, since high photon energies enable singlephoton excitations and ultrashort pulses allow timeresolved studies. Here we report the use of gasfilled hollowcore photonic crystal fibers (HCPCFs) for efficient ultrafast nonlinear optics in the ultraviolet. Soliton selfcompression of 400 nm pulses of (unprecedentedly low) ∼500 nJ energies down to sub6 fs durations is achieved, as well as resonant emission of tunable dispersive waves from these solitons. In addition, we discuss the generation of a flat supercontinuum extending from the deep ultraviolet to the visible in a hydrogenfilled HCPCF. Comparisons with argonfilled fibers show that the enhanced Raman gain at high frequencies makes the hydrogen system more efficient. As HCPCF technology develops, we expect these fiberbased ultraviolet sources to lead to new applications.
Towards an integrated AlGaAs waveguide platform for phase and polarisation shaping
G Maltese, Y Halioua, A Lemaitre, C GomezCarbonell, E Karimi, Peter Banzer, S Ducci
Journal of Optics
20(5)
05LT01
(2018)

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We propose, design and fabricate an onchip AlGaAs waveguide capable of generating a controlled phase delay of pi/2 between the guided transverse electric and magnetic modes. These modes possess significantly strong longitudinal field components as a direct consequence of their strong lateral confinement in the waveguide. We demonstrate that the effect of the device on a linearly polarised input beam is the generation of a field, which is circularly polarised in its transverse components and carries a phase vortex in its longitudinal component. We believe that the discussed integrated platform enables the generation of light beams with tailored phase and polarisation distributions.
Quantum theory of continuum optomechanics
Peter Rakich, Florian Marquardt
New Journal of Physics (20)
045005
(2018)

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We present the basic ingredients of continuum optomechanics, i.e. the suitable extension of cavityoptomechanical concepts to the interaction of photons and phonons in an extended waveguide. We introduce a realspace picture and argue which coupling terms may arise in leading order in the spatial derivatives. This picture allows us to discuss quantum noise, dissipation, and the correct boundary conditions at the waveguide entrance. The connections both to optomechanical arrays as well as to the theory of Brillouin scattering in waveguides are highlighted. Among other examples, we analyze the 'strong coupling regime' of continuum optomechanics that may be accessible in future experiments.
Testing for entanglement with periodic coarsegraining
Daniel S. Tasca, Łukasz Rudnicki, Reuben S. Aspden, Miles J. Padgett, Paulo H. Souto Ribeiro, Stephen P. Walborn
Continuous variables systems find valuable applications in quantum
information processing. To deal with an infinitedimensional Hilbert space, one
in general has to handle large numbers of discretized measurements in tasks
such as entanglement detection. Here we employ the continuous transverse
spatial variables of photon pairs to experimentally demonstrate novel
entanglement criteria based on a periodic structure of coarsegrained
measurements. The periodization of the measurements allows for an efficient
evaluation of entanglement using spatial masks acting as mode analyzers over
the entire transverse field distribution of the photons and without the need to
reconstruct the probability densities of the conjugate continuous variables.
Our experimental results demonstrate the utility of the derived criteria with a
success rate in entanglement detection of $\sim60\%$ relative to $7344$ studied
cases.
Residual and Destroyed Accessible Information after Measurements
When quantum states are used to send classical information, the receiver performs a measurement on the signal states. The amount of information extracted is often not optimal due to the receiver’s measurement scheme and experimental apparatus. For quantum nondemolition measurements, there is potentially some residual information in the postmeasurement state, while part of the information has been extracted and the rest is destroyed. Here, we propose a framework to characterize a quantum measurement by how much information it extracts and destroys, and how much information it leaves in the residual postmeasurement state. The concept is illustrated for several receivers discriminating coherent states.
Gauge invariant information concerning quantum channels
Łukasz Rudnicki, Zbigniew Puchała, Karol Zyczkowski
Motivated by the gate set tomography we study quantum channels from the perspective of information which is invariant with respect to the gauge realized through similarity of matrices representing channel superoperators. We thus use the complex spectrum of the superoperator to provide necessary conditions relevant for complete positivity of qubit channels and to express various metrics such as average gate fidelity.
We introduce a class of structured polychromatic surface electromagnetic fields, reminiscent of conventional optical axicon fields, through a judicious superposition of partially correlated surface plasmon polaritons. We show that such partially coherent axiconic surface plasmon polariton fields are structurally stable and statistically highly versatile with regard to spectral density, polarization state, energy flow, and degree of coherence. These fields can be created by plasmon coherence engineering and may prove instrumental broadly in surface physics and in various nanophotonics applications.
Stable Immobilization of Size‐Controlled Bimetallic Nanoparticles in Photonic Crystal Fiber Microreactor
Sebastian Ponce, Macarena Munoz, Ana M. Cubillas, Tijmen G. Euser, GuiRong Zhang, Philip St. J. Russell, Peter Wasserscheid, Bastian J. M. Etzold
The possibility of immobilizing ex situ‐synthesized colloidal bimetallic nanoparticles (NPs) of well‐defined characteristics inside hollow core photonic crystal fiber (HC‐PCF) microreactors is demonstrated. With the developed method, PtNi clusters remain strongly attached to the fiber core and can be used as active catalysts for the hydrogenation of an azobenzene dye. The study revealed that optical transmission exhibits a size‐dependent behavior, i.e., smaller NPs bring in less optical signal loss. Sufficient light transmission was achieved for all particle sizes. Furthermore, with these catalytic PCF microreactors, kinetic data can be obtained with a much lower amount of precious metals compared to a conventional batch reactor, opening a new pathway for in situ catalyst screening.
Highly Sensitive Luminescence Detection of Photosensitized Singlet Oxygen within Photonic Crystal Fibers
Gareth O. S. Williams, Tijmen G. Euser, Philip St. J. Russell, Alexander J. MacRobert, Anita C. Jones
Highly sensitive, quantitative detection of singlet oxygen (1O2) is required for the evaluation of newly developed photosensitizers and the elucidation of the mechanisms of many processes in which singlet oxygen is known or believed to be involved. The direct detection of 1O2 through its intrinsic phosphorescence at 1270 nm is challenging, because of the extremely low intensity of this emission, coupled with the low quantum efficiency of currently available photodetectors at this wavelength. We introduce hollow‐core photonic crystal fibers (HC‐PCF) as a novel optofluidic modality for photosensitization and detection of 1O2. We report the use of this approach to achieve highly sensitive detection of the luminescence decay of 1O2 produced by using two common photosensitizers, Rose Bengal and Hypericin, within the 60‐μm diameter core of a 15 cm length of HC‐PCF. We demonstrate the feasibility of directly detecting sub‐picomole quantities of 1O2 by using this methodology, and identify some aspects of the HC‐PCF technology that can be improved to yield even higher detection sensitivity.
Flying particle microlaser and temperature sensor in hollowcore photonic crystal fiber
Richard Zeltner, Riccardo Pennetta, Shangran Xie, Philip Russell
Whisperinggallery mode (WGM) resonators combine small optical mode volumes with narrow resonance linewidths, making them exciting platforms for a variety of applications. Here we report a flying WGM microlaser, realized by optically trapping a dyedoped microparticle within a liquidfilled hollowcore photonic crystal fiber (HCPCF) using a CW laser and then pumping it with a pulsed excitation laser whose wavelength matches the absorption band of the dye. The laser emits into coreguided modes that can be detected at the endfaces of the HCPCF. Using radiation forces, the microlaser can be freely propelled along the HCPCF over multicentimeter distances—orders of magnitude farther than in previous experiments where tweezers and fiber traps were used. The system can be used to measure temperature with high spatial resolution, by exploiting the temperaturedependent frequency shift of the lasing modes, and may also permit precise delivery of light to remote locations.
Topologicallyprotected travelingwave amplifier
Aashish Clerk, Martin Houde, Florian Marquardt, Vittorio Peano
Manipulation of Quenching in Nanoantenna–Emitter Systems Enabled by External Detuned Cavities: A Path to Enhance StrongCoupling
We show that a broadband Fabry Perot microcavity can assist an emitter coupled to an offresonant plasmonic nanoantenna to inhibit the nonradiative channels that affect the quenching of fluorescence. We identify the interference mechanism that creates the necessary enhanced couplings and bandwidth narrowing of the hybrid resonance and show that it can assist entering into the strong coupling regime. Our results provide new possibilities for improving the efficiency of solidstate emitters and accessing diverse realms of photophysics with hybrid structures that can be fabricated using existing technologies.
Active locking and entanglement in type II optical parametric oscillators
Joaquín RuizRivas, Germán J. de Valcarcel, Carlos NavarreteBenlloch
New Journal of Physics
20(023004)
(2018)

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Type II optical parametric oscillators are amongst the highestquality sources of quantumcorrelated light. In particular, when pumped above threshold, such devices generate a pair of bright orthogonallypolarized beams with strong continuousvariable entanglement. However, these sources are of limited practical use, because the entangled beams emerge with different frequencies and a diffusing phase difference. It has been proven that the use of an internal waveplate coupling the modes with orthogonal polarization is capable of locking the frequencies of the emerging beams to half the pump frequency, as well as reducing the phasedifference diffusion, at the expense of reducing the entanglement levels. In this work we characterize theoretically an alternative locking mechanism: the injection of a laser at half the pump frequency. Apart from being less invasive, this method should allow for an easier realtime experimental control. We show that such an injection is capable of generating the desired phase locking between the emerging beams, while still allowing for large levels of entanglement. Moreover, we find an additional region of the parameter space (at relatively large injections) where a mode with well defined polarization is in a highly amplitudesqueezed state.
Simple factorization of unitary transformations
Hubert de Guise, Olivia Di Matteo, Luis L. SánchezSoto
We demonstrate a method for general linear optical networks that allows one to factorize any SU(n) matrix in terms of two SU(n−1) blocks coupled by an SU(2) entangling beam splitter. The process can be recursively continued in an efficient way, ending in a tidy arrangement of SU(2) transformations. The method hinges only on a linear relationship between input and output states, and can thus be applied to a variety of scenarios, such as microwaves, acoustics, and quantum fields.
Polarimetric purity and the concept of degree of polarization
José J. Gil, Andreas Norrman, Ari T. Friberg, Tero Setälä
The concept of degree of polarization for electromagnetic waves, in its general threedimensional version, is revisited in the light of the implications of the recent findings on the structure of polarimetric purity and of the existence of nonregular states of polarization [J. J. Gil et al., Phys Rev. A 95, 053856 (2017)]. From the analysis of the characteristic decomposition of a polarization matrix R into an incoherent convex combination of (1) a pure state Rp, (2) a middle state Rm given by an equiprobable mixture of two eigenstates of R, and (3) a fully unpolarized state Ru−3D, it is found that, in general, Rm exhibits nonzero circular and linear degrees of polarization. Therefore, the degrees of linear and circular polarization of R cannot always be assigned to the single totally polarized component Rp. It is shown that the parameter P3D proposed formerly by Samson [J. C. Samson, Geophys. J. R. Astron. Soc. 34, 403 (1973)] takes into account, in a proper and objective form, all the contributions to polarimetric purity, namely, the contributions to the linear and circular degrees of polarization of R as well as to the stability of the plane containing its polarization ellipse. Consequently, P3D constitutes a natural representative of the degree of polarimetric purity. Some implications for the common convention for the concept of twodimensional degree of polarization are also analyzed and discussed.
Effect of anticrossings with cladding resonances on ultrafast nonlinear dynamics in gasfilled photonic crystal fibers
Francesco Tani, Felix Köttig, David Novoa, Ralf Keding, Philip Russell
Spectral anticrossings between the fundamental guided mode and corewall resonances alter the dispersion in hollowcore antiresonantreflection photonic crystal fibers. Here we study the effect of this dispersion change on the nonlinear propagation and dynamics of ultrashort pulses. We find that it causes emission of narrow spectral peaks through a combination of fourwave mixing and dispersive wave emission. We further investigate the influence of the anticrossings on nonlinear pulse propagation and show that their impact can be minimized by adjusting the corewall thickness in such a way that the anticrossings lie spectrally distant from the pump wavelength.
SatelliteBased QKD
Imran Khan, Bettina Heim, Andreas Neuzner, Christoph Marquardt
A global network of spacecraft and ground stations, distributing secret encryption keys by meansof quantum technology, could meet emerging and longterm threats to data security.
Mutual Unbiasedness in CoarseGrained Continuous Variables
Daniel S. Tasca, Piero Sánchez, Stephen P. Walborn, Łukasz Rudnicki
The notion of mutual unbiasedness for coarsegrained measurements of quantum continuous variable systems is considered. It is shown that while the procedure of “standard” coarse graining breaks the mutual unbiasedness between conjugate variables, this desired feature can be theoretically established and experimentally observed in periodic coarse graining. We illustrate our results in an optics experiment implementing Fraunhofer diffraction through a periodic diffraction grating, finding excellent agreement with the derived theory. Our results are an important step in developing a formal connection between discrete and continuous variable quantum mechanics.
Snowflake phononic topological insulator at the nanoscale
Christian Brendel, Vittorio Peano, Oskar Painter, Florian Marquardt
Physical Review B (Rapid Communications)
97(2)
020102
(2018)

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We show how the snowflake phononic crystal structure, which recently has been realized experimentally, can be turned into a topological insulator for mechanical waves. This idea, based purely on simple geometrical modifications, could be readily implemented on the nanoscale.
Broadband bright twin beams and their upconversion
Maria Chekhova, Semen Germanskiy, Dmitri Horoshko, Galiya Kitaeva, Mikhail Kolobov, Gerd Leuchs, Chris Phillips, Pavel Prudkovskii
We report on the observation of broadband (40 THz) bright twin beams through highgain parametric downconversion in an aperiodically poled lithium niobate crystal. The output photon number is shown to scale exponentially with the pump power and not with the pump amplitude, as in homogeneous crystals. Photon number correlations and the number of frequency/temporal modes are assessed by spectral covariance measurements. By using sumfrequency generation on the surface of a nonphasematched crystal, we measure a crosscorrelation peak with the temporal width of 90 fs.
Control of ultrafast pulses in a hydrogenfilled hollowcore photoniccrystal fiber by Raman coherence
Federico Belli, Amir Abdolvand, John Travers, Philip Russell
We present the results of an experimental and numerical investigation into temporally nonlocal coherent interactions between ultrashort pulses, mediated by Raman coherence, in a gasfilled kagomestyle hollowcore photoniccrystal fiber. A pump pulse first sets up the Raman coherence, creating a refractive index spatiotemporal
grating in the gas that travels at the group velocity of the pump pulse. Varying the arrival time of a second, probe, pulse allows a high degree of control over its evolution as it propagates along the fiber through the grating. Of particular interest are solitondriven effects such as selfcompression and dispersive wave (DW) emission. In the experiments reported, a DW is emitted at ∼300 nm and exhibits a wiggling effect, with its central frequency oscillating periodically with pumpprobe delay. The results demonstrate that a strong Raman coherence, created in a broadband guiding gasfilled kagome photoniccrystal fiber, can be used to control the nonlinear dynamics of ultrashort probe pulses, even in difficulttoaccess spectral regions such as the deep and vacuum ultraviolet.
Visualizing singlecell secretion dynamics with single protein sensitivity
Matthew Paul McDonald, André Gemeinhardt, Katharina König, Marek Piliarik, Stefanie Schaffer, Simon Völkl, Andreas Mackensen, Vahid Sandoghdar
Cellular secretion of proteins into the extracellular environment is an essential mediator of critical biological mechanisms, including celltocell communication, immunological response, targeted delivery, and differentiation. Here, we report a novel methodology that allows for the realtime detection and imaging of single unlabeled proteins that are secreted from individual living cells. This is accomplished via interferometric detection of scattered light (iSCAT) and is demonstrated with Laz388 cells, an Epstein Barr virus (EBV)transformed B cell line. We find that single Laz388 cells actively secrete IgG antibodies at a rate of the order of 100 molecules per second. Intriguingly, we also find that other proteins and particles spanning ca. 100 kDa1 MDa are secreted from the Laz388 cells in tandem with IgG antibody release, likely arising from EBVrelated viral proteins. The technique is general and, as we show, can also be applied to studying the lysate of a single cell. Our results establish labelfree iSCAT imaging as a powerful tool for studying the realtime exchange between cells and their immediate environment with singleprotein sensitivity.
Scalable Ion Trap Architecture for Universal Quantum Computation by Collisions
We propose a scalable ion trap architecture for universal quantum computation, which is composed of an array of ion traps with one ion confined in each trap. The neighboring traps are designed capable of merging into one single trap. The universal twoqubit SWAP−−−−−−√ gate is realized by direct collision of two neighboring ions in the merged trap, which induces an effective spinspin interaction between two ions. We find that the collisioninduced spinspin interaction decreases with the third power of two ions' trapping distance. Even with a 200 μm trapping distance between atomic ions in Paul traps, it is still possible to realize a twoqubit gate operation with speed in 0.1 kHz regime. The speed can be further increased up into 0.1 MHz regime using electrons with 10 mm trapping distance in Penning traps.
Quantum tomography enhanced through parametric amplification
E. Knyazev, Kirill Spasibko, Maria V. Chekhova, F. Ya Khalili
NEW JOURNAL OF PHYSICS
20
013005
(2018)

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Quantum tomography is the standard method of reconstructing the Wigner function of quantum states of light by means of balanced homodyne detection. The reconstruction quality strongly depends on the photodetectors quantum efficiency and other losses in the measurement setup. In this article we analyze in detail a protocol of enhanced quantum tomography, proposed by Leonhardt and Paul [1] which allows one to reduce the degrading effect of detection losses. It is based on phasesensitive parametric amplification, with the phase of the amplified quadrature being scanned synchronously with the local oscillator phase. Although with sufficiently strong amplification the protocol enables overcoming any detection inefficiency, it was so far not implemented in the experiment, probably due to the losses in the amplifier. Here we discuss a possible proofofprinciple experiment with a travelingwave parametric amplifier. We show that with the stateoftheart optical elements, the protocol enables high fidelity tomographic reconstruction of bright nonclassical states of light. We consider two examples: bright squeezed vacuum and squeezed singlephoton state, with the latter being a nonGaussian state and both strongly affected by the losses.
Tackling Africa’s digital divide
Martin P. J. Lavery, Mojtaba Mansour Abadi, Ralf Bauer, Gilberto Brambilla, Ling Cheng, Mitchell A. Cox, Angela Dudley, Andrew D. Ellis, Nicolas K. Fontaine, Anthony E. Kelly, et al.
Nature Photonics
12(5)
249252
(2018)
Innovations in ‘sustainable’ photonics technologies such as freespace optical links and solarpowered equipment provide developing countries with new costeffective opportunities for deploying futureproof telecommunication networks.
Majorization uncertainty relations for mixed quantum states
Zbigniew Puchała, Łukasz Rudnicki, Aleksandra Krawiec, Karol Życzkowski
Journal of Physics A: Mathematical and Theoretical
51(17)
(2018)
Majorization uncertainty relations are generalized for an arbitrary mixed quantum state ρ of a finite size N . In particular, a lower bound for the sum of two entropies characterizing the probability distributions corresponding to measurements with respect to two arbitrary orthogonal bases is derived in terms of the spectrum of ρ and the entries of a unitary matrix U relating both bases. The results obtained can also be formulated for two measurements performed on a single subsystem of a bipartite system described by a pure state, and consequently expressed as an uncertainty relation for the sum of conditional entropies.
Near optimal discrimination of binary coherent signals via atom–light interaction
Rui Han, János A Bergou, Gerd Leuchs
New Journal of Physics
20(4)
(2018)
We study the discrimination of weak coherent states of light with significant overlaps by nondestructive measurements on the light states through measuring atomic states that are entangled to the coherent states via dipole coupling. In this way, the problem of measuring and discriminating coherent light states is shifted to finding the appropriate atom–light interaction and atomic measurements. We show that this scheme allows us to attain a probability of error extremely close to the Helstrom bound, the ultimate quantum limit for discriminating binary quantum states, through the simple Jaynes–Cummings interaction between the field and ancilla with optimized light–atom coupling and projective measurements on the atomic states. Moreover, since the measurement is nondestructive on the light state, information that is not detected by one measurement can be extracted from the postmeasurement light states through subsequent measurements.
Threedimensional holographic optical manipulation through a highnumericalaperture softglass multimode fibre
Ivo T. Leite, Sergey Turtaev, Xin Jiang, Martin Siler, Alfred Cuschieri, Philip St. J. Russell, Tomas Cizmar
Holographic optical tweezers (HOT) hold great promise for many applications in biophotonics, allowing the creation and measurement of minuscule forces on biomolecules, molecular motors and cells. Geometries used in HOT currently rely on bulk optics, and their exploitation in vivo is compromised by the optically turbid nature of tissues. We present an alternative HOT approach in which multiple threedimensional (3D) traps are introduced through a highnumericalaperture multimode optical fibre, thus enabling an equally versatile means of manipulation through channels having crosssection comparable to the size of a single cell. Our work demonstrates realtime manipulation of 3D arrangements of microobjects, as well as manipulation inside otherwise inaccessible cavities. We show that the traps can be formed over fibre lengths exceeding 100 mm and positioned with nanometric resolution. The results provide the basis for holographic manipulation and other highnumericalaperture techniques, including advanced microscopy, through singlecorefibre endoscopes deep inside living tissues and other complex environments.
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