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.
Quantum Langevin approach to cavity quantum electrodynamics with molecules
We develop a quantum Langevin equations approach to describe the interaction between light and molecular systems modelled as quantum emitters coupled to a multitude of vibrational modes via a Holsteintype interaction. The formalism allows for analytical derivations of absorption and fluorescence profiles both in the transient and steady state regimes of molecules outside and inside optical cavities. We also derive expressions for the cavitymodified radiative emission branching ratio of a single molecule, cavity transmission in the strong coupling regime and Förster resonance energy transfer between donoracceptor molecules outside and inside optical cavities.
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.
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 spinsegmented manner, meaning that the far field of the scattered light exhibits laterally separated left and righthanded 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 spinsegmentation and, therefore, the sensitivity to small displacements of a scatterer. In particular, we experimentally achieve a change of the spinsplitting signal of 5% per nanometer displacement.
LongLived RefractiveIndex Changes Induced by Femtosecond Ionization in GasFilled SingleRing PhotonicCrystal Fibers
Johannes Köhler, Felix Köttig, Barbara Trabold, Francesco Tani, Philip 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.
Nonexponential decay of a giant artificial atom
Gustav Andersson, Baladitya Suri, Lingzhen Guo, Thomas Aref, Per Delsing
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.
Excitation of higherorder modes in optofluidic photonic crystal fiber
Andrei Ruskuc, Philipp Koehler, Marius A. Weber, Ana AndresArroyo, Michael Frosz, Philip Russell, Tijmen 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.
Super and subradiance of clock atoms in multimode optical waveguides
Laurin Ostermann, Clement Meignant, Claudiu Genes, Helmut Ritsch
The transversely confined propagating modes of an optical fiber mediate virtually infinite range energy exchanges among atoms placed within their field, which adds to the inherent free space dipoledipole coupling. Typically, the single atom free space decay rate largely surpasses the emission rate into the guided fiber modes. However, scaling up the atom number as well as the system size amounts to entering a collective emission regime, where fiberinduced superradiant spontaneous emission dominates over free space decay. We numerically study this super and subradiant decay of highly excited atomic states for one or several transverse fiber modes as present in hollow core fibers. As particular excitation scenarios we compare the decay of a totally inverted state to the case of Pi/2 pulses applied transversely or along the fiber axis as in standard Ramsey or Rabi interferometry. While a mean field approach fails to correctly describe the initiation of superradiance, a secondorder approximation accounting for pairwise atomatom quantum correlations generally proves sufficient to reliably describe superradiance of ensembles from two to a few hundred particles. In contrast, a full account of subradiance requires the inclusion of all higher order quantum correlations. Considering multiple guided modes introduces a natural effective cutoff for the interaction range emerging from the dephasing of different fiber contributions.
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˚.
Enhanced collective Purcell effect of coupled quantum emitter systems
David Plankensteiner, Christian Sommer, Michael Reitz, Helmut Ritsch, Claudiu Genes
arXiv:1811.03442
(2018)
Cavityembedded quantum emitters show strong modifications of free space radiation properties such as an enhanced decay known as the Purcell effect. The central parameter is the cooperativity C  the ratio of the square of the coherent cavity coupling strength over the product of cavity and emitter decay rates. For a single emitter, C is independent of the transition dipole moment and dictated by geometric cavity properties such as finesse and mode waist. In a recent work Phys. Rev. Lett. 119, 093601 (2017) we have shown that collective excitations in ensembles of dipoledipole coupled quantum emitters show a disentanglement between the coherent coupling to the cavity mode and spontaneous free space decay. This leads to a strong enhancement of the cavity cooperativity around certain collective subradiant antiresonances. Here, we present a quantum Langevin equations approach aimed at providing results beyond the classical coupled dipoles model. We show that the subradiantly enhanced cooperativity imprints its effects onto the cavity output field quantum correlations while also strongly increasing the cavityemitter system's collective Kerr nonlinear effect.
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.
Turning a molecule into a coherent twolevel quantum system
Daqing Wang, Hrishikesh Kelkar, DiegoMartin Cano, Dominik Rattenbacher, Alexey Shkarin, Tobias Utikal, Stephan Götzinger, Vahid Sandoghdar
Molecules are ubiquitous in natural phenomena and manmade products, but
their use in quantum optical applications has been hampered by incoherent
internal vibrations and other phononic interactions with their environment. We
have now succeeded in turning an organic molecule into a coherent twolevel
quantum system by placing it in an optical microcavity. This allows several
unprecedented observations such as 99\% extinction of a laser beam by a single
molecule, saturation with less than 0.5 photon, and nonclassical generation of
fewphoton superbunched light. Furthermore, we demonstrate efficient
interaction of the moleculemicrocavity system with single photons generated by
a second molecule in a distant laboratory. Our achievements pave the way for
linear and nonlinear quantum photonic circuits based on organic platforms.
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 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.
Interference effects in hybrid cavity optomechanics
Ondřej Černotík, Claudiu Genes, Aurelien Dantan
arXiv:1809.01420
(2018)
Radiation pressure forces in cavity optomechanics allow for efficient cooling of vibrational modes of macroscopic mechanical resonators, the manipulation of their quantum states, as well as generation of optomechanical entanglement. The standard mechanism relies on the cavity photons directly modifying the state of the mechanical resonator. Hybrid cavity optomechanics provides an alternative approach by coupling mechanical objects to quantum emitters, either directly or indirectly via the common interaction with a cavity field mode. While many approaches exist, they typically share a simple effective description in terms of a single force acting on the mechanical resonator. More generally, one can study the interplay between various forces acting on the mechanical resonator in such hybrid mechanical devices. This interplay can lead to interference effects that may, for instance, improve cooling of the mechanical motion or lead to generation of entanglement between various parts of the hybrid device. Here, we provide such an example of a hybrid optomechanical system where an ensemble of quantum emitters is embedded into the mechanical resonator formed by a vibrating membrane. The interference between the radiation pressure force and the mechanically modulated TavisCummings interaction leads to enhanced cooling dynamics in regimes in which neither force is efficient by itself. Our results pave the way towards engineering novel optomechanical interactions in hybrid optomechanical systems.
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.
Interferometric scattering microscopy reveals microsecond nanoscopic protein motion on a live cell membrane
Richard W. Taylor, Reza Gholami Mahmoodabadi, Verena Rauschenberger, Andreas Giessl, Alexandra Schambony, Vahid Sandoghdar
bioRxiv: http://dx.doi.org/10.1101/401133
(2018)
Much of the biological functions of a cell are dictated by the intricate motion of proteins within its membrane over a spatial range of nanometers to tens of micrometers and time intervals of microseconds to minutes. While this rich parameter space is not accessible to fluorescence microscopy, it can be within reach of interferometric scattering (iSCAT) particle tracking. Being sensitive even to single unlabeled proteins, however, iSCAT is easily accompanied by a large specklelike background, which poses a substantial challenge for its application to cellular imaging. Here, we show that these difficulties can be overcome and demonstrate tracking of transmembrane epidermal growth factor receptors (EGFR) with nanometer precision in all three dimensions at up to microsecond speeds and tens of minutes duration. We provide unprecedented examples of nanoscale motion and confinement in ubiquitous processes such as diffusion in the plasma membrane, transport on filopodia, and endocytosis.
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.
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.
Resonance inversion in a superconducting cavity coupled to artificial atoms and a microwave background
Juha Leppäkangas, Jan David Brehm, Ping Yang, Lingzhen Guo, Michael Marthaler, Alexey V. Ustinov, Martin Weides
We demonstrate how heating of an environment can invert the line shape of a driven cavity. We consider a superconducting coplanar cavity coupled to multiple artificial atoms. The measured cavity transmission is characterized by Fanotype resonances with a shape that is continuously tunable by bias current through nearby (magnetic flux) control lines. In particular, the same dispersive shift of the microwave cavity can be observed as a peak or a dip. We find that this Fanopeak inversion is possible due to a tunable interference between a microwave transmission through a background, with reactive and dissipative properties, and through the cavity, affected by biascurrent induced heating. The background transmission occurs due to crosstalk with the multiple control lines. We show how such background can be accounted for by a Jaynes or TavisCummings model with modified boundary conditions between the cavity and transmissionline microwave fields. A dip emerges when cavity transmission is comparable with background transmission and dissipation. We find generally that resonance positions determine system energy levels, whereas resonance shapes give information on system fluctuations and dissipation.
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)

Journal

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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.
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 Bykov, Shangran Xie, Richard Zeltner, Andrey Machnev, Gordon Wong, Tijmen Euser, Philip 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.
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.
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.
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 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
Physical Review A
97 (4)
042312
(2018)
Preprint

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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.
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.
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.
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.
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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