arXiv subjects

Stefan Rotter

Publications and source records attributed to Stefan Rotter.

At least 109 records · Page 6Linked to original sources

arXiv2016

Dynamically encircling exceptional points in a waveguide: asymmetric mode switching from the breakdown of adiabaticity

Physical systems with loss or gain feature resonant modes that are decaying or growing exponentially with time. Whenever two such modes coalesce both in their resonant frequency and their rate of decay or growth, a so-called "exceptional point" occurs, around which many fascinating phenomena have recently been reported to arise. Particularly intriguing behavior is predicted to appear when encircling an exceptional point sufficiently slowly, like a state-flip or the accumulation of a geometric phase. Experiments dedicated to this issue could already successfully explore the topological structure of exceptional points, but a full dynamical encircling and the breakdown of adiabaticity inevitably associated with it remained out of reach of any measurement so far. Here we demonstrate that a dynamical encircling of an exceptional point can be mapped onto the problem of scattering through a two-mode waveguide, which allows us for the first time to access the elusive effects occurring in this context. Specifically, we present experimental results from a waveguide structure that steers incoming waves around an exceptional point during the transmission process. In this way mode transitions are induced that make this device perfectly suited as a robust and asymmetric switch between different waveguide modes. Our work opens up new and exciting avenues to explore exceptional point physics at the crossroads between fundamental research and practical applications.

physics.optics↗

arXiv2016

Interaction-induced mode switching in steady-state microlasers

We demonstrate that due to strong modal interactions through cross-gain saturation, the onset of a new lasing mode can switch off an existing mode via a negative power slope. In this process of interaction-induced mode switching (IMS) the two involved modes maintain their identities, i.e. they do not change their spatial field patterns or lasing frequencies. For a fixed pump profile, a simple analytic criterion for the occurrence of IMS is given in terms of their self- and cross-interaction coefficients and non-interacting thresholds, which is verified for the example of a two-dimensional microdisk laser. When the spatial pump profile is varied as the pump power is increased, IMS can be induced even when it would not occur with a fixed pump profile, as we show for two coupled laser cavities. Our findings apply to steady-state lasing and are hence different from dynamical mode switching or hopping. IMS may have potential applications in robust and flexible all-optical switching.

physics.optics↗

arXiv2016

Spatio-temporal Control of Light Transmission through a Multimode Fiber with Strong Mode Coupling

We experimentally generate and characterize the eigenstates of the Wigner-Smith time-delay matrix, called principal modes, in a multimode fiber with strong mode coupling. The unique spectral and temporal properties of principal modes enable a global control of the temporal dynamics of optical pulses transmitted through the fiber, despite random mode mixing. Our analysis reveals that the well-defined delay time of the eigenstates are formed by multi-path interference, which can be effectively manipulated by the spatial degrees of freedom of the input wavefront. This study is essential to controlling the dynamics of wave scattering, paving the way for coherent control of pulse propagation through complex media.

physics.optics↗

arXiv2015

Engineering Long-Lived Collective Dark States in Spin Ensembles

Ensembles of electron spins in hybrid microwave systems are powerful and versatile components for future quantum technologies. Quantum memories with high storage capacities are one such example which require long-lived states that can be addressed and manipulated coherently within the inhomogeneously broadened ensemble. This broadening is essential for true multimode memories, but induces a considerable spin dephasing and together with dissipation from a cavity interface poses a constraint on the memory's storage time. In this work we show how to overcome both of these limitations through the engineering of long-lived dark states in an ensemble of electron spins hosted by nitrogen-vacancy centres in diamond. By burning narrow spectral holes into a spin ensemble strongly coupled to a superconducting microwave cavity, we observe long-lived Rabi oscillations with high visibility and a decay rate that is a factor of forty smaller than the spin ensemble linewidth and thereby a factor of more than three below the pure cavity dissipation rate. This significant reduction lives up to the promise of hybrid devices to perform better than their individual subcomponents. To demonstrate the potential of our approach we realise the first step towards a solid-state microwave spin multiplexer by engineering multiple long-lived dark states. Our results show that we can fully access the "decoherence free" subspace in our experiment and selectively prepare protected states by spectral hole burning. This technique opens up the way for truly long-lived quantum memories, solid-state microwave frequency combs, optical to microwave quantum transducers and spin squeezed states. Our approach also paves the way for a new class of cavity QED experiments with dense spin ensembles, where dipole spin-spin interactions become important and many-body phenomena will be directly accessible on a chip.

quant-ph↗

arXiv2015

Hybrid Quantum Systems with Collectively Coupled Spin States: Suppression of Decoherence through Spectral Hole Burning

Spin ensemble based hybrid quantum systems suffer from a significant degree of decoherence resulting from the inhomogeneous broadening of the spin transition frequencies in the ensemble. We demonstrate that this strongly restrictive drawback can be overcome simply by burning two narrow spectral holes in the spin spectral density at judiciously chosen frequencies. Using this procedure we find an increase of the coherence time by more than an order of magnitude as compared to the case without hole burning. Our findings pave the way for the practical use of these hybrid quantum systems for the processing of quantum information.

quant-ph↗

arXiv2015

General description of quasi-adiabatic dynamical phenomena near exceptional points

The appearance of so-called exceptional points in the complex spectra of non-Hermitian systems is often associated with phenomena that contradict our physical intuition. One example of particular interest is the state-exchange process predicted for an adiabatic encircling of an exceptional point. In this work we analyse this and related processes for the generic system of two coupled oscillator modes with loss or gain. We identify a characteristic system evolution consisting of periods of quasi-stationarity interrupted by abrupt non-adiabatic transitions, and we present a qualitative and quantitative description of this switching behaviour by connecting the problem to the phenomenon of stability loss delay. This approach makes accurate predictions for the breakdown of the adiabatic theorem as well as the occurrence of chiral behavior observed previously in this context, and provides a general framework to model and understand quasi-adiabatic dynamical effects in non-Hermitian systems.

quant-ph↗

arXiv2015

Percolating States in the Topological Anderson Insulator

We investigate the presence of percolating states in disordered two-dimensional topological insulators. In particular, we uncover a close connection between these states and the so-called topological Anderson insulator (TAI), which is a topologically non-trivial phase induced by the presence of disorder. The decay of this phase could previously be connected to a delocalization of bulk states with increasing disorder strength. In this work we identify this delocalization to be the result of a percolation transition of states that circumnavigate the hills of the bulk disorder potential.

cond-mat.mes-hall↗

arXiv2015

Constant-intensity waves and their modulation instability in non-Hermitian potentials

In all of the diverse areas of science where waves play an important role, one of the most fundamental solutions of the corresponding wave equation is a stationary wave with constant intensity. The most familiar example is that of a plane wave propagating in free space. In the presence of any Hermitian potential, a wave's constant intensity is, however, immediately destroyed due to scattering. Here we show that this fundamental restriction is conveniently lifted when working with non-Hermitian potentials. In particular, we present a whole new class of waves that have constant intensity in the presence of linear as well as of nonlinear inhomogeneous media with gain and loss. These solutions allow us to study, for the first time, the fundamental phenomenon of modulation instability in an inhomogeneous environment. Our results pose a new challenge for the experiments on non-Hermitian scattering that have recently been put forward.

physics.optics↗

arXiv2015

Analytical Study of Quantum Feedback Enhanced Rabi Oscillations

We present an analytical solution of the single photon quantum feedback in a cavity quantum electrodynamics system based on a half cavity set-up coupled to a structured continuum. The exact analytical expression we obtain allows us to discuss in detail under which conditions a single emitter-cavity system, which is initially in the weak coupling regime, can be driven into the strong coupling regime via the proposed quantum feedback mechanism [Carmele et al, Phys.Rev.Lett. 110, 013601]. Our results reveal that the feedback induced oscillations rely on a well-defined relationship between the delay time and the atom-light coupling strength of the emitter. At these specific values the leakage into the continuum is prevented by a destructive interference effect, which pushes the emitter to the strong coupling limit.

quant-ph↗

arXiv2015

Steady-state ab-initio laser theory for lasers with fully or nearly degenerate resonator modes

We investigate the range of validity of the recently developed steady-state ab-initio laser theory (SALT). While very efficient in describing various microlasers, SALT is conventionally believed not to be applicable to lasers featuring fully or nearly degenerate pairs of resonator modes above the lasing threshold. Here we demonstrate how SALT can indeed be extended to describe such cases as well, with the effect that we significantly broaden the theory's scope. In particular, we show how to use SALT in conjunction with a linear stability analysis to obtain stable single-mode lasing solutions that involve a degenerate mode pair. Our flexible and efficient approach is tested on one-dimensional ring lasers as well as on two-dimensional microdisk lasers with broken symmetry.

physics.optics↗

arXiv2015

Parity-time symmetry from stacking purely dielectric and magnetic slabs

We show that Parity-time symmetry in matching electric permittivity to magnetic permeability can be established by considering an effective Parity operator involving both mirror symmetry and coupling between electric and magnetic fields. This approach extends the discussion of Parity-time symmetry to the situation with more than one material potential. We show that the band structure of a one-dimensional photonic crystal with alternating purely dielectric and purely magnetic slabs can undergo a phase transition between propagation modes and evanescent modes when the balanced gain/loss parameter is varied. The cross-matching between different material potentials also allows exceptional points of the constitutive matrix to appear in the long wavelength limit where they can be used to construct ultrathin metamaterials with unidirectional reflection.

physics.optics↗

arXiv2015

Cumulants of Hawkes point processes

We derive explicit, closed-form expressions for the cumulant densities of a multivariate, self-exciting Hawkes point process, generalizing a result of Hawkes in his earlier work on the covariance density and Bartlett spectrum of such processes. To do this, we represent the Hawkes process in terms of a Poisson cluster process and show how the cumulant density formulas can be derived by enumerating all possible "family trees", representing complex interactions between point events. We also consider the problem of computing the integrated cumulants, characterizing the average measure of correlated activity between events of different types, and derive the relevant equations.

math.ST↗

arXiv2014

Invariance property of wave scattering through disordered media

A fundamental insight in the theory of diffusive random walks is that the mean length of trajectories traversing a finite open system is independent of the details of the diffusion process. Instead, the mean trajectory length depends only on the system's boundary geometry and is thus unaffected by the value of the mean free path. Here we show that this result is rooted on a much deeper level than that of a random walk, which allows us to extend the reach of this universal invariance property beyond the diffusion approximation. Specifically, we demonstrate that an equivalent invariance relation also holds for the scattering of waves in resonant structures as well as in ballistic, chaotic or in Anderson localized systems. Our work unifies a number of specific observations made in quite diverse fields of science ranging from the movement of ants to nuclear scattering theory. Potential experimental realizations using light fields in disordered media are discussed.

physics.optics↗

arXiv2014

Smooth optimal quantum control for robust solid state spin magnetometry

Nitrogen-vacancy centers in diamond show great potential as magnetic, electric and thermal sensors which are naturally packaged in a bio-compatible material. In particular, NV-based magnetometers combine small sensor volumes with high sensitivities under ambient conditions. The practical operation of such sensors, however, requires advanced quantum control techniques that are robust with respect to experimental and material imperfections, control errors, and noise. Here, we present a novel approach that uses Floquet theory to efficiently generate smooth and simple quantum control pulses with tailored robustness properties. We verify their performance by applying them to a single NV center and by characterising the resulting quantum gate using quantum process tomography. We show how the sensitivity of NV-ensemble magnetometry schemes can be improved by up to two orders of magnitude by compensating for inhomogeneities in both the control field and the spin transition frequency. Our approach is ideally suited for a wide variety of quantum technologies requiring high-fidelity, robust control under tight bandwidth requirements, such as spin-ensemble based memories involving high-Q cavities.

quant-ph↗

arXiv2014

Distribution of Orientation Selectivity in Recurrent Networks of Spiking Neurons with Different Random Topologies

Neurons in the primary visual cortex are more or less selective for the orientation of a light bar used for stimulation. A broad distribution of individual grades of orientation selectivity has in fact been reported in all species. A possible reason for emergence of broad distributions is the recurrent network within which the stimulus is being processed. Here we compute the distribution of orientation selectivity in randomly connected model networks that are equipped with different spatial patterns of connectivity. We show that, for a wide variety of connectivity patterns, a linear theory based on firing rates accurately approximates the outcome of direct numerical simulations of networks of spiking neurons. Distance dependent connectivity in networks with a more biologically realistic structure does not compromise our linear analysis, as long as the linearized dynamics, and hence the uniform asynchronous irregular activity state, remain stable. We conclude that linear mechanisms of stimulus processing are indeed responsible for the emergence of orientation selectivity and its distribution in recurrent networks with functionally heterogeneous synaptic connectivity.

q-bio.NC↗

arXiv2014

Non-Markovian dynamics of a single-mode cavity strongly coupled to an inhomogeneously broadened spin ensemble

We study the dynamics of a spin ensemble strongly coupled to a single-mode resonator driven by external pulses. When the mean frequency of the spin ensemble is in resonance with the cavity mode, damped Rabi oscillations are found between the spin ensemble and the cavity mode which we describe very accurately, including the dephasing effect of the inhomogeneous spin broadening. We demonstrate that a precise knowledge of this broadening is crucial both for a qualitative and a quantitative understanding of the temporal spin-cavity dynamics. On this basis we show that coherent oscillations between the spin ensemble and the cavity can be enhanced by a few orders of magnitude, when driving the system with pulses that match special resonance conditions. Our theoretical approach is tested successfully with an experiment based on an ensemble of negatively charged nitrogen-vacancy (NV) centers in diamond strongly coupled to a superconducting coplanar single-mode waveguide resonator.

quant-ph↗

arXiv2014

Mean-Field Analysis of Orientation Selectivity in Inhibition-Dominated Networks of Spiking Neurons

Mechanisms underlying the emergence of orientation selectivity in the primary visual cortex are highly debated. Here we study the contribution of inhibition-dominated random recurrent networks to orientation selectivity, and more generally to sensory processing. By simulating and analyzing large-scale networks of spiking neurons, we investigate tuning amplification and contrast invariance of orientation selectivity in these networks. In particular, we show how selective attenuation of the common mode and amplification of the modulation component take place in these networks. Selective attenuation of the baseline, which is governed by the exceptional eigenvalue of the connectivity matrix, removes the unspecific, redundant signal component and ensures the invariance of selectivity across different contrasts. Selective amplification of modulation, which is governed by the operating regime of the network and depends on the strength of coupling, amplifies the informative signal component and thus increases the signal-to-noise ratio. Here, we perform a mean-field analysis which accounts for this process.

q-bio.NC↗

arXiv2014

Protecting a Spin Ensemble against Decoherence in the Strong-Coupling Regime of Cavity QED

Hybrid quantum systems based on spin ensembles coupled to superconducting microwave cavities are promising candidates for robust experiments in cavity quantum electrodynamics (QED) and for future technologies employing quantum mechanical effects. Currently the main source of decoherence in these systems is inho- mogeneous spin broadening, which limits their performance for the coherent transfer and storage of quantum information. Here we study the dynamics of a superconducting cavity strongly coupled to an ensemble of nitrogen-vacancy centers in diamond. We experimentally observe for the first time, how decoherence induced by a non-Lorentzian spin distribution can be suppressed in the strong-coupling regime - a phenomenon known as "cavity protection". To demonstrate the potential of this effect for coherent control schemes, we show how appropriately chosen microwave pulses can increase the amplitude of coherent oscillations between cavity and spin ensemble by two orders of magnitude.

quant-ph↗