6,906 research outputs found
Tenfold way and many-body zero modes in the Sachdev-Ye-Kitaev model
The Sachdev-Ye-Kitaev (SYK) model, in its simplest form, describes k Majorana fermions with random all-to-all four-body interactions. We consider the SYK model in the framework of a many-body Altland-Zirnbauer classification that sees the system as belonging to one of eight (real) symmetry classes depending on the value of k mod 8. We show that, depending on the symmetry class, the system may support exact many-body zero modes with the symmetries also dictating whether these may have a nonzero contribution to Majorana fermions, i.e., single-particle weight. These zero modes appear in all but two of the symmetry classes. When present, they leave clear signatures in physical observables that go beyond the threefold (Wigner-Dyson) possibilities for level spacing statistics studied earlier. Signatures we discover include a zero-energy peak or hole in the single-particle spectral function, depending on whether symmetries allow or forbid zero modes to have single-particle weight. The zero modes are also shown to influence the many-body dynamics, where signatures include a nonzero long-time limit for the out-of-time-order correlation function. Furthermore, we show that the extension of the four-body SYK model by quadratic terms can be interpreted as realizing the remaining two complex symmetry classes; we thus demonstrate how the entire tenfold Altland-Zirnbauer classification may emerge in the SYK model
A phase-space approach to directional switching in semiconductor ring lasers
We show that a topological investigation of the phase space of a
Semiconductor Ring Laser can be used to devise switching schemes which are
alternative to optical pulse injection of counter-propagating light. To provide
physical insight in these switching mechanisms, a full bifurcation analysis and
an investigation of the topology is performed on a two-dimensional asymptotic
model. Numerical simulations confirm the topological predictions.Comment: 9 pages, 7 figure
Structure of Stochastic Dynamics near Fixed Points
We analyze the structure of stochastic dynamics near either a stable or
unstable fixed point, where force can be approximated by linearization. We find
that a cost function that determines a Boltzmann-like stationary distribution
can always be defined near it. Such a stationary distribution does not need to
satisfy the usual detailed balance condition, but might have instead a
divergence-free probability current. In the linear case the force can be split
into two parts, one of which gives detailed balance with the diffusive motion,
while the other induces cyclic motion on surfaces of constant cost function.
Using the Jordan transformation for the force matrix, we find an explicit
construction of the cost function. We discuss singularities of the
transformation and their consequences for the stationary distribution. This
Boltzmann-like distribution may be not unique, and nonlinear effects and
boundary conditions may change the distribution and induce additional currents
even in the neighborhood of a fixed point.Comment: 7 page
Domain wall in a chiral p-wave superconductor: a pathway for electrical current
Superconductors with p+ip pairing symmetry are characterized by chiral edge
states, but these are difficult to detect in equilibrium since the resulting
magnetic field is screened by the Meissner effect. Nonequilibrium detection is
hindered by the fact that the edge excitations are unpaired Majorana fermions,
which cannot transport charge near the Fermi level. Here we show that the
boundary between p_x+ip_y and p_x-ip_y domains forms a one-way channel for
electrical charge. We derive a product rule for the domain wall conductance,
which allows to cancel the effect of a tunnel barrier between metal electrodes
and superconductor and provides a unique signature of topological
superconductors in the chiral p-wave symmetry class.Comment: 6 pages, 3 figure
Excitability in semiconductor microring lasers: Experimental and theoretical pulse characterization
We characterize the operation of semiconductor microring lasers in an
excitable regime. Our experiments reveal a statistical distribution of the
characteristics of noise-triggered optical pulses that is not observed in other
excitable systems. In particular, an inverse correlation exists between the
pulse amplitude and duration. Numerical simulations and an interpretation in an
asymptotic phase space confirm and explain these experimentally observed pulse
characteristics.Comment: 9 pages, 10 figure
Exploring multi-stability in semiconductor ring lasers: theory and experiment
We report the first experimental observation of multi-stable states in a
single-longitudinal mode semiconductor ring laser. We show how the operation of
the device can be steered to either monostable, bistable or multi-stable
dynamical regimes in a controlled way. We observe that the dynamical regimes
are organized in well reproducible sequences that match the bifurcation
diagrams of a two-dimensional model. By analyzing the phase space in this
model, we predict how the stochastic transitions between multi-stable states
take place and confirm it experimentally.Comment: 4 pages, 5 figure
Topological insight into the non-Arrhenius mode hopping of semiconductor ring lasers
We investigate both theoretically and experimentally the stochastic switching
between two counter-propagating lasing modes of a semiconductor ring laser.
Experimentally, the residence time distribution cannot be described by a simple
one parameter Arrhenius exponential law and reveals the presence of two
different mode-hop scenarios with distinct time scales. In order to elucidate
the origin of these two time scales, we propose a topological approach based on
a two-dimensional dynamical system.Comment: 4 pages, 3 figure
Novel integrated tunable laser using filtered feedback for simple and very fast tuning
We present a novel integrated tunable laser based on filtered feedback, which combines a simple tuning method with ns switching speed
Integrated filtered-feedback tunable laser with enhanced control of feedback phase
Recently we presented a novel discretely tunable laser that consists of a Fabry-Perot laser which was forced to operate in single-mode condition by applying on-chip filtered feedback. The laser switches extremely fast (3 ns) and requires simple on/off control currents to switch the wavelength. In these first devices it was not possible to control the phase of the feedback light independently from the feedback intensity. In was solved by adding an extra electrode allowing us to control the phase separately. In this paper we present the new device and study the effect of the control ofthefeedbackphase in order to improve the performance ofthe original tunable laser concept
Fast integrated tunable laser using filtered feedback
A novel integrated tunable laser is presented which combines a simple tuning method with ns switching speed. The Photonic Integrated Circuit consists of a Fabry-Perot laser with deeply-etched DBR mirrors. The Fabry-Perot modes can be selected independently using an Arrayed Waveguide Grating and then re-injected into the laser cavity, forcing single mode operation at the wavelength of that mode. 4ns switching time as well as 15 dB SMSR is demonstrated on the prototype device
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