Unravelling-based (auto)control of back-action in atomic Bose–Einstein condensate

We present a novel feedback control method for quantum systems.Feedback does not affect the controlled system itself.Instead,it controls the unravelling of the quantum channel of interaction between the system and its environment.This interaction can be represented as a history of events.If their informational content is changed,their back-action on the system is also modified.Feedback action is trigged by the events,thus granting the system the degree of control over its own state.The efficiency of the proposed scheme is demonstrated on the example of two-mode atomic Bose-Einstein condensate,with one of its modes subject to phase-contrast imaging in a Mach–Zehnder interferometer.The histories of photocounts in the output channels of the interferometer are used for feedback.Its capabilities of state engineering are studied for different settings of the feedback loop and different numbers of events in the recorded histories.

Physics and applications of charged domain walls

The charged domain wall is an ultrathin(typically nanosized)interface between two domains;it carries bound charge owing to a change of normal component of spontaneous polarization on crossing the wall.In contrast to hetero-interfaces between different materials,charged domain walls(CDWs)can be created,displaced,erased,and recreated again in the bulk of a material.Screening of the bound charge with free carriers is often necessary for stability of CDWs,which can result in giant two-dimensional conductivity along the wall.Usually in nominally insulating ferroelectrics,the concentration of free carriers at the walls can approach metallic values.Thus,CDWs can be viewed as ultrathin reconfigurable strongly conductive sheets embedded into the bulk of an insulating material.This feature is highly attractive for future nanoelectronics.The last decade was marked by a surge of research interest in CDWs.It resulted in numerous breakthroughs in controllable and reproducible fabrication of CDWs in different materials,in investigation of CDW properties and charge compensation mechanisms,in discovery of light-induced effects,and,finally,in detection of giant two-dimensional conductivity.The present review is aiming at a concise presentation of the main physical ideas behind CDWs and a brief overview of the most important theoretical and experimental findings in the field.

Multiphoton ionization of standard optical fibers

Atoms ionization by the simultaneous absorption of multiple photons has found applications in fiber optics,where it leads to unique nonlinear phenomena.To date,studies of the ionization regime have been limited to gas-filled hollow-core fibers.Here,we investigate multiphoton ionization of standard optical fibers,where intense laser pulses ionize the atoms constituting the fiber structure itself,instead of that of the filling gas.We characterize material modifications produced by optical breakdown.Their formation affects laser beam dynamics over hours long temporal scales.The damage features are studied by means of optical microscopy and X-ray microtomography.In the framework of glass photonics,our results pave the way for a novel glass waveguide micromachining technique.

High-brightness all-fiber Raman lasers directly pumped by multimode laser diodes

High-brightness fiber laser sources usually utilize active rare-earth-doped fibers cladding-pumped by multimode laser diodes(LDs), but they operate in limited wavelength ranges. Singlemode-passive-fiber based Raman lasers are able to operate at almost any wavelength being pumped by high-power fiber lasers. One of the interesting possibilities is to directly pump graded-index(GRIN) multimode passive fibers by available high-power multimode LDs at 915–940 nm,thus achieving high-power Raman lasing in the wavelength range of 950–1000 nm, which is problematic for rare-earthdoped fiber lasers. Here we review the latest results on the development of all-fiber high-brightness LD-pumped sources based on GRIN fiber with in-fiber Bragg gratings(FBGs). The mode-selection properties of FBGs inscribed by fs pulses supported by the Raman clean-up effect result in efficient conversion of multimode pump into a high-quality output beam at 9 xx nm. GRIN fibers with core diameters 62.5, 85 and 100 μm are compared. Further scaling capabilities and potential applications of such sources are discussed.