arXiv subjects

O. G. Schmidt

Publications and source records attributed to O. G. Schmidt.

At least 19 recordsLinked to original sources

arXiv2025

An ultra-compact deterministic source of maximally entangled photon pairs

We present an ultra-compact source of maximally entangled on-demand photon pairs. Our results are based on coupling of single GaAs quantum dots that are embedded in monolithic micro-lenses to a single-mode fiber with directly attached to 3D-printed micro-optics (NA of 0.6) inside a cryogenic environment. This approach, which is geared towards future integration into industrial environments, yields state-of-the-art entangled photon pair creation performance while retaining flexibility and adjustability required for long-term operation of such a device - all while dramatically reducing the overall system footprint. We demonstrate near diffraction-limited performance and hyperspectral imaging utilizing a 3D-printed micro-objective with a full width at half maximum resolution limit of 604(16) nm when operating the system at a cryogenic temperature of 3.8 K. Furthermore, we prove that this system can be used to achieve single photon emission rates of 392(20) kHz at a 76 MHz pump rate and purities of 99.2(5) % using two-photon resonant excitation. Utilizing the exciton-biexciton emission cascade available in GaAs quantum dots under resonant excitation, near maximally entangled photon pairs with peak entanglement negatives 2n of 0.96(2) in a 4 ps time window, and 0.81(1) when averaged over one exciton lifetime, are demonstrated.

quant-ph↗

arXiv2020

Topological transitions in superconductor nanomembranes in a magnetic field with submicron inhomogeneity under a strong transport current

Under a strong transport current, the induced voltage in superconductor nanomembranes in a magnetic field with submicron inhomogeneity shows a pulse on a certain interval of the magnetic field. It is a manifestation of a wide phase-slip domain. The topological transition accompanying the phase-slip effect consists in the occurrence of two new loops of weak superconducting currents connecting two regions of superconducting screening currents, which are disconnected in case of vortex-chain dynamics. In the middle of the phase-slip domain, rapid dynamics of the superconducting order parameter consists in decoupling of a spontaneously nucleated vortex-antivortex pair, subsequent motion of a vortex and an antivortex in the opposite directions followed by their annihilation with an antivortex and a vortex from the adjacent pairs. The submicron-scale inhomogeneity of the magnetic field can be achieved through a direct patterning of the magnetic field applied to a planar membrane or using advanced nanostructuring, such as roll-up technology, focused ion-beam deposition or coating carbon nanotubes by superconducting materials. If the applied magnetic field is orthogonal to the axis of a microtube, which carries transport current in the azimuthal direction, the phase-slip regime is characterized by the vortex-antivortex lifetime of 10E-15 s versus 10E-12 s for disconnected vortex dynamics in the half-tubes. The phase-slip dynamics determines the voltage-magnetic field and voltage-current characteristics in nanoarchitectures with multiple disconnected loops of superconducting screening currents.

cond-mat.supr-con↗

arXiv2017

The complete micromagnetic characterization of asymmetrically sandwiched ferromagnetic films

The magnetic properties of ferromagnetic thin films down to the nanoscale are ruled by the exchange stiffness, anisotropies and the effects of magnetic fields. As surfaces break inversion symmetry, an additional effective chiral exchange is omnipresent in any magnetic nanostructure. These so-called Dzyaloshinskii-Moriya interactions (DMI) affect all inhomogeneous magnetic states either subtly or spectacularly. E.g., DMIs cause a chirality selection of the rotation sense and can fix the local rotation axis for the magnetization in domain walls (DW). But, they can stabilize also entirely different twisted magnetic structures. The chiral skyrmions a two-dimensional particle-like topological soliton is the ultimately smallest of these objects, which currently is targetted as a possible information carrier in novel spintronic devices. Observation and quantification of the chiral exchange effects provide for the salient point in understanding magnetic properties in ultrathin films and other nanostructures. An easy and reliable method to determine the DMI constant as materials parameter of asymmetric thin films is the crucial problem. Here, we put forth an experimental approach for the determination of the complete set of the micromagnetic parameters. Quasi-static Kerr microcopy observations of DW creep motion and equilibrium sizes of circular magnetic objects in combination with standard magnetometry are used to derive a consistent set of these materials parameters in polycrystalline ultrathin film systems, namely CrOx/Co/Pt stacks. The quantified micromagnetic model for these films identifies the circular magnetic objects, as seen by the optical microscopy, as ordinary bubble domains with homochiral walls. From micromagnetic calculations, the chiral skyrmions stabilized by the DMI in these films are shown to have diameters in the range 40-200nm, too small to be observed by optical microscopy.

cond-mat.mtrl-sci↗

arXiv2017

Measurement of the spin temperature of optically cooled nuclei and GaAs hyperfine constants in GaAs/AlGaAs quantum dots

Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid state quantum information technologies. While polarization of individual nuclear spins in diamond and SiC reaches 99% and beyond, it has been limited to 60-65% for the nuclei in quantum dots. Theoretical models have attributed this limit to formation of coherent "dark" nuclear spin states but experimental verification is lacking, especially due to the poor accuracy of polarization degree measurements. Here we measure the nuclear polarization in GaAs/AlGaAs quantum dots with high accuracy using a new approach enabled by manipulation of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed - the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework. Our findings unlock a route for further progress towards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin ensemble is used to achieve long qubit coherence. Moreover, GaAs hyperfine material constants are measured here experimentally for the first time.

cond-mat.mes-hall↗

arXiv2016

Enhanced transport length of spin-helical Dirac fermions in disordered 3D topological insulators

The transport length $l_\textrm{tr}$ and the mean free path $l_\textrm{e}$ are experimentally determined for bulk and surface states in a Bi$_2$Se$_3$ nanoribbon by quantum transport and transconductance measurements. We show that the anisotropic scattering of spin-helical Dirac fermions results in a strong enhancement of $l_\textrm{tr}$, which confirms theoretical predictions \cite{Culcer2010}. Despite strong disorder ($l_\textrm{e}\approx30$~nm), our result further points to the long-range nature of the scattering potential, giving a large ratio $l_\textrm{tr}/l_\textrm{e}\approx8$ that is likely limited by a finite bulk/surface coupling. This suggests that the spin-flip length could reach the micron size in disordered 3D topological insulator nanostructures with a reduced bulk doping, even if due to charge compensation.

cond-mat.mes-hall↗

arXiv2013

Non-integer optical modes in a Möbius-ring resonator

In-plane polarized light experiences a non-trivial topological evolution as it propagates resonantly in a Möbius ring resonator. The resultant geometric phase varies continuously when changing the light ellipticity, which leads to constructive interference for a non-integer number of wavelengths, and therefore to the occurrence of an arbitrary fractional number of optical modes. The geometric phase in Möbius-ring resonators is topologically robust and implies excellent intrinsic fault-tolerance.

physics.optics↗

arXiv2013

SiGe quantum dots for fast hole spin Rabi oscillations

We report on hole g-factor measurements in three terminal SiGe self-assembled quantum dot devices with a top gate electrode positioned very close to the nanostructure. Measurements of both the perpendicular as well as the parallel g-factor reveal significant changes for a small modulation of the top gate voltage. From the observed modulations we estimate that, for realistic experimental conditions, hole spins can be electrically manipulated with Rabi frequencies in the order of 100MHz. This work emphasises the potential of hole-based nano-devices for efficient spin manipulation by means of the g-tensor modulation technique.

cond-mat.mes-hall↗

arXiv2013

Statistical Properties of Exciton Fine Structure Splittings and Polarization Angles in Quantum Dot Ensembles

We propose an effective model to describe the statistical properties of exciton fine structure splitting (FSS) and polarization angle of quantum dot ensembles (QDEs). We derive the distributions of FSS and polarization angle for QDEs and show that their statistical features can be fully characterized using at most three independent measurable parameters. The effective model is confirmed using atomistic pseudopotential calculations as well as experimental measurements for several rather different QDEs. The model naturally addresses three fundamental questions that are frequently encountered in theories and experiments: (I) Why the probability of finding QDs with vanishing FSS is generally very small? (II) Why FSS and polarization angle differ dramatically from QD to QD? and (III) Is there any direct connection between FSS, optical polarization and the morphology of QDs? The answers to these fundamental questions yield a completely new physical picture for understanding optical properties of QDEs.

cond-mat.mes-hall↗

arXiv2013

An artificial atom locked to natural atoms

Single-photon sources that emit photons at the same energy play a key role in the emerging concepts of quantum information, such as entanglement swapping, quantum teleportation and quantum networks. They can be realized in a variety of systems, where semiconductor quantum dots, or 'artificial atoms', are arguably among the most attractive. However, unlike 'natural atoms', no two artificial atoms are alike. This peculiarity is a serious hurdle for quantum information applications that require photonic quantum states with identical energies. Here we demonstrate a single artificial atom that generates photons with an absolute energy that is locked to an optical transition in a natural atom. Furthermore, we show that our system is robust and immune to drifts and fluctuations in the environment of the emitter. Our demonstration is crucial for realization of a large number of universally-indistinguishable solid-state systems at arbitrary remote locations, where frequency-locked artificial atoms might become fundamental ingredients.

cond-mat.mes-hall↗

arXiv2013

Compositionally-modulated Si1-xGex multilayers with cross-plane thermal conductivity below the thin-film alloy limit

We describe epitaxial Ge/Si multilayers with cross-plane thermal conductivities which can be systematically reduced to exceptionally low values, as compared both with bulk and thin-film SiGe alloys of the same average concentration, by simply changing the thicknesses of the constituent layers. Ab initio calculations reveal that partial interdiffusion of Ge into the Si spacers, which naturally results from Ge segregation during growth, plays a determinant role, lowering the thermal conductivity below what could be achieved without interdiffusion (perfect superlattice), or with total interdiffusion (alloy limit). This phenomenon is similar to the one previously observed in alloys with embedded nanoparticles, and it stresses the importance of combining alloy and nanosized scatterers simultaneously to minimize thermal conductivity. Our calculations thus suggest that superlattices with sharp interfaces, which are commonly sought but difficult to realize, are worse than compositionally-modulated Si1-xGex multilayers in the search for materials with ultralow thermal conductivities.

cond-mat.mtrl-sci↗

arXiv2012

Evidence for Triplet Superconductivity in a Superconductor-Ferromagnet Spin Valve

We have studied the dependence of the superconducting (SC) transition temperature on the mutual orientation of magnetizations of Fe1 and Fe2 layers in the spin valve system CoO_x/Fe1/Cu/Fe2/Pb. We find that this dependence is nonmonotonic when passing from the parallel to the antiparallel case and reveals a distinct minimum near the orthogonal configuration. The analysis of the data in the framework of the SC triplet spin valve theory gives direct evidence for the long-range triplet superconductivity arising due to noncollinearity of the two magnetizations.

cond-mat.supr-con↗

arXiv2012

Semiconductor quantum dots with light-hole exciton ground state: fabrication and fine structure

Quantum dots (QDs) can act as convenient hosts of two-level quantum szstems, such as single electron spins, hole spins or excitons (bound electron-hole pairs). Due to quantum confinement, the ground state of a single hole confined in a QD usually has dominant heavy-hole (HH) character. For this reason light-hole (LH) states have been largely neglected, despite the fact that may enable the realilzation of coherent photon-to-spin converters or allow for faster spin manipulation compared to HH states. In this work, we use tensile strains larger than 0.3% to switch the ground state of excitons confined in high quality GaAs/AlGaAs QDs from the conventional HH- to LH-type. The LH-exciton fine structure is characterized by two in-plane-polarized lines and, ~400 micro-eV above them, by an additional line with pronounced out-of-plane oscillator strength, consistent with theoretical predictions based on atomistic empirical pseudopotential calculations and a simple mesoscopic model.

cond-mat.mes-hall↗

arXiv2012

On the nature of tunable hole g-factors in quantum dots

Electrically tunable g-factors in quantum dots are highly desirable for applications in quantum computing and spintronics. We report giant modulation of the hole g-factor in a SiGe nanocrystal when an electric field is applied to the nanocrystal along its growth direction. We derive a contribution to the g-factor that stems from an orbital effect of the magnetic field, which lifts the Kramers degeneracy in the nanocrystal by altering the mixing between the heavy and the light holes. We show that the relative displacement between the heavy- and light-hole wave functions, occurring upon application of the electric field, has an effect on the mixing strength and leads to a strong non-monotonic modulation of the g-factor. Despite intensive studies of the g-factor since the late 50's, this mechanism of g-factor control has been largely overlooked in the literature.

cond-mat.mes-hall↗

arXiv2012

Bending and wrinkling as competing relaxation pathways for strained free-hanging films

An equilibrium phase diagram for the shape of compressively strained free-hanging films is developed by total strain energy minimization. For small strain gradients Δε, the film wrinkles, while for sufficiently large Δε, a phase transition from wrinkling to bending occurs. We consider competing relaxation mechanisms for free-hanging films, which have rolled up into tube structures, and we provide an upper limit for the maximum achievable number of tube rotations.

cond-mat.mes-hall↗

arXiv2012

Universal recovery of the bright-exciton level-degeneracy in quantum dots without structural symmetry

The lack of structural symmetry which usually characterizes semiconductor quantum dots lifts the energetic degeneracy of the bright excitonic states and hampers severely their use as high fidelity sources of entangled photons. We demonstrate experimentally and theoretically that it is always possible to restore the excitonic degeneracy by the simultaneous application of large strain and electric fields, despite the fact that this possibility has fundamentally been doubted. This is achieved by using one external perturbation to align the polarization of the exciton emission along the axis of the second perturbation, which then erases completely the energy splitting of the states. This result, which holds for any QD structure, highlights the potential of combining complementary external fields to create artificial atoms meeting the stringent requirements posed by scalable semiconductor-based quantum-technology.

cond-mat.mtrl-sci↗

arXiv2011

Observation of spin-selective tunneling in SiGe nanocrystals

Spin-selective tunneling of holes in SiGe nanocrystals contacted by normal-metal leads is reported. The spin selectivity arises from an interplay of the orbital effect of the magnetic field with the strong spin-orbit interaction present in the valence band of the semiconductor. We demonstrate both experimentally and theoretically that spin-selective tunneling in semiconductor nanostructures can be achieved without the use of ferromagnetic contacts. The reported effect, which relies on mixing the light and heavy holes, should be observable in a broad class of quantum-dot systems formed in semiconductors with a degenerate valence band.

cond-mat.mes-hall↗

arXiv2011

Electronic and optical properties of quantum wells embedded in wrinkled nanomembranes

The authors theoretically investigate quantum confinement and transition energies in quantum wells (QWs) asymmetrically positioned in wrinkled nanomembranes. Calculations reveal that the wrinkle profile induces both blue- and redshifts depending on the lateral position of the QW probed. Relevant radiative transistions include the ground state of the electron (hole) and excited states of the hole (electron). Energy shifts as well as stretchability of the structure are studied as a function of wrinkle amplitude and period. Large tunable bandwidths of up to 70 nm are predicted for highly asymmetric wrinkled QWs.

cond-mat.mes-hall↗

arXiv2011

Manifestation of New Interference Effects in Superconductor/Ferromagnet Spin Valve

Superconductor/ferromagnet (S/F) spin valve effect theories based on the S/F proximity phenomenon assume that the superconducting transition temperature Tc of F1/F2/S or F1/S/F2 trilayers for parallel magnetizations of the F1- and F2-layers (TcP) are smaller than for the antiparallel orientations (TcAP). Here, we report for CoOx/Fe1/Cu/Fe2/In multilayered systems with varying Fe2-layer thickness the sign-changing oscillating behavior of the spin valve effect ΔTc=TcAP-TcP. Our measurements revealed the full direct spin valve effect with TcAP>TcP for Fe2-layer thickness dFe2<1 nm and the full inverse (TcAP =1 nm. Interference of Cooper pair wave functions reflected from both surfaces of the Fe2-layer appear as the most probable reason for the observed behavior of ΔTc.

cond-mat.supr-con↗