arXiv subjects

Manfred Bayer

Publications and source records attributed to Manfred Bayer.

At least 19 recordsLinked to original sources

arXiv2026

Semi-device-independent quantum randomness certification in semiconductor spin-noise measurements

Complex solid-state systems are promising platforms for scalable, high-bandwidth quantum random-number generation, yet certifying the quantum origin of their fluctuations remains difficult because the underlying microscopic dynamics are hard to model and validate. Here we demonstrate semi-device-independent quantum randomness certification from semiconductor spin noise, to our knowledge the first such certificate on any spin-noise source, without relying on a microscopic model of the spin system. The untrusted optical source is constrained by an experimentally tested mean-photon-number bound together with a declared analogue-range and per-sample clipping ceiling, while the trusted receiver is described as a calibrated, noisy, coarse-grained homodyne measurement. Using a semidefinite programme with rigorously controlled Fock-space truncation, we certify randomness that remains private against an adversary holding arbitrary quantum side information. Offline analysis yields certified entropy rates of $3.2$--$3.4$\,Gbit/s from a singly charged (In,Ga)As quantum-dot ensemble and $33$\,Mbit/s from $n$-GaAs in a spin-noise-matched detection mode. This exceeds the certified entropy rate of earlier spin-noise generators by more than two orders of magnitude, and the certificate tolerates a resolved per-symbol energy contribution from the solid-state emitter itself rather than requiring a near-vacuum input.

quant-ph↗

arXiv2026

Interaction between Rydberg Excitons in Cuprous Oxide Revealed through Resonant Second Harmonic Generation

We report experimental and theoretical investigations of interacting excitons of the yellow series in cuprous oxide (Cu$_2$O) with principal quantum numbers up to by means of second harmonic generation (SHG). Using picosecond pulsed laser excitation up to 10 GW/cm$^2$ peak intensity we observe a pronounced change of the spectra with increasing pump laser intensity: an energetic shift to lower absolute energies and a spectral broadening. The absolute intensities of the spectral lines scale for low powers with the square of the pump power, but saturates at higher powers. At still higher powers the SHG intensity is actually reduced. To explain these results quantitively, we developed a semi-classical theory of resonant SHG where the process of SHG is fully coherent. The excitons are assumed to be bosons interacting by a distance dependent potential giving rise to both the changes in spectral line shape and the saturation by a $\it{Rydberg}$ blockade. The concomitant measurement of two-photon absorption allows to derive quantitative values for the exciton-exciton interaction. While the results agree in order of magnitude with those calculated by state-of-the art atomic-like van der Waals interaction theory, the scaling with principle quantum number is quite different. As a possible screening by an electron-hole plasma created by three-photon absorption into blue and violet band states could be ruled out, our results point toward fundamental differences between excitons and atoms.

cond-mat.mes-hall↗

arXiv2026

Resonant cooling of nuclear spins by optically-oriented holes in MAPbI$_3$ perovskite crystals

Resonant cooling of nuclear spins by photogenerated spin-oriented holes is demonstrated for MAPbI$_3$ perovskite crystals. It is evidenced by Hanle-effect measurements under helicity-modulated excitation with variable frequency. The resonance position in magnetic field shifts toward higher fields with increasing modulation frequency. The invariance of the Hanle curve upon in-plane sample rotation is consistent with the involvement of $^{207}$Pb nuclei with spin $I = 1/2$, which do not exhibit quadrupolar splitting. The shape of the resonance feature in the Hanle curve reveals that the nuclear spins are cooled by carriers with a negative $g$-factor, consistent with holes. The resonance fields associated with the modulation frequencies exceed the half-width of the weakly localized hole contribution to the Hanle curve, indicating that strongly localized holes are the primary carriers responsible for the nuclear spin cooling.

cond-mat.mes-hall↗

arXiv2026

Hyperfine versus exchange interaction in the spin dynamics of spatially indirect excitons in CsPbI$_{3}$ perovskite nanocrystals

We study the dynamics of recombination, optical orientation, and optical alignment of excitons in ensembles of CsPbI$_{3}$ nanocrystals (NCs), synthesized in a glass matrix. In large NCs with size exceeding 16 nm, the low-energy photoluminescence is contributed by the emission of indirect in real space excitons formed by spatially separated electrons and holes, which are localized at the NC/glass interface. The recombination dynamics of an ensemble of such excitons extends from tens of nanoseconds to microseconds and exhibits a power-law dependence. Their optical alignment and optical orientation reveal a peculiar spin dynamics caused by excitons influenced by the exchange interaction, varying by orders of magnitude. We develop a theory of the polarized photoluminescence of triplet excitons, taking into account the interplay between the electron-hole exchange interaction, their Zeeman effect, and their hyperfine interaction with the nuclei. This model reveals that for the excitons with the smallest exchange splitting we reach the regime, where the exciton fine structure becomes dominated by the hyperfine interaction with the random nuclear spin fluctuations in the NCs.

cond-mat.mes-hall↗

arXiv2026

Layer-dependent Land\'e $g$-factors of electrons, holes, and excitons in two-dimensional Ruddlesden-Popper lead halide perovskites

Two-dimensional Ruddlesden-Popper lead halide perovskites provide a valuable platform for tailoring charge and spin properties through quantum confinement and reduced symmetry. While the electron and hole Land\'e $g$-factors in bulk lead halide perovskites exhibit a universal dependence on the band gap energy, their evolution in two-dimensional perovskites has remained largely unexplored. Here, the Zeeman splittings of electrons and holes in (PEA)$_2$MA$_{n-1}$Pb$_n$I$_{3n+1}$ perovskites with the number of inorganic layers ovarying in the range $n=1,...,8$ are measured by means of the spin-flip Raman scattering and time-resolved Kerr rotation magneto-optical techniques. A systematic evolution of the electron and hole $g$-factors with decreasing layer thickness, which deviates from the universal bulk behavior and reveals confinement-driven trends similar to those observed in perovskite nanocrystals, is found. The experimental results are in good qualitative agreement with empirical tight-binding calculations. The exciton $g$-factors are evaluated from the Zeeman splittings of the exciton resonances in reflectivity measured in pulsed magnetic fields up to 55~T. These results provide comprehensive insight into the spin properties of two-dimensional lead halide perovskites and establish them as a tunable platform for engineering spin-dependent phenomena in quantum-confined semiconductors.

cond-mat.other↗

arXiv2026

Millisecond spin relaxation times of distinct electron and hole subensembles in MA$_x$FA$_{1-x}$PbI$_3$ perovskite crystals

The unique combination of outstanding optical quality and attractive spin properties opens new avenues for optical spin control in hybrid organic-inorganic perovskite semiconductors. Using the optically detected magnetic resonance technique, we study the spins of electrons and holes in mixed-cation MA$_x$FA$_{1-x}$PbI$_3$ single crystals with $x = 0.4$ and 0.8. Multiple distinct spin subensembles with $g$-factors spanning from 2.9 to 3.6 for electrons and from 0.5 to 1.2 for holes are resolved, revealing diverse localization environments. We measure the longitudinal spin relaxation times, $T_1$, reaching 2 ms and remaining in the $\mu$s range even for weakly localized carriers at the cryogenic temperature of 1.6 K. The magnetic-field dependence of $T_1$ is dominated by the random nuclear (Overhauser) fields with strengths of $\sim 0.4-0.8$ mT for electrons and $\sim 4-12$ mT for holes, corresponding to $\mu$s-long correlation times of the hyperfine field determined by carrier hopping between shallow localization sites. The temperature dependence of $T_1$ reveals a weak localization potential of the charge carriers and shows a correlation between $T_1$ and the inhomogeneity of the spin ensemble. These results establish mixed-A-site perovskite single crystals as a promising solid-state platform with long-lived spin states for quantum information applications.

cond-mat.mes-hall↗

arXiv2026

Assessing the classicality of photon echo from excitons in lead halide perovskite nanocrystals

Photon echo (PE) spectroscopy is a powerful technique for probing decoherence mechanisms and charge carrier dynamics in semiconductor systems. Beyond traditional coherence measurements, characterizing the photon statistics of the echo signal is important for assessing its potential in quantum information applications and understanding the underlying quantum mechanical processes. Here, we study the photon statistics of PE signals generated by excitons in ensembles of lead halide perovskite CsPbI$_3$ nanocrystals at cryogenic temperature of 2 K using continuous-variable quantum state optical tomography based on homodyne detection. Pronounced Rabi oscillations of the PE amplitude allow us to evaluate the statistics for various pulse areas in the excitation sequence. The damping of the oscillations with increasing pulse area is attributed to spatial excitation inhomogeneity and excitation-induced dephasing. Despite the large ensemble of optically addressed excitons, the efficiency of generated PE signals is low which is attributed to the complex energy level structure of excitons and non-radiative recombination channels in CsPbI$_3$ nanocrystals. We analyze the statistical characteristics of PE via the second-order correlation function $g^{(2)}(0)$ and the characteristic function for different combinations of the areas of the excitation pulses. Our results show that $g^{(2)}(0) = 1$, and the characteristic function of the PE signal corresponds to classical behavior. The formation of photon echoes as well as $g^{(2)}(0) = 1$ at the echo time is reproduced by a quantized free-space multimode model. Despite the relatively low efficiency, the photon echo exhibits a high degree of coherence and minimal classical noise, consistent with Poissonian statistics.

cond-mat.mes-hall↗

arXiv2026

Millisecond-long electron spin lifetime in CsPbI$_3$ perovskite nanocrystals revealed by optically detected magnetic resonance

Perovskite nanocrystals are a convenient model system for optical spin orientation and manipulation. However, its real potential might be underestimated due to the incomplete knowledge on spin relaxation times, which are obscured by the limited sensitivity of measurement techniques as well as by the insufficient understanding of the spin relaxation mechanisms in perovskites. In this work, we study the spin relaxation of charge carriers in perovskite nanocrystals both experimentally and theoretically. We address the electron and hole spins in CsPbI$_3$ nanocrystals embedded in a glass matrix by the resonant spin inertia technique based on optically detected magnetic resonance. It allows us to determine the longitudinal spin relaxation time $T_1$ separately for electrons and holes, the $g$ factors, and the effective Overhauser field of the nuclear spin bath. At a temperature of 1.6 K, the $T_1$ time for electrons can be as long as 0.9 ms. We reveal the effect of the time-varying nuclear field fluctuations, which enhances the electron spin relaxation at low magnetic fields, and measure a rather long nuclear spin correlation time of about 60 $\mu$s. We develop a model of the spin relaxation in nanocrystals based on a two-LO-phonon Raman process, which explains the observed temperature dependence of the time $T_1$.

cond-mat.mes-hall↗

arXiv2026

Bright-dark exciton splitting in lead halide perovskite crystals accessed via quantum beats in photon echoes

Understanding the fine structure of excitons is crucial for optoelectronic and quantum photonic applications of lead halide perovskites. It is demonstrated that polarization-sensitive photon echo spectroscopy in magnetic field provides a powerful method to access coherent exciton dynamics and reveal their energy level structure, which is hidden by inhomogeneous broadening. Exciton quantum beats observed in both Faraday and Voigt geometries offer a precise probe of the energy splittings among the four 1$s$ exciton states, enabling determination of the fine structure and bright-dark splittings. Application of this technique to bulk mixed halide perovskite crystals FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ reveals a bright-dark exciton splitting of $\Delta_\mathrm{X}=0.46~$meV, along with electron and hole Land\'{e} $g$ factors $g_\mathrm{e}=3.38$ and $g_\mathrm{h}=-1.14$, respectively. The quantum beats persist on timescales of 20--50$~$ps, demonstrating remarkably robust spin and optical coherences at cryogenic temperature of 2$~$K. The decay of the quantum beats of the outer doublet is governed by dephasing due to dispersion of the bright-dark splitting of $\sim0.06~$meV caused by localization potential fluctuations, while dephasing in the bright exciton inner doublet originates from a small zero field splitting of $\sim0.035~$meV due to anisotropic potentials.

cond-mat.mes-hall↗

arXiv2026

Hyperfine interaction of electrons and holes with nuclei probed by optical orientation in MAPbI$_3$ perovskite crystals

Optical orientation of electron and hole spins by circularly polarized light is investigated for MAPbI$_3$ single crystals. The Hanle and polarization recovery effects measured in transverse and longitudinal magnetic fields, respectively, evidence the hyperfine interaction with nuclear spins as the main factor determining the spin dynamics of charge carriers at cryogenic temperatures. The parameters of the nuclear spin fluctuations within the carrier localization volume are evaluated. Dynamic polarization of the nuclear spins is demonstrated by the Overhauser field reaching 5 mT for acting on the electrons and -30 mT for acting on the holes.

cond-mat.mes-hall↗

arXiv2026

Millisecond spin coherence of electrons in semiconducting perovskites revealed by spin mode locking

Long spin coherence times of carriers are essential for implementing quantum technologies using semiconductor devices for which, however, a possible obstacle is spin relaxation. For the spin dynamics, decisive features are the band structure, crystal symmetry, and quantum confinement. Perovskite semiconductors recently have come into focus of studies of their spin states, notivated by efficient optical access and potentially long-living coherence. Here, we report an electron spin coherence time $T_2$ of the order of 1 ms, measured for a bulk FA$_{0.95}$Cs$_{0.05}$PbI$_3$ lead halide perovskite crystal. Using periodic laser pulses, we synchronize the electron spin Larmor precession about an external magnetic field in an inhomogeneous ensemble, the effect known as spin mode locking. It appears as a decay of the optically created ensemble spin polarization within the dephasing time $T_2^*$ of up to 20 ns and its revival during the spin coherence time $T_2$ reaching the millisecond range. This exceptionally long spin coherence time in a bulk crystal is complemented by millisecond-long longitudinal spin relaxation times $T_1$ for electrons and holes, measured by optically-detected magnetic resonance. These long-lasting spin dynamics highlight perovskites as promising platform for the quantum devices with all-optical control.

cond-mat.mtrl-sci↗

arXiv2026

Spin-dependent Raman and Brillouin light scattering on excitons in CsPbBr$_3$ perovskite crystals

The spin properties of excitons and charge carriers in CsPbBr$_3$ lead halide perovskite crystals are investigated by spin-dependent light scattering in magnetic fields up to 10 T. Spin-flip Raman scattering spectra measured under resonant excitation of exciton-polaritons show a rich variety of features provided by the Zeeman splittings of excitons and of electrons and holes interacting with the excitons. The magnitudes and anisotropies of their Land\'e $g$-factors are measured. A detailed consideration of the responsible mechanisms is presented and discussed in relation to the experimental data, in particular on the polarization properties of the Raman spectra. We consider several mechanisms for the combined spin-flip Raman scattering processes involving resident carriers and photoexcited excitons and suggest new ones, involving trions in the intermediate scattering state. A double electron spin-flip caused by the exciton interaction with two localized or donor-bound electrons is revealed. The spectral lines of Brillouin light scattering on exciton-polaritons shift in energy and become polarization-sensitive in magnetic field, evidencing the splitting of the exciton-polariton dispersion.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic switching of exciton lifetime in CrSBr

Exciton dynamics in layered magnetic semiconductors provide a sensitive probe of the interplay between spin order and light-matter interaction. Here, we study thin CrSBr layers using time-resolved photoluminescence spectroscopy in an external magnetic field, revealing a step-like reduction in the exciton lifetime from 11 to 7 ps, during the magnetization flip from the antiferromagnetic to the ferromagnetic phase. The reduction of the exciton lifetime in the ferromagnetic phase persists below the N\'eel temperature, as evidenced by its strong magnetic-field dependence that disappears in the paramagnetic phase. Ab initio calculations reveal a one-dimensional nature of free excitons accompanied by a pronounced change in the oscillator strength across the magnetic phase transition predicting a shorter radiative lifetime of free excitons in the antiferromagnetic phase of CrSBr contradicting the experimental observations. This discrepancy is explained by strong localization of excitons at low tempature. We show both experimentally and theoretically that the observed magnetic switching of the exciton lifetime is attributed to a larger exciton localization volume leading to a larger oscillator strength in the ferromagnetic phase. The results show that disorder-induced localization effects play a key role in exciton dynamics in CrSBr.

cond-mat.mes-hall↗

arXiv2026

Magnetoluminescence of ZnMnSe/BeMnTe heterostructures with type-II band alignment at millikelvin temperatures

The magneto-optical properties of a Zn$_{0.99}$Mn$_{0.01}$Se/Be$_{0.93}$Mn$_{0.07} $Te diluted magnetic semiconductor heterostructure with type-II band alignment are investigated at cryogenic temperatures down to 16 mK. The temperature of the Mn spin system, which at the lowest possible laser power reaches 270 mK, is evaluated from the giant Zeeman splitting of the direct exciton in the Zn$_{0.99}$Mn$_{0.01}$Se layers subject to an external magnetic field. The degree of circular polarization of the direct and indirect optical transitions, induced by the magnetic field, is a sensitive indicator for the laser heating of the Mn spin system. Evidence of spin glass formation in the Mn spin system of the Be$_{0.93}$Mn$_{0.07}$Te layers with the critical temperature of $T_{SG}=400$ mK is found.

cond-mat.mtrl-sci↗

arXiv2025

33 Gbit/s source-device-independent quantum random number generator based on heterodyne detection with real-time FPGA-integrated extraction

We present a high-speed continuous-variable quantum random number generator (QRNG) based on heterodyne detection of vacuum fluctuations. The scheme follows a source-device-independent (SDI) security model in which the entropy originates from quantum measurement uncertainty and no model of the source is required; security depends only on the trusted measurement device and the calibrated discretization, and thus remains valid even under adversarial state preparation. The optical field is split by a 90$^\circ$ optical hybrid and measured by two balanced photodiodes to obtain both quadratures of the vacuum state simultaneously. The analog outputs are digitized using a dual-channel 12-bit analog-to-digital converter operating at a sampling rate of 3.2 GS/s per channel, and processed in real time by an FPGA implementing Toeplitz hashing for randomness extraction. The quantum-to-classical noise ratio was verified through calibrated power spectral density measurements and cross-checked in the time domain, confirming vacuum-noise dominance within the 1.6 GHz detection bandwidth. After extraction, the system achieves a sustained generation rate of $R_{\rm net}= 33.92~\mathrm{Gbit/s}$ of uniformly distributed random bits, which pass all NIST and Dieharder statistical tests. The demonstrated platform provides a compact, FPGA-based realization of a practical heterodyne continuous-variable source-independent QRNG suitable for high-rate quantum communication and secure key distribution systems.

quant-ph↗

arXiv2025

Exciton spin structure in lead halide perovskite semiconductors explored via the spin dynamics in magnetic field

We theoretically investigate the spin structure and spin dynamics of excitons in bulk lead halide perovskite semiconductors with cubic, tetragonal, and orthorhombic crystal symmetry. The exciton spin structure and its modification by an external magnetic field are modeled for different regimes defined by the relative magnitude of the electron-hole exchange interaction (splitting between dark and bright states) and the Zeeman spin splitting. The effects of crystal symmetry and magnetic field orientation with respect to the crystal axes are considered for lead halide perovskite crystals with band gaps in the range 1.4 - 3.5 eV, having different ratios of electron and hole g-factors. For cubic symmetry, in a longitudinal magnetic field, our theory predicts quantum beats between the bright exciton states under linearly polarized excitation and detection, while the dark exciton remains optically inactive. In a transverse magnetic field, all exciton spin states become optically active and can be excited by circularly polarized light. Reduction of the crystal symmetry leads to a zero-field offset of the exciton Larmor precession frequencies, modifying the Zeeman splitting energy dependence on magnetic field. This theoretical framework allows for the extraction of the strength of the exchange interaction and the crystal symmetry. Experimentally, we measure the exciton spin coherence via time-resolved photoluminescence at a temperature of 1.6 K in longitudinal and transverse magnetic fields in orthorhombic MAPbI3 crystals. Polarization beats at the frequency of the bright exciton are observed in both configurations. Comparison with theory indicates that the excitons are in the strong exchange interaction regime, and the reduction of symmetry does not lead to a significant splitting of the exciton spin levels.

cond-mat.other↗

arXiv2025

Non-local synchronization of continuous time crystals in a semiconductor

Synchronization resulting in unified collective behavior of the individual elements of a system that are weakly coupled to each other has long fascinated scientists. Examples range from the periodic oscillation of coupled pendulum clocks to the rhythmic behavior in biological systems. Here we demonstrate this effect in a solid-state platform: spatially remote, auto-oscillating electron-nuclear spin systems in a semiconductor. When two such oscillators separated by up to 40 ${\mu}$m are optically pumped, their individually different frequencies lock to a common value, revealing long-range coherent coupling. For larger separations, the synchronization breaks. The interaction distance matches the electron spin diffusion length, identifying spin transport as the coupling-mediating mechanism and establishing phase coherence over mesoscopic distances. As a consequence, a wide-area optical pump drives all oscillators within the illuminated spot into a single synchronized state, despite their inhomogeneity. This synchronization accounts for the exceptional stability of the resulting auto-oscillations, enabling collective motion in distributed spin systems and paving the way toward coherent spin networks in spintronics.

cond-mat.mes-hall↗

arXiv2025

Rydberg excitons in Cu$_2$O at millikelvin temperatures

Rydberg excitons in the semiconductor Cu$_2$O have been observed in absorption experiments up to a principal quantum number of n = 28 at millikelvin temperatures [1]. Here, we extend the experimental parameter space by variing both temperature and excitation power. In particular, we show that the P excitons close to the band gap react more sensitively to an increase of the excitation power than states of the associated D exciton multiplet, even though the latter are located at comparatively higher energy. This finding is similar to the one observed when applying an external electric field, suggesting that the observed behavior arises from internal electric fields created by charged impurities that are optically ionized. At laser intensities below 1 $\mu$W/cm$^2$, absorption lines of excitons with n=29 are observed.

cond-mat.mes-hall↗