arXiv subjects

Stefan Faelt

Publications and source records attributed to Stefan Faelt.

11 recordsLinked to original sources

arXiv2026

Spin polarisation signatures of Fractionally Charged Skyrmions in Fractional Quantum Hall states

We investigate spin polarisation and low-energy excitations in fractional quantum Hall (FQH) states using cavity-polariton spectroscopy of high-mobility GaAs quantum wells. By measuring the optical coupling strength of interband Landau-level excitations over the range $1/3 \le \nu \le 1$, we extract the spin polarisation of the electron system as a function of filling factor. Complete suppression of the oscillator strength of the lowest energy excitation, characteristic of singlet trion formation in fully polarised systems, is reported for the first time in this regime. At large magnetic fields, fully polarised FQH states exhibit symmetric depolarisation away from their quantised fillings, analogous to Skyrmionic behaviour near $\nu=1$. The depolarisation follows an empirical law $S=\nu^*$, where $S$ is the number of spin flips per added magnetic flux quantum and $\nu^*$ the effective Composite Fermion filling factor. We interpret this behaviour as evidence for Minimal Fractionally Charged Skyrmions (MFCS) formed from bound spin-flip and quasiparticle excitations.

cond-mat.mes-hall↗

arXiv2024

Spin polarization of Quantum Hall states for filling factors 1 < v < 2 measured with microcavity polaritons

Spin polarization measurements were performed in three 2D Electron Gases in GaAs with densities n = 9.1, 7.2 and 6.5 x10^10 cm-2, in the quantum Hall regime. Full spin polarization at v = 1 surrounded by rapid depolarization due to Skyrmion formation was observed in all devices, consistent with past measurements. Depolarization of the v = 4/3, 8/5 states and repolarization of the v = 5/3 state was also measured, in remarkable agreement with a non-interacting, disorder-free Composite Fermion model. Optical power and temperature dependent measurements of the v = 1 state suggest a regime of non-linear optics.

cond-mat.mes-hall↗

arXiv2023

Spin-selective strong light-matter coupling in a 2D hole gas-microcavity system

The interplay between time-reversal symmetry breaking and strong light-matter coupling in 2D gases brings intriguing aspects to polariton physics. This combination can lead to polarization/spin selective light-matter interaction in the strong coupling regime. In this work, we report such a selective strong light-matter interaction by harnessing a 2D gas in the quantum Hall regime coupled to a microcavity. Specifically, we demonstrate circular-polarization dependence of the vacuum Rabi splitting, as a function of magnetic field and hole density. We provide a quantitative understanding of the phenomenon by modeling the coupling of optical transitions between Landau levels to the microcavity. This method introduces a control tool over the spin degree of freedom in polaritonic semiconductor systems, paving the way for new experimental possibilities in light-matter hybrids.

cond-mat.mes-hall↗

arXiv2020

A polariton electric field sensor

We experimentally demonstrate a dipolar polariton based electric field sensor. We tune and optimize the sensitivity of the sensor by varying the dipole moment of polaritons. We show polariton interactions play an important role in determining the conditions for optimal electric field sensing, and achieve a sensitivity of 0.12 V-m$^{-1}$-Hz$^{-0.5}$. Finally we apply the sensor to illustrate that excitation of polaritons modify the electric field in a spatial region much larger than the optical excitation spot.

cond-mat.mes-hall↗

arXiv2018

Strong interactions between dipolar polaritons

Nonperturbative coupling between cavity photons and excitons leads to formation of hybrid light-matter excitations termed polaritons. In structures where photon absorption leads to creation of excitons with aligned permanent dipoles, the elementary excitations, termed dipolar polaritons, are expected to exhibit enhanced interactions. Here, we report a substantial increase in interaction strength between dipolar polaritons as the size of the dipole is increased by tuning the applied gate voltage. To this end, we use coupled quantum well structures embedded inside a microcavity where coherent electron tunneling between the wells controls the size of the excitonic dipole. Modifications of the interaction strength are characterized by measuring the changes in the reflected intensity of light when polaritons are driven with a resonant laser. Factor of 6.5 increase in the interaction strength to linewidth ratio that we obtain indicates that dipolar polaritons could be used to demonstrate a polariton blockade effect and thereby form the building blocks of many-body states of light.

cond-mat.mes-hall↗

arXiv2017

Polaron-Polaritons in the Integer and Fractional Quantum Hall Regimes

Elementary quasi-particles in a two dimensional electron system can be described as exciton-polarons since electron-exciton interactions ensures dressing of excitons by Fermi-sea electron-hole pair excitations. A relevant open question is the modification of this description when the electrons occupy flat-bands and electron-electron interactions become prominent. Here, we perform cavity spectroscopy of a two dimensional electron system in the strong-coupling regime where polariton resonances carry signatures of strongly correlated quantum Hall phases. By measuring the evolution of the polariton splitting under an external magnetic field, we demonstrate the modification of electron-exciton interactions that we associate with phase space filling at integer filling factors and polaron dressing at fractional filling factors. The observed non-linear behavior shows great promise for enhancing polariton-polariton interactions.

cond-mat.mes-hall↗

arXiv2017

Optical probing of the intrinsic spin Hall effect in a high mobility GaAs p-doped quantum well

We report on the detection of the intrinsic spin Hall effect in a modulation doped Al-GaAs/GaAs/AlGaAs heterostructure bounded by a self-aligned pn-junction, fabricated by the cleaved edge overgrowth method. Light emission due to the recombination of electrons and spin-polarized holes was generated and mapped with a spatial resolution of one micrometer. An edge accumulated spin polarization of up to 11% was measured, induced solely by application of an electric Field perpendicular to the pn-junction. Using a quantum dot structure as light source, a linear dependence of the effective spin polarization, and with that the dominance of the spin Hall effect, with the electric field is seen. Spatially resolved spectroscopy from an epitaxially fabricated LED is demonstrated to be a valuable tool to probe the edge states of electron and hole gases in reduced dimensions.

physics.app-ph↗

arXiv2017

Squeezed thermal reservoirs as a resource for a nano-mechanical engine beyond the Carnot limit

The efficient conversion of thermal energy to mechanical work by a heat engine is an ongoing technological challenge. Since the pioneering work of Carnot, it is known that the efficiency of heat engines is bounded by a fundamental upper limit, the Carnot limit. Theoretical studies suggest that heat engines may be operated beyond the Carnot limit by exploiting stationary, non-equilibrium reservoirs that are characterized by a temperature as well as further parameters. In a proof-of-principle experiment, we demonstrate that the efficiency of a nano-beam heat engine coupled to squeezed thermal noise is not bounded by the standard Carnot limit. Remarkably, we also show that it is possible to design a cyclic process that allows for extraction of mechanical work from a single squeezed thermal reservoir. Our results demonstrate a qualitatively new regime of non-equilibrium thermodynamics at small scales and provide a new perspective on the design of efficient, highly miniaturized engines.

cond-mat.mes-hall↗

arXiv2016

Measurement of spin coherence using Raman scattering

Ramsey interferometry provides a natural way to determine the coherence time of most qubit systems. Recent experiments on quantum dots however, demonstrated that dynamical nuclear spin polarization can strongly influence the measurement process, making it difficult to extract the $T_2^*$ coherence time using optical Ramsey pulses. Here, we demonstrate an alternative method for spin coherence measurement that is based on first-order coherence of photons generated in spin-flip Raman scattering. We show that if a quantum emitter is driven by a weak monochromatic laser, Raman coherence is determined exclusively by spin coherence, allowing for a direct determination of spin $T_2^*$ time. When combined with coherence measurements on Rayleigh scattered photons, our technique enables us to identify coherent and incoherent contributions to resonance fluorescence, and to minimize the latter. We verify the validity of our technique by comparing our results to those determined from Ramsey interferometry for electron and heavy-hole spins.

quant-ph↗

arXiv2015

Generation of heralded entanglement between distant hole spins

Quantum entanglement emerges naturally in interacting quantum systems and plays a central role in quantum information processing. Remarkably, it is possible to generate entanglement even in the absence of direct interactions: provided that which path information is erased, weak spin-state dependent light scattering can be used to project two distant spins onto a maximally entangled state upon detection of a single photon. Even though this approach is necessarily probabilistic, successful generation of entanglement is heralded by the photon detection event. Here, we demonstrate heralded quantum entanglement of two quantum dot heavy-hole spins separated by 5 meters using single-photon interference. Thanks to the long coherence time of hole spins and the efficient spin-photon interface provided by self-assembled quantum dots embedded in leaky microcavity structures, we generate 2300 entangled spin pairs per second, which represents an improvement approaching three orders of magnitude as compared to prior experiments. Delayed two-photon interference scheme we developed allows for efficient verification of quantum correlations. Our results lay the groundwork for the realization of quantum networks in semiconductor nanostructures. Combined with schemes for transferring quantum information to a long-lived memory qubit, fast entanglement generation we demonstrate could also impact quantum repeater architectures.

cond-mat.mes-hall↗

arXiv2011

Quantum quench of Kondo correlations in optical absorption

The interaction between a single confined spin and the spins of a Fermionic reservoir leads to one of the most spectacular phenomena of many body physics -- the Kondo effect. Here we report the observation of Kondo correlations in optical absorption measurements on a single semiconductor quantum dot tunnel-coupled to a degenerate electron gas. In stark contrast to transport experiments, absorption of a single photon leads to an abrupt change in the system Hamiltonian and a quantum quench of Kondo correlations. By inferring the characteristic power law exponents from the experimental absorption line-shapes, we find a unique signature of the quench in the form of an Anderson orthogonality catastrophe, originating from a vanishing overlap between the initial and final many-body wave-functions. We also show that the power-law exponents that determine the degree of orthogonality can be tuned by applying an external magnetic field which gradually turns the Kondo correlations off. Our experiments demonstrate that optical measurements on single artificial atoms offer new perspectives on many-body phenomena previously studied exclusively using transport spectroscopy. Moreover, they initiate a new paradigm for quantum optics where many-body physics influences electric field and intensity correlations.

cond-mat.mes-hall↗