arXiv subjects

Vladimir Umansky

Publications and source records attributed to Vladimir Umansky.

At least 19 recordsLinked to original sources

arXiv2026

Emergent Chiral Metal Phase in Compressible Quantum Hall Fluids

We report transmitted conductance measurements between a source and reflection-less contacts connected to a compressible quantum Hall fluid with filling fraction $ν$. We observe that total sum of transmitted conductances universally approaches Hall conductance $ν(e^2/h)$. The universality of this sum rule is established experimentally across integer and fractional quantum Hall regimes, remaining invariant under variation in temperatures, sample geometries, material qualities and quasi-particle interactions. Chiral transport in compressible quantum Hall fluids, characterized by suppressed dissipative transport with a distinct handedness, is confirmed by floating contact measurements. Consequently, this sum rule emerges as a conduction law of the chiral metal phase. Theoretically, we argue that time-reversal-symmetry breaking under a strong magnetic field within this chiral metal gives rise unidirectional trajectories of carriers in a nearly flat potential landscape with point-like disorder potentials, a regime realized when the screening length is smaller than the magnetic length. Within this chiral framework, longitudinal resistance does not originate from bulk dissipation but primarily from the equilibration of electrochemical potentials at the contacts. Our study introduces a new paradigm of chiral transport across a broad class of gapless two-dimensional systems characterized by short-range screening and broken time-reversal symmetry.

cond-mat.mes-hall↗

arXiv2026

Observation of Time-Domain Braiding of Non-Abelian Anyons at $ν= 5/2$ State

Unlike elementary particles, which obey either bosonic or fermionic exchange statistics, certain quasiparticles, known as anyons, are predicted to exhibit Abelian or non-Abelian braiding statistics. While braiding Abelian anyons modifies the wavefunction by a 'statistical phase', braiding non-Abelian anyons implements a unitary transformation of the state within a degenerate subspace of states. Experimental evidence of non-Abelian braiding has thus far remained elusive. Here, we report a 'time-domain braiding' signature of non-Abelian anyons in the $ν= 5/2$ fractional quantum Hall state, by extending our previously demonstrated approach with Abelian anyons at $ν= 1/3$. Our approach is based on measurements of the current fluctuations arising from weak partitioning of a highly dilute one-dimensional edge mode. We independently probe the partition noise of the downstream charged mode and also that of the upstream neutral mode. These independent measurements agree with our theoretical predictions for 'time-domain braiding' of the downstream Abelian and the upstream non-Abelian anyons, respectively, in the 'particle-hole Pfaffian' topological order. Together, these results provide evidence for the presence of non-Abelian anyons.

cond-mat.mes-hall↗

arXiv2026

Constriction-induced modulation of charging energy in a quantum Hall cavity

Electronic Fabry-Pérot interferometers (FPIs) operating in the fractional quantum Hall regime are a key platform for probing anyonic braiding statistics, yet interpreting their interference signals is complicated by Coulomb charging effects, which are commonly treated as parasitic, static properties governed by the cavity's geometry and electrostatics. Here, using a gate-defined quantum Hall cavity tuned to the Coulomb-dominated regime, we demonstrate that the charging energy is in fact strongly and non-monotonically modulated by the magnetic field, varying by up to 60% over a range of only 100 mT. The effect appears exclusively when the quantum point contacts (QPCs) forming the cavity are weakly pinched off, i.e., in the strong cavity-to-lead coupling regime. By correlating the charging energy modulation with the QPC magneto-conductance, we attribute this behavior to field-dependent changes in local compressibility and electrostatic screening between the cavity and the leads, driven by the formation of incompressible fractional quantum Hall states within the constrictions. This result establishes QPC constrictions of quantum Hall cavities as active electrostatic elements rather than passive boundaries, revealing a dynamic screening mechanism, with direct consequences for the interpretation of interference measurements and the extraction of anyonic statistics.

cond-mat.mes-hall↗

arXiv2026

Coherence, long-range transport and nuclear polarization in a driven-dissipative dark exciton condensate

We report direct evidence for macroscopic coherence in a condensate of dark dipolar excitons in coupled quantum wells and show that its formation follows a non-equilibrium, driven-dissipative mechanism. The condensation transition is governed by gain-loss competition, in which the exceptionally long lifetime of dark excitons enables their dominance in mode selection. Condensate formation is revealed by photoluminescence darkening, changes in radiative recombination channels, and the emergence of long-range hydrodynamic transport manifested by propagation of density (sound) modes over millimeter-scale distances. The buildup of dark exciton density induces dynamic nuclear polarization, which closes the dark-bright exciton gap, Δ, via the Overhauser field. This leads to nuclear spin polarization across the entire mesa, far beyond the optically excited region, and produces pronounced hysteresis behavior. At Δ~ 0 the gap is locked and the condensate loss are minimal, resulting in a second threshold manifested as a photoluminescence blueshift. Coherence is revealed through interference between incident and boundary-reflected exciton currents, producing spatial modulation of the photoluminescence from the radiative reservoir and enabling extraction of the condensate coherence length. These results establish dark excitons as a platform for coherent quantum fluids in a driven-dissipative, strongly interacting regime with electrical tunability, bridging the physics of polariton condensates and matter-like excitonic systems.

cond-mat.mes-hall↗

arXiv2025

Anyonic Braiding in a Chiral Mach-Zehnder Interferometer

Fractional quantum statistics are the defining characteristic of anyons. Measuring the phase generated by an exchange of anyons is challenging, as standard interferometry setups -- such as the Fabry-Pérot interferometer -- suffer from charging effects that obscure the interference signal. Here, we present the observation of anyonic interference and exchange phases in an optical-like Mach-Zehnder interferometer based on co-propagating interface modes. By avoiding backscattering and deleterious charging effects, this setup enables pristine and robust Aharonov-Bohm interference without any phase slips. At various fractional filling factors, the observed flux periodicities agree with the fundamental fractionally charged excitations that correspond to Jain states and depend only on the bulk topological order. To probe anyonic statistics, we use a small, charged top-gate in the interferometer bulk to induce localized quasiparticles without modifying the Aharonov-Bohm phase; however, with introducing periodic phase slips. The magnitude of the observed phase slips and their signs align with the expected value at filling 1/3, but their direction shows systematic deviations at fillings 2/5 and 3/7. Control over added individual quasiparticles in this design is essential for measuring the coveted non-Abelian statistics in the future.

cond-mat.mes-hall↗

arXiv2025

Coherent Bunching of Anyons and their Dissociation in Interference Experiments

Aharonov-Bohm (AB) interference of fractional quasiparticles in the quantum Hall Effect generally reveals their elementary charge ($e^*$)[1-15]. Recently, our interferometry experiments with several particle states reported flux periods of $ΔΦ=(e/e^*)Φ_0$ the flux quantum) at moderate temperatures[16]. Here, we report interference measurements of particle-hole conjugated states at filling factor $ν=2/3, 3/5, 4/7$, revealing unexpected flux periodicities of $ΔΦ=Φ_0/ν$. The measured shot noise Fano factor (F) of the partitioned quasiparticles in each of the interferometers quantum point contacts (QPCs), was found to be, $F=ν$[17], and not that of the elementary charge, $F=e^*/e$[18,19]. These observations point to interference of bunched (clustered) elementary quasiparticles as coherent pairs, triples, and quadruplets, respectively. A small metallic gate (top gate, TG), deposited in the center of the interferometer bulk, forming an antidot (or a dot) when charged, thus introducing local quasiparticles at the (anti)dots perimeter. Surprisingly, such charging led to a dissociation of the bunched quasiparticles and thus recovered the conventional flux periodicity set by the elementary quasiparticles charge. However, the shot noise Fano factor (of each QPC) consistently remained at $F=ν$, possibly due to the neutral modes accompanied the conjugated states. The two observations - bunching and debunching (or dissociation) - are not expected by current theories. Similar effects may likely arise in Jains particle states (at lower temperatures) and at even denominator FQH states[20]

cond-mat.mes-hall↗

arXiv2024

Approaching ideal visibility in singlet-triplet qubit operations using energy-selective tunneling-based Hamiltonian estimation

We report energy selective tunneling readout-based Hamiltonian parameter estimation of a two-electron spin qubit in a GaAs quantum dot array. Optimization of readout fidelity enables a single-shot measurement time of 16 on average, with adaptive initialization and efficient qubit frequency estimation based on real-time Bayesian inference. For qubit operation in a frequency heralded mode, we observe a 40-fold increase in coherence time without resorting to dynamic nuclear polarization. We also demonstrate active frequency feedback with quantum oscillation visibility, single-shot measurement fidelity, and state initialization fidelity up to 97.7%, 99%, and over 99.7%, respectively. By pushing the sensitivity of the energy selective tunneling-based spin to charge conversion to the limit, the technique is useful for advanced quantum control protocols such as error mitigation schemes, where fast qubit parameter calibration with a large signal-to-noise ratio is crucial.

quant-ph↗

arXiv2024

True decoherence-free-subspace derived from a semiconductor double quantum dot Heisenberg spin-trimer

Spins in solid systems can inherently serve as qubits for quantum simulation or quantum information processing. Spin qubits are usually prone to environmental magnetic field fluctuations; however, a spin qubit encoded in a decoherence-free-subspace (DFS) can be protected from certain degrees of environmental noise depending on the specific structure of the DFS. Here, we derive the "true" DFS from an antiferromagnetic Heisenberg spin-1/2 trimer, which protects the qubit states against both short- and long-wavelength magnetic field fluctuations. We define the spin trimer with three electrons confined in a gate-defined GaAs double quantum dot (DQD) where we exploit Wigner-molecularization in one of the quantum dots. We first utilize the trimer for dynamic nuclear polarization (DNP), which generates a sizable magnetic field difference, $ΔB_\mathrm{z}$, within the DQD. We show that large $ΔB_\mathrm{z}$ significantly alters the eigenspectrum of the trimer and results in the "true" DFS in the DQD. Real-time Bayesian estimation of the DFS energy gap explicitly demonstrates protection of the DFS against short-wavelength magnetic field fluctuations in addition to long-wavelength ones. Our findings pave the way toward compact DFS structures for exchange-coupled quantum dot spin chains, the internal structure of which can be coherently controlled completely decoupled from environmental magnetic fields.

cond-mat.mes-hall↗

arXiv2024

Topological Thermal Hall Conductance of Even Denominator Fractional States

The even denominator fractional quantum Hall (FQH) states $ν=5/2$ and $ν=7/2$ have been long predicted to host non-abelian quasiparticles (QPs). Their present energy-carrying neutral modes are hidden from customary conductance measurements and thus motivate thermal transport measurements, which are sensitive to all energy-carrying modes. While past `two-terminal' thermal conductance ($k_{2t}T$) measurements already proved the non-Abelian nature of the $ν=5/2$ FQH state, they might have been prone to a lack of thermal equilibration among the counter-propagating edge modes. Here, we report a novel thermal Hall conductance measurement of the $ν=5/2$ and $ν=7/2$ states, being insensitive to equilibration among edge modes. We verify the state's non-Abelian nature, with both states supporting a single upstream Majorana edge mode (hence, a particle-hole Pfaffian order). While current numerical works predict a different topological order, this contribution should motivate further theoretical work.

cond-mat.mes-hall↗

arXiv2024

Giant Hyperfine Interaction between a Dark Exciton Condensate and Nuclei

We study the interaction of a dark exciton Bose-Einstein condensate with the nuclei in GaAs/AlGaAs coupled quantum wells and find clear evidence for nuclear polarization buildup that accompanies the appearance of the condensate. We show that the nuclei are polarized throughout the mesa area, extending to regions which are far away from the photoexcitation area, and persisting for seconds after the excitation is switched off. Photoluminescence measurements in the presence of RF radiation reveal that the hyperfine interaction between the nuclear and electron spins is enhanced by two orders of magnitude. We suggest that this large enhancement manifests the collective nature of the N-excitons condensate, which amplifies the interaction by a factor of sqrt{N}.

cond-mat.mes-hall↗

arXiv2024

Scaling tunnelling noise in the fractional quantum Hall effect tells about renormalization and breakdown of chiral Luttinger liquid

The fractional quantum Hall (FQH) effect provides a paradigmatic example of a topological phase of matter. FQH edges are theoretically described via models belonging to the class of chiral Luttinger liquid (CLL) theories [1 (Wen, 2007)]. These theories predict exotic properties of the excitations, such as fractional charge and fractional statistics. Despite theoretical confidence in this description and qualitative experimental confirmations, quantitative experimental evidence for CLL behaviour is scarce. In this work, we study tunnelling between edge modes in the quantum Hall regime at the filling factor $ν=1/3$. We present measurements at different system temperatures and perform a novel scaling analysis of the experimental data, originally proposed in Ref. [2 (Schiller et al., 2022)]. Our analysis shows clear evidence of CLL breakdown - above a certain energy scale. In the low-energy regime, where the scaling behaviour holds, we extract the property called the scaling dimension and find it heavily renormalized compared to naïve CLL theory predictions. Our results show that decades-old experiments contain a lot of previously overlooked information that can be used to investigate the physics of quantum Hall edges. In particular, we open a road to quantitative experimental studies of (a) scaling dimension renormalization in quantum point contacts and (b) CLL breakdown mechanisms at an intermediate energy scale, much smaller than the bulk gap.

cond-mat.mes-hall↗

arXiv2023

Measuring statistics-induced entanglement entropy with a Hong-Ou-Mandel interferometer

Despite its ubiquity in quantum computation and quantum information, a universally applicable definition of quantum entanglement remains elusive. The challenge is further accentuated when entanglement is associated with other key themes, e.g., quantum interference and quantum statistics. Here, we introduce two novel motifs that characterize the interplay of entanglement and quantum statistics: an 'entanglement pointer' and a 'statistics-induced entanglement entropy'. The two provide a quantitative description of the statistics-induced entanglement: (i) they are finite only in the presence of quantum entanglement underlined by quantum statistics; (ii) their explicit form depends on the quantum statistics of the particles (e.g., fermions, bosons, anyons). We have experimentally implemented these ideas by employing an electronic Hong-Ou-Mandel interferometer fed by two highly diluted electron beams in an integer quantum Hall platform. Performing measurements of auto-correlation and cross-correlation of current fluctuations of the scattered beams (following 'collisions'), we quantify the statistics-induced entanglement by experimentally accessing the entanglement pointer and the statistics-induced entanglement entropy. Our theoretical and experimental approaches pave the way to study entanglement in various correlated platforms, e.g., those involving anyonic Abelian and non-Abelian states.

cond-mat.mes-hall↗

arXiv2023

Heat Conductance of the Quantum Hall Bulk

The Quantum Hall Effect (QHE) is a prototypical realization of a topological state of matter. It emerges from a subtle interplay between topology, interactions, and disorder. The disorder enables the formation of localized states in the bulk that stabilize the quantum Hall states with respect to the magnetic field and carrier density. Still, the details of the localized states and their contribution to transport remain beyond the reach of most experimental techniques. Here, we describe an extensive study of the bulk's heat conductance. Using a novel 'multi-terminal' short device (on a scale of $10 μm$), we separate the longitudinal thermal conductance, $κ_{xx}T$ (due to bulk's contribution), from the topological transverse value $κ_{xy}T$, by eliminating the contribution of the edge modes. When the magnetic field is tuned away from the conductance plateau center, the localized states in the bulk conduct heat efficiently ($κ_{xx}T \propto T$), while the bulk remains electrically insulating. Fractional states in the first excited Landau level, such as the $ν=7/3$ and $ν=5/2$, conduct heat throughout the plateau with a finite $κ_{xx} T$. We propose a theoretical model that identifies the localized states as the cause of the finite heat conductance, agreeing qualitatively with our experimental findings.

cond-mat.mes-hall↗

arXiv2023

Anomalous Aharonov-Bohm Interference in the Presence of Edge Reconstruction

Interferometry is a vital tool for studying fundamental features in the quantum Hall effect (QHE). For instance, Aharonov-Bohm (AB) interference in a quantum Hall interferometer can probe the wave-particle duality of electrons and quasiparticles. Here, we report an unusual AB interference in a quantum Hall Fabry-Pérot interferometer (FPI), whose Coulomb interactions were suppressed with a grounded drain in the interior bulk of the FPI. In a descending filling factor from $ν=3$ to $ν\approx5/3$, the magnetic field periodicity, which corresponded to a single 'flux quantum,' agreed accurately with the enclosed area of the FPI. However, in the filling range, $ν\approx5/3$ to $ν=1$, the field periodicity increased markedly, apriori suggesting a drastic shrinkage of the AB area. Moreover, the modulation gate voltage periodicity decreased abruptly at this range. We attribute these unexpected observations to a ubiquitous edge reconstruction, leading to dynamical area changing with the field and a modified modulation gate-edge capacitance. These results are reproducible and support future interference experiments with a QHE-FPI.

cond-mat.mes-hall↗

arXiv2023

Partitioning of Diluted Anyons Reveals their Braiding Statistics

Correlations of partitioned particles carry essential information about their quantumness. Partitioning full beams of charged particles leads to current fluctuations, with their autocorrelation (namely, shot noise) revealing the particle' charge. This is not the case when the partitioned particle beams are diluted. Bosons or fermions will exhibit particles antibunching (due to their sparsity and discreteness). However, when diluted anyons, such as the quasiparticles in fractional quantum Hall states, are partitioned in a narrow constriction, their autocorrelation reveals an essential aspect of their exchange statistics: their braiding phase. Here, we describe detailed measurements of weak partitioned, highly diluted, one-dimension-like edge modes of the one-third filling fractional quantum Hall state. The measured autocorrelation agrees with our theory of braiding anyons in the time-domain (instead of braiding in space); with a braiding phase 2$θ$=2$π$/3, without any fitting parameters. Our work offers a relatively straightforward and simple method to observe the braiding statistics of other exotic anyonic states, such as non-abelian states, without resorting to complex interference experiments.

cond-mat.mes-hall↗

arXiv2023

Electron Pairing of Interfering Interface-Based Edge Modes

The remarkable Cooper-like pairing phenomenon in the Aharonov-Bohm interference of a Fabry-Perot interferometer (FPI)$\rm{-}$operating in the integer quantum Hall regime$\rm{-}$remains baffling. Here, we report the interference of paired electrons employing 'interface edge modes'. These modes are born at the interface between the bulk of the FPI and an outer gated region tuned to a lower filling factor. Such configuration allows toggling the spin and the orbital of the Landau level (LL) of the edge modes at the interface. We find that electron pairing occurs only when the two modes (the interfering outer and the first inner) belong to the same spinless LL.

cond-mat.mes-hall↗

arXiv2023

Probing two-qubit capacitive interactions beyond bilinear regime using dual Hamiltonian parameter estimations

We report the simultaneous operation and two-qubit coupling measurement of a pair of two-electron spin qubits that are actively decoupled from quasistatic nuclear noise in a GaAs quadruple quantum dot array. Coherent Rabi oscillations of both qubits (decay time $\approx$2 μs; frequency few MHz) are achieved by continuously tuning the drive frequency using rapidly converging real-time Hamiltonian estimators. By state conditional exchange oscillation measurements, we also observe strong two-qubit capacitive interaction (> 190 MHz). We show that the scaling of the capacitive interaction with respect to intra-qubit exchange energies is stronger than the bilinear form, consistent with recent theoretical predictions. We observe a high ratio (>16) between coherence and conditional phase-flip time, which supports the possibility of generating high-fidelity and fast quantum entanglement between encoded spin qubits using a simple capacitive interaction.

quant-ph↗

arXiv2022

The Role of Spin-Flip Collisions in a Dark Exciton Condensate

We show that a Bose-Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spin flipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperature-dependent blueshift of the exciton emission line. Measurements under in-plane magnetic field allows us to preferentially modify the bright excitons density, and determine their role in the system dynamics. We find that their interaction with the condensate leads to its depletion. We present a simple rate equations model, which well-reproduces the observed temperature, power, and magnetic field dependence of the exciton density.

cond-mat.mes-hall↗