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Fractional quantum Hall effect

The fractional quantum Hall effect (FQHE) is a quantum mechanical phenomenon observed in two-dimensional electron gases (2DEGs) at low temperatures and in the presence of strong perpendicular magnetic fields, where the Hall conductance exhibits quantized plateaus at rational fractional multiples of the fundamental quantum e^2/h, such as \nu = 1/3, $2/5, and other fractions with odd or even denominators.^[1] This effect reveals a novel state of matter characterized by strong electron-electron interactions that lead to the formation of quasiparticles with fractional electric charge, such as e/3, and exotic statistics intermediate between fermions and bosons, known as anyons.^[2] Unlike the integer quantum Hall effect, which arises from non-interacting electrons filling Landau levels, the FQHE requires Coulomb interactions to dominate, resulting in an incompressible quantum fluid that resists density changes and supports dissipationless edge transport.^[3] Discovered in 1982 by Daniel C. Tsui and Horst L. Störmer, in collaboration with Arthur C. Gossard, through experiments on high-mobility 2DEGs confined in GaAs/AlGaAs heterostructures at Bell Laboratories, the FQHE initially appeared as unexpected minima in the longitudinal resistivity and corresponding fractional plateaus in the Hall resistance at filling factors below 1/2.^[1] These observations, conducted at temperatures near 0.1 K and magnetic fields up to 12 T, defied explanations based on the integer effect and highlighted the role of interactions in high-purity semiconductor systems.^[1] In 1983, Robert B. Laughlin developed a theoretical framework using a trial many-body wavefunction, \psi(z_i) = \prod_{i<j} (z_i - z_j)^m \exp\left(-\sum_i |z_i|^2/4\ell^2\right), which accurately describes the ground state at principal fractions like \nu = 1/m (m odd integer) as an incompressible fluid pierced by correlated flux quanta, predicting the existence of fractionally charged quasiparticles and their braiding statistics.^[1] The FQHE has since been observed in various material systems, including graphene and bilayer structures, and more recently the fractional quantum anomalous Hall effect in moiré materials like graphene/hBN superlattices without external magnetic fields, extending to even-denominator states like \nu = 5/2 that may host non-Abelian anyons for fault-tolerant quantum computing.^[3]^[4] Its study has advanced condensed matter physics by exemplifying topological order, where ground-state degeneracy and quasiparticle properties are robust against perturbations, and has implications for precision resistance metrology and fundamental tests of quantum mechanics.^[5] The 1998 Nobel Prize in Physics was awarded to Tsui, Störmer, and Laughlin for this discovery and its explanation, underscoring its role in revealing collective quantum behaviors beyond single-particle pictures.^[2]

Fundamentals

Overview of the Quantum Hall Effect

The quantum Hall effect manifests in two-dimensional electron systems subjected to strong perpendicular magnetic fields at cryogenic temperatures, typically in the millikelvin range to minimize thermal broadening of energy levels.^[6] These systems include inversion layers at the Si/SiO₂ interface in metal-oxide-semiconductor field-effect transistors (MOSFETs) and, more commonly for high-precision studies, two-dimensional electron gases (2DEGs) confined at the interface of GaAs/AlGaAs heterostructures grown by molecular beam epitaxy. In such heterostructures, electrons from a doped AlGaAs layer transfer to the undoped GaAs channel, forming a high-density 2DEG (n ≈ 10^{11}–10^{12} cm^{-2}) with mobility exceeding 10^6 cm²/Vs, essential for observing sharp quantization features.^[7] Similar 2DEGs have been realized in graphene, where Dirac fermions exhibit relativistic dynamics under magnetic fields. In a perpendicular magnetic field B, the continuous electronic density of states in the 2DEG splits into discrete Landau levels due to cyclotron motion. The energy of the nth Landau level (n = 0, 1, 2, ...) is given by

E_n = \hbar \omega_c \left(n + \frac{1}{2}\right),

where \omega_c = eB / m^* is the cyclotron frequency and m^* is the effective electron mass.^[7] Each Landau level accommodates a degeneracy of eB / h electrons per unit area, leading to a filling factor \nu = n h / (e B), with n the areal electron density.^[7] As B increases or n decreases, \nu decreases, and when \nu is an integer, the Fermi level lies in the gap between filled and empty Landau levels, resulting in the integer quantum Hall effect (IQHE). In the IQHE, the Hall resistance R_{xy} exhibits plateaus at R_{xy} = h / (\nu e^2) for integer \nu = 1, 2, 3, \dots, while the longitudinal resistance R_{xx} vanishes at these plateaus, reflecting dissipationless edge transport and bulk insulating behavior. This quantization, precise to parts in 10^{10}, arises from the topological invariance of the filled Landau levels and has been verified in diverse 2D systems.^[6] Experimentally, IQHE is observed using samples patterned in the van der Pauw geometry, a cross-shaped configuration with four ohmic contacts for simultaneous measurement of current I, Hall voltage V_H, and longitudinal voltage V_L. For high-precision studies in high-mobility samples (\mu > 10^6 cm²/Vs), dilution refrigerators providing temperatures below 100 mK, superconducting magnets with B up to 20 T, and low-noise electronics are typically employed to resolve sharp quantized plateaus; the effect was first discovered in 1980 by Klaus von Klitzing using Si MOSFETs at approximately 1.5 K and B ≈ 10 T, establishing the foundational setup for subsequent studies.^[7] At higher magnetic fields, deviations from integer filling appear, signaling the emergence of fractional quantization.^[6]

Emergence of Fractional Quantization

The fractional quantum Hall effect (FQHE) extends the phenomenology of the integer quantum Hall effect by revealing quantized Hall resistance plateaus at non-integer filling factors \nu = p/q, where p and q are coprime integers with q > 1 and q initially odd.^[8] These plateaus occur in high-mobility two-dimensional electron systems under strong perpendicular magnetic fields and low temperatures, distinguishing the FQHE through its reliance on electron correlations beyond simple single-particle orbital filling.^[9] At these fractional filling factors, the Hall resistance R_{xy} takes precise quantized values given by R_{xy} = h / (\nu e^2).^[8] For instance, at \nu = 1/3, the observed plateau is R_{xy} = 3 [h](/page/H+) / e^2, representing a resistance approximately three times larger than the fundamental quantum [h](/page/H+) / e^2.^[8] Concurrently, the longitudinal resistance R_{xx} exhibits deep minima, often approaching zero within experimental resolution, signaling the formation of gapped, incompressible states where charge transport becomes dissipationless.^[8] This energy gap in R_{xx} persists over a finite range of magnetic fields and densities, underscoring the robustness of these fractional states against weak disorder.^[9] The emergence of FQHE requires conditions where electron-electron interactions dominate over the single-particle dynamics of the integer case. Specifically, the Coulomb interaction energy scale e^2 / (\epsilon \ell) must exceed the cyclotron energy \hbar \omega_c, with the magnetic length defined as \ell = \sqrt{\hbar / (e B)}.^[9] In typical semiconductor heterostructures, this regime is accessed at high magnetic fields (around 5–10 T) and millikelvin temperatures, where the ratio of interaction to cyclotron energy approaches or surpasses unity, enabling collective many-body effects.^[9] These fractional plateaus initially puzzled researchers, as they defied explanation within the non-interacting Landau level framework that successfully described integer fillings; simple orbital occupancy could not account for denominators like q = 3 without invoking strong correlations.^[8] The observed incompressibility suggested an underlying quantum liquid ground state, with low-energy excitations manifesting as quasiparticles or quasiholes carrying fractional electric charge e/q.^[10] For the \nu = 1/3 state, these excitations bear charge \pm e/3, providing a basic conceptual framework for the fractional quantization while highlighting the exotic nature of the system's elementary charges.^[10]

Historical Development

Initial Discovery and Experiments

The fractional quantum Hall effect was first observed in 1982 by Daniel C. Tsui, Horst L. Störmer, and Arthur C. Gossard at Bell Laboratories. Using high-mobility two-dimensional electron gases confined in GaAs/AlGaAs heterostructures grown by molecular beam epitaxy, they measured magnetotransport properties at temperatures below 4 K and magnetic fields around 10 T, revealing quantized Hall resistance plateaus at the filling factor ν = 1/3 alongside vanishing longitudinal resistance.^[8] These observations marked a departure from the integer quantum Hall effect, as the plateaus appeared at fractional values of the filling factor in the extreme quantum limit where only the lowest Landau level was occupied. Confirmation and extension of these findings came swiftly from the same group and others in 1983. A. M. Chang, M. A. Paalanen, D. C. Tsui, H. L. Störmer, and J. C. M. Hwang conducted low-temperature measurements (65–770 mK) on similar high-mobility GaAs/AlGaAs samples, observing sharp Hall plateaus at ν = 2/3 with activated transport behavior, where the activation energy peaked near integer and primary fractional fillings.^[11] Concurrently, H. L. Störmer, A. Chang, D. C. Tsui, J. C. M. Hwang, A. C. Gossard, and M. B. Serrano reported quantized plateaus at additional fractions, including ν = 3/5, suggesting multiple series of fractional states based on odd-integer denominators.^[12] To resolve weaker and more numerous fractional states, experimental techniques advanced throughout the 1980s with the adoption of dilution refrigerators achieving temperatures below 100 mK, enabling clearer resolution of plateaus that were broadened by thermal effects at higher temperatures. Cleaner samples with mobilities exceeding 10^6 cm²/V·s became essential, as disorder from impurities significantly smeared the quantized features and suppressed the effect. Developments in the late 1980s also revealed even-denominator fractional states, with initial hints of plateaus at fillings like ν = 1/4 emerging in high-quality samples under extreme conditions. The prominent even-denominator state at ν = 1/2, initially ambiguous due to its compressible nature in single layers, gained clear experimental identification in double-layer systems during the 1990s.^[13] Early experiments faced significant challenges, including the stringent requirements for sample quality to minimize scattering and achieve the necessary low disorder for fractional quantization to manifest. Additionally, edge effects in the Hall bar geometries could contribute to apparent broadening or shifts in resistance measurements, complicating the interpretation of bulk properties.^[8]

Major Theoretical Milestones

The theoretical understanding of the fractional quantum Hall effect (FQHE) began to take shape in 1983 with Robert B. Laughlin's seminal proposal, which introduced a gauge argument to explain the observed fractional Hall plateaus at filling factor ν=1/3. Laughlin posited that the ground state is a strongly correlated incompressible quantum fluid, characterized by a trial wavefunction that captures the correlations among electrons, leading to fractionally charged quasiparticle excitations with charge e/3. This framework shifted the view from perturbative treatments to a many-body correlated picture, resolving the puzzle of fractional quantization beyond simple band structure effects.^[10] Shortly thereafter, F. Duncan M. Haldane extended this idea in 1983 by developing a hierarchy concept for the FQHE states. Haldane proposed that daughter states arise from the condensation of quasielectrons or quasiholes from a parent state, such as the Laughlin state, into new incompressible fluids, generating a sequence of fractions like 2/5 or 2/7. This hierarchical structure provided a systematic way to account for the observed sequence of fractional fillings, emphasizing the role of quasiparticle interactions in building higher-order states.^[14] A major advance came in 1989 with J. K. Jain's introduction of the composite fermion model, which reinterpreted the FQHE as an integer quantum Hall effect of emergent composite fermions formed by binding electrons to an even number 2p of magnetic flux quanta. This mapping transforms the strongly interacting electron system into a Fermi sea of composite fermions at zero effective field for ν=1/2, explaining a broad range of observed fractions through integer fillings of these quasiparticles. The model unified the Laughlin and hierarchy pictures while predicting new states and transport properties.^[15] In the 1990s, further refinements addressed specific even-denominator and non-Abelian states. A mean-field theory for the half-filled Landau level at ν=1/2, developed by B. I. Halperin, Patrick A. Lee, and N. Read in 1993, described the composite fermion Fermi sea using Chern-Simons gauge fields, predicting compressible behavior with finite compressibility and longitudinal conductivity.^[16] Complementing this, Greg Moore and N. Read proposed in 1991 a Pfaffian wavefunction for the ν=5/2 state, introducing non-Abelian statistics for quasiparticles through p-wave pairing of composite fermions, which has implications for topological quantum computing.^[17] These works expanded the theoretical toolkit to include compressible phases and exotic braiding properties. Recent milestones up to 2025 have focused on numerical validations strengthening these models. Exact diagonalization studies have confirmed the energetic favorability of Laughlin and Jain states for small systems, with overlaps exceeding 0.99 between trial wavefunctions and exact ground states for ν=1/3 up to 20 electrons. Density matrix renormalization group (DMRG) methods have extended these validations to larger systems on cylinders, reproducing the hierarchy and revealing short-range correlations consistent with composite fermion predictions for ν=2/5.^[18] Additionally, experimental interferometry in the 2010s provided hints of anyonic braiding, such as phase shifts in Fabry-Pérot setups at ν=5/2 indicative of non-Abelian statistics, though full confirmation remains ongoing.^[19] In 2025, the ν=5/2 state was observed in trilayer graphene heterostructures with a remarkably large energy gap of up to several K, providing a more stable platform for future probes of its non-Abelian nature.^[20] These numerical and experimental efforts have solidified the theoretical foundations while guiding searches for robust non-Abelian phases.

Theoretical Models

Laughlin Wavefunctions and Trial States

In 1983, Robert B. Laughlin proposed a variational wavefunction to describe the ground state of the two-dimensional electron gas at filling factor \nu = 1/m, where m is an odd positive integer, providing a theoretical explanation for the observed fractional quantization in the quantum Hall effect.^[10] This ansatz captures the strong electron correlations by enforcing a relative angular momentum of at least m between any pair of electrons, ensuring they avoid each other as if each carries m units of magnetic flux.^[10] The Laughlin wavefunction in the lowest Landau level is expressed in complex coordinates z_j = x_j + i y_j (with magnetic length \ell) as

\Psi_m(\{z_i\}) = \prod_{i < j} (z_i - z_j)^m \exp\left( -\sum_k \frac{|z_k|^2}{4\ell^2} \right),

where the polynomial factor generates the required antisymmetry for fermions when m is odd, and the Gaussian ensures projection onto the lowest Landau level.^[10] This form yields zero interaction energy for short-range pseudopotentials that penalize relative angular momenta less than m, making it an exact ground state for such idealized interactions.^[10] For realistic Coulomb interactions, it serves as a highly accurate trial state, with properties like the correlation hole—where the electron density vanishes within a distance \sim \ell \sqrt{m} of any electron—emerging from the wavefunction's structure.^[10] The normalization of the Laughlin wavefunction and its density profile can be understood through a plasma analogy, where the logarithm of |\Psi_m|^2 maps to the electrostatic energy of a classical two-dimensional one-component plasma of charges at positions z_i, interacting logarithmically and confined by a uniform background.^[10] In this analogy, at inverse temperature \beta = m, the plasma reaches equilibrium with uniform density \nu = 1/m, justifying the filling factor and providing an intuitive picture of the incompressible fluid state.^[10] To describe excitations, Laughlin constructed quasihole states by inserting an additional flux quantum at a point \eta in the complex plane, modifying the wavefunction to

\Psi_m^{\text{qh}}(\{z_i\}; \eta) = \prod_i (\eta - z_i) \prod_{i < j} (z_i - z_j)^m \exp\left( -\sum_k \frac{|z_k|^2}{4\ell^2} \right).

^[10] These quasiholes carry fractional charge -e/m and create a depletion in electron density around \eta, with energy scaling as the logarithm of the system size, consistent with the incompressibility of the ground state.^[10] When two quasiholes are adiabatically braided around each other, they acquire a statistical phase of $2\pi / m beyond the Aharonov-Bohm contribution, demonstrating abelian anyonic statistics.^[10] While the Laughlin wavefunction exactly solves models with hard-core interactions projecting to relative angular momentum m, it approximates the true ground state for Coulomb repulsion, with variational energies closely matching numerical diagonalizations for \nu = 1/3.^[10] This ansatz was later generalized to form a hierarchy of states at other fillings by condensing quasielectrons or quasiholes into secondary Laughlin states.^[14]

Composite Fermion Construction

The composite fermion theory provides a unified theoretical framework for understanding the fractional quantum Hall effect (FQHE) by transforming the strongly interacting electrons in a magnetic field into weakly interacting composite fermions. In this approach, each electron is bound to an even number $2p of magnetic flux quanta through a singular gauge transformation, effectively attaching vortices to the electron wavefunction. This binding converts the original fermions into composite fermions, which experience a reduced effective magnetic field and behave as if they form integer quantum Hall states.^[15] The flux attachment is implemented by multiplying the electron wavefunction by a Jastrow factor \prod_{i<j} (z_i - z_j)^{2p}, where z_i are the complex coordinates of the electrons in the plane, and p is a positive integer. This attachment endows each composite fermion with an effective charge -e (same as the electron) but alters the statistical phase upon exchange due to the enclosed flux. The resulting effective magnetic field experienced by the composite fermions is B^* = B - 2p n \phi_0, where B is the external field, n is the electron density, and \phi_0 = h/e is the flux quantum. Consequently, the filling factor for composite fermions is \nu^* = \nu / (1 - 2p \nu), with \nu = n h / (e B) being the electron filling factor.^[21] Jain's construction maps the FQHE states to integer quantum Hall states of these composite fermions at filling \nu^*. Specifically, incompressible FQHE states occur at \nu = \nu^* / (2p \nu^* \pm 1), where \nu^* is an integer, allowing the composite fermions to occupy \nu^* filled effective Landau levels. This mapping explains the observed fractional Hall plateaus as manifestations of the integer quantum Hall effect in the transformed system. The corresponding trial wavefunction for the FQHE state is

\Psi_\nu = P_{\rm LLL} \left[ \prod_{i<j} (z_i - z_j)^{2p} \Psi_{\nu^*}^{\rm IQHE} \right],

where P_{\rm LLL} projects onto the lowest Landau level, and \Psi_{\nu^*}^{\rm IQHE} is the Slater determinant wavefunction for \nu^* filled Landau levels of non-interacting fermions. This form captures the correlations responsible for the FQHE and recovers the Laughlin wavefunction as a special case when \nu^* = 1.^[21] In the mean-field approximation, the composite fermion theory also describes compressible states, such as at \nu = 1/2 for p=1, where B^* = 0 and the composite fermions form a Fermi sea in zero effective field. This metallic state arises from the partial filling of the composite fermion Landau levels and provides a natural explanation for the observed even-denominator compressibility at half-filling.

Hierarchy of Fractional States

Principal Jain Sequences at Odd Denominators

The principal Jain sequences describe a series of fractional quantum Hall states at filling factors \nu = n/(2n+1), where n is a positive integer, emerging from the integer quantum Hall effect of composite fermions formed by attaching two units of magnetic flux to each electron.^[22] These states represent the primary odd-denominator fractions observed in the lowest Landau level, with prominent examples including \nu = 1/3 for n=1, \nu = 2/5 for n=2, and \nu = 3/7 for n=3.^[22] The composite fermion picture provides a unified framework for these states, mapping the strongly interacting electron system to non-interacting fermions in an effective magnetic field.^[15] In these states, the elementary quasiparticles carry a fractional charge of e/(2n+1), where e is the electron charge, consistent with the topological order of the ground state.^[22] For instance, at \nu = 1/3, quasiparticles have charge e/3, while at \nu = 2/5, the charge is e/5.^[15] This fractionalization arises from the flux attachment, leading to excitations that behave as anyons with Abelian statistics. The stability of these states is characterized by activation energy gaps \Delta, which separate the incompressible ground state from charged excitations and are typically in the range of 0.1 to 0.3 e^2/(\varepsilon \ell), where \varepsilon is the dielectric constant and \ell is the magnetic length.^[23] These gaps decrease with increasing n due to enhanced screening and reduced effective interaction strength in higher composite fermion Landau levels. Experimental observations confirm the sequence up to n \approx 10, such as states near \nu = 10/21, in high-mobility GaAs heterostructures. At the edges of these states, the low-energy excitations form a chiral Luttinger liquid, where fractional charges propagate along chiral modes without backscattering in the ideal case.^[24] For the principal sequence with n > 1, the edge consists of one charged mode carrying the Hall conductance and n-1 neutral modes, enabling fractional charge transport with non-Fermi liquid correlations.^[25] The construction with positive flux attachment (even number of fluxes, p=1) applies to \nu < 1/2, while states above half-filling, such as \nu = 2/3 = 1 - 1/3, are described by negative flux attachment or particle-hole conjugation, preserving the sequence symmetry across the \nu = 1/2 Fermi sea.^[26] Experimental confirmation of the principal sequence came through Hall resistance plateaus observed in the 1980s and 1990s in modulation-doped GaAs/AlGaAs heterostructures, with states at \nu = 2/5 and $3/7 reported as early as 1984–1987.^[27] The energy gaps were quantified via the temperature-activated peaks in the longitudinal resistance R_{xx}, showing minima that deepen with decreasing temperature and revealing the incompressible nature of these fractions.

Higher-Order and Even Denominator States

The Haldane-Halperin hierarchy provides a systematic framework for generating higher-order fractional quantum Hall states beyond the primary Laughlin and Jain sequences by positing that quasiparticles or quasiholes from a parent incompressible state condense into a new Laughlin-like state, forming daughter states at more complex rational filling factors.^[14]^[28] This iterative process begins with a parent state at filling ν₀ (typically an odd-denominator Laughlin state like ν=1/3) and involves attaching fluxes to quasiparticles, leading to a hierarchy of abelian topological phases characterized by their K-matrix description.^[29] The general filling factor in this construction is given by the continued fraction ν = 1 / [m ± 1/(2p₁ ± 1/(2p₂ ± ⋯))], where m is an odd integer, the p_i are positive integers, and the signs indicate the type of condensation (quasihole or quasielectron).^[29]^[30] Examples of such higher-order states include ν=2/7, obtained by condensing quasiholes of the ν=1/3 Laughlin state into a bosonic Laughlin state at effective filling 1/2, and ν=3/8, which arises in the second Landau level through a similar quasielectron condensation process from the same parent, exhibiting a small energy gap on the order of 10 mK.^[29] These states enrich the hierarchy by introducing additional levels of quasiparticle excitations, with their wavefunctions constructed via conformal field theory correlators that integrate over the positions of the condensed quasiparticles.^[29] The hierarchy thus predicts a dense spectrum of incompressible states at fractions like 4/11 and 5/13, which have been observed experimentally and fit within continued fraction expansions of the filling factor.^[29] Even-denominator fractional quantum Hall states represent a distinct class within this extended landscape, often arising outside the strict odd-denominator hierarchy and involving pairing instabilities or composite fermion descriptions. The prominent ν=5/2 state, observed in the second Landau level after filling the lowest level completely, is theorized to realize non-Abelian statistics through the Moore-Read Pfaffian wavefunction, given by

\Psi_{5/2} = \mathrm{Pf}\left( \frac{1}{z_i - z_j} \right) \prod_{i<j} (z_i - z_j)^2 \exp\left( -\sum_k \frac{|z_k|^2}{4\ell^2} \right),

where Pf denotes the Pfaffian antisymmetrizer, the product enforces pairing, and the Gaussian factor accounts for the magnetic length ℓ. This state hosts Ising anyons as quasiparticle excitations, enabling braiding operations with potential applications in topological quantum computing, though debates persist between the Pfaffian and its particle-hole conjugate, the anti-Pfaffian. At half-filling of the lowest Landau level, ν=1/2, the system forms a compressible state described as a Fermi sea of composite fermions, where electrons bind an even number of vortices to behave as fermions in zero effective field, resulting in gapless metallic transport rather than an incompressible fluid. This contrasts with gapped hierarchy states and underscores particle-hole symmetry considerations at half-filling, where conjugate states like ν=1/2 and ν=3/2 share topological properties but differ in stability due to Landau level mixing. Higher-order and even-denominator states generally feature smaller excitation gaps compared to principal sequences, rendering them more susceptible to disorder and Landau level mixing effects that can destabilize the topological order.^[29] Numerical studies in the 2020s, employing exact diagonalization and density matrix renormalization group methods on systems up to 20–30 electrons, have provided support for the Moore-Read Pfaffian at ν=5/2 under realistic Coulomb interactions, while highlighting competition with the anti-Pfaffian in disordered environments and interfaces between the two orders.^[31] Recent experiments as of 2025 have observed even-denominator states at ν = 3/10, 3/8, and 3/4 in ultrahigh-quality GaAs 2D hole systems, and developing states at ν = 1/6 and 1/8 in dilute GaAs electron systems, further enriching the hierarchy.^[32]^[33]

Experimental Evidence

Hall Resistance Measurements

Hall resistance measurements in the fractional quantum Hall effect (FQHE) are typically conducted using standard Hall bar geometries, where a two-dimensional electron gas is patterned into a long rectangular channel with multiple voltage probes along the sides and ends for multi-terminal transport characterization.^[34] In these setups, a small dc current is driven longitudinally along the channel, while the transverse Hall resistance R_{xy} and longitudinal resistance R_{xx} are measured as functions of perpendicular magnetic field B or gate voltage to tune the electron density.^[35] At low temperatures (typically below 100 mK) and high fields (above 5 T), R_{xy} exhibits quantized plateaus at values R_{xy} = \frac{h}{\nu e^2}, where \nu = p/q is the filling factor with integers p and q, accompanied by minima in R_{xx} approaching zero, confirming the incompressible nature of the FQHE states. The quantization of these plateaus has been verified with extraordinary precision, reaching relative uncertainties below $10^{-9} of h/e^2, enabling their use as resistance standards. Post-1990 advancements in cryogenic current comparator (CCC) bridges, which employ superconducting quantum interference devices (SQUIDs) to compare Hall resistances against integer quantum Hall references or the von Klitzing constant, have facilitated these high-accuracy measurements by nulling currents with ratios up to $10^8.^[36] For instance, direct comparisons between fractional states at \nu = 1/3 and integer plateaus demonstrate universality within parts per billion, underscoring the topological robustness of the FQHE. The temperature and magnetic field dependence of these resistances provides insight into the excitation gaps of FQHE states. Specifically, the peaks in R_{xx} at the plateau centers follow a thermal activation form R_{xx} \propto \exp(-\Delta / 2k_B T), where \Delta is the energy gap separating the ground state from quasiparticle excitations, and k_B is Boltzmann's constant; fitting this behavior yields \Delta values that decrease with increasing temperature or disorder. For the principal \nu = 1/3 state in GaAs heterostructures, experimental gaps are on the order of \Delta_{1/3} \approx 0.2 \, e^2 / \epsilon \ell, where \epsilon is the dielectric constant and \ell = \sqrt{\hbar / eB} is the magnetic length, though actual measured values are smaller (around 0.25 K or ~20 μeV) due to finite layer thickness and screening effects.^[37] These gaps, inferred from activation energies, also allow estimation of quasiparticle charges e^* = e/q through their scaling with Coulomb energy.^[37] Disorder from impurities or interface roughness plays a crucial role in broadening Landau levels into bands of extended and localized states, directly influencing the width of the quantized plateaus. In the scaling theory of the quantum Hall transition, extended states at critical energies enable dissipationless transport, while surrounding localized states trap electrons, leading to finite plateau widths proportional to the localization length \xi \propto |E - E_c|^{-\nu}, where E_c is the critical energy and \nu \approx 2.3 is the localization exponent observed in both integer and fractional regimes.^[38] Increased disorder narrows the extended state regions, widening plateaus but potentially suppressing fragile FQHE states if localization dominates.^[39] The observation of FQHE plateaus and associated gaps shows global consistency across diverse material systems, demonstrating the universality of the effect beyond traditional GaAs/AlGaAs heterostructures. In graphene encapsulated in hexagonal boron nitride, robust fractional states at \nu = 1/3, 2/3 emerge in multi-terminal Hall measurements up to fields of 35 T, with gaps comparable to GaAs.^[40] Similarly, by the 2020s, FQHE has been confirmed in InAs-based quantum wells and edge-confined structures, where high-mobility two-dimensional electron gases exhibit quantized R_{xy} at odd-denominator fractions like \nu = 1/3, 2/5, extending the phenomenon to narrower-bandgap semiconductors with potential for topological applications.^[41]

Interferometry and Quasiparticle Probes

One of the earliest direct probes of fractional charges in the fractional quantum Hall effect (FQHE) came from shot noise measurements in quantum point contacts. In 1997, experiments at filling factor \nu = 1/3 revealed excess noise consistent with quasiparticles carrying charge e/3, where e is the elementary electron charge, under Poissonian statistics. These results provided unambiguous evidence for the fractional nature of charge carriers, as the noise power scaled with the fractional charge rather than the full electron charge. Subsequent shot noise studies extended this to higher fractions, confirming the theoretical predictions of Laughlin quasiparticles. Fabry-Pérot interferometry emerged in the 2000s as a powerful tool to probe both the charge and interference patterns of FQHE quasiparticles through Aharonov-Bohm (AB) oscillations. In these devices, edge states are split and recombined around a central region acting as an interferometer, where the phase accumulated includes a term e^* \Phi / h, with e^* the quasiparticle charge and \Phi the magnetic flux. At \nu = 1/3, AB oscillations with a flux period corresponding to e^* = e/3 were observed, confirming the fractional charge via the interference visibility and phase evolution. This technique highlighted the coherent propagation of quasiparticles along edges, though early implementations faced limitations from disorder and finite temperature effects. Advances in the 2010s and 2020s shifted focus to Mach-Zehnder interferometers, which enable the measurement of anyonic braiding statistics by encircling quasiparticles in a two-arm setup. For abelian anyons at \nu = 1/3, these experiments captured the statistical phase of $2\pi/3 through interference patterns modulated by the enclosed quasiparticle number. At even-denominator fillings like \nu = 5/2, interferometry revealed hints of non-Abelian statistics, such as even-odd conductance patterns dependent on the parity of enclosed quasiparticles, suggesting topological degeneracy in the interference signal. Tunneling spectroscopy at FQHE edges has further corroborated fractional charges by examining current-voltage characteristics across point contacts. At \nu = 1/5, weak tunneling experiments detected quasiparticle charge e/5 through power-law scaling in the differential conductance, reflecting the anyonic nature of edge excitations. Despite these successes, interferometry in FQHE systems grapples with challenges like decoherence from environmental coupling and unwanted bulk-edge interactions, which can alter the effective interference area and suppress visibility. Recent graphene-based interferometers, leveraging the material's superior coherence and tunability, have yielded cleaner signals in 2023–2025 experiments, enabling sharper AB oscillations and braiding phase measurements at fractions like \nu = 2/5.^[42]^[43]^[44]

Significance and Applications

Topological Properties

The ground states of the fractional quantum Hall effect (FQHE) manifest topological order, a paradigm of quantum matter characterized by long-range entanglement in fully gapped, incompressible liquids without spontaneous symmetry breaking.^[45] This order arises from strong electron correlations in two dimensions under a perpendicular magnetic field, leading to robust topological properties invariant under smooth deformations.^[45] For Abelian FQHE states, such as the Laughlin state at filling factor \nu = 1/m, topological order is classified by the K-matrix formalism, where the simplest case features a scalar K = m that encodes the filling factor and quasiparticle braiding properties.^[45] A hallmark of this topological order is the ground state degeneracy, which depends on the system's topology. On a torus, the FQHE ground state at \nu = 1/q exhibits q-fold degeneracy, resulting from the adiabatic insertion of magnetic fluxes through the torus handles, which probes the anyonic statistics of quasiparticles.^[46] For example, the \nu = 1/3 Laughlin state shows threefold degeneracy on the torus.^[46] Quasiparticle excitations in topologically ordered FQHE states are anyons, obeying fractional statistics intermediate between bosons and fermions. In the Laughlin state at \nu = 1/q, quasiholes acquire a statistical phase \theta / \pi = 1/q upon exchange, as derived from the Berry phase accumulated during adiabatic transport around one another.^[47] Non-Abelian FQHE states, such as the Moore-Read Pfaffian at \nu = 5/2, host more exotic anyons like Ising-type quasiparticles, where braiding yields non-commuting unitary matrices—for instance, the exchange matrix includes elements akin to \sigma_x in the degenerate fusion space.^[48] The effective low-energy description of topological order in FQHE is provided by Chern-Simons gauge theory, where the Lagrangian takes the form

\mathcal{L} = \frac{k}{4\pi} \epsilon^{\mu\nu\lambda} a_\mu \partial_\nu a_\lambda + \bar{\psi} (i \gamma^\mu D_\mu - m) \psi,

with a statistical gauge field a_\mu coupled to matter fields \psi, and the level k determining the attached flux quanta that transmute electrons into composite particles realizing fractional statistics. Unlike superconductors, which involve U(1) symmetry breaking and gapless Goldstone modes, FQHE topological order preserves all symmetries of the Hamiltonian while maintaining an excitation gap to both charged and neutral modes, with emergent phenomena driven solely by inter-electron interactions.^[45]

Relevance to Quantum Technologies

The fractional quantum Hall effect (FQHE) holds significant promise for quantum technologies, particularly through its hosting of non-Abelian anyons that enable fault-tolerant topological quantum computing. In the proposed Moore-Read Pfaffian description of the ν=5/2 state, Ising anyons are predicted to emerge as quasiparticles whose braiding operations encode quantum information in a degenerate fusion space, effectively realizing Majorana-like zero modes that are robust against local noise and decoherence due to the topological protection of the ground state degeneracy.^[49]^[50] This non-Abelian statistics, akin to that in the Moore-Read Pfaffian state, allows for universal quantum computation via anyon braiding without the need for error correction at the physical qubit level.^[49] Further advancing this paradigm, the proposed particle-hole conjugate of the k=3 Read-Rezayi state at ν=2/5 is theorized to support parafermionic zero modes, which generalize Majorana modes and provide larger fusion spaces for more efficient encoding of logical qubits. Proposals for realizing and manipulating these anyons include interferometric readout schemes, where Fabry-Pérot interferometers detect braiding phases by measuring interference patterns of edge currents enclosing quasiparticles, enabling non-destructive qubit measurements.^[51] In the 2010s, experimental efforts by groups including Microsoft and collaborators at Freie Universität Berlin demonstrated progress toward anyon-based devices, such as hybrid setups combining FQHE edge states with superconductors to probe non-Abelian signatures in GaAs heterostructures.^[52]^[53] Despite these advances, key challenges persist, including the small energy gaps of approximately 0.1 K in FQHE states, which demand ultra-low temperatures and high magnetic fields, limiting practical scalability.^[54] Additionally, integrating large numbers of anyons while maintaining coherence and controlling braiding paths remains difficult due to disorder and finite-size effects in 2D systems. In the 2020s, hybrid semiconductor-superconductor platforms have shown progress in emulating FQHE-like non-Abelian states, using InAs nanowires proximity-coupled to superconductors to realize tunable Majorana modes at higher temperatures and without perpendicular fields.^[55] As of 2024, advances include the experimental realization of a fractional quantum anomalous Hall state in photonic systems, providing a dissipationless platform for topological quantum computing at higher temperatures and without perpendicular magnetic fields.^[56] Beyond computing, FQHE contributes to quantum metrology by providing precise standards for fractional charge quantization, as verified through shot noise measurements confirming e/3 quasiparticle charges with high accuracy, aiding resistance metrology traceable to fundamental constants.^[3] In graphene, valley-resolved FQHE states enable spintronic applications, where valley isospin textures support dissipationless spin currents and tunable ferromagnetism for valleytronic devices.^[57]

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