Ever since Milliken's 1 famous experiment it is well known that the electrical charge is quantized in units of the electronic chargee. For that reason, Laughlin's 2 theoretical prediction of the existence of fractionally charged quasi particles, put forward in order to explain the Fractional Quantum Hall (FQH) effect, is very counter intuitive. The FQH effect is a phenomenon that occurs in a Two Dimensional Electron Gas (2DEG) subjected to a strong perpendicular magnetic field. This effect results from the strong interaction among the electrons and consequently current is carried by the above mentioned quasi particles. We directly observed this elusive fractional charge by utilizing a measurement of quantum shot noise. Quantum shot noise results from the discreteness of the current carrying charges and thus is proportional to their charge, Q, and to the average current I, namely, S i =2QI . Our quantum shot noise measurements unambiguously show that current in a 2DEG in the FQH regime, at a fractional filling factor ν ν=1/3, is carried by fractional charge portions e/3; in agreement with Laughlin's prediction.The energy spectrum of a Two Dimensional Electron Gas (2DEG) subjected to a strong perpendicular magnetic field, B, consists of highly degenerate Landau levels with a degeneracy per unit area p=B/φ 0 , with φ 0 =h/e the flux quantum (h being Plank's constant). Whenever the magnetic field is such that an integer number ν (the filling factor) of Landau levels are occupied, that is ν=n s /p equals an integer (n s being the 2DEG areal density), the longitudinal conductivity of the 2DEG vanishes while the Hall conductivity equals νe 2 /h with very high accuracy. This phenomenon is known as the Integer Quantum Hall (IQH) effect 3 . A similar phenomenon occurs at fractional filling factors, namely, when the filling factor equals a rational fraction with an odd denominator q and is known as the Fractional Quantum Hall effect 4 . In contrast to the IQH effect, which is well understood in terms of non interacting electrons, the FQH effect can not be explained in such terms and is believed to result from interactions among the electrons, brought about by the strong magnetic field.Laughlin 2 had argued that the FQH effect could be explained in terms quasi particles of a fractional charge -Q=e/q. Although his theory is consistent with a considerable amount of the experimental data, no experiment directly showing the existence of the fractional charge exists. The early Aharonov-Bohm measurements 5 were proved to be in principle inadequate to reveal the fractional charge 6 . A more recent experiment based on resonant tunneling by Goldman and Su 7 was reproduced and interpreted differently by Franklin et al 8 . The difficulty in such experiments is that the results provide only the average charge per state and not the charge of individual particles. Quantum shot noise, on the other hand, probes the temporal behavior of the current and thus offers a direct way to measure the charge. Indeed, as early as in 1987, Tsui 9 sugges...

We report on the characterization of selectively doped GaAs/AlGaAs heterostructures, grown by an extremely clean molecular beam epitaxy system, which exhibit a Hall mobility of a two dimensional electron gas exceeding 10ϫ10 6 cm 2 /Vs for a wide range of undoped spacer layer thickness ͑50-100 nm͒. A maximum electron mobility of 14.4ϫ10 6 cm 2 /Vs was measured at 0.1 K in a structure with a 68 nm spacer thickness and an areal carrier density of 2.4ϫ10 11 cm Ϫ2. This is the highest electron mobility ever reported, leading to a momentum relaxation mean-free path of ϳ120 m. We present experiments that enable us to distinguish between the main scattering mechanisms. We find that scattering due to background impurities limits electron mobility in our best samples, suggesting that further improvement in structure quality is possible.

Ever since Milliken's 1 famous experiment it is well known that the electrical charge is quantized in units of the electronic charge -e. For that reason, Laughlin's 2 theoretical prediction of the existence of fractionally charged quasi particles, put forward in order to explain the Fractional Quantum Hall (FQH) effect, is very counter intuitive. The FQH effect is a phenomenon that occurs in a Two Dimensional Electron Gas (2DEG) subjected to a strong perpendicular magnetic field. This effect results from the strong interaction among the electrons and consequently current is carried by the above mentioned quasi particles. We directly observed this elusive fractional charge by utilizing a measurement of quantum shot noise. Quantum shot noise results from the discreteness of the current carrying charges and thus is proportional to their charge, Q, and to the average current I, namely, S i =2QI . Our quantum shot noise measurements unambiguously show that current in a 2DEG in the FQH regime, at a fractional filling factor ν ν=1/3, is carried by fractional charge portions e/3; in agreement with Laughlin's prediction.The energy spectrum of a Two Dimensional Electron Gas (2DEG) subjected to a strong perpendicular magnetic field, B, consists of highly degenerate Landau levels with a degeneracy per unit area p=B/φ 0 , with φ 0 =h/e the flux quantum (h being Plank's constant). Whenever the magnetic field is such that an integer number ν (the filling factor) of Landau levels are occupied, that is ν=n s /p equals an integer (n s being the 2DEG areal density), the longitudinal conductivity of the 2DEG vanishes while the Hall conductivity equals νe 2 /h with very high accuracy. This phenomenon is known as the Integer Quantum Hall (IQH) effect 3 . A similar phenomenon occurs at fractional filling factors, namely, when the filling factor equals a rational fraction with an odd denominator q and is known as the Fractional Quantum Hall effect 4 . In contrast to the IQH effect, which is well understood in terms of non interacting electrons, the FQH effect can not be explained in such terms and is believed to result from interactions among the electrons, brought about by the strong magnetic field.Laughlin 2 had argued that the FQH effect could be explained in terms quasi particles of a fractional charge -Q=e/q. Although his theory is consistent with a considerable amount of the experimental data, no experiment directly showing the existence of the fractional charge exists. The early Aharonov-Bohm measurements 5 were proved to be in principle inadequate to reveal the fractional charge The difficulty in such experiments is that the results provide only the average charge per state and not the charge of individual particles. Quantum shot noise, on the other hand, probes the temporal behavior of the current and thus offers a direct way to measure the charge. Indeed, as early as in 1987, Tsui 9 suggested that the quasi particle's charge could in principle be determined by measuring quantum shot noise in the FQH regime. However, no theory...