F. E. Camino

Realization of a Laughlin quasiparticle interferometer: Observation of fractional statistics

We report experimental realization of a novel Laughlin quasiparticle interferometer, where quasiparticles of the 1/3 fractional quantum Hall fluid encircle an island of the 2/5 fluid. Interference fringes are observed as conductance oscillations, as in the Aharonov‐Bohm effect. For the first time, we observe an Aharonov‐Bohm superperiod of five magnetic flux quanta (5h/e) through the island. The corresponding 2e charge period is confirmed in calibrated gate experiments. These results are interpreted in terms of the fractional statistical phase acquired by a quasiparticle of the 1/3 fluid encircling the quasiparticles of the 2/5 fluid.

Aharonov-Bohm superperiod in a laughlin quasiparticle interferometer

We report an Aharonov-Bohm superperiod of five magnetic flux quanta (5h/e) observed in a Laughlin quasiparticle interferometer, where an edge channel of the 1/3 fractional quantum Hall fluid encircles an island of the 2/5 fluid. This result does not violate the gauge invariance argument of the Byers-Yang theorem because the magnetic flux, in addition to affecting the Aharonov-Bohm phase of the encircling 1/3 quasiparticles, creates the 2/5 quasiparticles in the island. The superperiod is accordingly understood as imposed by the anyonic statistical interaction of Laughlin quasiparticles.

e/3 Laughlin quasiparticle primary-filling nu=1/3 interferometer.

We report experimental realization of a quasiparticle interferometer where the entire system is in 1/3 primary fractional quantum Hall state. The interferometer consists of chiral edge channels coupled by quantum-coherent tunneling in two constrictions, thus enclosing an Aharonov-Bohm area. We observe magnetic flux and charge periods h/e and e/3, equivalent to creation of one quasielectron in the island. Quantum theory predicts a 3h/e flux period for charge e/3, integer statistics particles. Accordingly, the observed periods demonstrate the anyonic statistics of Laughlin quasiparticles.

Quantum transport in electron Fabry-Perot interferometers

We report experiments on Fabry-Perot electron interferometers in the integer quantum Hall regime. The GaAs/AlGaAs heterostructure devices consist of two constrictions defined by etch trenches in 2D electron layer, enclosing an approximately circular island. The interferometer is formed by counterpropagating chiral edge channels coupled by tunneling in the two constrictions. Interference fringes are observed as conductance oscillations, similar to the Aharonov-Bohm effect. Front gates deposited in etch trenches allow to fine-tune the device and to change the constriction filling f relative to the bulk filling. Quantum-coherent conductance oscillations are observed on the f = 1 - 4 plateaus. On plateau f we observe f conductance oscillations per fundamental flux period h/e. This is attributed to the dominance of the electron-electron Coulomb interaction, effectively mixing Landau level occupation. On the other hand, the back-gate charge period is the same (one electron) on all plateaus, independent of filling. This is attributed to the self-consistent electrostatics in the large electron island. We also report dependence of the oscillation period on front-gate voltage for f = 1, 2 and 4 for three devices. We find a linear dependence, with the slope inversely proportional to f for f = 1 and 2.

Transport in the Laughlin quasiparticle interferometer: Evidence for topological protection in an anyonic qubit

We report experiments on temperature and Hall voltage bias dependence of the superperiodic conductance oscillations in the novel Laughlin quasiparticle interferometer, where quasiparticles of the 1/3 fractional quantum Hall fluid execute a closed path around an island of the 2/5 fluid. The amplitude of the oscillations fits well the quantum-coherent thermal dephasing dependence predicted for a two point-contact chiral edge channel interferometer in the full experimental temperature range 10.2<T<141 mK. The temperature dependence observed in the interferometer is clearly distinct from the behavior in single-particle resonant tunneling and Coulomb blockade devices. The 5h/e flux superperiod, originating in the anyonic statistical interaction of Laughlin quasiparticles, persists to a relatively high T~140 mK. This temperature is only an order of magnitude less than the 2/5 quantum Hall gap. Such protection of quantum logic by the topological order of fractional quantum Hall fluids is expected to facilitate fault-tolerant quantum computation with anyons.

Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells

We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of 4 meV to zero, at a field of 35 kVcm. This decrease, due to the fieldinduced deformation of the exciton wave function, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separation. However, a new theory that considers the modification of screening with that separation is needed to account for the observed dependence on excitation power of the individual energies of the two exciton components.

Flux-period scaling in the Laughlin quasiparticle interferometer

We report experiments on electron interferometer devices in the quantum Hall regime, where edge channels circle a two-dimensional (2D) electron island. The main confinement is produced by etch trenches, into which front-gate metal is deposited. We find a linear dependence of the Aharonov-Bohm period on gate voltage for electrons (integer filling

Low-field quantum Hall transport in an electron Fabry-Perot interferometer: Dependence of constriction filling on front-gate voltage

f=1

Experimental realization of Laughlin quasiparticle interferometers

) and for Laughlin quasiparticles (fractional 2/5 embedded in 1/3). The capacitance of a large 2D electron island with respect to the front gates is approximately proportional to the island radius. Comparing the experimental data for the integer and the fractional fillings and for two samples, we find the magnetic field period and its slope scale with the radius of the Aharonov-Bohm orbit. Analysis of the directly measured integer and fractional slope data allows us to determine the interferometer area in the fractional regime, and thus the Laughlin quasiparticle flux period of

Primary-Filling e/3 Quasiparticle Interferometer

5h∕e