non-abel FI

Non-Abelian Phases in the Fractional Quantum Hall Regime

The fractional quantum Hall effect (FQHE) occurs in a two-dimensional electron gas (2DEG) subjected to a perpendicular magnetic field at low temperature. It is now understood to arise from strong electron-electron interactions. In transport experiments the FHQE is characterized by Hall resistance quantized to rational fractional values of h/e2 and vanishingly small longitudinal resistance. Quasi-particle excitations in the FQHE are called anyons. This is because anyons are neither fermions nor bosons, but rather acquire a phase upon particle exchange. In certain exotic FQHE states (e.g. ν=5/2 and ν=12/5) the excitations are theorized to be even more bizarre. Excitations for these states are believed to obey non-Abelian braiding statistics; repeated exchange of two particles does not simply induce a phase but rather results in a unitary transformation of the wavefunction within a degenerate manifold. The topology of such states potentially makes them immune to local perturbations and may allow for the construction of topologically protected qubits. Topologically protected qubits may be less susceptible to decoherence, a problem that plagues most solid-state approaches to quantum computing.

While theoretically exciting, we still don’t know much about the n=5/2 and 12/5 states including whether they do in fact support non-Abelian excitations. This is because both states are very fragile and easily disturbed by disorder. The 5/2 and 12/5 states are only seen in the highest quality samples. We have recently demonstrated GaAs 2DEGs that display exceptionally robust states at 5/2 and 12/5. Our group and our collaborators use these samples to study basic physical phenomena and test their utility as a platform for topological quantum computing.



Engineering Heterostructures for High Fidelity Spin Qubits

Nanostructures such quantum dots fabricated on modulation-doped AlGaAs/GaAs heterostructures are widely used in spin-based approaches to quantum computing. Charge noise in these devices, however, limits gate fidelity. A quiet electrostatic environment is therefore essential for further progress.

It is known that one major contribution to charge noise is the tunneling of electrons from the surface gates to the 2D electron gas via intermediate trap sites. In order to reduce this tunneling, our research is focused along two lines. First, we are examining new heterostructure designs and fabrication recipes in which we attempt to reduce both the ease of tunneling from the surface as well as the density of traps in the modulation doping layer. Second, we are also investigating undoped, accumulation-mode heterostructure designs since it is believed that Si donors in the modulation doping layer could mediate electron tunneling from the surface. Since these accumulation mode devices necessarily require the use of dielectric layers, one major thrust of the project is to compare the noise level in devices with different dielectric materials and deposition methods.

John's device

The charge noise is characterized using laterally defined quantum point contacts (QPCs) to detect charge fluctuations by operating the QPC in transition regions between neighboring conductance plateaus where the conductance is very sensitive to the local electrostatic environment. By monitoring current fluctuations in these devices, we can then quantify the noise level and make meaningful comparisons between different heterostructure designs and device fabrication recipes.


Ultra-High Mobility 2DEGs and 2DHSs in GaAs Grown by Molecular Beam Epitaxy

A major thrust in the Quantum Semiconductor Systems group is growth of extremely high quality GaAs/AlGaAs heterostructures. One metric of quality is 2D mobility, which can now exceed 30 x 106 cm2/Vs at low temperatures. At low temperature mobility is limited by imperfections in the grown sample. Imperfections include intentionally introduced charged impurities, unintentional background charged impurities and structural defects.

Our efforts are focused in 3 areas: improved MBE vacuum conditions, source material purity, and heterostructure design.

Vacuum Quality: The Purdue MBE chamber has a base pressure ~ 1 x 10-12 Torr. Samples are loaded through 2 chambers and undergo two separate heat treatments before moving into the growth chamber. Our team works with vendors to design components that minimize power dissipation in an effort to keep the vacuum pristine.

Source Material Purity: Evidence strongly indicates that source material, particularly gallium, is a primary source of residual background impurities. We have undertaken a program to further refine gallium used in our system beyond what is commercially available.

Heterostructure Design: We explore novel heterostructure designs and intentionally introduce specific types of disorder in order to understand the relationships between heterostructure design, disorder and the strength of the fractional quantum Hall effect.



Novel Devices with Non-Polar m-plane GaN/AlGaN and Lattice-Matched AlInN/GaN heterostructures

Our work in the III-Nitride material system is focused on exploiting its unique physical properties to produce novel light sources based on intersubband transitions. Due to the large conduction band offsets available in Al(In)GaN/GaN heterostructures, intersubband transitions can span the technologically important near-IR (~1.5microns) to far-IR (~100microns) spectral range. Demonstration of a quantum cascade laser in this wavelength regime is a primary goal. Material quality still limits device performance. The large lattice mismatch between GaN, AlN, and InN requires careful strain management and mitigation of deleterious effects induced by structural defects. In addition the low crystal symmetry of the wurtzite phase supports large spontaneous and piezoelectric polarization fields which can create unwanted electric fields in optical structures. To combat these issues our group explores novel growth schemes including the use of lattice-matched AlInN/GaN heterostructures to reduce strain and growth on non-polar m-plane substrates in which polarization fields are eliminated. Our group studies on the structural, morphological and electrical properties of the material we grow by molecular beam epitaxy. We collaborate with the Malis group to study intersubband optical properties.



Quantum Computing with Majorana Fermions in Hybrid Semiconductor/Superconductor Systems

Recently we have launched Station Q Purdue - signifying our strong collaboration with Microsoft Research Station Q.  We are constructing a new deposition system that combines MBE growth of high spin-orbit coupling III-V semiconductors with superconducting metals to engineer hybrid materials capable of hosting Majorana fermions.  We actively collaborate with the experimental groups in Copenhagen, Delft, and Sydney in this effort. We anticipate that our new system will be operational in the summer of 2016.