Scientific Interests

We focus on the electronic phenomena in low-dimensional matter, topologically non-trivial systems, and quantum coherent superconducting degrees of freedom. This lab cross-pollinates ideas from these and other areas to develop new scientific insights and technologies.

Novel superconducting qubits

A semiconductor nanowire (white) with left and right regions partially coated in a superconductor (blue), forming a Josephson weak link. A spinful quasiparticle (ball + arrow) is trapped in the middle region and determines the supercurrent flowing through the weak link (connected arrows traversing the device). Magnetic flux through a loop (not shown) determines the phase drop φ across the weak link.

Microwave technology made possible by superconductors has led to a prospective quantum computing architecture based on Josephson tunnel junctions. A key aspect of this architecture, known as circuit quantum electrodynamics (cQED), is the flexibility in the design of electrodynamical modes and their interconnects. Here’s some reading on that (in no particular order): link 1 || link 2 || link 3 || link 4

The flexibility of cQED makes it useful for connecting to new degrees of freedom. In other words, cQED can help us discover and develop next-generation quantum devices. We are interested in pursuing this thread. So far, we (as part of the team at Qulab) have used this ability to demonstrate the Andreev spin qubit. For pedagogical introductions to Andreev bound states, see this thesis (focusing mostly on short junctions) and this thesis (big PDF, focusing more on longer junctions with spin-orbit effects).

The Andreev spin qubit encodes quantum information in the spin of a quasiparticle trapped in a superconducting device. As shown schematically in the image below, this quasiparticle has a spin-dependent supercurrent, due to spin-orbit coupling. This means we can meaningfully separate the spin states’ energies with tiny magnetic fields (much smaller than Earth’s magnetic field), which is convenient. Crucially, it can have a spin-dependent coupling to microwave-frequency modes, which we used for the read-out of information from the qubit. The fact that the connection is made via current rather than electric or magnetic dipoles means that Andreev states can be coupled to things like microwave resonators much more strongly than conventional spin qubits can. As an added bonus, these spin states have the potential to be highly anharmonic: the two spin states of interest are well-separated in energy from all the other states of the system.

Zooming out, Andreev qubits occupy a niche where strong couplings come in small, anharmonic packages, all of which are handy characteristics for a qubit. We will continue to explore this research area and its extensions both theoretically and in the lab.

Topologically nontrivial systems

Topology is a powerful mathematical concept that has lately gained widespread use in the categorization of quantum systems and the prediction of potentially useful new phenomena. Topologically non-trivial systems exist in both natural materials and designed structures.

Topological insulators

Topological insulators are materials whose electronic quantum ground state has special distinction relative to “trivial” insulators. This results in unique, conducting electronic modes at their boundaries which can be the building block towards topological quantum computers. In the past, we (as part of the PJH group) revealed a 2D topological insulator phase in monolayer WTe2. Unique to this material is an additional low-density 2D superconducting phase that is accessible by electrostatic gates, which we also discovered. The presence of both these electronic phenomena brings forth the possibility to connect helical modes with superconductivity all within the same material, suggesting a unique path to Majorana bound states.

Artist’s conception of a monolayer of WTe2 in which there exists a junction between the helical edge states of the insulating phase (left) and the superconducting phase (right).

Josephson circuits

Topological concepts can be applied in different contexts. We showed how to bring the physics of topological semimetals to quantum electrodynamical states of Josephson junctions with our work on Weyl Josephson Circuits. We used concepts of symmetry topology, and “parametric dimensionality” (one dimension per control parameter) to show how the circuit’s microwave modes can develop topological “Weyl points” that can be manipulated on-demand. The circuit at the right can simulate Weyl semimetals with broken inversion symmetry.

We continue to explore how the physics of non-traditional materials may be realized in Josephson tunnel junctions. This will help us understand experimental phenomena in more complex mesoscopic devices as well as lead to new applications.

Heterostructuring for new correlated electronic states

Upper: large twist angles lead to small-scale superlattices and hardly perturb the band structure. Middle: medium twist angles (around 2 degrees) change the band structure appreciably. Lower: a special, small angle around 1.1 degrees flattens the bands. Superconductivity has been discovered in this and other situations.

Twisting van der Waals layers for strongly correlated electronic phases

Van der Waals heterostructures introduce a completely new twist to heterostructure design — that of the relative angle of the crystals in the lattice. A small rotation of two equivalent lattices results in a long-wavelength moiré pattern (see left panels in the image at the right).

This clean method of generating a superlattice can lead to new physics, including the formation of new band structures. In a special case that is called the “magic angle”, the bands become so flat that electron-electron interaction effects become dominant, resulting in a strongly correlated electronic system. In particular, we (as part of the PJH group) found that a new, unexpected insulating state forms at *half-filling* of these bands. In the midst of this flat band, we found novel strongly-coupled superconducting states.

New phases of electrostatically-tunable matter generated by this twisting technique continue to be discovered at a rapid pace, and our understanding of these systems continues to deepen. Read recent perspectives of leaders in this field here.

Plasmonic superconductivity in van der Waals bilayers

Superconductivity mediated by the Coulomb interaction, a concept long-discussed (since 1964!), has never been confirmed in the lab. We proposed that van der Waals bilayers of very different materials (graphene and transition metal dichalcogenides) are a uniquely capable platform for generating, even designing, Coulombic superconductivity in the lab.

Materials discovery

2D materials have been a source of exciting physics since the beginning of this century. However, with normal fabrication techniques, only graphene and the semiconducting transition metal dichalcogenides have been fruitful. This is because many other potentially interesting 2D materials are sensitive to air and degrade if exposed (left panel in the figure below). An important technique developed in the 2010s is the protection of novel 2D materials from air by sealing them while in a protected environment (middle panel), enabling us to inspect their properties without degradation. In the future, we will seek to interface different types of materials and discover new phenomena and functionalities (right panel).

vdw_hetero1

Improving scientific communication: the Virtual Science Forum

Valla is a founding member of the Virtual Science Forum, an organization begun in August 2019 with the mission to make scientific communication more accessible. We organize our own seminar series and offer beginning-to-end support for organizing an academic event of your own. It could be a seminar series, a workshop, or a tutorial. We prioritize upholding a safe, accessible, inclusive, and equitable online environment for all across the spectrum of identities and lived experiences. We encourage participation, either as speakers, organizers, or audience, of all members of the scientific community. We are also always looking to add to our team of contributors. Please contact me, join our github, or engage in our open gitter chat if you are interested to get involved!

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